One of the most common roofing systems we see is called a modified bitumen roof. Modified Bitumen is rolled out on flat roofs.
Polymer-modified bitumen or modified bitumen (MB) sheet membranes were developed in Europe in the early 1960s and have been in use in the U.S. since the mid 1970s. Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. MB roof systems typically are installed as a two-ply system and almost always are fully adhered.
Source Everybody Needs a Roof
Modified is often damage by hail when the hail penetrates the granules and allows water to reach the underside of the material.