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Glossary

Plywood

2-4-1®
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-A
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-B
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.
Adhesive
See Glue
AFG-01
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.
Air-Dried
See Seasoning
Anchor Bolt
Bolts that tie the sill plate and thus the frame of a structure to its foundation.
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.
B-B
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.
B-D
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.
Back-Priming
Application of a coat of primer to the back of a panel. Cabinet doors should be back-primed to prevent warping.
Backstamp
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
Batten
A thin, narrow strip of plywood or lumber used to conceal or protect a joint between adjoining pieces of lumber or plywood.
Beam
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.
Bevel
To cut panel edges or ends at an angle to make smooth mating joints between panels.
Blocking
Light lumber strips nailed between major framing members to support edges of structural panels where they meet.
Blow
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.
Boat
Patch. - See Repairs
Bond
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.
Bow
Distortion of a structural wood panel so that it is not flat lengthwise. - See also Cup
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.
Bridging
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.
Brushed
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
Bundle
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.
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.
Camber
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.
Cantilever
That portion of a structural beam which extends or “cantilevers” beyond the end support, and whose end is not supported.
Caulk
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 Core Gap
Center-To-Center
See On-Center and Clear Span
Chamfer
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
Checking
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.
Chord
Any of the outside members of a truss connected and braced by web members. Also, may refer to perimeter members of a panel diaphragm.
Circular Plug
See Repairs
Class I, II
See B-B Plyform
Clear Span
Distance between inside faces of supports.
Code
See Model Code
Column
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.
COM-PLY®
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.
Component
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
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
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.
Crawl Space
A space often about two feet high beneath a house floor allowing access to plumbing or wiring. - See also PIRF
Cripple
Any vertical framing member cut less than full length, as in cripple studs under a window opening.
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.
Crossband (Cores)
In plywood, the veneer layers with grain direction perpendicular to that of the face plies.
Cup
Crosswise distortion of a structural wood panel from its flat plane. - See also Bow (Distortion across panel).
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
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.
Deflection
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.
Delamination
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.
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.
Diaphragm
Elements of a building that provide shear strength to withstand wind and earthquake loads.
Dogbone Plug
Repairs
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
Ductile Materials
Materials that can be subjected to large strains before rupturing. Ductile materials are capable of absorbing shock or energy.
Eave
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.
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 Spacing
See Panel Spacing
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.
Edgemark
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.
Embossed
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.
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
Engineered 24” Framing
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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.
Expansion
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.
Face
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 Grain
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Face-Checking
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.
Fascia
Wood or plywood trim used along the eave or the gable end of a structure.
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.
Filler
A material for filling nail holes, checks, cracks or other blemishes in surfaces of wood before application of paint, varnish or other finishes.
Finishes
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.
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 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.
Flange
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).
Flashing
See Z Flashing
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
Footing
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.
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.
FRP
See “Fiberglass-Reinforced Plastic”
FRT
See “Fire-Retardant-Treated”
Furring
Process of leveling parts of a ceiling, wall or floor by means of wood strips, called furring strips, before adding panel cover.
Girder
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.
Glue
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.
Glue-Nailed (Nail-Glued)
{terms_and_definitions}
Glued Floor System
See APA Glued Floor System
Glueline
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).
Glulam
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.
Grade
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
Grain
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.
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.
Groove
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.
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).
Growth Ring
{terms_and_definitions}
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.
Hardwood
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.
HDO
See High Density Overlay
Header
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.
Heartwood
The nonactive core of a tree distinguishable from the growing sapwood by its usually darker color and greater resistance to rot and decay.
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.
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)
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.
I-Beam
See I-JOIST
I-Joist
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.
Identification Index Former term for Span Rating
See Span Rating
IIC
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.
Inner Plies
All plies of a plywood panel except face and back.
INR
See Impact Noise Rating
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
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.
Joist
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.
Kerf
A slot made by a saw; the width of a saw cut.
Kiln-Dried
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
Knot
Natural growth characteristic of wood caused by a branch base imbedded in the tree trunk.
Knothole
Void produced when a knot drops out of veneer.
Laminated Veneer Lumber (LVL)
Structural wood elements constructed of veneers laminated together with their fibers oriented in a parallel direction.
