There are a number of different types of “built-up roofs” that are often simply referred to as flat roofs. They include: tar & gravel, single ply membrane, multi-ply membrane, protected membrane roofs and insulated membrane roof assembly.
Flat roofs aren’t in fact flat really but rather have a low slope to them for drainage purposes. They are considered flat when their “pitch” or slope is: 2/12 or less. In other words, when the rise of the roof is 2” (or less) for every 12” of run. As you might well imagine then, it’s key that the roof covering be “sealed” or water-tight and provide for the efficient evacuation of water from the roof.
Most commonly these flats are found on commercial buildings, however “tar & gravel” roofs and some membrane roofing materials are also found on residential roofs. They can be used in conjunction with sloped roofs on the same home or building as well.
Tar & gravel roofs have multiple plies of roofing felts with asphalt applied between them. The term “tar” in this type of system comes from coal tar that was used on these roofs. Over the final layer of asphalt, gravel is broadcast to provide wear protection and reduce UV degradation. Some roofers may use roll roofing instead for the final layer however the life span (about 5 to 10 years) is far less than gravel (about 10 to 20 years for 4 ply and 5 to 10 years for 2 ply). In fact, some use gravel on top of other types of roofing systems to provide surface protection and to act as a UV inhibitor also (see left picture). In either case, if the gravel is displaced / not evenly distributed then the materials beneath are subject to damage by sunlight or otherwise. Other factors affecting the deterioration of the tar and gravel roofs are largely associated with its installation, maintenance, amount / distribution of gravel, and, sunlight. Over the years, some continue to apply tar as a stop-gap measure to extend the life of the roof. Eventually however, the tar cracks / deteriorates (see middle picture) creating the potential for water / moisture penetration into the building. Further, sometimes the tar will bubble-up or blister (see right picture) making it prone for water entry as well.
If you are going to buy, or already own, a home / building with a tar & gravel roof, protect yourself…get it inspected.
More information about other Roofing Systems.