Stevens EP™ brand of TPO is the world's first reinforced TPO (thermoplastic polyolefin) based roofing membrane. It is formulated from a proprietary compound of ethylene propylene rubber and polypropylene. Stevens EP™ membranes exceed the ASTM D-6878-03 specification for TPO roofing. They are environmentally friendly roofing materials from cradle to grave that provide outstanding weatherability, fire and chemical resistance. The seams and details of a Stevens EP™ roof are hot-air welded to create a bond stronger than the material itself.