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.
While asphalt is most commonly used in a built-up roof system, plies of felt can also be installed with cold adhesive. High performance asphalt such as SEBS can also be used. Felts used are Type IV or Type VI with better manufacturer warranties provided on the Type VI. Typical felts are fiberglass, but polyester can also be used. Gravel can be substituted with lava rock or other products when color or design requirements dictate.
When installing a built-up roof, a roofing kettle is parked beside the building with a propane tank. A cast iron pipe is connected and run to the top of the building. The asphalt is heated by propane in the kettle and then pumped to the roof. The roofers then spread it on the roof while rolling felts into it, then spreading gravel in it. The process creates smoke and fumes requiring ventilation and AC intakes to be closed. Once completed and surfaced, the felts are hidden and the roof looks beautiful.
For more information on built-up roof systems, please call us at 1-800-276-6344.