A plywood generally uses 1/2-inch to 3/4-inch-thick sheets of plywood that have one rough side (which faces down) and one smooth side, which faces up. The sheets are generally 4 x 8 or 4 x 12 feet in size. Plywood is made by gluing together multiple thin layers of solid wood at 90-degree angles and pressing them tightly as the glue dries to form a very strong structural sheet. In the 1950s, this material quickly replaced solid wood planking as a subfloor material in homes.
Standard plywood can be used for subfloors, but it is more common to use tongue-and-groove sheets that interlock along the edges. The recommended thickness of the plywood subfloor is governed by the spacing of the joists. Some experts suggest that 15/32-inch plywood should be standard if the underlying floor joists are spaced 16 inches apart or less, but slightly thicker 3/4-inch plywood should be used for joists spaced further apart. However, check with your local code authorities for the official recommendations.
OSB (oriented strand board) is made from large, flat chips arranged in as many as 50 layers, which are adhered together with glues and pressed into sheets. Because it makes use of leftover wood material, OSB tends to be cheaper than plywood, and it is more uniform, since it has none of the knots and other defects that may be present in plywood. OSB came into use as a subfloor material in the 1970s.
The type of OSB used for subfloors generally is made with tongue-and-groove edges (T & G). The standard subfloor for joists spaced up to 16 inches apart is 23/32-inch T & G sheets. Some local codes may require 1-inch thick sheets for joists up to 24 inches apart; for these, you may need to buy from a construction lumber yard since general home-improvement centers may not stock them.
