Plywood’s Built Strong Following
Since 1905, when plywood was developed in St. John, Ore., it has changed little. But its use in home building has radically changed the design and structure of homes.
Here’s a review of the types and applications of plywoods available.
Softwood-veneer plywood is most widely used in construction and industrial applications. It’s made by cross-laminating veneers such as pine, spruce, fir and hemlock. Running the grains at right angles gives plywood its strength. It comes in 4-by-8-foot panels; one-half, three-eighths and three-quarter-inch thicknesses are common.
During manufacture, wood veneers are bonded with an adhesive under a hot press. Depending on the adhesive, the plywood is graded for interior or exterior use. Interior-grade plywood is made with moisture-resistant glue. Exterior-grade uses 100% waterproof glue.
Plywood is graded by the quality of the veneer 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 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 one-eighth-inch wide and knotholes up to a one-half-inch diameter. C-grade (regular) allows tight knots and knotholes to a 1 1/2-inch diameter plus synthetic repairs and defects that do not impair strength.
D-grade permits knots and knotholes to a 2 1/2-inch diameter, synthetic repairs and stitching repairs.
Hardwood-veneer plywood is a quality interior panel for furniture and cabinet construction. The face and back plies are hardwood such as birch, oak, walnut and 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-by-10-foot panels, in one-half, three-quarter and 1-inch thicknesses.
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 come in three-eighths, one-half, five-eighths and three-quarter-inch thicknesses.