MB's are a roof system developed by taking the qualities of a BUR system along with the qualities of a single-ply roof. A standard MB roof system should have a base sheet or ply, similar to the BUR system, then a waterproofing sheet similar to single-ply.
To break it down, a modified bitumen membrane is composed primarily of polymer-modified bitumen reinforced with one or more plies of fabric such as polyester, fiberglass, or a combination of both. Factory surfacing, if applied, includes mineral granules, slag, aluminum, or copper.
Modified bitumen's generally use a traditional waterproofing medium -- asphalt -- modified with atactic polyproplylene (APP), styrene (SBS), synthetic rubber, or other agents that will enhance the asphalt properties.
Properly installed and mainatained MB roof systems should last 25+ years.