Gravel roofs (also known as tar and gravel roofs) are a common sight on commercial buildings throughout southeast Wisconsin (and everywhere else!). Ever wonder why gravel is put on a roof? Two reasons: 1) to weigh down the roofing material of single-ply flat roofs so the wind can’t lift it up, and 2) to protect the underlying layers of a built-up roof. The small gravel pebbles are embedded in a layer of tar to keep them in place.