Plywood
Plywood is a structural wood panel product manufactured from thin sheets of wood veneer that are cross-laminated—meaning the grain direction of each layer is oriented at right angles to the adjacent layers—and bonded together under heat and pressure using waterproof adhesives.[1] This construction confers exceptional dimensional stability, preventing warping and shrinking that plague solid wood, along with a high strength-to-weight ratio that enables it to withstand impacts, chemicals, and fluctuating environmental conditions better than many alternative materials.[1] Developed through industrial processes refined in the 19th and 20th centuries, plywood's modern origins trace to a 1865 U.S. patent by inventor John Mayo, which described layering thin wood sheets with adhesives to form durable panels, though rudimentary laminated wood techniques date back millennia to ancient civilizations like the Egyptians around 2600 BC.[2][3] By the early 20th century, standardized production emerged, with the first 4 ft by 8 ft sheets introduced in 1928 for building and furniture applications, and associations like the APA – The Engineered Wood Association, formed in 1933, establishing quality standards that propelled its widespread adoption.[4] The manufacturing process begins with debarking and conditioning logs, followed by rotary peeling into veneers, drying, grading, adhesive application, and hot-pressing into panels, optimizing wood fiber use and yielding a product stronger per unit weight than solid lumber.[5][6] Plywood's versatility supports diverse applications, from structural sheathing in walls, roofs, and floors to concrete formwork, siding, marine construction, and furniture components, where its rigidity and resistance to splitting provide advantages over dimensional lumber in load-bearing and finishing contexts.[1] Environmentally, it represents efficient resource utilization, requiring less raw timber than equivalent solid wood volumes while maintaining renewability through managed forestry.[7] Available in thicknesses from 5/16 inch to over an inch and in softwood or hardwood varieties, plywood's engineered properties have made it indispensable in construction and manufacturing, balancing cost, performance, and sustainability.[1]
History
Origins and Early Uses
The concept of plywood originated with ancient lamination techniques, with archaeological evidence indicating the use of glued wood veneers as early as 2600 BC in Egypt, where scarce timber prompted craftsmen to layer thin sheets crosswise for enhanced durability in furniture, coffins, and small watercraft.[8][3] These early panels conserved resources while improving structural integrity against splitting, as cross-graining resisted warping better than solid wood.[9] Traces of such laminated artifacts have been recovered from pharaonic tombs, underscoring practical engineering rather than ornamental intent.[10] Similar rudimentary lamination appeared in other ancient civilizations, though less extensively documented; for instance, around the 1st millennium AD in China, thin wood shavings were produced via primitive slicing methods, potentially for layered applications, but without the systematic cross-ply adhesion seen in Egyptian examples.[8] In Europe during the Renaissance, veneering—applying thin wood slices to substrates—emerged for decorative furniture by the 1600s, laying groundwork for stronger composites, yet these lacked the multi-layer bonding defining true plywood.[4] The transition to formalized early plywood occurred in the late 18th century, with British inventor Samuel Bentham patenting a veneer production machine and gluing process in 1797, enabling efficient multi-ply boards for shipbuilding and cabinetry.[11] By the mid-19th century, American patents like John K. Mayo's 1865 grant advanced steam-powered veneer peeling and adhesive bonding, facilitating curved furniture components and prototypes for broader structural uses, though production remained artisanal and limited by adhesive quality.[12] These innovations built on ancient principles but introduced mechanization, marking plywood's shift from sporadic craft to proto-industrial material primarily for furniture and lightweight framing.[13]Industrial Development
Industrial production of plywood began in the mid-19th century, driven by mechanized veneer slicing. In 1797, English inventor Samuel Bentham patented a steam-powered machine for producing continuous veneers from logs, enabling efficient thin-sheet fabrication essential for layered plywood construction.[4] This innovation addressed limitations of manual cutting, which had restricted output to small-scale artisanal applications since ancient times. Subsequent advancements, including Immanuel Nobel's rotary lathe in the late 19th century, allowed for the continuous peeling of uniform veneers, scaling up material availability for commercial glue-lamination processes.[13] The first U.S. patent for plywood, granted to John K. Mayo on December 26, 1865, described a method of cementing veneers under pressure to form durable panels resistant to warping.