Synonymous with APA RATED STURD-I-FLOOR 48 oc. 2-4-1 is a 1-1/8-inch-thick all-veneer panel with an Exposure 1 durability classification. It's designed for single-floor applications over 2x supports spaced 32 inches on center or over 4x supports 48 inches on center. 2-4-1 may also be used in Heavy Timber roof construction. Available as specified with square edge or tongue-and-groove joint.

303® Specialty Siding: 

See APA Rated Siding


A sanded plywood panel with A-grade face and back plies and D-grade inner plies. Bonded with interior or exterior glue. Commonly used for cabinets, built-ins, furniture, partitions and other interior or protected applications where a smooth surface or appearance quality on both sides is important.

A-A Exterior: 

An Exterior-type sanded plywood panel with A-grade face and back plies and C-grade inner plies. Bonded with exterior glue. Commonly used for fences, built-ins, signs, boats, cabinets, commercial refrigerators, shipping containers, tanks, tote boxes, ducts and other exterior or high moisture applications where a smooth surface or appearance quality on both sides is important.


A sanded plywood panel with A-grade face, B-grade back and D-grade inner plies. Bonded with interior or exterior glue. Commonly used as a substitute for A-A where the appearance of one side is less important.

A-B Exterior: 

An Exterior-type sanded plywood panel with A-grade face, B-grade back and C-grade inner plies. Bonded with exterior glue. Commonly used as a substitute for A-A Exterior where the appearance of one side is less important.

A-C Exterior: 

An Exterior-type sanded plywood panel with A-grade face, C-grade back and C-grade inner plies. Bonded with exterior glue. Commonly used for soffits, fences, boxcar and truck linings, farm buildings, tanks, commercial refrigerators and other high-moisture applications where the appearance or smoothness of only one side is important.

A-D: A sanded plywood panel with A-grade face, D-grade back and D-grade inner plies. Bonded with interior or exterior glue. Commonly used for paneling, built-ins, shelving, partitions and other interior or protected applications where the appearance or smoothness of only one side is important.

Abrasive Material: 

The mineral substance coating on a sanding belt that removes material from the board.

Acrylic Resin:

An ingredient of water-base (latex) paints and stains. Synthetic resin with excellent weathering characteristics. Acrylics can be colorless and transparent, or pigmented.

Acrylic Resin:

An ingredient of water-base (latex) paints and stains. Synthetic resin with excellent weathering characteristics. Acrylics can be colorless and transparent, or pigmented.


Design strategy that allows for multiple future uses in a space as needs evolve and change. Adaptable design is considered a sustainable building strategy as it reduces the need to resort to major renovations or tearing down a structure to meet future needs.


Material introduced into a panel during the manufacturing process which imparts a particular property. Additives include preservatives, water repellents, and fire retardant, but not binders.


A substance (glue) capable of bonding material together via surface attachment - such as a laminate to a panel.


See Glue


Any substance that is used to bond one surface to another surface by attachment. Adhesives include adhesive bonding primers, adhesive primers, adhesive primers for plastics, and any other primer.

Adhesives — Dry Film Bonding Adhesives

Dry film bonding adhesives are non-tacky elastometric materials coated on a release liner and sold in roll form. The film adhesive is bonded to the edge backing material with low temperature heat and low pressure. The adhesive backed material is then heat-bonded at a higher temperature and thermo-set to the application surface.

Adhesives — Epoxy

A two-component thermosetting adhesive typically used for laminating medium and heavy gauge vinyls. Epoxy adhesives are generally bonded 1:1 (resin to hardener) by volume and are roll-coated to either the backside of the vinyl web or to the board surface. Wet lamination is followed by stack curing the panels from one to three days at temperatures above 50 degrees F. Solvent containing epoxies typically have better green strength (wet tack) than 100% solid systems.

Adhesives — Heat Seal Adhesives

These are systems are dry coatings on the back of flexible laminates. They are heat sealed to panels by rollers or quick presses at low temperatures and low pressures. They are applied and dried by the laminate producer, using water or solvent based polymers. Heat seal adhesives differ from hot melts in chemistry. They do not reflow upon heating. Also, they do not change with age. They adhere to most surfaces and they form a very tough bond.

Adhesives — Hot Melt

An adhesive which is a 100% solids thermoplastic and is applied molten to form a bond upon cooling. Hot melts differ from conventional liquid adhesives because they set by cooling, rather than by absorption or evaporation. In practice, papers are pre-coated with hot melt by the manufacturer; the hot melt is later reactivated by heat when the paper is laminated to substrate on the laminating line.

Adhesives — Polyurethane Dispersions

Polyurethane Dispersions (PUD’s) are aqueous dispersions of fully reacted urethane polymers containing hydrophilic anionic, cationic or nonionic groups noted for their high performance properties, excellent adhesion, chemical resistance, outstanding toughness and low pressure flexibility.
PUD’s are used for the membrane pressing of vinyl films and veneer to an MDF core. PUD’s can be used in conjunction with a hardener for higher heat resistance, if required.
PUD’s are usually applied by spraying on the MDF, air dried and then mated with the vinyl film or veneer in a membrane press. Typical membrane press temperatures range from 158 to 194 degrees F with cycle times of 20 to 120 seconds.

Adhesives — Pressure Sensitive Adhesives

Pressure sensitive adhesives are viscoelastic materials coated on a release liner and sold in roll or sheet form. The adhesive is sticky to the touch and can be applied to most surfaces with light roll pressure. Pressure sensitive adhesives bond through intermolecular forces of attraction between like or unlike surfaces, which resist separation.
The highest performing, longest aging adhesives are usually cross-linked high molecular weight acrylic materials. Pressure sensitive adhesive backed veneers are easy to apply and require no liquid adhesives.

Adhesives — Solvent Borne

An adhesive containing polymetric materials dissolved in volatile organic solvents. A small percentage of the cross-linker is added to obtain certain desired performance properties (i.e., higher heat resistance). This type of adhesive is typically used on a “hot line” laminator where it is applied in 2 coats to the board surface, dried and then heat activated prior to hot rolling the laminate to the substrate. Solvent borne adhesives offer good coatability, high heat resistance and excellent bond strength when laminating 2 mil and 4 mil solid vinyl films. When applied to composite substrates, solvent borne adhesives create little to no grain raise.

Adhesives — Water Borne

Water borne adhesives include both thermosetting urea formaldehyde systems, as well as formulated synthetic latex (usually vinyl latex) types. These adhesives are generally used for paper laminating, where the adhesive is applied to the web and/or board surface and tacks up through one or more heated rolls that combine the paper to the board. Water based vinyl acetate/ethylene (VAE) copolymer adhesives are often used for laminating 2 mil and 4 mil vinyl films to various board surfaces.

Aerosol Adhesive

An adhesive packaged as an aerosol product in which the spray mechanism is permanently housed in a non-refillable can designed for hand-held application without the need for ancillary hoses or spray equipment. Aerosol adhesives include special purpose spray adhesives, mist spray adhesives and web spray adhesives.


A performance specification developed by APA for glues recommended for use in the APA Glued Floor System. AFG-01 requires that glues applied at the job site be sunlight resistant, strong under many moisture and temperature conditions, and able to fill gaps.

Aggregate-Coated Panel

A panel with stone chips embedded in a resin coating.

Agrifiber Board

A composite panel product derived from recovered agricultural waste fiber from sources including, but not limited to, cereal straw, sugarcane bagasse, sunflower husk, walnut shells, coconut husks, and agricultural prunings. The raw fibers are processed and mixed with resins to produce panel products with characteristics similar to those derived from wood fiber. The products must comply with the following requirements:
  • The product is inside of the buildings waterproofing system.
  • Composite components used in assemblies are to be included (e.g., door cores, panel substrates, etc.)
  • The product is part of the base building systems.


See Seasoning

Air-dried Lumber

Lumber that has reached its equilibrium moisture content by being exposed to air.

Anchor Bolt

Bolts that tie the sill plate and thus the frame of a structure to its foundation.

ANSI A208.2-1994

The American National Standards Institute standard which sets forth requirements and test methods for dimensional tolerances, physical and mechanical properties and formaldehyde emissions for medium density fiberboard (MDF). Methods of identifying products conforming to the standard are specified. Property requirements are described in metric and imperial units. A summary of this standard is available in the Composite Panel Association’s “Buyers & Specifiers Guide” which may be viewed on-line at ( The complete standard is available from the Composite Panel Association, or from ANSI at (

APA - The Engineered Wood Association

The trade organization representing the majority of the North American wood panel manufacturers, as well as I-Joist, glulam and structural composite lumber manufacturers. The Association has three main functions: 1) research to improve wood structural panel and other engineered wood products and construction systems, 2) quality inspection and testing to assure the manufacture of high quality products that meet the appropriate product standard, and 3) education and promotion of engineered wood products and building systems.

APA Glued Floor System

A floor system developed by APA in which a single layer of APA RATED STURD-I-FLOOR panels (or subflooring in the case of double-layer construction) is glue-nailed to wood joists. The bond is so strong that floor and joists behave like an integral unit, greatly increasing floor stiffness and greatly reducing floor squeaks and nail popping. Only construction adhesives conforming to APA specification AFG-01 are recommended for use with the system.

APA Performance Rated Panels®

Panel products developed by APA, such as APA RATED SHEATHING, APA RATED STURD-I-FLOOR and APA RATED SIDING, designed and manufactured to meet performance criteria for specific end-use applications. APA Performance Rated Panels can be manufactured as conventional veneered plywood, as composites (veneer faces bonded to reconstituted wood cores), or as mat-formed panels (including waferboard and oriented strand board.) The trademarks on APA Performance Rated Panels include a Span Rating denoting the maximum recommended spacing of supports over which the panel should be placed for the designated end use, and the exposure durability classification of the panel.

APA Rated Sturd-I-Floor®

An APA Performance Rated Panel designed and manufactured specifically for residential and other light frame single-floor (combined subfloor-underlayment) applications for use under carpet. APA RATED STURD-I-FLOOR can be manufactured with Span Ratings of 16, 20, 24, 32 and 48 oc, in thicknesses ranging from 19/32 to 1-1/8 inch, and in three exposure durability classifications - Exterior, Exposure 1 and Exposure 2. Panels are available with either square edges or tongue-and-groove edges as specified. APA RATED STURD-I-FLOOR 48 oc plywood, commonly called 2-4-1, is also used in heavy timber roof construction.

APA Rated® Sheathing

An APA Performance Rated Panel designed and manufactured specifically for residential and other light frame wall sheathing, roof sheathing and subflooring applications. APA RATED SHEATHING can be manufactured with Span Ratings of 12/0, 16/0, 20/0, 24/0, 24/16, 32/16, 40/20 and 48/24, in thicknesses ranging from 5/16 to 3/4 inches, and in three exposure durability classifications-Exterior, Exposure 1 and Exposure 2.
A grade designation covering APA proprietary siding products. Commonly used, in addition to siding, for fencing, soffits, wind screens and other exterior applications. Can be used for interior paneling. Can be manufactured as conventional veneered plywood, as a composite or as oriented strand board siding. Both panel and lap siding are available. Special surface treatment such as V-groove, channel groove, deep groove (such as APA Texture 1-11), brushed, rough sawn and texture-embossed (MDO). Span Rating (stud spacing for siding qualified for APA Sturd-I-Wall applications) and face grade classification (for veneer-faced siding) indicated in trademark.

APA Sturd-I-Wall®

A construction system in which APA Rated Siding panels or lap are attached directly to studs (single wall) or over nonstructural wall sheathing, such as fiberboard, gypsumboard or rigid foam insulation. APA Siding bearing a Span Rating of 24 oc in the trademark can be applied vertically direct to studs spaced 24 inches on center. Siding with a Span Rating of 16 oc can be used vertically direct to studs 16 inches on center. Panels with either Span Rating can be applied direct to studs 24 inches on center with face grain horizontal provided horizontal joints are blocked.

APA Trademark

APA is an approved quality supervision and testing agency for softwood plywood and structural wood panels. Typical trademarks of APA member-manufactured products are shown throughout these courses. Some engineered wood products bear the APA EWS trademark. Engineered Wood Systems is a related corporation of APA.

Appearance Grade

Defines the surface finish of a glulam beam. Architectural and Industrial are the most common appearance grades. Premium grade beams are available as custom orders. The structural quality of glulam beams has no relation to the appearance grade specified.

Assembly Recycled Content

Includes the percentages of post-consumer and pre-consumer content. The determination is made by dividing the weight of the recycled content by the overall weight of the assembly.


Test methods published by the American Society for Testing Materials ( which may be used by MDF mills for quality control purposes.


A sanded plywood panel with B-grade face and back and D-grade inner plies. Bonded with interior or exterior glue. Utility panel for interior or protected applications.

B-B Exterior

An Exterior-type sanded plywood panel with B-grade face and back and C-grade inner plies. Bonded with exterior glue. Utility panel with solid paintable surface both sides.

B-B Plyform®

Concrete form grades with high re-use factor. Sanded both sides and mill-oiled unless otherwise specified. Special restrictions on species. Also available in HDO for very smooth concrete finish, in STRUCTURAL I (all plies limited to Group I species), and with special overlays. EXPOSURE DURABILITY CLASSIFICATION: Exterior.

B-C Exterior

An Exterior-type plywood panel with sanded B-grade face, C-grade back and C-grade inner plies. Bonded with exterior glue. Utility panel for farm service and work buildings, boxcar and truck linings, containers, tanks, agricultural equipment, as a base for exterior coatings, etc.


A plywood panel with sanded B-grade face, D-grade back and D-grade inner plies. Bonded with interior or exterior glue. Utility panel for backing, sides of built-ins, industry shelving, slip sheets, separator boards, bins, etc.


Application of a coat of primer to the back of a panel. Cabinet doors should be back-primed to prevent warping.


A non-decorative laminate used on the back of composite panels to protect them from changes in humidity. Backers are often used to balance laminated panel construction with the objective of preventing warping or cupping.


The approved agency mark on the back of a panel. All unsanded and touch-sanded panels, and panels with A or B faces on one side only, carry the APA trademark on the panel back. - See also APA Trademark and Edgemark

Balanced Construction

A composite panel which is laminated in a fashion (usually two-sided) that resists warping when subjected to uniformly distributed changes in humidity.

Basis of Design (BOD)

Includes design information necessary to accomplish the owner’s project requirements, including system descriptions, indoor environmental quality criteria, other pertinent design assumptions (such as weather data), and references to applicable codes, standards, regulations and guidelines.


A thin, narrow strip of plywood or lumber used to conceal or protect a joint between adjoining pieces of lumber or plywood.


Normally a horizontal or sloping member of glulam that is designed to carry vertical loads. Simple Span: a member that is supported at both ends. Continuous: a single member which is supported at more than two bearing locations. Cantilever: a member which has one or both supports away from the ends; one of which overhangs its support.

Bearing Stress

The compressive stress exerted on an external surface of a member. This stress is commonly the stress occurring at a point of support, such as at a beam hanger.

Bending Strength

The ability of a member, such as a beam, to resist the tendency to break when exposed to external forces such as roof or floor loads. The strength is achieved by the resisting couple action of the tension and compression stresses at the top and bottom of the beam.


To cut panel edges or ends at an angle to make smooth mating joints between panels.


The substance (generally resin) that bonds the fiber together in composite panels.

Bio-based Products

Products that use biological, agricultural or renewable materials such as soy, wheat, bamboo, biodiesel, etc.


Something when left alone, breaks down and can be absorbed into the eco-system.


The application of binder and additives to fiber in the manufacturing of composite panels.


