tar and gravel
A roof covering with gravel or small stones laid over a tar layer, commonly used on flat roofs.
Tar and gravel is a common roofing material, especially for flat roofs (including common variants) which uses a base layer of tar made from organic resins or coal byproducts, with a layer of gravel or small stones (such as smooth river stones) on top to ballast the tar. It is heavier than a simple tar roof, and sometimes requires structural reinforcement.
Gravel is commonly considered to add aesthetic appeal to a tar roof, making it more attractive if people have to look down on it. It also serves as a flame retardant.
The stone layer is part of the covering arrangement, which distinguishes this material from exposed tar roofing. The gravel does not form a continuous stone slab; it is a layer of separate small stones over the tar.