Commonly referred to as “tar and gravel” roofs, built-up roof systems have been used in the U.S. for more than 100 years. Known as BUR, these systems are made of alternating layers of bitumen and reinforcing fabrics that create a finished membrane.
The number of plies in a cross-section is the number of plies on a roof. For example, the term “four plies” means a four-ply roof membrane construction. Sometimes, a base sheet—used as the bottommost ply—is mechanically fastened. Built-up roofs are considered fully adhered if applied directly to roof decks or insulation.
This low-slope roofing system continues to be the standard after all these years and is most likely the type of system that is on your current roof.
Polymer-modified bitumen or modified bitumen (MB) sheet membranes have been in use in the U.S. since the mid-1970s.
These roof membranes are made of reinforced fabrics that serve as carriers for hot polymer-modified bitumen. Much like BUR membranes, MB roof system membranes are composed of multiple layers. They’re usually installed as a two-ply system and are almost always are fully adhered.
This system’s flexible, almost rubber-like quality makes it perfect for buildings that show signs of movement.
There are two types of MB roofing membranes:
SBS polymer-modified bitumen membranes are commonly are installed in hot moppings of asphalt or cold adhesive. Some are self-adhering, which means they contain an adhesive backing.
APP polymer-modified bitumen membranes are typically heat-welded or torch-applied.
