Built-up roof membranes (BUR) consist of multiple layers (plies) of membrane reinforcement, commonly referred to as “felts” (typically Type IV or VI glass or #15 organic felt with oxidized bitumen, polyester reinforcement may be used with Styrene Ethylene Butylene Styrene (SEBS) modified asphalt) that are installed in a shingle method with alternative layers of bitumen and are finished with a surfacing layer. Built-up roofing membranes can be installed as part of uninsulated, conventional, or protected roofing systems.
The procedure for installing the membrane following preparation of the substrate is basically as follows:
- Application of the asphalt bitumen.
- Application of the felts.
- Installation of the membrane flashings.
- Installation of protective surfacing.
The first step when installing a BUR system is the application of the asphalt bitumen. The bitumen is used to adhere the felts to their substrate and to each other. The asphalt provides the waterproofing properties to the roof membrane and correct application is critical. The asphalt bitumen typically used in BUR roof systems is oxidized Type 1, Type 2, or Type 3 asphalt, SEBS modified asphalt, or a cold-applied adhesive. Asphalt is heated in a kettle or tanker and then applied by mop or mechanical spreader. Cold-applied adhesives typically are solvent-borne asphalts that do not have to be heated in a kettle or tanker.
The number of plies of felt used will either be two, three, or four (or more in some applications) depending upon the project specifications and requirements. The most common BUR application is a “four-ply” built up roof which denotes the use of four plies of reinforcement used in the membrane construction.
The surfacing layer may use a variety of materials, including mineral aggregate (gravel, slag or mineral granules) embedded into a flood coat of asphalt, mineral surfaced sheet adhered to the membrane with asphalt or adhesive, or a field- or factory-applied coating (aluminum or elastomeric coatings). The reflectivity of built up roofs depends on the surfacing material used.