GAFGLAS Ply 4 Ply Sheet roofing felt has a high tensile strength and a high degree of dimensional stability, resulting in a roof system with greater resistance to splitting and cracking. It is constructed with a strong glass fiber mat that is coated with the finest quality asphalt, and is designed with optimum porosity to ensure a sound, watertight membrane. The optimum porosity of the felt provides ready escape of air while allowing proper asphalt penetration at the time of mopping.