The Firestone Ply IV Sheet is asphalt impregnated, glass fiber mat reinforced roofing ply which has been designed to provide superior durability and ease of application. It is designed for use in the construction of Firestone Asphalt Built-Up Roof Systems. This Sheet can be used in conjunction with Firestone Modified Bitumen Base and Cap Sheets to build Hybrid Systems. Firestone Ply IV Sheet may be used as a ply in Modified Bitumen and Built-Up Roofing Systems.