Wood is by nature anisotropic. That is, it exhibits different properties when its characteristics are measured in different directions. Wood can be measured longitudinally, radially, and tangentially, with the bending strength of the longitudinal measure being 15 to 40 times as great a its tangential measure. Because plywood is engineered from plies alternating at 90-degree orientations, the bending strength become equalized in all directions.
Plywood is one of the most ancient of building materials dating back to the Egyptian empire around 3500 B.C. It has been surmised that Egyptian architects created the material due to a lack of quality wood products. Thin sheets of inferior wood was glued crosswise until the desired thickness was achieved, then higher quality veneers were glued to the exterior to give the appearance of fine wood. The discovery of the material's improved structural quality was an accident.
Throughout history plywood has been used in countless applications from furniture making, to cabinetry, to dwelling construction. The type of function of the application dictates the strength required of the plywood to be used. Plywood designed for outdoor or marine use will be engineered to resist water and retain its strength even in high humidity conditions. High strength plywood for industrial uses may be constructed of mahogany rather than the more common pine or spruce. Plywood used for flooring may be 1-inch thick, while veneers for fine furniture may be as little as 1/6-inch.
Plywood is divided into 5 grades based on its appearance and strength. Grade A is the highest grade. Grade A plywood has a smooth surface, is paint-able and has few patches. Grade B is solid but may have some knots and minor splits in the surface. Grade C may have more knots and splits but is still structurally sound. Grade C Plugged falls between C and B with splits and cracks that are not as pronounced as Grade C. Grade D plywood could have knots up to 3 inches. Plywood frequently has different quality surfaces on the front and back and may be referred to a "A-C" or "B-C" to describe the grading of each side.
The strength of the plywood is determined by a number of different factors including the type of wood each veneer is cut from; the type of adhesive used to glue the veneers together; the amount of pressure used to form the boards; and the number of plies. Higher grade plywood will resist cutting and will hold nails well. Lower grade plywood may be strong enough for interior structural use, but is not appropriate for finished use.
Plywood is valued for its high strength and low cost compared to most common solid woods. Contemporary methods of plywood production use both pressure and heat to force the throughout the wood fibers of each veneer resulting in a tight bond. Because each layer is crossed at 90 degrees from the previous layer the plywood exhibits strength flexibility and stiffness qualities in all directions. This cross layering of the veneers also help eliminate warping and splitting making plywood and excellent choice for construction purposes.