First discovered as a roofing material in Europe in the 1960s and used for roofing in American construction since 1975, bitumen is a form of asphalt that is applied to roofs in long sheets of black material. When bitumen is "modified," additional polymers such as atactic polypropylene or styrene butadiene styrene are applied to the surface of the bitumen sheets. This modification helps the bitumen remain watertight and gives it a rubber or plastic-like quality.
Modified bitumen is offered with a smooth surface or with granulation, depending on the manufacturer. With granulation, the surface of the rolled-out bitumen is coated with white granules to give it a more rough appearance. You also have the option to purchase modified bitumen in different colors so that the roof matches the overall colors of the building.
Granulated modified bitumen roofing is compatible with any flat or low-slope commercial or residential roof. In the construction industry, low slope is defined as a roof with a pitch, or slope, of 2-12 or less, which is approximately a 10 degree angle. A 1-12 pitch, which closely resembles a flat roof, is roughly 5 degrees.
Granulated modified bitumen may be applied with a hot torch, "hot mopped" with a brush or cold-adhered. The easiest option, which has become popular In America, is a peel-and-stick method that allows the installer to roll out a sheet that adheres to the roof as it rolls. This method not only is the easiest, but also is the safest, as it reduces any burn risk from hot mopping or torching and no fumes are created in the process.
Granulated modified bitumen roofing is an inexpensive alternative to shingles or tiles, but the lifespan is significantly shorter, with some roofs needing repairs within a 10-year period. Water leaks may also be difficult to find at times. On the plus side, an existing modified bitumen roof may be repaired with coatings over the life of the roof.