A spray foam roof system can be installed on a built up roof, either smooth or gravel surfaced. The key determining factor is that a built up roof is permanently adhered to the roof deck and underlying surfaces. To prep a built up roof for spray foam application, all the gravel must be removed and the surface must be cleaned.
What is Built Up Roofing?
Built-up roofing, also called BUR, is the most common roofing material used on low-slope roofs. It is composed of alternating layers of reinforcing fabric and bitumen (asphalt) and is finished with a top layer of aggregate, such as stone or gravel. BUR is preferred for low-slope, or “flat,” roofs because it creates a continuous sealed surface. By contrast, roof shingles are not sealed and require a fairly steep roof slope to shed water effectively.
Types of Built-Up Roofing
Built-up roofing is nothing new. The concept been around for over 100 years, although the material and its installation certainly have evolved during that time. Modern built-up products incorporate a rigid insulation layer for improved energy efficiency.
Most built-up roofing is comprised of three parts: bitumen material, ply sheets, and one or more surfacing materials. The bitumen can be either “hot,” meaning it is heated so that it liquefies during installation, or it can be “cold,” which is more like an adhesive and is not heated. Cold built-up roofing can be sprayed or applied with a squeegee. It does not give off toxic fumes during application and is not dependent on weather. It also offers better performance than hot built-up roofing.
The ply sheets of built-up roofing are special fabrics that are reinforced with fiberglass or organic materials. Each ply sheet layer is laid over hot or cold bitumen to bond it to the roof. Ply sheets are commonly produced in a standard width of about 36 inches.
Surfacing materials form the top layer and may consist of small stones or finer gravel, depending on the application. This layer provides a finished look and helps protect the layers below from sunlight and damage from flying or falling debris. It also makes the roof safe to walk on. One special type of built-up roofing, called ballasted roofing, uses large stones (about 1 1/2 to 2 1/2 inches in diameter) for the surface layer. With a ballasted system, the lower layers are not adhered or fastened to the roof structure, and the heavy surface layer keeps the roofing in place.
Reroofing with spray foam is also possible on these other commercial roofing system types.