Fir Plywood – Species preferred by most contractors. • 1/2” CD is 4-ply. Many of our competitors do not stock 1/2” fir plywood. • 5/8” CD is 5-ply. Most of our competitors stock 4-ply. • 3/4” T&G is Sturdi-floor underlayment grade • 1/4” AC is solid core for use as underlayment and it is stamped accordingly.