The building envelope or building enclosure includes the roof, walls, and windows and is one of the most critical parts of a building. Its role is more than to be aesthetically pleasing. The envelope must protect occupants from the weather and help occupants be comfortable, dry, and safe. Roofs are affected by snow, wind, rain, solar radiation, etc. Thus, the roof is the part of the building that is most exposed to outside conditions. Roof membranes are subject not only to sudden temperature changes but also to accelerated aging because of ultraviolet radiation, acid rain, and mechanical stress caused by wind. As a result, many different type of roofing materials and systems have been developed over the many years. Rubber, rubber-like and polymeric materials play an important role in helping the roof resist the harsh environmental conditions. This chapter gives an overview of the various roofing systems (past and present), their properties and application method, as well as the various ASTM standards associated with them.