Light lumber strips nailed between major framing members to support edges of structural panels where they meet.


A localized delamination caused by steam pressure build-up during a hot pressing process. This can result from excessive moisture, adhesive (glue) spread, and/or press temperatures.


A localized delamination caused by steam pressure buildup during the hot pressing operation. The steam may result from high moisture content of the veneer, excessive glue spread, or high press temperatures.

Board Foot

A unit of lumber measure that is 1” thick by 12” wide by 12” long. A board 1” by 6” by 2’ equals 1 board foot. A board 2” by 12” by 12” equals 2 board feet.

Board Foot

The basic unit of measurement for lumber. One board foot is equal to a 1-inch board, 12 inches in width and 1 foot in length. Thus, a 10-foot long, 12-inch wide and 1-inch thick piece would contain 10 board feet. When calculating board feet, nominal sizes are assumed.


Patch. - See Repairs


To glue together, as veneers are “bonded” to form a sheet of plywood. Pressure is applied to keep mating parts in proper alignment. Most glues used in panel manufacture require both heat and pressure to cure properly.

Bond Classification

Exposure ratings for APA structural wood panels designated in APA trademarks as Exterior, Exposure 1, Exposure 2, or Interior.
  • Exterior panels have a fully waterproof bond and are designed for applications subject to permanent exposure to the weather or to moisture.
  • Exposure 1 panels have a fully waterproof bond and are designed for applications where long construction delays may be expected prior to providing protection, or where high moisture conditions may be encountered in service. Exposure 1 panels are made with the same exterior adhesives used in Exterior panels. However, because other compositional factors may affect bond performance, only Exterior panels should be used for permanent exposure to the weather.
  • Exposure 2 panels (identified as Interior type with intermediate glue under PS 1) are intended for protected construction applications where only moderate delays in providing protection from moisture may be expected.
  • Interior panels or panels which lack further glueline information in their trademarks are manufactured with interior glue and are intended for interior applications only.

Borer Holes

Voids made by wood-boring insects, such as grubs or worms.


A log live sawn and kept together in the order of sawing.


The deviation from flatness along the length of the panel.


Distortion of a structural wood panel so that it is not flat lengthwise. - See also Cup


A form of warp that is an end-to-end curve along the length of the board.

Box Beam

A beam built of lumber and plywood in the form of a long hollow box which will support more load across an opening than will its individual members alone. Lumber members form the top and bottom (flanges) of the beam, while the sides (webs) are plywood.


Short wood or metal braces or struts placed crosswise between joists to help keep them in alignment. Bridging may be solid or crossed struts. Most building codes no longer require bridging of floor joists.

Broken Grain

A (leafing, shelling, grain separation) separation on veneer surface between annual rings.


An APA 303 Siding surface treatment. Brushed or relief-grain surfaces accent the natural grain pattern to create striking textures. Difficult to paint or stain. - See APA Rated Siding

Building Code

A set of regulations governing construction in a particular political subdivision, such as a city or county. The building code spells out certain requirements pertaining to such criteria as lumber strength, values, grades, and spans.


The process of rounding an edge of a board used as shelving, stadium seating, stepping, etc.


A unit or stack of wood panels held together for shipment with bands. Stack size varies throughout the industry, with the average stack running about 30 to 33 inches high. A bundle 30 inches high, for example, contains 120 sheets of 1/4-inch panels, 80 sheets of 3/8-inch panels, or 60 sheets of 1/2-inch panels.


A wartlike growth that forms on a tree and that, when sliced, produces extremely disoriented grain patterns that are quite attractive.

Butt Joint

A straight joint in which the interface is perpendicular to the panel face. An end butt joint is perpendicular to the grain.

Butt Joint

The joint formed when two parts are fastened together without overlapping. For end-to-end joints, use a nailing strip. For corner joints, nail directly into panel if it is at least 3/4-inch thick. If panel is thinner than 3/4-inch, use a reinforcing block.

C-C Plugged Exterior

An Exterior-type touch-sanded plywood panel with C-Plugged-grade face, C-grade back and inner plies. Bonded with exterior glue. Commonly used for severe moisture conditions, exterior balconies and decks, refrigerated or controlled atmosphere rooms, and boxcar and truck floors.

C-D Plugged

A touch-sanded plywood panel with C-Plugged-grade face, D-grade back and inner plies. Bonded with interior or exterior glue. Used for built-ins, cable reels and walkways.


The measurement of board thickness. Also refers to the tool used to measure thickness or diameter.


The curvature built into a beam (in a direction opposite to the extended deflection) to prevent it from appearing to sag under a loaded condition.


A log that has been debarked and sawn square.


That portion of a structural beam which extends or “cantilevers” beyond the end support, and whose end is not supported.

Carbon Footprint

A measure of the amount of carbon dioxide emitted through the combustion of fossil fuels. A carbon footprint is often expressed as tons of carbon dioxide or tons of carbon emitted, usually on an annual.

Carbon Neutral

A scenario where the net discharge of carbon dioxide into the atmosphere is zero. Carbon neutrality can be achieved by planting enough trees so that CO2 emissions as a result of combustion would be offset by CO2 absorption by the plants. In the presence of water and light, trees convert CO2 into sugar and oxygen thru the process of photosynthesis. The average tree absorbs 10 kg (22 lbs) of CO2 per year. Carbon neutral is also referred to as “net zero carbon”.


A drying defect where the surface of wood dries faster than the wetter inner core; this causes permanent set and stresses that release when the board is cut.


Waterproof sealant used to fill joints or seams. Caulks are available as putties, ropes, or compounds extruded from cartridges.

Center (Centers)

Inner ply or plies of a plywood panel whose grain runs parallel with that of the face and back plies.

Center Gap

(See crossband gap)

Center Gap

See Core Gap


See On-Center and Clear Span

Center-to-Center Spacing

The spacing between structural members determined by measuring from the center of one to the center of the next, e.g. “16-inches o.c.” Chamfer - A bevel or slope created by slicing off the square edge or end of a piece of wood or other material.


Inner layers whose grain direction runs parallel to that of the outer plies. May be of parallel laminated plies.

Certified Wood

Wood originating from forests with audited and certified sustainable forestry practices such as protecting trees for future needs as well as wildlife habitat, streams and soil.


The process or actual document tracking a wood product from the forest through processing and manufacture to a vendor or consumer, verifying that the wood is from a certified forest.


The flat surface created by slicing off the square edge or corner of a piece of wood or panel.

Channel Groove

An APA 303 Siding texture consisting of shallow grooves cut into panel faces during manufacture. - See APA Rated Siding


A visible “wavy” condition across the surface width of a panel created when the panel is sanded. Chatter marks are parallel to one another, usually between 1/4” and 1/2” apart, and perpendicular to the sanding direction.


A lengthwise separation of wood fibers, usually extending across the rings of annual growth, caused chiefly by strains produced in seasoning.


Wood exposed to alternating moist and dry conditions eventually develops open cracks or “checks.” Reduce checking by sealing panel edges before installation to minimize moisture absorption, and by using a priming coat or resin sealer on the surfaces.

Chip Load

The amount of material removed by each cutting tooth of a saw blade, router, or shaper, as it moves through the material being cut.


Defect created during cutting/machining when material is torn-away from the top or bottom panel edges.


Any of the outside members of a truss connected and braced by web members. Also, may refer to perimeter members of a panel diaphragm.


The top or bottom member of a truss, to which the web members are attached.

Circular Plug

See Repairs

Class I, II

Term used to identify different species group combinations of B-B concrete form panels.

Class I, II

See B-B Plyform

Clear Span

Distance between inside faces of supports.

Climb Cutting

A machining technique in which the cutting tool rotates in the same direction as the material being cut is traveling.

Coating Equipment

The purpose of coating equipment is to provide a thin layer of adhesive that will bond an overlay to a substrate. Adhesive is applied to the board, to the web or to both. There are a variety of coating systems.
When using a water based adhesive, part of the moisture is flashed off before laminating takes place. This aids in minimizing or eliminating fiber-pop or telegraphing after lamination.


See Model Code


Normally a vertical member that is designed to carry loads from a beam: Concentrically Loaded: when the resultant load acts parallel to the axis of the member and is applied at its centerline. Eccentrically Loaded: when the resultant load acts parallel to the axis of the member but is applied away from its centerline such as along its side.


APA proprietary trade name for APA member-produced composite panels. - See APA Performance Rated Panels and Composite Panel

Combination Number

The identification used to describe the type of lamination layup in the glulam member, the associated allowable design stresses, its intended application, and if the lumber used was visually or mechanically graded.


A process that occurs prior to building occupancy during which the performance of the building systems are checked and adjusted if necessary, in order to ensure that they are operating as intended by the design and that the owner’s operational needs are met. Cradle to Cradle—Refers to the closed loop cycle of products that have a perpetual life through their ability to be completely recycled or decomposed without environmental degradation at the end of their initial use life.


(mixed 1 and 2 common) are boards that have too many defects to be FAS or Select.


For plywood applications, a glued and/or nailed structural assembly of plywood and lumber, such as a box beam or stressed-skin panel. Also describes prefabricated building sections in panelized construction.

Composite Panel

A veneer-faced panel with a reconstituted wood core. - See APA Performance Rated Panels and COM-PLY

Composite Panel Association (CPA)

An association of North American producers of MDF, Particleboard, and Agrifiber panels located at 18928 Premiere Court, Gaithersburg, Maryland 20875. (

Composite Wood

A product consisting or wood or plant particles or fibers bonded together by a synthetic resin or binder (i.e., plywood, particle-board, OSB, MDF, composite door cores). For the purposes of LEED requirements, products must comply with the following conditions:
  • The product is inside of the building’s waterproofing system.
  • Composite wood components used in assemblies are included (e.g., door cores, panel substrates, plywood sections of I-beams).
  • The product is part of the base building systems.

Compression Parallel To Grain

A measurement of the internal compressive stress induced in a wood member when a load is applied to the end of the piece. This would normally be thought of as the stress that occurs at a column.

Compression Perpendicular To Grain

A measurement of the internal compressive stress induced in a wood member when a load is applied to the edge or face of the piece. This is also referred to as bearing stress for a joist, beam or similar piece of wood as it bears or supports a load. The load tends to compress the fibers, and it is thus necessary that the bearing area be sufficient to prevent crushing.

Concentrated Load

See Loads

Concrete Form

Mold into which concrete is poured to set. Plywood provides tough, durable, easy-to-handle, split-resistant and lightweight concrete forms. It can be bent for curved forms and liners, and its natural insulating properties help moderate temperature variations for more consistent curing. Almost any APA trademarked Exterior-type plywood can be used in concrete formwork applications, but PLYFORM is specifically manufactured for that purpose. - See also B-B Plyform


A type of tree that’s characterized by needle-like or scale-like foliage, usually evergreen.

Construction (panel construction)

Term referring to detailed manner in which veneers are assembled and/or thickness of veneer used, e.g., “4-ply construction,” “3-layer construction,” “1/10 inch face and back,” etc.

Continuous Laminates

The typical construction of a continuous laminate consists of: a melamine-impregnated alpha-cellulose overlay on top of a decorative surface paper, superimposed over one or more phenolic or melamine resin impregnated papers. The laminate is formed on a continuous, double belt press at pressures of 125-570 psi and temperatures of 275 to 300 degrees F. The thickness is determined by the number of layers of Kraft papers and the resulting amount of resin that is absorbed, is normally 1/32” thick. The surface finish is achieved by using and transferring the texture from a steel caul plate or release paper.


Sometimes referred to as “crossband.”

Core (Cores)

In conventional plywood, inner plies whose grain runs perpendicular to that of the outer plies. In composite panels, a layer of reconstituted wood.

Core Gap (Center Gap)

An open veneer joint extending through, or partially through, a plywood panel. Product Standard PS 1 specifies that the average of all gaps shall not exceed 1/2 inch, and that every effort be made to produce closely butted core joints.

Core Separation

Delamination of the core, often at the center line, caused by blows or insufficient internal bond.

Cradle to Grave

Refers to the life cycle of a product — the materials, energy and environmental impact involved — from its beginnings (extraction) through the end of its usefulness (disposal). Sometimes applied specifically to hazardous waste.

Crawl Space

A space often about two feet high beneath a house floor allowing access to plumbing or wiring. - See also PIRF


Any vertical framing member cut less than full length, as in cripple studs under a window opening.


A form of warp that is an end-to-end curve along the edge of the board.

Cross Cutting

Sawing wood across the grain. Because the wood in structural wood panels is either cross-laminated or randomly oriented, any cut made in a structural wood panel is a cross cut. Always use a cross cut saw when hand- or power-sawing structural wood panels.


Inner layers whose grain direction runs perpendicular to that of the outer plies. May be of parallel laminated plies. Sometimes referred to as core.

Crossband (Cores)

In plywood, the veneer layers with grain direction perpendicular to that of the face plies.

Crossband Gap and Center Gap

An open joint extending through or partially through a panel, which results when crossband or center veneers are not tightly butted.


The highly figured wood that occurs where a limb joins a trunk; the grain swirls dramatically where the wood fibers have crowded and twisted together.


A phenomenon in which the center of the panel is thicker along its length than at the two long edges.


Deviation from a straight line across the width of the panel.


Crosswise distortion of a structural wood panel from its flat plane. - See also Bow (Distortion across panel).


A form of warp that is an edge-to-edge curve across the face of the board.

Curved Panel

Stressed-skin or sandwich panels curved to various degrees of arc. Used in roof construction.

Dado Joint

Joint formed by intersection of two boards, one of which is notched with a rectangular groove.

Dead Load (D.L.)

See Loads


A type of tree where the leaves fall off every autumn; typically a hardwood, but not always. Some hardwoods in tropical regions keep their leaves all year long.

Decorative Foils

Cellulose papers weighing between 40 and 140 grams per square meter in an untreated state. Impregnation with melamine thermoplastic resins can add 20 to 40 grams per meter - depending on the basis weight of the paper. Foils require an adhesive for lamination.

Decorative Foils

Cellulose papers weighing 40 to 140 grams per square meter, untreated. The papers may be impregnated with melamine thermosetting resins or left untreated. Treating may add 20 to 40 grams of weight. Decorative foils require an adhesive for lamination.
These papers are referred to as “finished foils” in Europe. In the U S they are referred to as melamine papers, intermediate weight foils or impregnated foils. Impregnation has a direct effect on the internal bond strength of the paper, as well as the porosity and machinability of the laminated material.

Decorative Foils — Prerimpregnated

These papers are treated during the paper making process. They are usually treated with melamine and/or Urea formaldehyde. Additionally, they are usually treated with acrylic, which allows the sheet to remain flexible after the resins are fully cured. The resins are calendered. These papers can be chemically embossed and they may be available with pre-applied hot melt adhesives.

Decorative Foils — Unimpregnated

Usually untreated papers; although a small amount of resin may be added. These papers are usually top coated with a varnish.

Decorative Panel

A plywood panel grade with rough-sawn, brushed, grooved or striated faces. May be Interior or Exterior type. Common uses include paneling, built-ins, accent walls, counter facings and displays. Exterior type uses include siding, gable ends and fences. Check with manufacturer for specific Exterior application recommendations, which vary with particular products.

Defects, Open

Irregularities such as splits, open joints, knotholes, or loose knots, that interrupt the smooth continuity of the veneer.


The measurement of board/panel “sag” between supports when a load is applied - such as with shelving.


Bending of a structural wood panel or framing member between supports under an applied load. For glulam beams, deflection is the vertical displacement that occurs when a beam is loaded, generally measured at positions between supports or at the end of a cantilever.

