Built up Roof  is a very common type of roof covering often used on flat roofs in the city. It is composed of multiple layers of a felt membrane held in place with a mopped on with a liquefied asphalt slurry. With periodic maintenance recoating , approximately every 5 years, this roof can be very durable. If the coating is somehow breached, water penetration will cause blisters, delamination, and underlayment damage.