Built-up roofs or BURs (also called a tar and gravel roof) is a well-established and popular style of flat roof that is made up of alternating layers of roofing felt or fiberglass and asphalt that is applied with heat, which combines to make a long-lasting sturdy roofing system.

The gravel embedded on the top coat of asphalt (the flood coat) protects the underlying layers from the elements, including ultraviolet rays and natural threats like roof hail damage or heavy rains.

Various types of BURs require a different type and weight of gravel depending on the underlying layers, the structure of the building, and the climate of the location.

Even though the asphalt waterproofs the roof (highly important for flat roofs where water and moisture can build up instead of draining off as it does on a sloped roof), the gravel layer extends the life of the roof.

Note that the “gravel” is usually some sort of aggregate material like pea gravel, slag, or mineral granules that are pea-sized and about a quarter-inch thick, and serve the purpose of satisfying the Class A surface burning designation.