Deflection Limit

The maximum amount the beam is permitted to deflect under load. Different deflection limits are normally established for live load and total load.


A visible separation between plies that would normally receive glue at their interface and be firmly contacted in the pressing operation. Wood characteristics, such as checking, leafing, splitting, and broken grain, are not to be construed as delamination. See corresponding definition for those terms. For purpose of reinspection, areas coinciding with open knotholes, pitch pockets, splits, and gaps and other voids or characteristics permitted in the panel grade are not considered in evaluating ply separation of Interior type panels bonded with interior or intermediate glue. In evaluating Interior panels bonded with exterior glue (Exposure 1), delamination in any glueline shall not exceed three square inches except where directly attributable to defects permitted in the grade as follows: Delamination associated with: Knots and knotholes - shall not exceed the size of the defect plus a surrounding band not wider than 3/4 inch. In evaluating Exterior type panels for ply separation, the area coinciding with the grade characteristics noted in paragraph (a) are considered, and a panel is considered delaminated if visible ply separation at a single glueline in such an area exceeds three square inches. All other forms of permissible defects - shall not exceed the size of the defect.


Separation between plies or within reconstituted wood due to adhesive bond failure. Separation in area immediately over or around a permitted defect does not constitute delamination.


The weight of a panel as measured in pounds per cubic foot, or in kilograms per cubic meter.

Density Profile

Gradient density of a panel from face to face.


A concave area in the surface of the panel.

Design Value

A measurement of strength in lumber, involving the basic properties of wood. These are: fiber stress in bending (Fb), tension parallel to grain (Ft), horizontal shear (Fv), compression perpendicular to grain (FcI), and modulus of elasticity (E).

Design Values

Allowable stress values as they are established for each glulam beam, described in terms of Bending (Fb), Horizontal Shear (Fv), Modulus of Elasticity (E) and other stresses.


Elements of a building that provide shear strength to withstand wind and earthquake loads.

Dimension Lumber

Lumber that is from two inches up to, but not including, five inches thick, and that is two or more inches in width. Dimension also is classified as framing, joists, planks, rafters, etc.

Direction of Grain

Usually refers to the linear direction of a wood grain pattern, or the direction in which a composite panel has moved through a sander.

Dogbone Plug


Double Wall

A light frame wall construction system consisting of exterior finish siding, such as APA RATED SIDING, applied over structural wall sheathing-typically APA RATED SHEATHING. - See APA STURD-I-WALL


Seasoned, usually to a moisture content of less than 19%.

Dubbing of the End (End Snipe)

Tapering of the edge of a panel as it enters and/or exits a sander.

Ductile Materials

Materials that can be subjected to large strains before rupturing. Ductile materials are capable of absorbing shock or energy.

Dulling Effect

A low-gloss area on a wet-coated panel usually caused by an incompatible finishing material, a soft panel surface, or excessive pre-heating of the panel prior to coating.


The edge of a roof that extends beyond or overhangs a wall. The underside of an eave may form an open soffit. Textured panels, applied face down to eave rafters as roof sheathing, gives open soffits a decorative finished surface.


Little or no impact on the native eco-system.

Ecological Footprint

The area of land and water needed to produce the resources to entirely sustain a human population and absorb its waste products with prevailing technology. The concept of an ecological footprint is used as a resource management and community-planning tool.

Edge Banding

A laminate material which provides a protective decorative surface for panel edges.

Edge Sealing

Application of a coating (e.g., sealant, paint) to the edges of a structural wood panel to reduce its water absorption. Edge seal before painting the panel surface if panel edges will be exposed to repeated wetting and drying.

Edge Snipe (Rollover)

A narrow tapered condition along the edge of a panel that may be caused when the sander belt width exceeds the panel’s width. Snipe is difficult to spot visually, but can be measured with a caliper.

Edge Spacing

See Panel Spacing

Edge Splits

Wedge-shaped openings in the inner plies caused by splitting of the veneer before pressing.

Edge Support

Support, such as panel clips or lumber blocking, installed between framing members at structural wood panel edges to transfer loads from one panel to the other across the joint. Panels with tongue-and-groove edges can be used in many applications without additional edge support.

Edge Treatment

Edge finishing method, such as banding with wood or plastic, or filling with putty or spackle.

Edge Void

A panel defect in which the edge or end of an inner ply has split or broken away during manufacture, leaving a gap in the edge of the plywood panel.

Edgebanding Materials — Laminated Vinyl

This edgebanding utilizes vinyl laminated to vinyl, ABS or a paper backer. The carrier is generally rigid clear or colored PVC that is .010” to .030” thick. The surface is usually a reverse printed or solid color vinyl that is .002” to .008” thick. This edgbanding is rigid enough for automatic edgebanders and is suitable for straight-line or softform applications.

Edgebanding Materials — Melamine Edgebanding

The term “melamine edgebanding” covers a broad range of paper edgebanding materials, including: single layer printed products, laminated foils and continuous melamine laminates. It is produced in master logs and can be slit to virtually any width. Usually found in Europe, melamine edgebanding is an economical pre-glued and automatic product. It is suitable for straight-line, contour and softform applications.

Edgebanding Materials — Metallic Edgebanding

This category includes textured metals (brushed, polished, matte, etc.) or metallic transfer foils laminated to a backer in a master log. The material can be slit to virtually and width. This decorative edgebanding may be used as an inlay.

Edgebanding Materials — Polyester Laminate Edgebanding

Decorative papers, often matching HPL designs and colors, are impregnated with polyester resins and laminated to a variety of backers. This edgebanding is produced in logs and can be slit to any width. A range of finishes are available. Polyester edgebanding is produced in light weight and heavy weight versions. The heavy weight version is excellent for straight-line, contour and softform automatic edgebanding applications.

Edgebanding Materials — PVC

An extruded or calendered thermoplastic edgebanding made of polyvinyl chloride; used to match vinyl, paper, paint or HPL. Calendered PVC is manufactured in wide logs and slit to size. Extruded PVC is manufactured to exact width. PVC is available in numerous colors, patterns and woodgrains. A wide range of widths, thicknesses and surface textures are available. PVC is usually top coated with a UV cured resin for protection. PVC is used in straight-line, as well as contour automatic edgebanding applications. PVC is not recommended for softform applications.

Edgebanding Materials — Real Wood Veneers

These real wood veneers include a variety of domestic and imported hardwood (and some softwood) species that are available in different cuts (flat cut, rift cut, quarter cut, etc.). These veneers are sliced from 1/25” to 1/50” thin. They are available with plain, paper or fleece backs; with varying degrees of flexibility. The veneers may be finger or butt jointed, to produce continuous edgebanding. Veneer edgbanding is suitable for straight-line, contour and softform automatic applications.

Edgebanding Materials — Reconstituted Wood Strips

Man-made veneers manufactured in Europe or Asia. Light colored woods (Usually Ayous, Obeece, Koto or Italian poplar) are peeled, clipped, dyed and then re-glued into a rectangular block that is molded for the different cuts (flat, quartered, etc). The block is then re-sliced into leaves and slit to varying widths for edgebanding. These veneers can be produced into paper back or fleece back strips or coils for straight line, contour or softform automatic edgebanding applications.

Edgebanding Materials — Substrates

See composites glossary.


APA trademark stamped on the panel edge. Appears on sanded grades with B-grade or better veneer faces, PLYRON, MARINE, and panels with overlaid surfaces on both sides.

Embodied Energy

Total energy used to create a product, including the energy used in mining or harvesting, processing, fabricating, and transporting the product.


A panel surface treatment. Heat and pressure against a master pattern impress a variety of textured effects into panel surfaces, which remain smooth and paintable.


A process in which the panel surface is given a “relief” effect with a patterned pressure plate in a press.

End Checks

A drying defect caused by the ends of the boards drying faster than the rest of the wood; can usually be prevented by sealing the end grain.

End Grain

The end of a piece of wood exposed when the wood fibers are cut across the grain. All structural wood panel edges are end grain, and should be finished accordingly.

End Spacing

See Panel Spacing


The process of sealing the ends of the boards to prevent checking caused by unrestrained evaporation of moisture.

Energy Conservation Measures (ECMs)

Installation or modification of equipment or systems for the purpose of reducing energy use and/or costs. (LEED definition)

Energy Efficiency

Ratio of energy output of a conversion process or of a system to its energy input.

Engineered Lumber/Wood

Composite wood products made from lumber, fiber or veneer, and glue. Engineered wood products can be environmentally preferable to dimensional lumber, as they allow the use of waste wood and small diameter trees to produce structural building materials. Engineered wood products distribute the natural imperfections in wood fiber over the product, making them stronger than dimensional lumber. This allows for less material to be used in each piece, another environmental benefit. Potential environmental drawbacks with engineered wood include impacts on indoor environmental quality due to offgassing of chemicals present in binders and glues, and air and water pollution related to production.

Environmental Impact Statement

A document required of federal agencies by the National Environmental Policy Act for major projects or legislative proposals significantly affecting the environment. A tool for decision making, it describes the positive and negative effects of the undertaking and cites alternative actions.

Environmentally Preferable

Products or services that have a lesser or reduced effect on the environment.

Equilibrium Moisture Content

Any piece of wood will give off or take on moisture from the surrounding atmosphere until the moisture in the wood comes to equilibrium with that in the atmosphere. The moisture content of wood at the point of balance is called the equilibrium moisture content and is expressed as a percentage of the weight of the oven-dried wood.

Equilibrium Moisture Content (EMC)

The state at which the panel neither gains nor loses moisture given the relative humidity and temperature of the surrounding atmosphere.


Moisture absorption causes wood to expand. Spacing between panel edges and ends is recommended to allow for any possible panel swelling.

Exposure 1

See Bond Classification

Exterior Glue

A 100 percent waterproof adhesive bonding all Exterior, Exposure 1 and most Interior-type panels. For applications subject to temporary exposure to moisture during construction, specify Exposure 1, Exposure 2 or Interior type with exterior glue. For permanent exposure to weather or moisture, use only Exterior-type panels. - See also Bond Classification

Exterior Type

PS 1 term for plywood manufactured for permanent outdoor or marine use and bonded with 100 percent waterproof adhesives - See Bond Classification

Extreme Fiber In Bending

A measure of the stress applied to the parallel fibers of a piece of wood under loaded conditions. When a load is applied to a piece of wood it causes the wood to bend, producing tension in the fibers on one side and compression in the fibers on the opposite side.


The face of the plywood panel; the side of a panel that is of higher veneer quality on any panel whose outer plies (front and back) are of different veneer grades; either side of a panel where the grading rules draw no distinction between faces. The face ply of a panel; the outer veneer on the face of a panel.


The highest-grade side of any veneer-faced panel that has outer plies of different veneer grades. Also, either side of a panel where grading rules draw no distinction between faces. For example, the face of an A-C panel is the side with the A-grade outer ply. Both sides of an A-A or B-B panel are referred to as faces.

Face Blisters

A dark densified area on the panel surface that may rupture or break upon cutting or machining.


Partial separation of wood fibers parallel to grain in the wood or veneer surfaces of panels caused chiefly by the strains of weathering and seasoning.


First and Second. Mixed domestic hardwood lumber grade is the highest grade of hardwood lumber. In most species a board must be 6” or wider, 8’ or longer. Walnut and butternut are the exceptions.


Wood or plywood trim used along the eave or the gable end of a structure.


A broad, flat, horizontal surface, sometimes used to cover a joint, or as the outer edge of a cornice. Also facia.

Feed Rate

The speed at which material is cut/machined - usually measured in feet or meters per minute.

Fiber Raise (Fiber Pop)

A situation in which the face fibers of a composite panel have raised above the surrounding surface - usually caused by moisture absorption.

Fiberglass-Reinforced Plastic (FRP)

A tough, nonscuff plywood coating made of glass fibers combined with resins. These coated panels (composite) are used in truck and trailer bodies, containers and concrete forms. Seamless panels 40 feet long and longer can be produced as trailer sidewalls or roofs.


A type of washboardlike figure that occurs in some species of wood with wavy grain.


The pattern on a wood’s surface, resulting from the combination of its natural features and the way the log was cut.


A high-solids finishing material used to fill small voids or pits in panel surfaces and edges.


A material for filling nail holes, checks, cracks or other blemishes in surfaces of wood before application of paint, varnish or other finishes.


A method of joining two pieces of lumber end-to-end by sawing into the end of each piece a set of projecting “fingers” that interlock. When the pieces are pushed together, this forms a strong glue joint.


Stains, paints or sealers which protect, color or enhance the natural beauty of structural wood panels.
  • Exterior finishes primarily protect siding and maintain its appearance. They minimize the weathering action which roughens and erodes the surface of unfinished wood. Different finishes give varying degrees of protection so the type, quality, quantity and application must be considered to achieve the desired performance. All exterior panel edges should be sealed if the panels will be painted or stained. Sealing while panels are stacked is easiest. Exterior finishes recommended for structural wood panels include semi-transparent stain, solid-color stain and acrylic latex paint.
  • Interior finishes: Preparation is minimal. Overlaid (MDO and HDO) plywood needs no preparation; sanded and textured grades require only touch-sanding. Recommended interior finishes include oil base paint, latex paint, stain and sealer.

Fire-Rated Systems

Wall, floor and roof construction of specific materials and designs that has been tested and rated according to fire safety criteria (e.g., flame spread rate and fire resistance). Testing and approval are performed by agencies such as Underwriters Laboratories, Inc. A one-hour rating, for example, means that an assembly similar to that tested will neither collapse nor transmit flame or high temperature for at least one hour after a fire starts. Plywood is an approved material in a number of fire-rated designs.


A chemical applied to lumber or other wood product to slow combustion and flame spread.

Fire-Retardant-Treated (FRT)

Chemical treatment of wood and plywood to retard combustion. Plywood is pressure-impregnated with fire retardant chemicals mixed in water in accordance with American Wood Preservers Association Standard AWPA C27. NOTE: Span Ratings and load capacities are based on untreated panels, and may not apply following fire-retardant treatment. Obtain structural performance characteristics of FRT panels from the company providing the treatment and redrying service.

Flame Spread

The rate at which flame spreads along the surface of a material as measured in a standard testing procedure. The rating is expressed in numbers or letters as they relate to finishing requirements or building codes. Plum Creek Super-Refined MDF2?, like all unrated MDF products, carries a “class C” rating.

Flame Spread

The spread of fire along the surface of a material. Flame spread ratings are expressed in numbers or letters and are used in building code interior finish requirements.


Top and bottom longitudinal members of a beam or I-joist. Plywood box beams are fabricated with lumber flanges (top and bottom) and plywood webs (sides).


See Z Flashing

Flat Panel Laminating Equipment — Cold Press

The cold press system has the advantage of being a smaller investment and utilizes less complicated technology but it does require a longer pressing time. This system is usually used for HPL and other phenolic backed laminates.
Adhesive (PVA) is applied with a glue spreader onto the substrate. Laminate is placed onto the substrate. The substrate and laminate unbonded panels are stacked and placed into the cold press, where pressure is applied for 30 to 60 minutes.

Flat Panel Laminating Equipment — Continuous Belt Roll Laminating System

The continuous belt roll laminating system is a more sophisticated equipment arrangement. Panels (particleboard, MDF, hardboard, etc.) are fed into the line, continuously, end to end. The overlays are applied from rolls to the adhesive coated substrate, on one or both sides, prior to bonding. Depending upon the adhesive system used, the process (bonding) may be completed by using either hot or cold calendar rolls.

Flat Panel Laminating Equipment — Edge Foilers

These are single or double sided machines that typically shape, sand and foil to a straight or profiled edge. The foil is transferred by heated silicone wheels, using pressure to match the desired edge profile. MDF is the most common substrate used with edge foiling.