[14] Commercial industrialization accelerated in the early 20th century, particularly in the U.S. Pacific Northwest, where abundant softwood resources supported factory-scale operations. By 1905, the industry emerged with dedicated mills producing glued panels for applications like furniture and early automotive components, showcased at the Lewis and Clark Centennial Exposition.[3] Standardization advanced in 1928 with the introduction of the 4 ft by 8 ft (1.2 m by 2.4 m) sheet size, optimizing handling and transport for building trades.[4] A pivotal breakthrough occurred in 1934 when chemist James Nevin at Harbor Plywood Corporation developed phenolic resin adhesives yielding fully waterproof bonds, expanding plywood's viability for exterior and marine uses previously dominated by solid lumber.[15] This innovation, tested rigorously against boiling water exposure, underpinned the formation of the Douglas Fir Plywood Association in 1933 to promote uniform standards amid growing output.[8] By the late 1930s, U.S. production exceeded demand for domestic markets, with exports rising and wartime applications foreshadowing further mechanization.[16]Post-War Expansion and Innovations
Following World War II, the plywood industry in the United States experienced significant expansion driven by the post-war economic boom and housing construction surge. In 1944, production stood at 1.4 billion square feet from 30 mills, but by 1954, output had nearly tripled to approximately 4 billion square feet from 101 mills.[8] This growth continued, with production reaching 7.8 billion square feet between 1959 and 1964, surpassing earlier forecasts.[8] The expansion was fueled by increased demand for affordable, versatile building materials in residential and commercial construction, as well as furniture and consumer goods. Plywood's strength-to-weight ratio and ease of use made it ideal for sheathing, subflooring, and prefabricated components during the suburban housing boom.[17] By the 1950s, plywood had become a standard material in home construction, supporting rapid urbanization and infrastructure development.[18] Innovations in adhesives and manufacturing processes further enabled this growth. Post-war availability of low-cost synthetic resins, leveraging expanded petroleum infrastructure, improved bond durability and weather resistance, allowing for exterior and structural applications.[19] Adhesive and technology advancements facilitated the use of Southern pine and other species, culminating in the first Southern pine plywood mill in 1964.[8] These developments expanded production capabilities beyond traditional Douglas fir, broadening plywood's utility in diverse climates and uses.[8]Composition and Properties
Material Components
Plywood is constructed from multiple thin layers of wood veneer, termed plies, which are bonded cross-grain with adhesives to enhance strength and stability. These veneers are derived from logs processed via rotary peeling, slicing, or sawing, yielding sheets typically 1 to 3 millimeters thick.[20] Softwood veneers, commonly sourced from species such as Douglas fir, pine, spruce, hemlock, redwood, and cedar, predominate in structural plywood for construction due to their abundance, uniform properties, and cost-effectiveness. Hardwood veneers, including birch, oak, maple, poplar, cherry, and larch, are frequently employed in face and back plies for decorative or higher-grade panels, offering superior surface quality and resistance to wear.[21][22][21] Adhesives, primarily thermosetting synthetic resins, constitute the binding component, applied between plies under heat and pressure to form durable cross-laminated structures. Urea-formaldehyde (UF) resins are standard for interior plywood, providing strong bonds at ambient curing temperatures but with limited hydrolytic stability. Phenol-formaldehyde (PF) resins, used in exterior grades, deliver waterproof performance through higher curing temperatures and greater chemical resistance to moisture. Melamine-urea-formaldehyde (MUF) and other modifications bridge interior and exterior needs, balancing strength, cost, and emission profiles.[23][24][20] Certain plywood variants incorporate alternative core materials, such as lumber cores from hardwood strips for improved screw-holding or composite cores like medium-density fiberboard (MDF) or particleboard for dimensional stability and reduced weight, though all-veneer construction remains prevalent for optimal shear strength. Additives, including extenders, fillers, or biocides, may be integrated into adhesives or veneers to mitigate defects, enhance bonding, or provide rot and insect resistance, particularly in treated panels.[25][20]Mechanical and Physical Properties
