1. A layer or thickness, as of building or roofing paper. 2. A layer of wood in plywood. 3. A layer of felt in a built-up roofing membrane; a four-ply membrane would have at least four plies of felt at any vertical cross section cut through the membrane; the exposure of any felt may be computed by dividing the felt width minus 2 inches by the number of plies, thus, the exposed surface of a 36 inch wide felt in a four-ply built-up roof membrane should be 8-1/2 inches. 4. A layer or thickness of yarns used in carpet; if the pile yarn is described as 4-ply, it means that each tuft is made of 4 yarns spun together.