A.Clayton DeKorne responds: According to Tom Kositsky, a technical representative for the American Plywood Association, you would not need two layers of plywood for any structural purpose, as long as you use APA-trademarked Sturd-I-Floor panels with a 24-inch on-center span rating. (A complete explanation of the APA ratings, and guidelines for the use and installation of plywood, can be found in the APA’s Residential and Commercial Design and Construction Guide. Single copies are available free from the APA, 7011 S. 19th St. W., Tacoma, WA 98466; 206/565-6600).
Sturd-I-Floor-rated panels are designed as combination subfloor and underlayment panels, usually intended to go under a carpet and pad. In general, a panel intended for use as "subfloor" is designed as a structural member, with a span rating (such as 16, 20, or 24 inches on-center) that is clearly marked on the grade stamp (see illustration,...
to read the full article.