What Type of Plywood for Floors?

It's not just what you see on the surface that determines whether your floor will stand the test of time. You should think of your floor as a set of building blocks. If any of the lower blocks are unsteady, the final block won't be secure. In other words, get your subfloor right before worrying about your ceramic tile, hardwood or laminate.

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.

You install a plywood subfloor directly on top of the house's joists. You apply construction adhesive directly on the joists with a caulk gun, then set the plywood panel on the construction adhesive with the tongue facing the groove of the next panel. Tongue-and-groove plywood panels have ridges on one edge and slots on the other, making it easier to fit the panels together. This stops the edges of the plywood from moving and squeaking after insulation. Many tongue-and-groove panels self-gap to avoid buckling. If you're not using tongue-and-groove panels, place your panels with a 1/8-inch gap at all edges and ends to leave room for natural expansion.

When you've positioned your plywood panel, it should sit halfway over the joist, next to the next panel. You tap the panel into position with a sledgehammer and a tapping block, working back and forth across the entire area to get a tight fit. When the panel is tight, tap in two set nails, one at each corner, then use a pneumatic framing nailer to fasten the panel to the joists.

You fit the finish floor on top of the installed subfloor. Sometimes, you lay an underlayment on top of the subfloor. This is a thin layer of foam, felt, cement board, fiberboard or plywood. An underlayment provides a smoother surface for the finish floor than the subfloor does. It can also increase stability, muffle sounds and act as a moisture barrier.

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.

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.

Alternatives to plywood and reinforcements for it include cement board, OSB (oriented-stranded board) and Sturd-I-Floor. Cement board works well for tile finish floors because it's smooth and strong enough to take a lot of water, which may affect the strength of plywood. It isn't a subfloor by itself and typically goes on top of a plywood underlayment. 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.