These products are factory-made composite sheet goods consisting of bitumen (asphalt or coal tar) modifier(s), and reinforcement(s). “Modified Bitumens” encompass a variety of bitumen-based materials that can be differentiated by thickness, modifier type and reinforcement type(s). Bitumens exhibit the thermoplastic quality of being softened by heat. Modified bitumen membranes comprise their own generic category defined by bitumen base and are often installed as a multiply system. The application method may be by torching (heat-welding), hot-mopping, or cold-adhesive. The membrane system may be held in place on the roof by fully adhering the base sheet, mechanically attaching the base sheet or by using ballast.
There are two main types of “modifiers” used to make sheet membranes: APP and SBS. APP (atactic polypropylene) and SBS (styrene-butadiene-styrene) refer to the type of polymers that are added to the membrane. There are a variety of reinforcements that are used, including, but not limited to, fiber glass mats and scrims and polyester mats and scrims.