Polymer-modified bitumen or modified bitumen (MB) sheet membranes were developed in Europe in the early 1960s and have been in use in the U.S. since the mid-1970s. Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. Modified Bitumen Roof systems typically are installed as a two-ply system and almost always are fully adhered.
There are two types of MB roofing membranes:
- SBS polymer-modified bitumen membranes commonly are installed in hot moppings of asphalt (similar to BUR systems) or cold adhesives. Some SBS modified membranes are self-adhering; this is, they contain an adhesive backing.
- APP polymer-modified bitumen membranes typically are heat-welded or torch-applied. Consumers should be cautioned that the NRCA (National Roofing Contractors Association) does not recommend torch-applying a modified bitumen membrane sheet directly to a wood deck.
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