HOME IMPROVEMENT : Going Beneath the Surface to Appreciate Plywood

Since 1905 when plywood was first developed in St. John, Ore., it has changed little--but its use in home building has changed the design and structure of homes radically.

Here’s a review to help you understand the types and applications of plywoods that are available:

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-foot-by-8-foot panels with 1/2-inch, 3/8-inch, and 3/4-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% 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 1/8-inch wide and knotholes up to 1/2-inch diameter. C-grade (regular) allows tight knots and knotholes to 1 1/2-inch diameter plus synthetic repairs and defects that do not impair strength. D-grade permits knots and knotholes to 2 1/2-inch diameter, synthetic repairs and stitching repairs.

Some plywood and reconstituted panels are performance rated by the American Plywood Assn. These are stamped with a number such as 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 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-foot-by-10-foot panels 1/2-inch, 3/4-inch 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.