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INFORMATION SERIES: Flat Roofs

There are numerous types 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. Asphalt roll roofing consists of the same basic material as asphalt shingle; however, it is produced in rolls 36 inches wide. The exposure may vary from a 50% exposure (50% overlap) to a 90% maximum exposure on steeper applications. The surface usually has a granular coating but can also have a smooth finish (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. The normal service life of this type roofing is 8-12 years.	As they age, single ply and built-up flat or low slope roof surfaces exhibit signs of wear such as blistering, cracking or bubbling of the roof membrane. Alligatoring is a deficiency whereby the roof covering develops deep cracks with the appearance of alligator skin. Once the roof membrane is broken, leakage is imminent, although it may not be readily apparent in the living area. Consequently. this condition can rapidly increase in severity and cause consequential damage to the insulation, roof framing or interior surfaces. There are also numerous metal roofing materials of various types and designs which are usable on low slope or flat roofs. Most require very little maintenance and have a relatively long service life. The seams and connectors (or fasteners) are generally the only weak points in this type roof system and should be checked periodically. Standard maintenance also includes periodic recoating with a suitable material compatible with any existing coating or pursuant to manufacturer's recommendations. Water ponding on a roof will increase the rate of roof wear and/or chance of leakage. Design criteria sometimes allows minor ponding; however, drainage or evaporation of any water should occur within a day of a rain storm. 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. When the surface of a flat or slope roof is worn and requires replacement, the existing roofing material in most cases should be removed down to solid decking. Since the design of this type roof usually restricts visibility of structural members, removal of the surface affords the only opportunity to check the condition of the roof framing. Removal of the old, worn surfaces will also increase the life of the new roofing. Built-up roofing materials are designed specifically for flat or low slope application. Tar or asphalt saturated roofing felts are installed in 2-5 layers (plies) with a tar or asphalt mastic between alternate layers. The more plies present, the longer the expected service life. The expected service life of this type roofing varies from 10-25 years depending on quality, installation and maintenance. A gravel or slag covering is sometimes applied to flat or low slope built-up roofs to provide some protection to the roof surface from the sun and other elements as well as to act as a ballast for the roofing material or panel type deck insulation. Uniform coverage should be maintained to prevent exposure of the roof surface and premature wear.

Single ply membranes are a popular roofing for flat or low slope applications due to the lower cost and relative ease of application compared to other low slope roofing materials. As the name implies, the roofing is installed in single layers with minimal overlap. Various types of adhesives. solvents or heating devices are used to seal seams. There are many different types and qualities of single ply membranes with new products becoming available every year. Consequently, it is very difficult to determine the quality and expected life of this type roof based solely on appearance. The HouseMaster Technical Staff recommends that home buyers, when possible, obtain specific installation and maintenance information from the owner or roofer. In general, this type roofing has an8-15 year service life.

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