Lamination Layup
The physical arrangement of different grades of laminations throughout the depth of a glulam member.
Laminations
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.
Lap
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.
Lay-Up
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.
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.
Live Loads (L.L.)
See Loads
Loads
{terms_and_definitions}
Lumber Core
Plywood manufactured with a core composed of lumber strips. The face and back (outer) plies are veneer.
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.
MDO
See Medium Density Overlay
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
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
Modulus of Elasticity (MOE)
{terms_and_definitions}
Moisture Barrier
See Vapor Barrier
Moisture Content
The amount of water contained in the wood, usually expressed as a percentage of the weight of oven-dry wood.
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.
Nail-Glued
See Glue-Nailed
Nails
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.
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
Noncertified
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.
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.
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.
Overhang
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)
P & TS
Plugged and touch-sanded face of a plywood or composite panel.
Paint
See Finishes
Panel Clip
Specially shaped metal device for supporting panel edges to reduce differential deflection in roof construction.
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
Paneling
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.
Patch
See Repairs
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
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 Streak
A localized accumulation of pitch in wood cells in a more or less regular streak.
Plate
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.
Plug
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).
Ply
A single veneer in a panel.
Plyform
See B-B Plyform
Plyron
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.
Prefabricated
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
Prefinished
A ready-to-use panel with factory-applied finish - paint, overlays or coatings.
Preframed
Panelized building in which wall, floor or roof sections are framed and sheathed at the factory.
Preprimed
A panel with a factory-applied primer or undercoat needing only final finish after installation.
Preservatives
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.
Pressure-Preservative Treated
Wood treated with preservative or fire retardants by pressure-injecting treating solutions into wood cells.
Primer
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.
PRP
See APA Performance Rated Panels
Purlin
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.
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 Resistance
{terms_and_definitions}
Racking Strength
A measure of the ability of a structure, such as a wall, to withstand horizontal forces acting parallel to the structure.
Radius
A dimension that is commonly used as a means of describing the camber requirements in a glulam beam, as in radius of curvature.
Rafter
Sloping supporting member of a roof immediately beneath the sheathing.
Raised Grain
See Grain Raise
Rated Sheathing
See APA Rated Sheathing
Rated Siding
See APA Rated Siding
Repairs
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.
Resawn
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.
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.
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.
Ripping
Sawing wood in the direction of the grain.
Rotary Peel
See Peeler Log
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
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.
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.
Sapwood
Living wood of pale color near the outside of a log. Under most conditions, sapwood is more susceptible to decay than heartwood.
Scarf Joint
An angled or beveled joint in plywood splicing pieces together. The length of the scarf is 5 to 12 times the thickness.
Screws
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.
Seasoning
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.
Seasoning
Shear Wall - See Diaphragm
Sheathing
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
Shim
See Repairs
Shiplap
Jointing in which ends or edges are notch-milled to overlap and form a rabbet joint.
Shop Cutting Panel
{terms_and_definitions}
Siding
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
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
Soffit
The underside of the roof overhang. Structural wood panels are often used as finishing materials for soffits.
Softwood
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.
Solid Core
See Plugged Core
Sound Transmission Class
See STC
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.
Species Group
See Group Number
Stains
See Finishes
STC
Sound Transmission Class. A measure of the ability of a wall or floor assembly to reduce noise transmission.
Stitching
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.)
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.
Stringer
A lumber member supporting a series of cross members. Frequently applied to stair supports.
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.
Stud
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.
Sturd-I-Floor
See APA Rated Sturd-I-Floor
Subflooring
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.
Synthetic Repairs
See Repairs
T-Beam
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.
Telegraphing
Show-through on a smooth overlaid plywood panel surface of underlying grain or defects.
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.
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
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.
Truss
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.
Underlayment
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.
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.
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.
Veneer
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.
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.
Void
See Core Gap
Waferboard
Panels manufactured from reconstituted wood wafers, as opposed to strands, bonded with resins under heat and pressure like OSB. - See also Oriented Strand Board
Wainscot
The wooden lining of the lower part of an interior wall.
Waler
Horizontal timbers used to brace concrete form sections.
Warping
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.
Web
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
Wicking
The tendency of wood to draw moisture up through its cells by capillary action in the direction of the grain.
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.
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.