Plywood's mechanical properties are characterized by orthotropic behavior resulting from the cross-laminated veneers, which distribute stresses more evenly across the panel compared to solid wood, reducing splitting and warping under load.[26][27] The modulus of rupture (MOR) in bending, a key measure of flexural strength, typically ranges from 30 to 70 MPa for structural softwood plywood, depending on species, number of plies, and adhesive type, with higher values parallel to the face grain.[26][28] Tensile strength parallel to the grain averages 20-50 MPa, while compression strength parallel to the surface is around 30-50 MPa, enhanced by the layered structure that resists buckling.[28] In-plane shear strength, critical for applications like flooring, often exceeds 10 MPa, benefiting from the interlocking grain orientations.[29] The modulus of elasticity (MOE), indicating stiffness, varies from 6 to 12 GPa parallel to the face grain for common softwood plywood, dropping to 2-5 GPa perpendicular due to inherent wood anisotropy, though cross-lamination mitigates overall directional weakness.[28][26] These properties are tested under standards like ASTM D3043 for structural panels, where plywood outperforms oriented strand board in bending and tension for equivalent thicknesses.[1] Fatigue resistance is notable in cyclic loading scenarios, such as in bridges, where cross-graining prevents progressive failure modes seen in isotropic materials.[27] Physical properties include a density of 400-700 kg/m³, influenced by wood species and veneer thickness, with softwood plywood averaging 500-600 kg/m³ and hardwoods higher, directly correlating with load-bearing capacity.[28][30] Plywood is hygroscopic, absorbing moisture up to 10-15% equilibrium content in humid conditions, but exhibits lower swelling (0.2-0.5% per percent moisture change) than solid wood due to balancing layers constraining expansion.[31] Thermal expansion coefficients are low at approximately 6 × 10⁻⁶ /°C in the plane for balanced panels, minimizing dimensional changes over temperature ranges of -20°C to 60°C.[32] Thermal conductivity mirrors that of the base wood, around 0.1-0.15 W/m·K for softwoods, increasing with density and moisture.[33]Performance Under Stress
Plywood exhibits orthotropic mechanical behavior due to its cross-laminated veneer structure, where alternating grain directions enhance resistance to splitting, warping, and in-plane shear compared to solid wood, while providing directional stiffness and strength primarily parallel to the face veneers.[26] [34] This layering distributes stress across plies, with outer veneers bearing the majority of tensile and compressive loads in bending, resulting in more uniform performance than isotropic solid wood, though overall values are lower than clear wood specimens.[26] Under bending stress, plywood's modulus of rupture (MOR), a measure of ultimate strength, typically ranges from 33.72 to 42.61 MPa, while the modulus of elasticity (MOE) indicates stiffness at 6.96 to 8.55 GPa; these values derive from static bending tests per ASTM standards and vary with wood species, density, and moisture content.[26] Tension strength parallel to the surface exceeds perpendicular strength, as cross-plies contribute minimally to longitudinal load-bearing, leading to failure initiated at veneer edges or knots.[26] [34] Compression parallel to the face parallels bending performance, with buckling risks in thin panels under edgewise loading, mitigated by thicker cross-plies.[26] Shear performance distinguishes plywood, with rail shear strengths of 26.2 to 39.3 MPa in the plane, bolstered by glue lines and orthogonal grains that resist horizontal sliding; glue-line shear, however, is lower at 1.3 to 5.6 MPa, sensitive to adhesive quality and defects like voids.[26] Factors such as veneer thickness, number of plies (e.g., 5–9 for structural grades), and environmental exposure degrade properties: moisture above 12% reduces MOR by up to 50% via softening of wood and adhesive bonds, while defects like knots or slope-of-grain deviations amplify stress concentrations, dropping effective strength per Hankinson's formula for angled loading.[26] [34]| Property | Typical Range | Direction/Notes | Source |
|---|---|---|---|
| MOE (Bending) | 6.96–8.55 GPa | Face grain parallel | [26] |
| MOR (Bending) | 33.72–42.61 MPa | Ultimate strength, elastic limit | [26] |
| Rail Shear | 26.2–39.3 MPa | In-plane, enhanced by cross-graining | [26] |
| Glue-Line Shear | 1.3–5.6 MPa | Perpendicular to plane, adhesive-dependent | [26] |
Manufacturing Process
Veneer Preparation
Veneer preparation in plywood manufacturing commences with log debarking and conditioning to produce thin wood sheets suitable for layering. Logs, typically sourced from softwood or hardwood species and cut to lengths of 2 to 4 meters, undergo mechanical debarking to remove outer bark, minimizing contamination and preserving log integrity during subsequent processing.[21] Conditioning follows debarking, involving immersion in hot water vats or exposure to steam at temperatures of 60–82°C (140–180°F) for 1–6 hours, depending on log diameter, species, and desired veneer quality; this softens lignin and hemicellulose, enhancing wood plasticity for uniform peeling while reducing lathe knife wear and veneer defects like splits or checks.