A built-up roof consists of multiple plies of felt with interplies of asphalt. Usually three or four plies of felt are used. Asphalt is the waterproofing in the system and felt is used to keep it from separating. The system is then covered in several ways, including the following:
- No Surfacing: Sometimes the felt is left exposed and no surfacing is applied. This is acceptable temporarily during construction, but a surfacing should always be applied. Leaving an exposed surface will begin to deteriorate the felts from the top. While there are multiple layers of felt, leaving it exposed will shorten the life of the roof.
- Emulsion or Aluminum Coating: Aluminum coating mixed with an emulsion was formerly popular because it allowed for a low cost surfacing with highly reflective properties. Over time, the aluminum would weather and disappear.
- Asphalt Floodcoat: Sometimes called a glaze, the felts are covered with a flood of asphalt. This results in ‘alligatoring,’ where the asphalt develops cracks and separates.
- Gravel: A popular surfacing for a built-up roof is gravel. The gravel is embedded in a thick floodcoat of asphalt. Not all of the gravel sinks into the asphalt, leaving a top layer of loose gravel. Once complete, the gravel protects the roof from sun, rain and physical damage.
- Hybrid: This method involves installing a modified bitumen or granulated sheet over the plies. While the built-up system is prized for its tensile strength, its ability to hold strong against building movement, the modified bitumen cap sheet allows flexibility. The hybrid system provides the best of both.