Built-up roofing (BUR), also called tar and gravel roofing, consists of multiple layers of felt or fiberglass alternated with hot asphalt or tar, creating a strong, thick membrane typically 3/8 to 1/2 inch thick. This traditional system provides excellent durability and waterproofing. The process begins with installation of a base sheet (heavy felt or fiberglass) attached to the deck. Hot asphalt is then mopped onto the deck and the base sheet is bedded into it. Successive layers of felt are laid down and mopped with hot asphalt, each layer overlapped to create a shingles effect. Typically, three to four plies (layers) are installed. Additional ply layers increase durability; 4-ply systems last longer than 3-ply systems. Surface treatment with hot asphalt and mineral gravel (aggregate) provides UV protection and creates a fire-resistant surface. The gravel also provides thermal mass, moderating temperature swings. The complete system weight is 15 to 25 pounds per 100 square feet. Installation requires careful attention to temperature management; asphalt must be heated to 350-375°F for proper application. Poor temperature control results in asphalt that is too thick and does not flow, or too thin and does not bond properly. Roof penetrations require flashing that extends through the BUR and is integrated with the membrane layers. The system provides excellent resistance to foot traffic and weathering. Maintenance involves periodic removal of gravel and inspection for deterioration. Recoating with asphalt and new gravel can extend life significantly. Proper drainage is essential as water will accumulate on a flat roof if drainage is inadequate.