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