[21][35] Additives such as sodium sulfite may be introduced to further facilitate softening, particularly for hardwoods.[36] Peeling occurs on a spindleless or spindle lathe, where the conditioned log—centered and rotated at high speed—is pressed against a stationary knife, yielding continuous veneer ribbons 0.5–3 mm thick; the process peels from the outer layers inward, with core remnants repurposed as lumber or particleboard feedstock.[21][36] Resulting veneers are clipped to standard widths, typically 1.2–1.5 meters, and scanned for defects such as knots or wormholes, which are trimmed to ensure structural uniformity.[21] Drying reduces veneer moisture content to 2–12% via high-velocity hot air ovens or roller dryers operating at 150–200°C for seconds to minutes, preventing glue adhesion failures and dimensional instability in final panels; over-drying risks brittleness, while under-drying promotes delamination.[21][36] Dried veneers are graded by quality—face, back, or core—and stored under controlled humidity to maintain readiness for gluing.[21]Gluing and Assembly
Glue application in plywood production involves spreading adhesives onto the surfaces of dried veneers using automated glue spreaders, which deposit a thin, uniform layer to ensure effective bonding during subsequent pressing.[21] The primary adhesives are synthetic resins, including urea-formaldehyde (UF) for interior applications offering moisture resistance but not waterproofing, and phenol-formaldehyde (PF) for exterior grades providing boil-proof durability due to their thermosetting properties under heat and pressure.[21][37] Melamine-urea-formaldehyde (MUF) variants enhance bond strength and reduce formaldehyde emissions compared to standard UF while maintaining moderate water resistance.[38] Assembly, or layup, follows gluing, where veneers are stacked in an odd number of plies—typically three or more—with the grain direction of adjacent layers oriented perpendicular to one another, typically at 90 degrees, to distribute stresses evenly and confer anisotropic strength approximating isotropy.[21][39] The outermost face and back veneers are often higher-quality, while inner core veneers may be lower-grade for cost efficiency, with automated layup lines aligning sheets precisely to minimize defects like misalignment or air pockets.[5] This cross-graining exploits the natural tensile strength along wood fibers while mitigating weaknesses perpendicular to them, as verified by mechanical testing standards.[21] Layup occurs rapidly in continuous production to prevent glue from setting prematurely, with assembly times controlled to optimize open time—typically 10-30 minutes for UF adhesives—before transfer to hot presses.[40]Pressing and Finishing
After the assembly of veneered layers with adhesive, the plywood panel undergoes hot pressing to cure the resin and form a consolidated product. Hydraulic multi-opening presses apply uniform pressure and heat to the stacked panels, typically ranging from 1.0 to 3.5 MPa, though higher values up to 10-12 MPa may be used for denser woods like poplar.[41][42][43] Pressing temperatures generally fall between 100°C and 160°C, selected to activate thermosetting resins such as phenol-formaldehyde without degrading the wood fibers.[41][44] The duration varies with panel thickness, wood species, and resin type, often 2 to 7 minutes total, or approximately 30-60 seconds per millimeter of thickness to ensure adequate heat transfer to the core.[41][45] Prior to hot pressing, a cold pre-pressing step may flatten the veneers and distribute adhesive evenly.[20] This process expels excess moisture and air, minimizing voids while achieving bondline thickness of about 0.1-0.2 mm.[46] Post-pressing, panels are cooled to stabilize dimensions and prevent warping, often under controlled conditions to manage residual internal stresses from uneven moisture gradients during heating. Edges are then trimmed to precise dimensions using circular saws, removing excess material and ensuring squareness for standard sizes like 4x8 feet.[39][47] Sanding follows, typically on both faces with industrial belt or roller sanders to achieve uniform thickness tolerance (e.g., ±0.2-0.5 mm for structural grades) and a smooth surface for further applications.[21][48] This step removes surface irregularities, adhesive squeeze-out, and minor defects, with grit progressing from coarse (e.g., 80-120) to fine (e.g., 150-220) for grades requiring aesthetic quality.[47][49] Finishing may include optional overlays, such as phenolic films for exterior use, or coatings applied via rollers or vacuum impregnation to enhance durability and appearance, though uncoated panels are common for interior structural roles.[21] Panels undergo visual and mechanical inspection for defects like delamination or voids before grading and packaging.[39] These operations ensure compliance with standards like PS 1-09 for structural plywood, where bond quality directly influences shear strength and moisture resistance.[50]Classification and Types
By Wood Species and Source
