Built Up Roofing System
BUR systems generally are composed of alternating layers of bitumen and reinforcing fabrics, called roofing felts or ply sheets, that create a finished membrane. BUR systems are considered fully adhered if applied directly to roof decks or insulation.
The bitumen layers typically used in BUR roof systems can be asphalt, hot rubberized asphalt, or cold-applied adhesive. Solid asphalt is heated in a kettle to become a liquid for ease of application. A layer of hot bitumen asphalt is applied to the substrate with a mop or mechanical spreader. Cold-applied adhesives typically are solvent-based asphalts that don’t have to be heated in a kettle or tanker. Then a layer of reinforcing fabric is laid on top. This is considered one layer or ply. This process is repeated to add additional layers until the desired number of plies is achieved (e.g., 4-ply). The redundancy of the repeated layers of material creates a waterproof seal.
The top layer of a BUR roofing system consists of a flood coat of asphalt with aggregate embedded. Aggregates such as gravel, calcite, or mineral granules can be used. Higher reflectivity (SRI-value) can be achieved by choosing calcite or other light-coloured aggregate. Perimeter flashings, which are the transitions from the horizontal flat roof surface to vertical surfaces such as walls or parapets, typically consist of two-ply modified bitumen adhered with either asphalt, adhesive, or torched in-place.
When complete the BUR system is considered a homogeneous reinforced membrane. These multiple layers provide the longevity and durability for which BUR systems are known.