View Full Version : plywood for subfloor questions
03-06-2012, 10:21 AM
I found there are different plywood on the market: sanded plywood, sheathing plywood, pine plywood, fir plywood, subfloor plywood, domestic plywood, and ....
I am using it on my basement floor. which is good to me? the subfloor plywood? the sheathing is for wall only?
The plywood for sheathing has to meet certain shear-stress strength and weather resistance standards, since it's structurally used for keeping the wall & roof framing from racking under wind loads, as well durability under high humidity cycling. The stuff for subfloors has flex & rigidity requirements for spanning between joists (requirements you don't have, if you're putting the ply directly atop rigid foam resting on concrete, and fully supported across its area rather than only on 1.5" strips 16"-48"on center).
IIRC at the maximum spacing between joists for any subflooring it is only allowed to flex 1/360th of the span width at 100lbs per square foot live loading to meet spec, (and the strength of wooden flooring is allowed to be used for thinner subfloor if the flooring is perpendicular to the joists) but since there is NO span in your application that spec is moot. 100lbs live loading anywhere on a 7/16" or half-inch sheathing fully supported by XPS and concrete won't have ANY appreciable deflection compared to the (16"/360=) 0.044" allowed for subflooring for 16" o.c. joists.
The nail-retention of subflooring or sheathing is also a factor, but since sheathing is designed to handle the weight & stresses of wind-on-siding, it's more than sufficient for flooring fasteners. Plywood used in cabinetry is geared more toward absolute flatness and surface smoothness. It could be used in this low mechanical-stress application, but t & g or ship-lap sheathing is more appropriate here.