Polymer modified bitumen (MB) sheet membranes were developed in Europe in the early 1960's and have been in use in the U.S. since the mid 1970's. Commonly referred to as a 2nd generation built up roof, it is lightweight, available with mineral granule surfacing or smooth. This system provides a cost competitive alternative to conventional built-up roofing. There are two types of modified bitumen membranes, SBS (styrene-butadiene-styrene) and APP (attactic polypropelene). Modified bitumen membranes can be installed with a common torch method, hot asphalts or cold-applied adhesives. A typical manufacturer warranty ranges from 10-20 years.