The built-up asphalt roof waterproof layer is really multiple layers of asphalt and felts (usually 3-5 layers). Built-up roofs are usually covered with small rocks (gravel) if the slope is not steep or the wind is not strong. The main advantage of these roofs is the redundant layers of waterproofing. The disadvantage to the owner is the cost and the use of torches and kettles. The cost is high due to the cost of the asphalt (competing with roads and road repairs). Other factors that drive up the price is that there are fewer roofers that want to install these types of roofs, which are tough on the installers, and fewer roofers have the knowledge to install these kinds of roofs.