Flat Panel Laminating Equipment — Edge Laminating Equipment

The function of edge laminating equipment is to provide a finished edge to a panel or a potion of a panel that has been surface laminated. A product with the porosity of particleboard may need to be edgebanded with a band thick enough to conceal the voids between the particleboard. Sometimes homogeneous cores with heavier density, such as MDF, can be banded with standard edge treatments or heat transfer foils.

Flat Panel Laminating Equipment — Edgebanders

Edgebanders apply PVC, HPL, melamine, polyester, solid wood and wood veneers to the edges of panels. Types of edgebanders include: single and double-sided machines, hot air, PVC, contour, straight-line and softforming variations. Edgebanders can be manual or automatic. They can apply edgings with or without coated adhesives. Many larger edgebanders have front end tenoning capabilities for sizing and shaping.

Flat Panel Laminating Equipment — Heat Reactive Roll Laminating Equipment

Heat reactive equipment utilizes decorative overlays that have a dry adhesive already applied to their backs. The laminating machine contains heated rollers which activate the adhesive, roll the overlay onto the substrate and apply pressure while the bond is created. There are no curing ovens required in this process. Bonding and curing are rapid.

Flat Panel Laminating Equipment — Hot Platen Press

Hot platen presses are internally heated single opening or multi-opening presses with flat or molded platens. The pressing times and the temperatures are determined by the adhesive, chemical and/or heat reactions. The thickness and composition of the core influence the pressing times and temperatures, as well.
A glue spreader is usually used to apply common adhesives. The adhesives used are urea formaldehyde, PVA, PVAC and phenolic resins.

Flat Panel Laminating Equipment — Low Pressure Press (TFM)

This system earned its name from the distinction between high pressure lamination, which utilizes pressures between 700-1400 PSI, and low pressure lamination, which utilizes pressures between 350 and 400 PSI. Low pressure lamination is a process whereby the resin saturated films are pressed onto the substrate (particleboard or MDF) with a pressure of 350+ PSI and a temperature of about 350-400 degrees F.
The resin is the bonding material. The resin liquefies under heat and pressure and is thermofused directly to the core. The resins form a firm cross-linked thermo-set material. The surface texture of the final product is set by the caul plates (or sometimes release papers), which are fixed to the hot press platens. The most common resin is melamine. Polyester and phenolic surface films are sometimes pressed. Phenolic resin films are mainly used on concrete form.

Flat Panel Laminating Equipment — Membrane Press

Membrane press technology, formerly known as the vacuum press system, is used for the lamination of molded (three-dimensionally shaped) substrates with veneers, vinyls and other materials. The adhesives are applied either to the core or to the laminating materials. The pressing process is performed with a silicone or rubber membrane, which forms the laminating material under pressure and heat over the molded substrate. In a membrane press, heat is applied to both the top and bottom. Pressure is applied only to the top and a vacuum is drawn from the bottom. The pressure used to complete the laminating process is about 105 lbs per Square inch.
The adhesives used for veneers are primarily PVA and urea formaldehyde. Vinyls are usually bonded with water based polyurethane, which is dried after application and reactivated during the pressing process.

Flat Panel Laminating Equipment — Profile Wrappers

Profile wrap machines apply an overlay to a preformed substrate, typically in pre-shaped molding form. The substrate is usually MDF but can be particleboard, solid wood, extruded plastic or metal. A flexible overlay, such as paper, vinyl, wood veneer or a metallic foil is applied to the surface of a molding. These machines usually utilize hot melt or PVA adhesives.

Flat Panel Laminating Equipment — Softforming Edgebanders

Single and double sided softforming edgebanders are capable of applying either a flat or a shaped edge, such as an ogee. Most softformers have sizing and shaping stations, which mill the edge prior to the application of the edgebanding. Softform edgebanders can utilize either PVA or hot melt adhesive systems.

Flat Panel Laminating Equipment — Surface Foiling Equipment

This equipment takes a heat transfer foil and transfers it to the surface of a smooth substrate. The foil is transferred from the carrier film through heat and pressure. Because the foil is very thin, it is important that the surface is extremely clean of dust and other lose particles.
There are three additional foiling machines that can cover board surfaces, as well as their edges: bluff cut machines, random foilers and molding foilers.

Flat Panel Laminating Equipment — Vacuum Press

Vacuum presses are used for lamination of molded (three-dimensionally shaped) substrates with vinyls and other materials. The adhesives are applied to either the core or to the laminating materials. The pressing process uses a silicone or rubber membrane, which forms the laminating material under pressure and heat over the molded substrate.
In a vacuum press, heat is applied to the top of the press, while a vacuum is drawn from the bottom. Since only the atmospheric pressure is involved, the laminating process operates in the range of 26 lbs per square inch at sea level.
Vinyls are usually bonded with water based polyurethane, which is dried after application and reactivated with heat during pressing.

Flitch Matched or Book Matched

Sequentially sawn lumber from the same log. 4/4, 5/4, 6/4, etc: Is the thickness given in fractions. It implies that the lumber is rough sawn 1/16”+ over the stated fraction. 4/4 = 1-1/16” to 1-1/8” and should finish to 13/16” 5/4 = 1-5/16” to 1-3/8” and should finish to 1-1/16”


The degree by which a material will compress before being penetrated by a cutting tool.

Foam Core

Center of a plywood “sandwich” panel. Liquid plastic foamed into all spaces between the plywood panels serves to both insulate and support the component skins. Or plywood skins are pressure-glued to both sides of rigid plastic foam boards or billets. - See Sandwich Panel


Thin decorative paper laminates with a melamine topcoat for durability.


The base for foundation walls, posts, chimneys, etc. The footing is wider than the member it supports, and distributes the weight of the structure to the ground over a larger area to prevent settling.


A reactive organic compound (HCOH) used to manufacture binders.


It is a flammable, poisonous, colorless gas with a suffocating odor. Formaldehyde is prepared commercially by passing methanol vapor mixed with air over a catalyst, e.g., hot copper, to cause oxidation of the methanol; it is also prepared by the oxidation of natural gas. It has been identified as a carcinogen.


MDF mill equipment that forms the furnish into a pressable mat.

Frame Construction

Construction in which the structural parts are wood or dependent on a wood framework for support. Typically, lumber framing is sheathed with structural wood panels for roofs, walls and floor. The classification of frame construction remains the same in building codes even when masonry covering is applied on exterior walls.


Lumber used for structural members in a house or other building. The skeleton to which roofs, floors, and sides are attached.


See “Fiberglass-Reinforced Plastic”


See “Fire-Retardant-Treated”


Stands for the Forest Stewardship Council, see sustainable forestry.


The blended wood fiber and binders used in composite panel manufacturing.


Process of leveling parts of a ceiling, wall or floor by means of wood strips, called furring strips, before adding panel cover.


A large horizontal beam which supports interior walls or joists. Most wood frame houses have a lengthwise center girder that supports the joists and floor panels.


Many adhesives, preferably in conjunction with nails or other fasteners, produce strong joints in plywood construction. Type depends on purpose and exposure of finished product.

Glued Floor System

See APA Glued Floor System


The adhesive joint formed between veneers in a plywood panel or between face veneers and core in a composite panel (primary glueline), or between lumber and structural wood panel parts in an assembly such as a component (secondary glueline).


Short for glued-laminated structural timber - large beams fabricated by bonding layers of specially selected lumber with strong, durable adhesives. End and edge jointing permit production of longer and wider structural wood members than are normally available. Glulam timbers are used with structural wood panels for many types of heavy timber construction.


A shorthand version of Glue Laminated. Glue Laminated is a process in which individual pieces of lumber or veneer are bonded together with an adhesive to make a single piece, with the grain of each piece running parallel to the grain of each of the other pieces.


Refers to the letter-graded quality of veneers used in plywood manufacture (N, A, B, C-Plugged, C and D), or to particular panels, e.g., A-A, Underlayment, etc. - See also Veneer Grade


A designation of the quality of a log or wood product such as lumber, veneer, or plywood.

Grade Name

Term used to identify panels having special characteristics and/or requirements as described under Section 3.6, such as Marine, Decorative, and Underlayment.


A stamp or symbol indicating the grade, quality and/or intended use of a piece of lumber, plywood, or other wood product. To be recognized as “grade marked,” the product must bear an official stamp issued by a grading agency and applied by a qualified grader, or it must be accompanied by a certificate attesting to the grade.

Grading Rules

A set of criteria by which to judge various pieces of lumber or panels in terms of strength, appearance, and suitability for various uses. Regional grading agencies draw up rules for grading based on the voluntary product standards issued by the U.S. Bureau of Standards.


The natural growth pattern in wood. The grain runs lengthwise in the tree and is strongest in that direction. Similarly, grain usually runs the long dimension in the face and back veneers of a plywood panel, making it stronger in that direction. Structural wood panels should therefore usually be applied with the long dimension perpendicular to or across supports.


The direction of wood fibers in a tree or piece of wood with the respect to the axis of the trunk.


A general term referring to the arrangement, appearance, and direction of wood fibers. Among the many types of grain are fine, coarse, straight, curly, open, flat, vertical, and spiral.

Grain Raise

The condition on the surface of a plywood panel resulting from harder or denser wood fibers swelling and rising above softer surrounding wood.


A term that is widely used to describe a building and site that is designed in an environmentally sensitive manner, i.e. with minimal impact to the environment.


Unseasoned; not dry. Lumber with a moisture content of 19% or more.

Green Building

The intent to use holistic design strategies and environmentally friendly products and practices in building construction (including demolition and construction waste management), remodeling and repair, to minimize environmental impact.

Green Building

A building that minimizes impact on the environment through resource (energy, water, etc.) conservation and contributes to the health of its occupants. Comfortable, aesthetically pleasing and healthful environments characterize green buildings.

Green Design

A design, usually architectural, conforming to environmentally sound principles of building, material and energy use. A green building, for example, might make use of solar panels, skylights and recycled building materials.

Green Development

A sustainable approach to real estate development that incorporates such environmental issues as: efficient and appropriate use of land, energy, water, and other resources; protection of significant habitats, endangered species, archeological treasures and cultural resources; and integration of work, habitat and agriculture. Green development supports human and natural communities and cultural development while remaining economically viable for owners and tenants.

Green Power

Electricity generated from renewable energy sources (solar, wind, biomass, geothermal, and hydroelectricGreen Design - a design, usually architectural, conforming to environmentally sound principles of building, material and energy use. A green building, for example, might make use of solar panels, skylights, and recycled building materials.

Greenhouse Effect

Greenhouse gases in the earth’s atmosphere permit solar radiation to pass through but prevent most of the reflected infrared radiation from the earth’s surface and lower atmosphere from escaping into outer space. This process occurs naturally and has kept the earth’s average surface temperature at approximately 60°F. Life on earth would not be possible without the natural greenhouse effect, but environmental scientists are concerned about the increased emissions of greenhouse gases from human activities, leading to climate change and its consequential adverse effects.

Greenhouse Gases

Any gas that absorbs infrared radiation in the earth’s atmosphere. Common greenhouse gases include water vapor, carbon dioxide (CO2), methane (CH4), nitrogen oxides (NOx), ozone (O3), chlorofluorocarbons (CFCs), halogenated fluorocarbons (HCFCs), perfluorinated carbons (PFCs), hydrofluoro-carbons (HFCs) and Sulfur Hexafluoride (SF6). Carbon dioxide, methane and nitrogen oxides are of particular concern due to their long residence time in the atmosphere.


The act of overstating one’s commitment to environmental activity or making misleading claims about the environmental aspects of products, services or technology to garner good publicity.

Grit Marks

Linear abrasions caused by the sanding process.

Grit Size

A reference to the coarseness of the abrasive material on a sanding surface. The lower the grit number, the coarser the abrasive material.


One of the surface treatments frequently given to textured plywood in which a series of narrow, parallel channels are cut into the surface of the panel. Grooving is available in a variety of widths and spacings on several surface textures.


Term used to classify species covered by this Standard. Species covered by this Standard are classified as Groups 1, 2, 3, 4, and 5. See Table 1 for listing of species in individual groups and the reference in Section 2 for product use information.

Group Number

Plywood is manufactured from over 70 species of softwood. These species are classified according to strength and stiffness under manufacturing standard PS 1 into Groups 1 through 5. Group 1 woods are the strongest. The group number of a particular panel is determined by the weakest (highest numbered) species used in face and back (except for some thin panels where strength parallel to face grain is unimportant).

Gusset Plate

A piece of plywood connecting lumber members of a truss or other frame structure. Gussets may be applied to one or both sides of the joint. Plywood is used because of its great strength and split-resistance.


A general term referring to hot pressed engineered wood panels made with refined wood and a lignin binder. Additives may be introduced during the manufacturing process to impart certain properties such as stiffness and hardness. The density range is roughly 55 to 75 pounds per cubic foot.


A measure of a panel’s resistance to surface indentation, or impact resistance, which is stated in pounds, and is related to panel density.


Wood of the deciduous or broadleaved trees - oak, maple, ash, walnut - as distinct from the softwood of the coniferous or needleleaved trees - pine, fir, spruce, hemlock. The term has only a general reference to actual wood hardness. Construction and industrial plywood may use either variety. See SOFTWOOD.


Wood cut from the broad-leaved, mostly deciduous trees that belong to the botanical group Angiospermae.


A general term referring to any variety of broad-leaved, deciduous trees, and the wood from those trees. The term has nothing to do with the actual hardness of the wood; some hardwoods are softer than certain softwood (evergreen) species.


See High Density Overlay


For panels, a cross member placed between studs or joints to support loads over openings for stairways, chimneys, doors, etc. For glulam beams, a beam which is used to support walls and/or floor and roof joists that run perpendicular to it.


A beam fitted between trimmers and across the ends of tailpieces in a building frame; a horizontal support at the top of an opening.


Nonactive core of a log generally distinguishable from the outer portion (sapwood) by its darker color.


The nonactive core of a tree distinguishable from the growing sapwood by its usually darker color and greater resistance to rot and decay.


Mature wood that forms the spine of the tree.

Heat Transfer Foils

A panel laminating system involving the transfer of a complete coating surface from a carrier film to a substrate by means of heat and pressure.

Heavy Timber

A building code designation for a particular type of construction with good fire endurance. Heavy Timber is widely recognized as comparable to one-hour construction. A panel roof deck of 1-1/8-inch tongue-and-grooved plywood with exterior glue over 4-inch-wide supports meets the Heavy Timber requirements and provides the same fire performance as nominal 2-inch tongue-and-groove lumber decking.

Heavy White Pocket

May contain a great number of pockets, in dense concentrations, running together and at times appearing continuous; holes may extend through the veneer but wood between pockets appears firm. At any cross section extending across the width of the affected area, sufficient wood fiber shall be present to develop not less than 40 percent of the strength of clear veneer. Brown cubicle and similar forms of decay which have caused the wood to crumble are prohibited.

High Density Overlay (HDO)

Exterior-type plywood finished with a resin-impregnated fiber overlay to provide extremely smooth hard surfaces that need no additional finishing and have high resistance to chemicals and abrasion. The overlay material is bonded to both sides of the plywood as an integral part of the panel faces. Used for concrete forms, cabinets, highway signs, counter-tops and other punishing applications.
See also Medium Density Overlay (MDO)

High Pressure Laminate (HPL)

Built-up sheet laminate constructed of multiple layers of kraft paper saturated with phenolic resin, a decorative layer impregnated with melamine resin, and a clear or tinted thin overlay heavily saturated with melamine resin. The layers are bonded together under heat and pressure to form a very durable surface for use in applications such as counter tops or panel edges.

