A flat or low slope roof is often not a suitable surface for shingles or other roofing materials because moisture can’t flow off them. One solution to this is a built-up, or tar and gravel, roof, where many layers of hot asphalt or bitumen, tar paper and a waterproof membrane are applied and then covered with gravel. The gravel both protects these layers and acts as a ballast.
Advantages - Tar and gravel roofs will last between 10 and 20 years and are rated by how many layers are applied during installation. Typically, a roof with 3 to 5 layers of bitumen and felt is considered standard. This type of roof coating is lower in cost compared to other flat roof solutions and offers resistance to heat conduction between the interior and exteriors of your home, which saves on heating and cooling costs.