P&TS underlayment is a structural plywood panel that can be nailed directly over the floor joists in 19/32" or thicker panels, saving time and cost in floor construction. Engineered for strength and durability; its cross-laminated veneer construction resists warp, splits, puncturing, and impact damage. - ¾ in. Exterior Fir Underlayment T&G, 4 ft. x 8 ft.