Modified bituminous roofing systems are available in two materials which are commonly used today for low-sloped roofing applications: Atactic Polypropylene (APP) and Styrene-Butadiene-Styrene (SBS). The primary purpose for both of these materials is to provide the weatherproofing cover for the roof assembly. Crude bitumen is a hydrocarbon created naturally from petroleum, and modified bitumen is a blend of other organic materials with a polymer additive of either atactic polypropylene or styrene butadiene styrene. Atactic polypropylene is a plastic additive that gives rigidity and tear resistance to the final product, and styrene-butadiene-styrene is a rubber additive that gives more resilient benefits. These blends are then applied to a base material of polyester or fiberglass. Sheet asphalt paving (mastic asphalt concrete) makes up the majority of bituminous products used in the U.S. Even though bitumen is sometimes referred to as tar or tar-like in its consistency, it is not to be confused with tar. Tar is a product manufactured from coal and is not used as a part of this roofing system.