An advancement over traditional tar and gravel roofs, but less modern than modified asphalt roofing systems, conventional asphalt (bitumen) built-up roofing (BUR) for low-slope applications consists of conventional asphalt, formed around a synthetic reinforcement layer, to create a durable, dependable, and less expensive commercial roofing solution.
Conventional asphalt roofing comes in roll form and is typically formed by alternating from 2-5 layers of substrate and asphalt to create a multi-ply system that meets desired performance, cost, and warranty needs.
Conventional roofing for low-slope commercial applications is typically built-up roofing (BUR), accomplished by layering multiple sheets to achieve desired performance, budget, or warranty needs. Combinations typically include a base layer (base sheet), one or more inner layers (ply sheets), and a top layer (cap sheet).
The middle layers of multiply arrangements are called ply sheets that sit between the base sheet, which is affixed to the roof, and the cap sheet which is the topmost sheet in the built-up membrane.
The top layer of multiply arrangements is called the cap sheet. It often various slightly from base and ply sheets most noticeably because it is often topped with granules for extra impact resistance and UV protection.