Many commercial buildings or even common houses have gravel flat-roofs. This type of roof is composed of layers of asphalt, hot tar and roofing felt, lined with light-color gravel. Usually, this final layer of gravel protects the roof against sun damage and maintain a reasonable temperature in the summer and requires flat roof repair occasionally. It usually lasts more than 20 years.
Gravel roofs may also have other components (besides basic gravel), such as fiberglass and hot-applied asphalt, that creates a durable roof system. This one is called built-up roof (BUR) and it is an excellent weatherproofing, protecting the roof against storms, hail, and fire. It can also protect the final layer of the roof from degradation and blistering. This BUR aggregate usually has a fixed and predetermined dimension for all types of flat roofs, which is about a quarter-inch thick.
Another style of gravel roofs is ballast, usually used for single-ply flat roofs. Ballast stones are usually one to two inches thick ns its main purpose is to weigh down the roofing materials. But, besides that, it is a good insulator for the membrane and the other materials underneath and protects the roof from strong winds. These particularities make gravel roofs based on ballast more energy-efficient.