Modified Bitumen roof systems are composed of multiple layers. They are most commonly applied as a two-ply system and are practically always entirely adhered.
There are two types of Modified Bitumen roofing membranes:
SBS are usually installed in hot mopping of asphalt or cold adhesive and certain are self-adhering. They tend to impart a rubberized quality to asphalt.
APP are heat welded or torch applied. They tend to impart a plasticized quality to asphalt.