Plywood is classified by the species of wood used in its veneers, which influences mechanical properties such as strength, density, and resistance to warping. Softwood plywood, derived from coniferous species like pine (Pinus spp.), spruce (Picea spp.), and Douglas fir (Pseudotsuga menziesii), typically exhibits lower density (around 400-500 kg/m³) and is suited for structural applications due to its availability in large panels and cost-effectiveness.[51] These species provide adequate shear strength for sheathing and subflooring, with bending strength varying by specific tree; for instance, spruce plywood often shows modulus of elasticity values of 8-10 GPa in structural grades.[26] Hardwood plywood utilizes veneers from deciduous trees such as birch (Betula spp.), oak (Quercus spp.), and beech (Fagus spp.), yielding higher density (600-800 kg/m³) and superior surface quality for finish work. Birch plywood, for example, demonstrates higher bending strength (up to 50-60 MPa) compared to softwoods, attributed to the tighter grain and greater fiber density, making it preferable for cabinetry and furniture.[52] Beech veneers further enhance plywood's tensile strength, outperforming poplar in shear and modulus properties due to inherent wood fiber characteristics.[53] Species selection impacts glue adhesion and overall panel stability, with hardwoods generally requiring more precise drying to minimize defects. Tropical plywood incorporates species from regions like Southeast Asia, including meranti (Shorea spp.) and lauan (Shorea spp.), offering enhanced density and uniformity over softwoods, with strength properties supporting marine and exterior uses. These panels achieve higher evenness in layers and resistance to delamination, though sourcing raises concerns over sustainable harvesting; certified tropical plywood from managed forests maintains comparable performance to temperate hardwoods.[54] The source of wood—whether domestic temperate forests or imported tropical—also affects traceability and environmental impact, with U.S.-sourced softwoods often adhering to stricter regulations than unregulated imports.[26] Variations in species-specific properties necessitate matching plywood type to application demands, as evidenced by empirical testing showing birch and alder outperforming aspen in load-bearing scenarios.[53]By Construction and Bonding
Plywood classification by construction and bonding encompasses the arrangement of veneers or core materials in its layered structure, as well as the adhesive type determining moisture resistance and bond durability. Construction methods influence dimensional stability, machinability, and load-bearing capacity, while bonding types dictate suitability for interior, exterior, or severe wet environments. These categories are defined in standards such as U.S. Voluntary Product Standard PS 1-19 for structural plywood, which specifies bond performance through boil tests and shear strength requirements, and European Norm EN 636, which assigns classes based on exposure conditions.[55][56] Bonding classifications primarily revolve around adhesive performance under moisture. Interior-bonded plywood uses urea-formaldehyde or similar resins providing sufficient dry strength but failing under prolonged wetting, as the glue lines delaminate when boiled for over an hour in standard tests; it suits enclosed, dry applications like cabinetry interiors. Exterior-bonded plywood employs phenolic or resorcinol-formaldehyde adhesives that maintain integrity after extended boiling or cyclic wetting-drying, enabling use in sheathing, siding, or framing exposed to weather. Marine-bonded variants utilize premium waterproof glues, such as resorcinol or epoxy, passing rigorous submersion and shear tests for boatbuilding or high-humidity structures where delamination risks structural failure. Intermediate bonds, blending interior and exterior properties, appear in some softwood panels for occasional moisture exposure.[57][58][57] Construction types vary by core composition and ply configuration to optimize specific performance traits. All-veneer construction stacks thin, cross-grained wood veneers throughout, yielding uniform thickness and flexibility ideal for bending or curved forms, though it may exhibit slight telegraphing of inner plies under finish. Lumber-core plywood features a central panel of edge-glued solid wood strips (typically 1/4 to 3/4 inch thick) sandwiched between face, back, and crossband veneers, enhancing screw-holding strength—up to 3-5 times that of veneer-core equivalents—and machinability for furniture or doors, but increasing weight and cost. Engineered-core options replace the center with particleboard or medium-density fiberboard (MDF) for superior flatness and reduced expansion (under 0.5% in humidity swings versus 1-2% for veneer-core), suiting painted panels or overlays, though they compromise edge strength and require exterior bonding for wet areas to prevent core swelling. Multi-ply constructions, often 5-9 layers in structural grades, distribute stresses evenly via alternating grain directions, achieving span ratings like 24/16 (24/16 pounds per square foot live/dead load) under PS 1-19 deflection limits.[59][59][58]Specialty Variants
