There are numerous type roofing materials specifically designed for use on flat or low slope roofs. Due to their design, flat roofs are more prone to leakage than steeper roofs and proper installation is essential. Leaks, when they do occur, are also more difficult to locate and correct. Since low slope/flat roofs are often neither readily accessible nor visible, they are often neglected. However, since they generally require more maintenance than most steep slope roofs they must be checked on a regular basis.
Low-slope and Flat Roofs
Most roofing used on moderate and steep-slope roofs, such as shingles, is installed in an overlapping manner to shed water. Roofing used on low-slope and flat roof surfaces needs to be installed in a waterproof manner, as the minimal slope will result in slower water runoff or even ponding in many cases. For this reason, roofing products used on steeper slopes are generally not suited for the lower slopes, or at least they cannot be installed in the same manner. Fortunately, there are many products specifically designed for use on low-slope or flat roofs.
Because of the slower runoff or potential for ponding, low-slope and flat roofs are more prone to leakage than steeper roofs and proper installation is essential. They also require more maintenance than most steep-slope roofs and must be checked on a regular basis. But since low-slope and flat roofs are often neither readily accessible nor visible to homeowners, maintenance tends to be neglected until it is too late. When leaks do occur, they are also more difficult to locate and correct.
One traditional roof product used for low-slope roofs is roll roofing. This material is similar to asphalt shingle; however, it is produced in rolls 36 inches wide and can have a smooth or granule surface. This roofing can be installed with various exposures, from a 50% exposure (50% overlap) to a 90% maximum exposure on steeper applications. Roll roofing is also commonly used as a valley liner, with or without metal flashing. With normal aging, seams may lift and require re-sealing at any time. This type roof has a relatively short service life, sometimes failing in less than 10 years.
Built-up roofing is another traditional low-slope and flat roofing. For years, this was the premier roofing product, although much more costly and difficult to install than roll roofing. When installed properly, it provides a watertight roofing membrane with a relatively long service life. The earliest built-up roofing was made of tar-saturated felts installed in 2-5 layers (plies) with a coating of hot tar between alternate layers. Subsequently, asphalt felts and coatings were used. The expected service life of this type roofing varies from 15-40 years depending on the quality of the materials and installation. A gravel or slag covering is often applied to built-up roofs to provide protection for the roof surface from the heat of the sun, and surface damage, as well as to act as ballast for the roofing material or panel- type insulation that may be installed on the deck. Uniform gravel coverage should be maintained to prevent exposure of the roof surface and premature wear.
The relatively short service life of roll roofing, and the high cost and difficult application process required for built-up roofing eventually led to the development of fiberglass- reinforced asphalt roofing, as well as many other synthetic roofing materials, commonly referred to as single-ply roofing. With names like EPDM, CVPC, modified-bitumen, and SBS, these products are now the most popular roofing for low-slope and flat applications. Strips of this type roofing are installed in a single layer with minimal overlap and sealed with various types of adhesives, solvents or heating devices depending on the makeup of the roofing. There are many different types and qualities of single-ply membranes; consequently, it is very difficult to determine the quality and expected life of this type roof based solely on appearance. When possible, homebuyers should obtain specific installation and maintenance information from the owner or roofer. In general, manufacturer warranties run 10-20 years for this type roof, but installation quality and maintenance will be a factor in the actual service life.
As they age, single-ply and built-up roof surfaces eventually exhibit signs of wear such as blistering, bubbling and cracking of the roof membrane, and separation of seams. As with any roofing, flashing wear or damage is also a concern. Once a low-slope or flat roof exhibits excessive wear or damage, the potential for leakage exists. Although signs of leakage may not be readily apparent within the living area of a house, if the roof is no longer watertight, it will leak. Conditions can also rapidly increase in severity in a short time. The result can be significant consequential dam-age to the insulation, roof framing or interior surfaces. When the surface of a low-slope or flat roof is worn and requires replacement, the existing roofing material generally must be removed down to a solid deck.
There are also numerous metal roofing products that can be used on low-slope and flat roofs. Most of these require very little maintenance and have a relatively long service life. The seams and connectors (or fasteners) are generally the only weak points with many metal roof systems and should be checked periodically. Standard maintenance may also include periodic recoating with a material compatible with any existing coating or pursuant to manufacturer's recommendations.
Water ponding on any roof will increase the rate of roof wear and/or chance of leakage. Standard design criteria sometimes allow minor ponding; however, with residential construction drainage or evaporation of any water should occur within a day of a rainstorm. Whenever possible, measures should be taken to reduce any significant ponding. In some cases ponding may cause or be the result of roof framing settlement.