Built-Up Roof (BUR)
The traditional hot-tar-and-gravel roof is built from three or more plies of waterproof material alternated with hot tar and ballasted by a layer of smooth river stone. Once made of tar paper, these types of roofs gradually are using more-advanced materials such as fiberglass membranes.
Torch Down Roof
Sheets of modified bitumen are torched and adhere to the roofing surface. Once the layers reach the right temperature, seams are melted together to create a waterproof seal. The membrane layers of a tourch down roof are made of an asphalt compound called bitumen that is modified with either rubber or plastic.
Alterations and retrofits.