Marine plywood is formulated for environments with high moisture exposure, employing waterproof phenolic adhesives and veneers selected for density and rot resistance, typically from species such as Douglas-fir, Western larch, or tropical hardwoods like meranti.[60] All plies must achieve a B-grade or higher, permitting small knots but prohibiting voids or defects that could compromise structural integrity under wet conditions.[60] It adheres to specifications like BS 1088, which mandates specific tropical hardwood veneers without core gaps, ensuring delamination resistance in marine applications such as boat hulls.[61] ![De Havilland DH-98 Mosquito][float-right]Aircraft plywood prioritizes an exceptional strength-to-weight ratio, constructed from thin birch or mahogany veneers—often as few as three to five plies totaling 0.4 mm thick—laminated with high-quality adhesives to meet aviation demands for lightness and rigidity.[62] Grades such as A-A feature defect-free surfaces on both faces, while A-B allows minor imperfections on one side, with production emphasizing uniform grain and minimal weight variation for applications like the De Havilland Mosquito's airframe, which relied on molded plywood for its fuselage.[62] Domestic variants may incorporate African mahogany over poplar or basswood cores, pressed to comply with military specifications like MIL-P-6070 for structural aircraft use.[63] Fire-retardant plywood undergoes chemical impregnation during manufacturing to form a char barrier that limits flame spread and smoke development, typically achieving Class A or B ratings per ASTM E84 surface burning tests.[64] Treatments are applied under pressure to penetrate all veneers, followed by kiln drying to below 19% moisture content, enabling use in interior assemblies where building codes require reduced combustibility without altering the wood's appearance significantly.[65] Flexural strength is evaluated via ASTM D5516 for exposure to elevated temperatures, ensuring the material retains load-bearing capacity longer than untreated plywood in fire scenarios.[66] Flexible or bending plywood features cross-grained veneers of uniform thickness, often from hardwoods like lauan or meranti, allowing radii as tight as 2-3 inches without cracking when steamed or mechanically formed.[67] Available in barrel or column variants sized 4x8 feet at 3/8-inch thickness, it incorporates a central cross-ply for stability, suited for curved architectural elements such as arches or furniture components that demand conformability over straight panels.[68]
Standards, Dimensions, and Quality
Sizing and Thickness Standards
Plywood panels are produced in standardized nominal sizes and thicknesses to facilitate interchangeability in construction and manufacturing, though actual dimensions may vary slightly due to manufacturing processes like sanding and moisture content. In the United States, the Engineered Wood Association (APA) establishes performance-rated standards under APA PS 1, with common panel lengths and widths including 4 feet by 8 feet (1.22 m × 2.44 m), 4 feet by 9 feet, and 4 feet by 10 feet.[1] Thicknesses are designated by performance categories such as 5/16 inch, 3/8 inch, 15/32 inch, 1/2 inch, 19/32 inch, 5/8 inch, 23/32 inch, 3/4 inch, 7/8 inch, 1 inch, and 1-1/8 inch, corresponding to nominal values before final processing.[1] Actual thicknesses often differ from nominal; for instance, nominal 3/4-inch plywood typically measures 23/32 inch (about 0.719 inches) after sanding to meet span ratings and tolerances specified in APA standards.[69] Thickness tolerances in APA PS 1 allow for variations based on panel grade and construction, with structural panels required to meet specific limits for flatness, thickness swell, and edge alignment to ensure load-bearing performance.[70] Internationally, the International Organization for Standardization (ISO) governs dimensions through standards like ISO 1954:2013, which specifies tolerances for length, width, thickness, squareness (maximum deviation of 0.2% of the smaller dimension or 2 mm), and edge straightness (maximum bow of 0.2% of panel length). Common metric panel sizes align with imperial equivalents, such as 1220 mm × 2440 mm, while thickness tolerances for sanded plywood are often calculated as a maximum of (nominal thickness × 0.03 mm) + 0.2 mm and a minimum of -0.3 mm to -0.5 mm depending on grade.[71][72] The following table summarizes common nominal thicknesses and their typical actual measurements in U.S. APA-rated softwood plywood:| Nominal Thickness (inches) | Performance Category | Typical Actual Thickness (inches) |
|---|---|---|
| 1/4 | 5/16 | 0.281 - 0.344 |
| 3/8 | 3/8 | 0.344 - 0.406 |
| 15/32 | 15/32 | 0.438 - 0.500 |
| 1/2 | 1/2 | 0.469 - 0.531 |
| 19/32 | 19/32 | 0.563 - 0.625 |
| 5/8 | 5/8 | 0.594 - 0.656 |
| 23/32 | 23/32 | 0.688 - 0.750 |
| 3/4 | 23/32 | 0.719 |
| 7/8 | 7/8 | 0.844 - 0.906 |
| 1 | 1 | 0.938 - 1.000 |
| 1-1/8 | 1-1/8 | 1.063 - 1.125 |