High Pressure Laminate (HPL)

HPL is constructed by sandwiching a decorative surface paper between a melamine impregnated alpha-cellulose overlay and a layered stack of phenolic resin impregnated Kraft papers. The sandwiched papers are pressed at temperatures exceeding 265 degrees F and pressures of 1,000 and 1,400 psi. The thickness is determined by the number of layers of Kraft papers and the resulting amount of resin that is absorbed. The surface finish is achieved by using and transferring the texture from a steel caul plate or release paper.

High Pressure Laminate (HPL) — Heat Transfer Rolls

These foils involve the transfer of a complete coating system from a carrier film to a substrate by means of heat and pressure. The foils are gravure printed in reverse sequence on a Mylar film. The release coat is printed first, followed by the pattern of a wood grain print, the ground coat and the adhesive. These foils provide both a decorative effect, as well as a protective layer.

Homogeneous Panel

A composite panel with similar fiber quality and/or density throughout its thickness.


A drying defect that occurs when the lumber undergoes severe case-hardening in the early stages of drying; appears as deep, internal checks.

Hook or Rake Angle

Degrees of angle on a cutting tool affecting the ease with which it penetrates the material being machined.

Horizontal Shear

The measure of the resistance of the shearing stress along the longitudinal axis of a piece of wood. When a load is applied to a piece of wood supported at each end, there is a stress over each support that tends to slide the fibers across each other horizontally. The internal force that resists this action is the horizontal shear strength of the wood. The shearing action is maximum at the mid-depth of a simple span beam at the supports.

Hotmelt Adhesive

A 100% solid thermoplastic substance used in a variety of gluing and laminating processes. Unlike air-dried adhesives, hotmelts cure as they cool.




An “I”-shaped engineered wood structural member. The product is prefabricated using sawn or structural composite lumber flanges and wood structural panel webs, bonded together with exterior type adhesives.


A beam whose cross section resembles the letter “I”; one in which the top and bottom flanges (such as 2x4s) are connected by thinner material (such as plywood or OSB).

Identification Index Former term for Span Rating

See Span Rating


See Impact Insulation Class

Impact Insulation Class

Values which rate the capacity of floor assemblies to control impact noise such as footfalls. FHA requirements (and some local building codes) specify minimum acceptable ratings.

Impact Noise Rating

Values for floor assembly impact sound transmission, now replaced by IIC classification.

Impact Resistance

See “Hardness.”

Indoor Air Quality (IAQ)

Indoor air that contains no known contaminants at harmful concentrations and with which a substantial majority of the people exposed to the air do not express dissatisfaction. Good indoor air quality inside a building results from:
  • Introducing an appropriate amount of outside air into the building through the HVAC systems
  • locating outside air intakes so that the outside air introduced into the HVAC systems is of the best possible quality
  • proper filtration
  • proper air distribution
  • proper removal of indoor pollutants
  • proper commissioning of the building and its building systems.

Indoor Air Quality/Indoor Environmental Quality

Refers to the quality of indoor air for occupants as influenced by the exchange rate of fresh air, off-gassing of interior products, efficiency and design of HVAC systems.

Inner Plies

Plies other than face or back plies in a panel construction. Sub-face, sub-back, crossband and center are classed as inner plies.

Inner Plies

All plies of a plywood panel except face and back.


See Impact Noise Rating

Insulation Value

A measure of a material’s resistance to heat flow/transfer stated as “R Value.”

Integrated Design

A holistic process that considers the many disparate parts of a building project, and examines the interaction between design, construction, and operations to optimize the energy and environmental performance of the project. The strength of this process is that all relevant issues are considered simultaneously in order to “solve for pattern” or solve many problems with one solution. The goal of integrated design is developments that have the potential to heal damages environments and become net producers of energy, healthy food, clean water and air, and healthy human and biological communities.


An essential concept in sustainable building. Viewing a building as a system allows the discovery of synergies and potential tradeoffs or pitfalls with design choices. An integrated design approach helps maximize synergies and minimize unintended consequences.

Interior Glue

A moisture-resistant, but not waterproof, adhesive used in the manufacture of some Interior-type plywood panels.

Interior Type

PS 1 term for plywood manufactured for indoor use or construction subjected to only temporary moisture. - See Bond Classification

Internal Bond

Overall measure of panel integrity illustrating how durably the furnish is bonded together. “I.B.” is generally expressed in pounds per square inch and represents the force perpendicular to the panel surface required to pull a standard test sample apart.

Jointed Core

Core veneer that has had edges machined square. Gaps between pieces of core shall not exceed 3/8 inch, and the average of all gaps in the panel shall not exceed 3/16 inch.

Jointed Inner Plies

Crossband and center veneer that has had edges machine-squared to permit tightest possible layup.


Horizontal framing member of a floor, ceiling or flat roof. Structural wood panels are commonly used for subflooring and underlayment or single-layer flooring (APA RATED STURD-I-FLOOR) over floor joists. APA RATED SHEATHING is typically used over roof joists.


A piece of lumber two to four inches thick and six inches wide, used horizontally as a support for a ceiling or floor. Also, such a support made from steel, aluminum, or other material.

Juvenile Wood

The wood in every tree that forms within its first 10 years or so; usually has undesirable characteristics such as low strength and shrinkage along the grain.


The width of a saw cut.


A slot made by a saw; the width of a saw cut.


A heated chamber of a building used to dry lumber; humidity and air circulation are constantly monitored and adjusted as the wood dries.

Kiln Dried After Treatment (KDAT)

Treated lumber that has been seasoned in a kiln to a predetermined moisture content following the treating process.


Wood dried in ovens (kilns) by controlled heat and humidity to specified limits of moisture content. Veneers are kiln dried before lay-up. - See also Seasoning

Kiln-dried Lumber

Lumber that has dried in a kiln to a specific moisture content.


Natural characteristic of wood that occurs where a branch base is embedded in the trunk of a tree. Generally the size of a knot is distinguishable by a difference in color of limbwood and surrounding trunkwood. Natural characteristic of wood that occurs where a branch base is embedded in the trunk of a tree. Generally the size of a knot is distinguishable by a difference in color of limbwood and surrounding trunkwood abrupt change in growth ring width between knot and bordering trunkwood diameter of circular or oval shape described by points where checks on the face of a knot that extend radially from its center to its side experience abrupt change in direction.


Natural growth characteristic of wood caused by a branch base imbedded in the tree trunk.


The section of a branch or limb that has been overgrown by expanding girth of a tree; may be loose or tight.


A branch or limb embedded in a tree and cut through in the process of manufacturing. Knots are classified according to size, quality and occurrence. In lumber, the size classifications are: Pin knot, one not over 1/2-inch in diameter; Small, a knot larger than 1/2-inch but not over 3/4-inch; Medium, larger than 3/4-inch but not over 1 1/2-inches; Large, over 1 1/2-inches in diameter. Laminated Veneer Lumber - LVL - Structural wood members constructed of veneers laminated to make a “flitch” from which pieces of specific sizes can be trimmed.


Void produced when a knot drops out of veneer.


Voids produced by the dropping of knots from the wood in which they are originally embedded.


(n) A decorative overlay. (v) To bond layers of material together with adhesive.

Laminated Veneer Lumber (LVL)

Structural wood elements constructed of veneers laminated together with their fibers oriented in a parallel direction.

Laminating Equipment — Board Drying Equipment

Drying equipment is used to dry and cure any filling agent used to make the substrate smooth. The most frequently used systems are: high intensity infrared, high velocity air and UV curing oven.

Laminating Equipment — Board Preheating Equipment

This equipment is generally used in cold climates. The purpose is to stabilize the entire board surface for successful, consistent laminating, year-round.

Laminating Equipment — Board Resonating

Resonating occurs when a panel (PB, MDF, etc.) is passed under high pressure between two polished steel rollers that are heated to relatively high temperatures. The process causes the two resins (specie and added) to become liquid for the instant of pressure. The resin is calendered under the influence of the polished steel rolls and pressure. Crushing of the substrate’s top wood fiber cells occurs during this process.
Resonating improves sanding grit effectiveness or eliminates sanding altogether. This process can be used prior to laminating on either wet or dry systems.

Laminating Equipment — Cleaning Equipment

The purpose of the panel cleaner is to remove dust and chips from the board surface after sanding. This process usually involves a sweeper vacuum type of equipment. Brushing equipment may be used, in order to separate small particles from the board.

Laminating Equipment — Filling Equipment

The equipment is used to fill void areas on the board surface. Filling requires oven curing and sanding of the filled surface to insure a successful bond between the board and the overlay. The most common types of filling are ultra violet (UV) curable, waterbase and solvent base.

Laminating Equipment — Sanding Equipment

Sanding is used to prepare a substrate’s surface for lamination. It is particularly important when particleboard and MDF are used, in conjunction with thinner overlays. The two primary types of sanding equipment used in flat panel lamination are: Contact drum sanders and platen head wide belt sanders. Drum and stroke sanders, although available, are generally not used in flat panel lamination.
Sanding the substrate is usually completed at the beginning of the preparation process. If it is necessary to fill the board, a second sanding may take place, in order to insure that the board is totally smooth before sanding.

Laminating Equipment — Surface Preparation Equipment

The purpose of this equipment is to properly prepare particleboard, MDF and other substrates for successful lamination. When correctly used, this equipment produces a product surface that will be and dust free. Some applications require the board to be warmed before lamination. This will limit telegraphing of dust and fibers.

Lamination Layup

The physical arrangement of different grades of laminations throughout the depth of a glulam member.


Individual pieces of lumber that are glued together end to end for use in the manufacture of glued laminated timber. These end-jointed laminations are then face bonded together to create the desired member shape and size.


A condition where the veneers are so placed that one piece overlaps the other.


To position adjacent objects so that one surface extends over the other. Term may designate a lap siding technique, in which each panel or piece overlaps the edge of the next lower panel. A shiplap joint unites two panels when half the thickness of each is cut away so that the two pieces fit together with outer faces flush.


A thin, narrow wooden strip, used as a backing for wall plaster or other materials.


The step in structural wood panel manufacture in which veneers or reconstituted wood layers are “stacked” in complete panel “press loads” after gluing and before pressing. Also the construction of the panel.


A layer is a single veneer ply or two or more plies laminated with parallel grain direction. Two or more plies laminated with grain direction parallel is a “parallel laminated layer.”


In plywood a layer consists of one or more adjacent plies having the wood grain in the same direction. For instance, four ply panels always have three layers with both core plies at right angles to the faces. These two plies are one layer and each face is another. In composite panels, the reconstituted wood portion is one layer and each face is another.


An acronym for Leadership in Energy and Environmental Design. LEED is a point-based rating system developed by the US Green Building Council that evaluates the environmental performance from a “whole building” perspective over its life cycle, providing a definitive standard for what constitutes a green building according to six categories:
  • Sustainable Sites
  • Water Efficiency
  • Energy and Atmosphere
  • Material Resources
  • Indoor Environmental Quality
  • Innovation and Design Process
Buildings evaluated by LEED are rated as certified, silver, gold, or platinum. There are a total of 69 LEED credits available in the six categories: 26 credits are required to attain the most basic level of LEED certification; 33 to 38 credits are needed for Silver; 39 to 51 credits for Gold; 52 to 69 credits for the Platinum rating.

Life Cycle Assessment

An objective process of determining the impact of a product or activity on the environment through analyzing the entire cycle of a product, technology, service, etc. For products, it would include extraction, manufacture, transportation, installation, maintenance, use and ultimate disposal or recycle.

Life-Cycle Cost (LCC)

The total cost of acquiring, owning, operating and disposing of a building or building system over its entire useful life. LCC includes the cost of land acquisition, construction costs, energy costs, the cost to maintain, service and repair the building and its systems, costs of system replacement, financing costs, and residual or salvage value at the end of the building’s useful life.

Light White Pocket

Advanced beyond incipient or stain stage to point where pockets are present and plainly visible, mostly small and filled with white cellulose; generally distributed with no heavy concentrations; pockets for the most part separate and distinct; few to no holes through the veneer.

Linear Expansion / Contraction

Percent change in the length and width of a panel when exposed to a 50% to 80% change in relative humidity.

Liquid Adhesive Roll Laminating Equipment

The roll laminating equipment consists of one (sometimes two or three) heated roll combining stations, where continuous rolls of paper or vinyl film are laminated to the substrates. The liquid adhesive is applied to the board, to the web or both. The lamination and bond are created as paper or film is pulled from an unwind stand and is married to the substrate as they meet and pass through the rotary roll combining station.
The equipment can be designed for single or double sided lamination.

Live Loads (L.L.)

See Loads

Local/Regional Materials

Building products manufactured and/or extracted within a defined radius of the building site. For example, the US Green Building Council defines local materials as those that are manufactured, processed and/or extracted within a 500-mile radius of the site. Use of regional materials is considered a sustainable building strategy due to the fact that these materials require less transport, reducing transportation-related environmental impacts. Additionally, regional materials support local economies, supporting the community goal of sustainable building.


Any number of panels considered as a single group for evaluating conformance to this Standard.

Low Basis Weight Paper

Often referred to as “micropapers” or “rice papers” that may range in weight from 20 to 30 grams per square meter, and which are sometimes pre-impregnated with resin.

Low Basis Weight Papers

Sometimes referred to as micro-papers or rice papers. These papers range in weight from 23 to 30 grams and are sometimes preimpregnated with resin. Acrylic, polyester and other resins can be added during the paper making process to improve the internal bond strength of the paper. The paper is then printed and then coated with polyurethane, urea, polyester acrylic or melamine resins.
Low basis weight papers are usually divided into two categories; standard and industrial. Standard grade papers contain a lower amount of resin in the base paper and offer an economical laminate for use on lower wear surfaces; such as wall paneling. Industrial grade papers have a higher resin content and greater internal bond strength.

Low Pressure Laminate (LPL)

A pre-printed or solid-colored decorative paper saturated with melamine resin which, under heat and pressure, is bonded to a panel surface without the need for additional adhesive. The resulting durable surface is featured in a broad range of products from kitchen cabinetry to laminate flooring.


Building materials and finishes that exhibit low levels of “offgassing,” the process by which VOCs (Volatile Organic Compounds) are released from the material, impacting health and comfort indoors and producing smog outdoors. Low (or zero) VOC is an attribute to look for in an environmentally preferable building material or finish. See “Volatile Organic Compound (VOC)” for more information.

Lumber Core

Plywood manufactured with a core composed of lumber strips. The face and back (outer) plies are veneer.

Manufactured Housing

Housing units partially or completely built in a factory.

Marine Grade

Plywood panels manufactured with the same glueline durability requirements as other Exterior-type panels but with more restrictive veneer quality and manufacturing requirements. The grade is particularly suitable for marine applications where bending is required, as in boat hulls.


Refined fiber as it is formed and conveyed to the press.

Material Safety Data Sheet (MSDS)

A compilation of information required under the OSHA Communication Standard on the identity of hazardous chemicals, health, and physical hazards, exposure limits, and precautions. Section 311 of SARA requires facilities to submit MSDSs under certain circumstances.

MDF (Medium Density Fiberboard)

A composite wood fiberboard, used for cabinetry and other interior applications. MDF containing urea formaldehyde can contribute to poor indoor air quality.


