If one roof system type could be thought of as ubiquitous, built-up roofing would certainly have that honor. Since their inception more than 100 years ago, built-up roofs have earned a tried-and-true reputation. But even this workhorse has limitations that should be considered when choosing a roof system.
Built-up roofs are preferred when there is heavy traffic or the potential for mechanical abuse on the roof, because the roof membrane tends to be thicker and more substantial than other systems. Gravel surfacing makes the roof highly resistant to normal traffic. Multiple layers mean that there is redundancy in the roof. Built-up roofing is familiar, so if a facility is located in an area with a limited choice of roofing contractors, it is almost certain that at least one will be competent to handle the installation and repair of a built-up roofing system.
Care must be taken with a built-up roof if wide-open expanses of roof exist or if there is movement within the structure itself. In a traditional built-up roofing system, the ability to stretch and relax because of thermal or structural movement is limited to about 10%. Where the building is very long, additional curbs must be built into the roof to keep the area of expansion and contraction within manageable limits. This may become a problem if these curbs must be positioned to not interfere with drainage.
Cold-weather installation raises other issues. Cold temperatures adversely affect the installation of the roof because the material is not flexible at low temperatures, and low asphalt temperatures will cause non-adhesion of the reinforcing felts.
If long pipe lengths are required to reach the roof from the kettle, the asphalt can cool too much before it reaches the application point, causing poor installation.
Built-up roofs can also be installed with cold adhesives that solve many of the temperature-at-application-point problems associated with hot asphalt. Ease of materials delivery is also a consideration in choosing a cold-applied system as the materials can be brought up a freight elevator and delivered to the roof. If odor or VOCs are a consideration, most cold adhesives flash off solvents as they cure.
Built-up roofs are generally not the best choice for re-cover systems because they are heavier than most other types of roof systems, and the added weight may preclude a re-cover. Built-up roofing should not be chosen when the roof is subject to heavy coatings of grease and oil as hydrocarbon materials will dissolve the waterproofing asphalt.
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