Modified Bitumen systems consist of asphalt with added modifiers for durability. These are either one- two- or three ply-systems, with two-ply being the most common. They are factory manufactured and combine the redundancy of built up roofing with the ease of application of a single-ply system.
- Styrene Butadiene Styrene (SBS) systems, usually installed with hot asphalts or cold adhesives
- Atactic Polypropylene (APP) systems, almost always installed using the torch method
Both SBS and APP systems can be reinforced with glass or polyester.
Advantages of a Modified Bitumen sytem include:
- Factory-manufactured rolls of roofing materials ensures quality control
- Expansion and contraction with movement of the building
- Variety of surface finishes to enhance resistance to oxidation and damaging UV rays