See Medium Density Overlay

Medium Density Fiberboard (MDF)

An engineered wood product made from mechanically refined wood fibers combined with resin, which are bonded together under heat and pressure. Its typical density range is 40 to 50 pounds per cubic foot (640 - 880 kg/m3). The durable homogeneous construction of MDF resists warping, cracking and splitting - offering unparalleled design flexibility where intricate shaping and finishing are required. Some of the more common uses of MDF include furniture, cabinetry, millwork, store fixtures and laminate flooring. As with solid wood, the nature of MDF can vary significantly between manufacturers, based on wood species and production technology.

Medium Density Overlay

Exterior-type plywood finished with an opaque resin-treated fiber overlay to provide a smooth surface ideal as a paint base. Recommended for siding and other outdoor applications, and for built-ins, signs and displays, furniture, etc. Available without grooving, with V-grooves, or in T1-11 or reverse board-and-batten grooving. - See also High Density Overlay (HDO) and APA Rated Siding

Melamine Resin

A resin formulation used in the saturation of paper overlays which adheres to the panel substrate in hot press lamination.


Lumber that has been remanufactured into door and window parts or decorative trim. Generally made from the shop grades of Ponderosa Pine, Sugar Pine, White Fir, Douglas Fir, or Western Hemlock.

Miter Joint

A joint formed by fitting together two pieces of lumber or panels that have been cut off at a 45° angle.

Model Code

A building code developed by a regional federation of building officials. These are continually reviewed and updated by committees of building officials. Model codes in the United States are the Uniform Building Code (UBC), published by the International Conference of Building Officials; the Standard Building Code (SBC), published by the Southern Building Code Congress International; and the National Building Code (NBC), published by the Building Officials and Code Administrators International (BOCA). Members of these three code organizations comprise the National Evaluation Service. - See National Evaluation Service

Modular Housing

A type of housing in which major components are assembled in a factory and then shipped to the building site to be joined with other components to form the finished structure. The components are usually uniform incremental sizes, permitting some flexibility of design while maintaining the structure of individual elements. Sometimes called “prefabricated” or “prefab” housing by laymen; these terms are avoided by the industry because of negative connotations.

Modulus of Elasticity (MOE)

The pounds per square inch measure of a panel’s resistance to deflection when loaded as a simple beam.

Modulus of Rupture (MOR)

The pounds per square inch measure of the maximum breaking strength of a board when loaded as a simple beam.

Moisture Barrier

See Vapor Barrier

Moisture Content

The amount of water in the panel expressed as a percentage of dry weight.

Moisture Content

The amount of water contained in the wood, usually expressed as a percentage of the weight of oven-dry wood.

Moisture Content

The amount of water in a piece of wood expressed as a percentage of the green weight minus the dry weight times 100, divided by the green weight.

Mottling Effect

An irregular appearance in an area (or entire surface) of a finished panel which may be caused by coarse or raised fiber, heavy application of coating material, improper drying of coating material, or incompatible solvents. Also known as “orange peel.”

Nail Popping

Flooring nails sometimes appear to “pop” up so that nail head impressions are visible on the surface of the finished floor covering. Shrinkage of floor joist away from the nail shank after installation exposes the head. When floor members are dry, make sure fasteners are flush with or below floor surface just prior to installation of thin floor covering such as tile, linoleum or vinyl. Fasteners should be set if green framing will present nail popping problems upon drying. Do not fill nail holes.


See Glue-Nailed


Nails commonly used for residential construction include:
  • Common and box nails: 16 penny (d) common and box, for general framing. 8d and 10d common and box nails, for toenailing. 6d and 8d common and box nails, for subfloor, wall and roof sheathing. Size depends on thickness of structural wood panel sheathing.
  • Scaffold nails: 8d and 10d most common, for scaffolds, bracing and any temporary fastening that must later be removed.
  • Siding nails: Nonstaining nails of size specified for siding thickness.
  • Casing and finish nails: 4d, 6d and 8d most common, for exterior and interior trim and installation of siding and paneling where large nailheads should not show.
  • Roofing nails: A special type, commonly available. Size depends on thickness of roofing and deck material.
  • Drywall nails: 4d to 6d size depends on drywall thickness; for 1/2-inch drywall use 4d drywall nails.
For underlayment and finish floor: Special nail types with greater holding power than ordinary varieties are also available. For hardwood strip flooring, use either 8d hardwood nails or 2-1/2-inch hardened, spiral-threaded (screw-shanked) nails. For 1/2-inch and thinner Underlayment grade plywood (over subflooring), use 3d ring-shanks. For 19/32 through 3/4-inch Sturd-I-Floor panels, use any of the 4d deformed-shank nails illustrated below.
For 1/4-inch panels use 3/4-inch or 1-inch brads, 3d finish nails, or (if no objection to heads showing) 1-inch blue lath nails. For exterior application, use galvanized or coated nonstaining nails or fasteners.
Predrilling is occasionally necessary in careful work where nails must be very close to panel edges. Select a drill bit of slightly smaller diameter than the nail to be used.
Space nails about 6 inches apart for most work. Closer spacing is necessary only with thin panels which might otherwise buckle slightly between nails.

National Evaluation Service (NES)

An arm of the Council of American Building Officials sponsored jointly by the three major American model code organizations - the International Conference of Building Officials (ICBO); the Southern Building Code Congress International (SBCCI); and the Building Officials and Code Administrators International (BOCA). NES studies applications for new products, and publishes evaluation reports recommending approval by its three constituent members.


The industry designation for No added urea formaldehyde, referring to adhesives used in the manufacturing process of composite panels and plywood.

Net Zero

The concept or goal of to have a carbon neutral building—one with a net energy consumption of zero—producing no carbon dioxide emissions and thus no negative impact on the environment.

Noise-Rated Systems

Construction designed to reduce sound transmission. Various plywood construction systems tested both in laboratories and buildings meet or exceed requirements.

Nominal Dimension

Full designated dimension. For example, a nominal 2 inch by 4 inch stud may measure 1-1/2 inch x 3-1/2 inch when surfaced. It is a commercial size designation, subject to acceptable tolerances. - See also Sized for Spacing

Nominal Dimensions

Dimensions based on rough-cut(unplanned) softwoods; a 2x4 is nominally 2” x 4”—It’s actually 1-1/2” x 3-1/2”.

Nominal Measure

The nominal or common-named sizes of lumber, usually expressed in terms of the nearest inch regardless of actual surface, or net, sizes.

Nominal Thickness

Full “designated” thickness. For example, 1/10 inch nominal veneer is 0.10 inch thick. Nominal 1/2 inch thick panel is 0.50 inch thick. Also, commercial size designation, subject to acceptable tolerances.


Structural panels not included in Product Standard PS 1, or covered under various Performance Standards, and which may bear the mark of the manufacturer rather than a recognized testing agency, such as APA.

O & ES

Oiled and edge-sealed. Surfaces of concrete form panels are lightly coated with oil and the edges sealed if specified.


Release of volatile chemicals from a product or assembly. Many chemicals released from materials impact indoor air quality and occupant health and comfort. Offgassing can be reduced by specifying materials that are low- or no-VOC and by avoiding certain chemicals (e.g., urea formaldehyde) entirely. Controlling indoor moisture, and specifying pre-finished materials, can also reduce offgas potential.

Old Growth

Biologically, a stand of timber that is near its climax; such trees may be 200 years old or more. In timber management planning, old growth also refers to timber that is older than the rotation age planned for future forests; this definition may include trees that are 100 years of age, or less.

On-Center (O.C.)

On-center spacing, meaning the distance from the center of one structural member to the center of the adjacent member, as in the spacing of studs, joists, rafters, nails, etc.


Refers to the degree with which finishing material masks the underlying substrate. For example, high opacity indicates more complete coverage.

Open Defects

Irregularities such as splits, open joints and knotholes that interrupt the smooth continuity of veneer.

Oriented Strand Board (OSB)

Structural wood panels manufactured from reconstituted, mechanically oriented wood strands bonded with resins under heat and pressure. Oriented strand material may be produced as the center layer of composite panels, or may be cross-laminated in layered panels.

Oriented Strand Board (OSB)

A structural panel made of narrow strands of fiber oriented lengthwise and crosswise in layers, with a resin binder. Depending on the resin used, OSB can be suitable for interior or exterior applications.


The end of a beam that extends beyond its support as in the eave overhang of a house.

Overlaid Plywood

Plywood panels with factory-applied, resin-treated fiber faces on one or both sides. Term may also apply to metal and other overlaid panels. - See High Density Overlay (HDO) and Medium Density Overlay (MDO)


A thin layer of paper, veneer, foil, or other laminate material.

P & TS

Plugged and touch-sanded face of a plywood or composite panel.


See Finishes


A portable platform used as a base for storing, stacking, and transporting goods in a unit.

Panel Clip

Specially shaped metal device for supporting panel edges to reduce differential deflection in roof construction.

Panel Product

Any variety of wood products such as plywood, particleboard, hardboard, oriented strand board, or waferboard, sold in sheets or panels. Although sizes vary, a standard size for most panel products is 4x8 feet.

Panel Spacing

The gap left between installed panels in a structure. Panels in floor, wall or roof deck construction should be spaced to allow for any possible expansion due to changing moisture absorption levels. Proper spacing helps prevent buckling and warping. - See also Sized for Spacing


Wood panels joined in a continuous surface, especially decorative panels for interior wall finish. Textured plywood in many varieties is often used as interior paneling either in full wall sections or accent walls. - See APA Rated Siding for textured plywood used as paneling.

Panelized Construction

Building components fabricated in wall, floor, or roof sections, etc., to be assembled into a completed structure at the building site. Panelized construction speeds erection and cuts on-site labor costs. It offers the high quality available through controlled factory production and inspection procedures.


An engineered wood product made from a combination of wood particles and fibers bonded together with a synthetic resin under heat and pressure. Particleboard is available in a variety of thicknesses, sizes and densities and is used in wide range of products where dimensional stability and strength are critical. Particleboard’s machinability and face integrity make it especially well-suited for laminating applications.


See Repairs


Inserts of sound wood or synthetic material in veneers or panels for replacing defects. “Boat” patches are oval-shaped with sides tapering in each direction to a point or to a small rounded end; “Router” patches have parallel sides and rounded ends. “Sled” patches are rectangular with feathered ends.

Peeler Log

A specially selected softwood log used to produce veneer. Peelers are debarked, then lathe-turned against a long knife blade which slices off a thin, continuously unwinding sheet of veneer then clipped to size, dried, graded, repaired and laminated into plywood panels.

Performance Rated Panels®

See APA Performance Rated Panels

Performance Standard

A standard applying to panels such as APA RATED SHEATHING, APA RATED STURD-I-FLOOR and APA RATED SIDING. Panels manufactured to meet APA performance standards must satisfy rigorous, exacting performance criteria. - See also Product Standard and APA Performance Rated Panels

Permanent Wood Foundation

See Wood Foundation

Phenol Formaldehyde

A water-resistant thermosetting resin used to bond softwood plywood, OSB, and some exterior or moisture-resistant grades of composite panels.


A sharpened or pointed stake, post, or pale, usually used as fencing.

PIRF (Perimeter-Insulated Raised Floor System)

Crawl space foundation-floor system where insulation is applied only to the inside of the perimeter foundation stem wall. The resulting system saves construction costs and gives superior energy performance.

Pitch Pocket

A well-defined opening between rings of annual growth, usually containing, or which has contained, pitch, either solid or liquid.

Pitch Streak

A localized accumulation of resin in coniferous woods which permeates the cells forming resin soaks, patches, or streaks.

Pitch Streak

A localized accumulation of pitch in wood cells in a more or less regular streak.


The small, soft core occurring in the center of the tree trunk.


Small voids caused when fibers are torn out of the panel edge or surface during machining or sanding.

Plain Sawn

Lumber sawn parallel to the grain.


In wood frame construction, the horizontal lumber member on top and/or bottom of the exterior wall studs which ties them together and supports the studs or rafters.


The rigid heated surface within a composite panel press that comes into contact with the panel surfaces.


See Repairs

Plugged Core

Inner ply construction of solid C-Plugged veneer pieces. Gaps between pieces of core should not exceed 1/2-inch per Products Standard PS 1. - See Jointed Core

Plugged Crossbands

Under Face (PCUF) A designation denoting a SANDED PANEL of special construction, making it suitable for use as an UNDERLAYMENT, for example A-C (PCUF).

Plugged Inner Plies

(Also referred to as solid inner plies.) - Refers to C Plugged crossband and centers and additional limitations, as given in 3.8.1.


Sound wood of various shapes, including among others, circular and dog-bone, for replacing defective portions of veneers. Also synthetic plugs used to fill openings and provide a smooth, level, durable surface. Plugs usually are held in veneer by friction until veneers are bonded into plywood.


A single veneer lamina in a glued plywood panel. (See also layer.)


A single veneer in a panel.


See B-B Plyform


A plywood panel manufactured with a hardboard face for an extra-smooth painting and tough wearing surface. May be Interior or Exterior type. Interior PLYRON is available with a standard, tempered or treated hardboard surface and is manufactured of D-grade veneer except the ply directly under the hardboard surface, which must be C-grade. Exterior PLYRON is available with a tempered or treated surface and is manufactured with C-grade plies throughout. PLYRON is ideal for work surfaces, fixtures, built-ins, cabinets and doors, underlayment and industrial uses.


Plywood is a flat panel built up of sheets of veneer called plies, united under pressure by a bonding agent to create a panel with an adhesive bond between plies as strong as or stronger than, the wood. Plywood is constructed of an odd number of layers with grain of adjacent layers perpendicular. Layers may consist of a single ply or two or more plies laminated with parallel grain direction. Outer layers and all odd numbered layers generally have the grain direction oriented parallel to the long dimension of the panel. The odd number of layers with alternating grain direction equalizes strains, reduces splitting, and minimizes dimensional change and warping of the panel.


A flat panel made up of a number of thin sheets, or veneers, of wood in which the grain direction of each ply, or layer, is at right angles to the one adjacent to it. The veneer sheets are united, under pressure, by a bonding agent.

Polyester Laminate

Decorative paper impregnated with polyester resin which is laminated to a backer for flat or contoured applications.


The permeability of the panel’s surface or core to liquid coatings and/or adhesives.

Post-consumer Recycled Content

Use of materials discarded by households, commercial or industrial facilities, and thereby diverted from the waste stream.

Post-formable HPL

High pressure laminate designed to be hot-formed around a radius where a curved design is preferred over a square edge.


These papers are treated after they have been manufactured. They are usually treated with melamine and/or urea formaldehyde. Additionally, they are usually treated with acrylic, which allows the sheet to remain flexible after the resins are fully cured. With post-impregnated papers, the paper fibers are encapsulated with resin. The voids and air spaces in the paper are filed with resin. These papers may be top coated and they may have hot melt adhesives applied.

Post-impregnated Paper

Paper treated (after it has been manufactured) with resin such as urea formaldehyde, melamine, or acrylic which allows the paper to remain flexible - even with the resin fully cured. These papers are easily topcoated and are available with a pre-applied hotmelt adhesive.

Postforming Equipment — Conventional Postforming

There are three ways of putting a soft, radiused edge on a panel. Softforming and profiling are two of the methods. The third method is conventional postforming. This is the process where a laminated surface is formed to a shaped substrate material. There are two types of machines that are used for conventional postforming; stationary and through-feed.
Stationary machines will utilize a heated bar which follows the pre-shaped substrate. Contact adhesives or PVAC glues can be used with this machine. Stationary machines can use a variety of substrates and are easy to set up and operate.
Through-feed postforming machines are used where higher production rates are required. These postformers incorporate more automated stations or zones (sizing, adhesive applications, activation, forming and trimming).

Postforming Equipment — Direct Postforming

Direct postforming involves the same processes as conventional postforming, with the exception being that the substrate is pre-shaped on the postforming machine. These machines require fewer processing stations than conventional postforming machines.

