Plywood is composed of thin sheets of veneer, or plies, arranged in layers to form a panel. Plywood always has an odd number of layers, each one consisting of one or more plies, or veneers.
How is Plywood Manufactured?
In the plywood manufacturing process, a log is turned on a lathe and a long knife blade peels the veneer. The veneers are clipped to a suitable width, dried, graded, and repaired if necessary. Next the veneers are laid up in cross-laminated layers. Sometimes a layer will consist of two or more plies with the grain running in the same direction, but there will always be an odd number of layers, with the face layers typically having the grain oriented parallel to the long dimension of the panel.
Adhesive is applied to the veneers which are laid up. Laid-up veneers are then put in a hot press where they are bonded to form panels.
Wood is strongest along its grain, and shrinks and swells most across the grain. By alternating grain direction between adjacent layers, strength and stiffness in both directions are maximized, and shrinking and swelling are minimized in each direction.
How does Plywood Compare with OSB?
Manufacturers of oriented strand board (OSB) and plywood claim both products work well. The issue for most builders who choose between plywood and OSB is
durability. OSB looks like "a bunch of wood chips glued together". Detractors are quick to say, "OSB falls apart". This opinion has a familiar tone. Plywood suffered the same criticism not too long ago. Delamination of early plywood sheathings gave plywood a bad name. Not many builders share that view today.
All model building codes use the phrase "wood structural panel" to
describe the use of plywood and OSB. Codes recognize these two materials as
the same. Likewise, the Engineered Wood Association, the U.S. agency
responsible for approving more than 75% of the structural panels used in
residential construction, treat OSB and plywood as equals in their published
performance guidelines. And wood scientists agree that the structural
performance of OSB and plywood are equivalent under controlled, dry conditions.
There really is little evidence to indicate one product is better than the other. It comes down to personal preference of the builder and specific applications where one may outperform the other.