Reply by Don Newton

As Lee said keep the sheets size to 4×4 or 2×4 as recommended by the NWFMA. I would at least seal the underside of the plywood and the edges. Stagger your joints when laying the plywood and allow a minimum of 1/8” gap between panel edges. There is an “Underlayment Grade” plywood that is not T & G. The underlayment grade doesn’t have any voids in the core.

-- Don, Pittsburgh