Pre-consumer Recycled Content

Use of materials that have been diverted from the waste stream during the manufacturing process.

Pre-impregnated Paper

Paper treated (while it is being manufactured) with resin such as urea formaldehyde, melamine, or acrylic which allow the paper to remain flexible - even with the resin fully cured. These papers are easily printed or heat embossed, and are available with a pre-applied hotmelt adhesive.


A cold press in the MDF manufacturing process following the “former” that consolidates the fiber prior to hot pressing.


Either the premature curing of resin before hot-pressing, or a reference to incomplete sanding of a composite panel.


In housing, all parts constructed or fabricated at the factory so that final construction only involves assembling and uniting standard parts at the job site. Commonly abbreviated as prefab. - See Panelized Construction


A ready-to-use panel with factory-applied finish - paint, overlays or coatings.


Panelized building in which wall, floor or roof sections are framed and sheathed at the factory.


A panel with a factory-applied primer or undercoat needing only final finish after installation.


Any substance applied to wood that helps it resist decay, rotting, or harmful insects.


Products which prevent wood deterioration due to weather exposure, excessive moisture or insect attack. Treatments range from chemical pressure-impregnation, as for wood foundations, to application of paints or sealers.


Heated platens in the MDF manufacturing process which consolidate the pre-pressed mat into a panel.

Pressure-Preservative Treated

Wood treated with preservative or fire retardants by pressure-injecting treating solutions into wood cells.


A process of impregnating lumber or other wood products with various chemicals, such as preservatives and fire-retardants, by forcing the chemicals into the structure of the wood using high pressure.

Primary Grit Marks

Linear scratches in the face of a panel from coarse sanding which are not removed by subsequent finish sanding.


An undercoat applied to bare wood as a sealer and base for paint. - See Finishes

Product Standard

An industry product manufacturing or performance specification. APA trademarks carrying the PS 1 or PS 2 mark are identification by the manufacturer that the panel has been produced in conformance with U.S. Product Standard PS 1 for Construction and Industrial Plywood or Voluntary Product Standard PS 2, Performance Standard for Wood-based Structural-Use Panels. PS 1 is a detailed manufacturing specification and alternate performance standard developed cooperatively by the softwood plywood industry and the U.S. Department of Commerce. PS 1 requirements and a supplementary set of APA specifications help ensure that plywood manufactured by APA member mills maintains its consistently high quality. PS 2 is a similar standard without the detailed manufacturing specification that relies on performance testing to assure that the structural panels meet realistic, rigorous standards.


The three-dimensional shape of a machined panel edge or face.


See APA Performance Rated Panels


Pounds of force per square inch of panel surface.


Subframing which supports roof decking where larger beams are main structural supports. Also a secondary structural framing member such as a joist or rafter that is normally supported by walls or primary beams.

Quatersawn Lumber

Vertical grained lumber.

R Value

A measurement of thermal resistance, or ability to retard heat transmission. Used to compute insulating effectiveness.

Rabbet Joint

A joint formed by cutting a groove in the surface or along the edge of a board, plank or panel to receive another piece.

Racking Strength

A measure of the ability of a structure, such as a wall, to withstand horizontal forces acting parallel to the structure.


A dimension that is commonly used as a means of describing the camber requirements in a glulam beam, as in radius of curvature.


Sloping supporting member of a roof immediately beneath the sheathing.

Railroad Tie

A piece of industrial lumber used to support rails on a roadbed. In Britain and other countries, a “Sleeper”.

Raised Grain

See Grain Raise

Random Lengths

Lumber of various lengths, usually in even two-foot increments. Lumber offered as random-length will contain a variety of lengths which can vary greatly between manufacturers and species. A random-length loading is presumed to contain a fair representation of the lengths being produced by a specific manufacturer.

Rapidly Renewable

Materials are considered to be an agricultural product, both fiber and animal, that takes 10 years or less to grow or raise. And to harvest in an ongoing and sustainable fashion. (MR 6).

Rated Sheathing

See APA Rated Sheathing

Rated Siding

See APA Rated Siding


a ribbon-shaped strand of wood cells that extends from the inner bark to the pith perpendicular to the axis of a tree trunk; rays appear as fleck on quartersawn surfaces of some species.

Recycled Content

Term used by the LEED rating system to reference materials that contain pre- and post-consumer recovered material introduced as a feed stock in a material production process, usually expressed as a percentage.

Regionally Extracted Materials

Term used by the LEED rating system to reference materials sourced from within 500 miles of a project site (thereby reducing environmental impact from transportation of materials). for use in this credit, must have their source as a raw material from within a 500-mile radius of the project site. (MR 5.2).

Regionally Manufactured Materials

For use in credit MR 5.1 and MR 5.2, must be assembled as finished product within a 500-mile radius of the project site. Assembly, as used for this credit definition, does not include on-site assembly, erection or installation of finished components, as in structural steel, miscellaneous iron or systems furniture. (MR 5.1) (MR 5.2).

Relative Humidity

A ratio comparing the amount of water vapor present in the air to the amount which saturated air would hold at the same temperature.


Any patch, plug, or shim.


Any patch, plug or shim in a veneer. A patch is a sound wood insert to replace a defect in veneer. Boat patches are oval shaped with sides tapering to points or small rounded ends. Router patches have parallel sides and rounded ends. Sled patches are rectangular with feathered ends.
A plug may be a circular or dogbone shaped wood patch, or a synthetic filler of fiber and resin to fill openings and provide a smooth, level, durable surface. A shim is a long narrow wood or synthetic repair not more than 3/16 inch wide. Various other shapes of plugs or patches may be encountered. PS 1 specifies sizes, shapes and numbers of allowable patches in given veneer grades.


See Rough Sawn

Resilient Floor Covering

Any of the vinyl or asphalt-base floor coverings (tile or sheet) with enough “give” to resist deformation or denting from dropped objects. Resilient floor coverings installed over APA STURD-I-FLOOR or UNDERLAYMENT panels with “sanded face” provide smooth, stiff floors for comfortable walking.

Resin Spots

Hard pieces of dark or black material in the face layer of an MDF panel composed of resin and wood dust.

Reverse Board And Batten

An APA 303 Siding surface treatment. Deep, wide grooves cut into textured siding surfaces during manufacture create striking, sharp shadow lines. - See APA Rated Siding

Ridge Beam

The top horizontal member of a sloping roof, against which the ends of the rafters are fixed or supported.

Ridge Beam

A horizontal timber to which the tops of rafters are fastened. Also called a Ridge Board or a Roof Tree.

Rift Lumber

Diagonally grained lumber.

Rigid Frame

Structural member functioning like an arch, comprised of studs and rafters fastened with plywood gussets. Rigid frame construction eliminates the need for ceiling or tie members.


Sawing wood in the direction of the grain.

Roof Truss

An engineered building component supporting the roof in place of rafters. Roof trusses are usually constructed in a triangular shape with a number of interconnected pieces that spread a load evenly across the truss.

Rotary Peel

See Peeler Log

Rough Grain

Grain characteristics which prevent sanding to a smooth surface.

Rough Lumber

Lumber which has not been dressed or surfaced but has been sawn, edged, and trimmed.

Rough Sand

A rough area of a panel that was not sanded with the finish heads.

Rough Sawn

A decorative APA Siding treatment imparting a rough, rustic appearance by saw-scoring the surface of a panel during manufacture. Same as resawn. - See APA Rated Siding

Router Patch

See Repairs


The revolutions per minute speed of a motor or cutting tool.


An unsanded corner or edge of the panel (which is thicker than the rest of the panel) that may appear discolored.


Surfaced One Face


Surfaced Two Faces


Surfaced Two Faces and Straight Line Ripped One Edge


Surfaced Two Faces and Straight Line Ripped Two Edges


A situation in which the panel face layer or laminate has been sanded off exposing the core.

Sanded Panels

Interior or Exterior plywood panels factory-sanded for applications where smoothness and appearance are important. These panels - with N, A or B-grade faces - are ideal for furniture, cabinets, doors, fences, signs, etc. Sanded panels save time because they may be finished with little or no preparation.

Sander Hesitation

Concave linear “troughs” across the width of a panel caused when the panel stops under a moving sander head.

Sander Skip

A portion of the panel surface which wasn’t sanded, usually appearing as a rough concave area.

Sandwich Panel

A section of layered construction (as of walls) made up of high-strength plywood faces, or “skins,” attached to both sides of low-density core materials such as plastic foam or honeycomb paper fillers.


The living wood of lighter color occurring in the outer portion of a log. Sometimes referred to as “sap.”


Living wood of pale color near the outside of a log. Under most conditions, sapwood is more susceptible to decay than heartwood.


New wood surrounding the denser heartwood.

Saturated Papers

Decorative papers generally weighing between 60 to 120 grams per square meter which are saturated with melamine or polyester resin, and partially cured at the point of its manufacture. Final curing occurs during hot press lamination.

Saturated Papers

These papers usually weigh between 60 and 130 grams. These papers are saturated with reactive resins and are partially cured. Final curing occurs at the time of hot press lamination. This is when the resins form a hard cross-linked thermo-set material; thus creating a permanent bond with the substrate. The decorative paper is similar to that used for high pressure laminate.

Saturated Papers — Melamine (Thermofused Melamine, TFM or Low Pressure Laminate)

The paper is impregnated with a melamine or a urea-melamine resin system; then partially cured to a “B” stage. The resin is fully cured when the paper is pressed at 300-400 psi, at a temperature of 300-400 degrees F.

Saturated Papers — Polyester

The resin is introduced to the paper during an impregnating process. The impregnated paper is then dried. The resin is fully cured when the paper is pressed at 175-200 psi, at a temperature of 275-300 degrees F.

Scarf Joint

An angled or beveled joint in plywood splicing pieces together. The length of the scarf is 5 to 12 times the thickness.


The standardized test measure of force in pounds required to extract a screw from the face or edge of a panel.


Use flat head wood screws for attaching structural wood panels where nails will not provide sufficient holding power. Sizes shown below are minimum; use longer screws where work permits. Lubricate screws with soap if they are hard to drive. If used for sheathing, use same spacing as recommended for nails.


Removal of moisture from wood to improve its serviceability, usually by air drying - drying by air exposure without artificial heat - or kiln drying - drying in a kiln with artificial heat. Plywood veneers are seasoned before lay-up and gluing into panels.


Shear Wall - See Diaphragm


A clear board that is too short or narrow to be FAS.


A lumber defect that is a lengthwise separation of wood, usually along the growth rings.


The structural covering, usually of wood panels or boards, on the outside surfaces of framing. It provides support for construction, snow and wind loads and backing for attaching exterior facing materials such as wall siding, roof shingles or underlayment in double-layer floors. APA RATED SHEATHING is recommended for conventional applications. - See APA Rated Sheathing


Plywood, waferboard, oriented strand board, or lumber used to close up side walls, floors, or roofs preparatory to the installation of finish materials on the surface. The sheathing grades are also commonly used for pallets, crates, and certain industrial products.


A long, narrow repair of wood or suitable synthetic not more than 3/16 inch wide.


See Repairs


Jointing in which ends or edges are notch-milled to overlap and form a rabbet joint.

Shop-cutting Panel

A shop-cutting panel is one which has been rejected as not conforming to a standard grade because of deficiencies, other than adhesive bond quality, which prevents it from meeting the requirements of this Standard. Blistered panels are not considered as coming within the category of “shop-cutting” panel. Localized delamination may occur as a result of a deficiency. However, shop-cutting panels may be suitable for cut-up use where cutting eliminates the deficiency in the portion of the panel salvaged. Such a panel must be identified with a separate mark as specified in 6.2.1.

Sick Building Syndrome

Indoor air quality is compromised because of inadequate ventilation or because of chemical or biological contaminants. Building occupants experience negative health or comfort effects.


See APA Rated Siding

Sill Plate

The lowest framing member of a structure, resting on the foundation and supporting the floor system and the uprights of the frame.

Single Floor

A single-layer structural wood panel flooring system combining subflooring and underlayment. - See APA Rated Sturd-I-Floor

Single Floor

Single Wall - See APA Strud-I-Wall


A structure constructed at the site where it is to remain.

Sized For Spacing

A notation in APA RATED SHEATHING and RATED STURD-I-FLOOR trademarks indicating panels may be trimmed during manufacture to length and width tolerances of +0, 1/8 inch. This trimming is designed to encourage proper panel spacing. - See Panel Spacing

Sized For Spacing

Sled Patch - See Repairs


Straight Line Rip One Edge


Straight Line Rip Two Edge

Snake Tracks

Serpentine raised marks on the face of the panel running along its length caused by buildup or skips on the sander belt.


The underside of the roof overhang. Structural wood panels are often used as finishing materials for soffits.


Wood of the coniferous or needleleaved trees - pine, fir, spruce, hemlock - as distinct from the hardwood of the deciduous or broadleaved trees - oak, ash, maple, walnut. The term has only a general reference to actual wood hardness. Construction and industrial plywood and other panel products may use either variety, but are more commonly manufactured of softwoods.


Wood cut from coniferous trees belonging to the botanical group Gymnospermae.


A general term referring to any variety of trees having narrow, needle-like or scale-like leaves, generally coniferous. The term has nothing to do with the actual softness of the wood; some “softwoods” are harder than certain “hardwood” species.

Solid Core

See Plugged Core

Solvent Borne Adhesives

Solutions of polymers, volatile organic solvents, and crosslinking materials designed to obtain specific properties in the laminating process, such as a heat-resistance bond.

Sound Transmission Class


Southern Yellow Pine

A species group, composed primarily of Loblolly, Longleaf, Shortleaf, and Slash Pines. Various subspecies are also included in this group. This group refers to the southeastern United States, from Texas to Virginia.


An attractive dark brown or black stain in some woods caused by decay.

Span Rating

A set of numbers used in marking sheathing and combination subfloor-underlayment (single floor) grades of plywood as described in 3.8.5. Formerly called Identification Index.

Span Rating

The number that appears in the trademark on APA RATED STURD-I-FLOOR, APA RATED SHEATHING and APA RATED SIDING panels. Two numbers separated by a slash (e.g., 24/0, 32/16, 48/24) appear on APA RATED SHEATHING. The left-hand number is the maximum recommended center-to-center spacing of supports in inches when the panel is used for roof sheathing with long dimension across supports (unless the strength axis is otherwise identified). The right-hand number is the maximum center-to-center spacing of supports in inches when the panel is used for subflooring with long dimension across supports. When a panel is applied as wall sheathing, the left-hand number applies to stud spacing. A rating of 24 oc or more means the panel can be applied to studs spaced 24 o.c. A rating less than 24 oc means the panel can be applied to studs spaced 16 o.c. APA RATED SHEATHING panels may be applied as wall sheathing either vertically or horizontally. In all cases the panel should be applied continuous over three or more supports.
The single-number Span Ratings on APA RATED STURD-I-FLOOR panels (16, 20, 24, 32 or 48 oc) denote maximum recommended center-to-center spacing between floor joists with panels laid with long dimension across three or more supports.
Similarly, the single-number Span Ratings on APA RATED SIDING panels are the maximum recommended center-to-center spacings of studs (16 or 24 o.c.) when the panel is applied vertically direct to studs (or over nonstructural wall sheathing such as fiberboard, gypsum or rigid foam insulation sheathing). All RATED SIDING panels may be applied horizontally direct to studs spaced 16 or 24 inches on center, provided horizontal joints are blocked. When RATED SIDING is used over APA RATED SHEATHING or lumber, the Span Rating refers to the maximum recommended spacing of vertical rows of nails rather than studs.
Panels with a given Span Rating may be manufactured in more than one thickness, and vice versa, because of varying panel compositions and configurations.


