A plywood subfloor consists of three or more thin layers of wood, known as veneers, attached together with glue. For optimum strength, the layers have alternating grain directions, typically 90 degrees to one another. Plywood is extremely versatile, very stable, has a high strength to weight ratio and does not corrode.
Grade of Plywood
The letters A, B, C, D and X grade plywood for quality, with A being the highest quality. Be aware that these letter grades don't represent structural properties, only visual characteristics. Plywood has two ratings: The first letter rates the quality of the front of the panel, and the second rates the quality of the back of the panel. Panels graded A are free of small knots and defects. B-graded panels may have small defects, C-graded panels may have open knots, holes and discolorations, and D-graded panels may have several defects. It's possible for a panel to have an A-graded front and a D-graded back. A scrap wood panel get an X-grade.
"Interior," "exterior," "structural" and "marine" are classifications of plywood. Interior plywood is ideal for rooms not exposed to moisture, such as bedrooms, and makes a good subfloor for carpeting, tiles, vinyl, engineered wood laminate flooring and hardwood flooring. Exterior plywood is a better choice for bathrooms, kitchens and any other rooms exposed to water and moisture. Structural plywood is good for rooms with heavy furniture or high traffic, such as living rooms, dining rooms and entryways. Marine plywood is the strongest and most water-resistant type of plywood, making it a good choice for bathrooms and basements.
Thickness of Plywood
If you're installing a subfloor in a new home, you should refer to your local building code for guidelines. The thickness of your plywood subfloor depends on the joist spans. The National Wood Flooring Association recommends a minimum plywood panel thickness of 7/8-inch for joist spans of 19.2 to 24 inches, and a minimum plywood panel thickness of 5/8-inch for joist spans of 16 inches or less. If your plywood subfloor is too thin, it might not cope with the load, and it might creak or squeak when you walk on it.
Plywood Alternatives
Alternatives to plywood include cement board, OSB (oriented-stranded board) and Sturd-I-Floor. Cement board works well for tile finish floors because it's strong enough to take a lot of water, which may affect the strength of plywood. Fiber cement board is a smoother type of cement board. OSB is a single-layer composite wood, made from shredded wood strands joined with wax, that stands up well against moisture and works as underlayment. Sturd-I-Floor is a specialized engineered wood panel designed for single-layer flooring installations on upper floors, where you lay carpet directly over the subfloor. Sturd-I-Floor is more rigid than either traditional plywood or OSB.
