The built-up roofing system, otherwise known as BUR has been the staple of flat roofing systems for well over 100 years. BUR is comprised of multiple layers of ply sheets, squeezed together using a waterproof material. There can be as many as 5 layers pressed into this application. This is then topped with a gravel, coating, or a cap sheet for additional protection. These roofs are known for their durability, but can sometimes require maintenance due to the amount of structure needed to reinforce their large weight.