Built-up roofs consist of the following basic parts: felts and bitumen (which fuse into a laminate) and are covered by a mineral aggregate. When a buit-up roof is properly applied, the facility has a flexible roof whose strength is sufficient enough to withstand normal expansion and contraction with temperature change. Felts stabilize the bitumen. The bitumen is the waterproofing element. The aggregate protects the bitumen from damage due to solar radiation. It also offers some resistance to wind-uplift and hail. A properly designed and installed built-up roof with an aggregate surface can hold up to minor localized ponding water which lasts for a day or two (Griffin & Fricklas, 2006).
Built-up roofs may be either aggregate or smooth-surfaced. The major difference is the smooth surfaced is water-shedding rather than water-resistance. All membrane roofs should have a water drainage system in place, but as mentioned above, the aggregate surface can tolerate minor, intermittent ponding (Griffin & Fricklas, 2006).
As with all systems, built-up roofs have problems. The top four problems are as follows:
Blisters occur in varying degrees of severity. This can range from spongy spots, which are barely dectable, to large bloated humps extending several square feet in area. A limited number of small blisters usually only require repair, however larger blisters may easily be puntured by foot traffic, dropped tools, equipment loads, etc. Blisters increase the vulnerability of the roof to degradation drastically. This can significantly reduce the surface life of the roof. All blisters occur due to a void in the mopping bitumen between felt plies or between the substrate and the membrane. In the majority of cases this void will date to the application of the hot bitumen (Griffin & Fricklas, 2006).
Splitting is usually noticed in late winter (February/March), when snow and ice thaw. Splitting causes almost immediate leaks. This is often due to the glass-like nature of the roofing system. BUR membranes also have high levels of thermal contraction in cold weather, contracting nearly five times as much as the same length of steel (Griffin & Fricklas, 2006). This roofing type becomes very brittle and elastic at subfreezing temperatures and semifluid and plastic at moderate to high temperatures. A BUR behaves in a completely different manner in warmer climates than in cooler ones. This is an important consideration when selecting a roofing system.
Ridging/Wrinkling can appear as parallel lines about insulation board joints, forming what resembles picture frames (rectangles). In some cases the ridges look like long blisters. Although the visual may be similar, the mechanics and causes are different. Ridges form from felts which have absorbed moisture and swelled or from edge curling of an underlying insulation board. The wet-dry cycle of the felts causes repeatitive flexing of the membrane, eventually causing cracks to form. Moisture seems to be the sole cause of ridging (Griffin & Fricklas, 2006).
Slippage is the relative lateral movement between plies (Griffin & Fricklas, 2006, p. 256). This can appear as random wrinkles, similar to a poorly laid carpet. Slippage can expose the base sheet, transforming the membrane from a multiple ply system to a single ply. This leaves the base sheet exposed to weather, among other destructive elements. This, according to the National Roofing Contractors Association, is not a very common occurrence, but can be costly to rectify.
Griffin, C. W., & Fricklas, R. L. (2006). Manual of low-slope roof systems. (4th ed.). New York: McGraw-Hill.