GAFGLAS? Ply 4 Ply Sheet roofing felt is constructed with a strong glass fiber mat that is coated with the finest quality asphaIt, and is designed with optimum porosity to ensure a sound, watertight membrane. GAFGLAS? Ply 4 Ply Sheet is used as a ply felt in the construction of built-up roofs. It is designed for use in all climate zones and is appropriate for use as a base sheet, as a ply felt, and in the construction of flashing membranes.