Softwood-veneer plywood is most widely used in construction and industrial applications. It's made by cross-laminating an odd number of wood veneers such as pine, spruce, fir and hemlock. Running the wood grains at right angles to each other gives plywood its strength. It comes in 4 x 8-foot panels with , , and inch common thicknesses.
During manufacture, wood veneers are bonded with an adhesive under a hot press. Depending on the adhesive used, the plywood is graded for interior or exterior use. Interior-grade plywood is made with moisture-resistant glue. Exterior-grade uses 100 percent waterproof glue.
Plywood is graded by the quality of the veneer used on the face and back surfaces, as well as by the type of adhesive. The highest quality veneer grades are N and A. These have smooth, defect-free surfaces. Grades C and D are the lowest. N-grade natural finish is all sapwood or heartwood free of open defects and with no more than six well-matched wood repairs per panel.
A-grade is similar with no more than 18 neatly made repairs parallel to the grain. B-grade has a solid surface with circular repair plugs and tight knots permitted to 1-inch diameter and minor splits allowed.
C-grade (plugged) permits synthetic repairs, splits up to -inch wide and knotholes up to inch diameter. C-grade allows tight knots and knotholes to 1 inch diameter plus synthetic repairs and defects that do not impair strength.
D-grade permits knots and knotholes to 2 inch diameter, synthetic repairs and stitching repairs.
Some plywood and reconstituted panels are performance rated by the American Plywood Association (APA). These are stamped with a number like 24/16 which means they can be used in walls over studs on 24-inch centers and floors over 16-inch centers.
Hardwood-veneer plywood is a quality interior panel for furniture and cabinet construction. The face and back plys are hardwood-like birch, oak, lauan mahogany, walnut an ash with softwood interior plies.
Lumbercore plywood is the highest quality with hardwood face and back laminated to a center core of hardwood strips. It's used for desk and table tops, and comes in 4 x 10-foot panels , and 1 inch thick.
Medium-density overlay (MDO) is exterior plywood veneered with an opaque resin-treated fiber overlay suitable for painting. It's used for exterior signs and soffits as well as kitchen and bathroom cabinets. MDO exterior plywood siding is also available.
High-density overlay (HDO) is similar to MDO but tougher and more abrasion-resistant. It's used to build concrete forms and for counter tops and workbenches. Both MDO and HDO com in , , , and inch thick.
Plyron is a trade name for plywood with a hardboard face and back used for utility cabinets, doors and underlayment for carpet or tiles. It comes in interior and exterior grades , and inch thick.
Plywood sidings are offered with a broad range of veneer face grades, surface patterns and textures where the natural beauty of the wood shows. Reconstituted wood panels:
Particle board is the most widely used and known reconstituted panel. This non-veneered panel is made from wood chips bonded with resins under heat and pressure. Industrial grade and underlayment are the two most common types. Industrial particle board has a fine finish and is used in furniture and cabinets under a plastic laminate. Underlayment particle board goes over a subfloor and under carpet or tile. Standard thicknesses are , and inch.
Wafer board is similar but made from wood wafers. Its distinctive wafer pattern makes it popular for cabinets, storage bins and craft projects. Construction uses include wall or roof sheathing, subflooring and underlayment. Sheets are available in 7/16, and -inch thicknesses.
Oriented strand board (OSB) is like wafer board but has thin wood strands instead of wide wafers. The strands are mechanically oriented in perpendicular layers like plywood veneer grains and then bonded with phenolic resin. Considered to be one of the strongest reconstituted panels, OSB is used like wafer board. Besides standard 4 x 8-foot sheets, larger panels up to 8 x 24 feet can be ordered in thicknesses from to .
Hardboard is made by interlocking refined wood fibers and then compressing them into thin, hard, impact-resistant sheets with heavy pressure rollers. The well-known Masonite is a hardboard. It's used where low weight but not high strength is needed for such things as cabinet backs. Use hardboard where good finishing qualities are needed.
Tempered hardboard has been treated to improve stiffness, hardness and finishing properties. It can be used outside for shutters and door panels. It comes in 1/16, and inch thicknesses.
Fiberboard, also called grayboard, is a non-veneered structural panel used for carpet underlayment, sound deadening, exterior siding, wall sheathing and protective floor cover-up. Standard panels are 4 x 8 but the Homasote Co. also offers 4 x 10, 4 x 12 and 8 x 12 sheets and inch thick.
Composite plywood is a softwood-veneer-faced panel with a particleboard or oriented-strand core. Known by the trade name Comply, it's used like softwood-veneer plywood and comes in the same sizes and thicknesses.
