A built-up roofing consists of plies or layers of roofing felt bonded together on site with hot bitumen. A protective surface coating of gravel or slag is sometimes embedded in a heavy top coating of hot bitumen. It is laid down to conform to the roof deck, and to protect all angles formed by the roof deck with projecting surfaces, and forms a single-unit flexible waterproofed membrane fastened to the deck by cementing and nailing. The simple principle on flat roofs is to turn the membrane up to form a skirting or base flashing on the vertical surfaces, making a large watertight tray. The only outlets from this tray are the roof drains to dispose of water.