A category of biological classification; a class of individuals having common attributes and designated by a common name. “Species” is always properly used with the “s” when referring to trees or other biological classifications.

Species Group

See Group Number


A defect that’s caused by a fungus living in a tree, which appears as small white pits or spots.


A narrow, turned piece of wood.


Lengthwise separation of wood fibers completely through the veneer, caused chiefly by the manufacturing process or handling.


A separation of wood fibers that extends completely through a piece of lumber, usually at the ends.


The degree to which a compressed MDF panel returns to its original uncompressed state.


Canadian woods of similar characteristics that have been grouped for production and marketing. The S-P-F species have moderate strength, are worked easily, take paint readily, and hold nails well. They are white to pale yellow in color. The largest volume comes from Eastern Canada (Saskatchewan and east), where the principal species in the group are: Red Spruce, Black Spruce, Jack Pine, and Balsam Fir. The principal species of the group originating in Western Canada (British Columbia and Alberta) are White Spruce, Engelmann Spruce, Lodgepole Pine, and Alpine Fir. Some lumber production in the New England States also is marketed as Spruce-Pine-Fir (south).


Panels featuring right-angled corners or equal corner-to-corner diagonal measurements.


See Finishes

Starved Glueline

Insufficient adhesive spread in the lamination process.


Sound Transmission Class. A measure of the ability of a wall or floor assembly to reduce noise transmission.


The difference in height between adjoining panels due to thickness variation.


A piece of wood, typically _” square, that’s inserted at regular intervals between layers of green wood to assist the drying process.

Sticker Stain

Sometimes called shadow, it’s a stain that forms under the stickers in a stack of drying wood.


In plywood manufacture, a method for temporarily sewing thin strips of veneer together into sheets before laminating them into panels. Stitching serves no structural function, but allows conservation of formerly wasted veneer scraps.

Stock Glulam

Glulams which are manufactured to common, standard dimensions and characteristics, and kept in inventory for immediate job site delivery. (May be cut to customer-specified lengths.)


See “Pitch streak.”

Stressed-Skin Panel

An engineered structural panel assembly for roof deck or floor applications built of plywood sheets glued to framing members. The quick-covering assembly has greater load carrying capacity than would its individual members if installed separately.


A lumber member supporting a series of cross members. Frequently applied to stair supports.


A stripe or ribbon pattern that occurs when woods with interlocked grain (which slopes in alternate directions) are quartersawn.

Structural I

Unsanded grade for use where shear and cross-panel strength properties are of maximum importance, such as panelized roofs and diaphragms. All plies in Structural I plywood panels are special improved grades and panels marked PS 1 are limited to Group 1 species. Other panels marked Structural I Rated qualify through special performance testing. Manufactured with Exterior or Exposure 1 durability classifications.


The basic vertical framing members of walls, usually 2x4s. Studs are traditionally spaced 16 inches on center, sometimes 24 inches as in the Engineered 24” Framing System.


A framing member, usually cut to a precise length at the mill and designed to be used in framing building walls with little or no trimming before it is set in place. Studs are most often 2x4s, but 2x3s, 2x6s, and other sizes are also included in the stud category; studs may be of wood, steel, or composite material.


See APA Rated Sturd-I-Floor

Sub-face (Sub-back)

The ply adjacent to the exposed face (or back) of a parallel laminated outer layer.


APA RATED SHEATHING panels applied directly over floor joists which will receive an additional underlayment layer. Structural wood panels provide strength and stiffness. They also reduce the number of floor joints as compared with board sheathing.


The platform upon which adhesive, laminate and/or other finishing material is applied.

Surfaced Checks

A drying defect that occurs when the surface dries too quickly in relation to the core.


Meeting the needs of the current generation without compromising the ability of future generations to meet their needs. (Definition from the Bruntland Commission Report, UN, 1987.) SUSTAINABILITY refers to the concept that new development must meet the needs of the present without compromising those of the future. Sustainability is measured in three interdependent dimensions: the environment, economics, and society—often referred to as the triple bottom line.

Sustainable Design

An integrated, whole building design approach that reduces negative impact on the environment. It strives to use resources wisely and minimize waste while improving the indoor air quality for the comfort and health of building occupants.

Sustainable Forestry

The practice of managing forest resources to meet the long-term forest product needs of humans while maintaining the biodiversity of forested landscapes. The primary goal is to restore, enhance and sustain a full range of forest values- economic, social, and ecological. (MR 7).


Thickness increase in a panel due to moisture absorption or wetting.

Synthetic Repairs

See Repairs


Beam resembling a “T” in cross section. Several side-by-side T-beams acting as a unit may form a floor. This principle accounts for the increased stiffness of glued floors.


Show-through on a smooth overlaid plywood panel surface of underlying grain or defects.

Tensile Strength

The maximum pounds per square inch of longitudinal stress a material can resist without tearing apart.

Tension Parallel To Grain

A measurement of the strength of wood when tension is applied in the same direction as that of the wood grain.

Tension Perpendicular To Grain

A measurement of the strength of wood when tension is applied across the direction of the wood grain. Glulam members would be designed to avoid inducing tension perpendicular to grain stresses.


The size of the cells in wood, described as ranging from coarse to fine; often confused with grain.

Texture 1-11

APA trade name for a special RATED SIDING panel 19/32 or thicker with 3/8-inch-wide vertical grooves typically spaced 4 or 8 inches on center. Shiplapped edges maintain pattern continuity when installed. - See APA Rated Siding

Textured Plywood

Panels with a variety of machined surface textures. Available in Exterior type with fully waterproof glueline for siding and other outdoor uses and for interior wall paneling. - See APA Rated Siding

Thermally Fused Melamine

Paper saturated with melamine resin which is thermally fused to the substrate.

Thermofoiled Lamination

A finishing process in which a flexible laminate, such as vinyl film, is vacuum-formed over a three-dimensional surface in a heated press.

Thermoplastic Resins

Adhesives that cure at room temperature and soften upon exposure to heat.

Thermosetting Resins

Adhesives that cure in a hot press via chemical crosslinking to form rigid bonds that are not re-softened by subsequent exposure to heat.


Thickness variation within a panel, or between panels.

Tongue-And-Groove Joint

A system of jointing in which the rib or tongue of one member fits exactly into the groove of another. A specially designed APA tongue-and-groove panel edge joint is particularly efficient in transferring the load across the joint. Some APA RATED STURD-I-FLOOR T&G panels measure 47-1/2 inches across the face.

Touch-Sanded Panels

Structural wood panels “sized” to uniform thickness by light surface sanding during manufacture. Sander skips are admissible. Normally applied to C-Plugged faces.


A sizing operation consisting of a light surface sanding in a sander. Sander skips to any degree are admissible.


Wood products infused or coated with any variety of stains or chemicals designed to retard fire, decay, insect damage or deterioration due to weather.

Triple Bottom Line

According to the World Business Council for Sustainable Development, “Sustainable development involves the simultaneous pursuit of economic prosperity, environmental quality and social equity. Companies aiming for sustainability need to perform not against a single, financial bottom line, but against [this] triple bottom line.”.


A combination of members usually arranged in triangular units to form a rigid framework for supporting loads over a span. Parallel chord trusses are also used for floor and roof supports.


A form of warp where one corner of a board is not aligned with the others.

Unbalanced Construction

Use of two different laminates or finishing materials on the front and back of a panel that respond unequally to changes in moisture, thus increasing the risk of warp.


A material applied over subflooring and directly beneath nonstructural finish flooring, such as tile or carpeting. Wood panel underlayment provides a smooth surface for finish flooring and excellent puncture and indentation resistance. - See also Subflooring, Plugged Crossband Under Face and Underlayment Grade

Underlayment C-C Plugged Exterior

An Exterior grade underlayment panel with a touch-sanded C-Plugged face ply. Common uses include underlayment in conditions of severe moisture or humidity (bathrooms, kitchens), refrigerator or controlled atmosphere storage rooms, exterior balconies and decks, pallet bins, tanks, boxcar and truck floors and linings, and open soffits.

Underlayment Grade

PS 1-designated, touch-sanded Interior panels designed as a base for finish flooring such as carpeting (and tile or linoleum when specified with a sanded face) and installed over structural subflooring such as APA RATED SHEATHING. These panels are manufactured with either interior or exterior glue - the latter designed for applications subject to long construction delays or similar moisture exposure. Underlayment panels are identified by Group number.

Uniform Load

Weight distributed evenly across a shelf or panel.

Unsanded Panels

Interior or Exterior sheathing grade panels designed for utility applications and left unsanded for greater stiffness, strength and economy.

Unsupported Length

The distance between the end supports, or intermediate bracing of a column or beam.

Urea Formaldehyde (UF)

Interior grade thermosetting resin commonly used in the composite panel manufacturing process.

Vapor Barrier

A material (such as plastic film) which controls moisture transmission through walls and other building elements. Often combined with insulation to control condensation. A vapor barrier should be installed on the warm side of walls.


Thin sheets of wood of which plywood is made. Also referred to as “plies” in the glued panel.


A thin sheet of wood laminated with others under heat and pressure to form plywood, or used for faces of composite panels. Also called ply.


Wood peeled, sawn, or sliced into sheets of a given constant thickness and combined with glue to produce plywood or laminated-veneer lumber. Veneers laid up with the grain direction of adjoining sheets at the right angles produce plywood of great stiffness and strength, while those laid up with grains running parallel produced flexible plywood most often used in furniture and cabinetry. White Wood - 1. Wood products intended for treating, but not yet treated. 2. A designation applied to a number of species, such as White Fir.

Veneer Grade

The standard grade designations of softwood veneer used in panel manufacture. The six grades are:
  • Special order “natural finish” veneer - Select all heartwood or all sapwood. Free of open defects. Allows some repairs.
  • Smooth and paintable. Neatly made repairs permissible. Also used for natural finish in less demanding applications.
  • Solid surface veneer. Router or sled repairs and tight knots permitted.
  • Plugged - Improved C veneer with splits limited to 1/8 inch in width and knotholes and borer holes limited to 1/4 inch by 1/2 inch.
  • Knotholes to 1 inch. Occasional knotholes 1/2 inch larger permitted providing total width of all knots and knotholes within a specified section does not exceed certain limits. Limited splits permitted. Minimum veneer grade permitted in Exterior type plywood.
  • Permits knots and knotholes to 3 inches in width, and 1/2 inch larger under certain specified limits. Limited splits permitted.

Vinyl Film

A flexible polyvinyl chloride laminate used for decorative surfacing which may be either clear or solid-colored.

Vinyl Films — 2 Mil Reverse Printed Rigid Film

Unembossed, extruded or calendered rigid PVC. The print design and ground coat are printed on the back of the film in reverse order (reverse print). Used for paneling, Kitchen cabinets, furniture and Mobile homes.

Vinyl Films — Sandwich Film

Calendered, semirigid two-ply laminate. This opaque base film is top printed and a clear overlay is laminated over the top. It is designed for mitre folding and flat sheet lamination. It is sometimes available with a scuff-resistant coating. This film ranges from 4.0 to 8.0 mils in thickness.

Vinyl Films — Semirigid Clear Film

Calendered semirigid PVC. This film is reverse printed on the back of the film. It can be embossed and can be coated with scuff-resistant coatings. This film ranges from 4.0 to 8.0 mils in thickness. This film can be mitre folded.

Vinyl Films — Solid Color Film

Calendered, semirigid film that is custom color matched in a variety of hues. This film is integrally colored and can be top printed and/or embossed. Top printed film is used in mobile homes, RV’s, commercial paneling and moveable walls. Solid color film is used in furniture, fixtures and displays. Kitchen cabinets and Office furniture. This film is sometimes available with a scuff-resistant coating. This film ranges from 3.5 to 8.0 mils in thickness.

Vinyl Films — Thermoformed Overlay Films

Calendered or extruded solid color rigid film in single-ply and two-ply construction. Gauges range from .010” to .030”. It may be printed in woodgrain or decorative patterns. Film is sometimes embossed and may be coated with scuff and stain resistant coatings. Primers for adhesion are available. This film is designed for thermoforming with heat and pressure in a bladder press or vacuum forming process.

Vinyl Films — Wrapping Films

Calendered or extruded rigid vinyl films in gauges from .005” to .010”. Film may be printed in woodgrain or decorative patterns. It may be embossed and may be coated for scratch and stain resistance. Primers may be added to promote adhesion. This film is designed for wrapping profiles and can be flat laminated and miterfolded.


See Core Gap

Volatile Organic Compounds (VOCs)

Carbon compounds emitted as gases from some solids or liquids, containing chemicals with adverse health effects. Concentrations are up to ten times higher indoors than outdoors.


Panels manufactured from reconstituted wood wafers, as opposed to strands, bonded with resins under heat and pressure like OSB. - See also Oriented Strand Board


The wooden lining of the lower part of an interior wall.


Horizontal timbers used to brace concrete form sections.


Thin to open areas in veneer sheets that result from outer log surface irregularities. Generally, only veneer peeled from the outer log surface will contain wane. Some wane areas may contain bark inclusions. For grading, wane is classed as an open defect.


The presence of bark or a lack of wood from any cause along the edge or corner of a piece of lumber.


Bending, twisting or turning of a panel due to unbalanced construction, exposure to excessive moisture, or other unfavorable conditions.


Any deviation of the face or edge of a board from flatness, or any edge that is not at right angles to the adjacent face or edge; the most common forms of warp are bow, cup, twist, and crook.


Bending or twisting from a straight line. An improperly seasoned piece of lumber may warp when exposed to heat or moisture. To reduce the possibility of warping, protect wood panels from dampness or moisture and follow APA spacing recommendations. Painting and water-repellent dips will minimize moisture absorption. Sealing all edges and back-priming also reduces the chances of warping in cabinet doors.

Water Repellents

Wood preservatives with water-resistant properties.

Waterborne Adhesives

Water-soluble adhesives such as urea formaldehyde and vinyl acetate commonly used in paper lamination.

Waterproof Adhesive

For purposes of this Standard, glue capable of bonding plywood in a manner to satisfy the exterior performance requirements given herein.


The structural wood panel component of an I-joist, the web is the vertical section located between the two flanges. Also see Box Beam and Truss

White Pocket

A form of decay (Fomes pini) that attacks most conifers but has never been known to develop in wood in service. In plywood manufacture, routine drying of veneer effectively removes any possibility of decay surviving. (Admissible amounts of white pocket permitted by this Standard were established through a 2-year research project at the U.S. Forest Products Laboratory.)


The tendency of wood to draw moisture up through its cells by capillary action in the direction of the grain.

Wood failure (percent)

The area of wood fiber remaining at the glueline following completion of the specified shear test. Determination is by means of visual examination and expressed as a percent of the test area.

Wood Foundation

A residential and light frame foundation system utilizing pressure-preservative-treated plywood panels and wood framing in place of poured concrete footings and masonry or poured concrete walls. The system is commonly known as the Permanent Wood Foundation (PWF). Permanent Wood Foundation basements are warmer, tighter, drier, and more leak-resistant than conventional basements. The system can often be installed on a prepared site in less than half a day in nearly any weather, speeding construction and reducing costs. The PWF is also applicable to crawl-space foundation construction.

Wood Veneers

Solid wood laminate in a “flat,” “quarter,” “rift” or “rotary” cut.

Z Flashing

A Z-shaped piece of galvanized steel, aluminum or plastic installed at horizontal joints of plywood siding to prevent water from entering wall cavity.

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