All roofs are designed to be either watershedding (steep roofs) or waterproof (flat roofs) Flat roofs use membrane roofing systems. They can be a s simple as tar and gravel or as advanced as the “torch on” modified bitumen or single ply systems . All roofs slope to a drain somewhere. They can be in the middle of the roof or at the edges in the form of gutters or scuppers. This guide offers information on the variety of flat roofing systems available and how to recognize the pros and cons of each.
A good roofing system depends on the following; a good roof design, quality materials for the design and installation process, and professional installation practices. When one or more of these factors is left out or modified, the result is a less than adequate roof.
Flat roofing systems can be installed on buildings with roof slopes from dead flat up to 3 in 12, but the norm is usually in the 1/8″ per foot to 1/4″ per foot range.
Roll Roofing: Sometimes called selvage roofing, roll roofing consists of the same material as asphalt shingles (asphalt impregnated felts covered with granules). The surface can be completely covered or only 50% covered with granules. It is installed as a single ply and has a limited lifetime expectancy of between 5 and 10 years. Roll roofing is installed in long strips with little overlap, and because it expands and contracts with temperature changes, it buckles and wrinkles. Though in some cases the entire system is cemented in place, often only the seams and edges are secured.
Single Ply Roofing: This type of roofing system is fairly new to North America. Some are a modified bitumen (asphalt) base while others are plastic (primarily PVC) or synthetic rubber. Some are torched or tarred down, and some are glued or mechanically fastened with strips or buttons. They can also be laid loose and held in place with gravel. The life expectancy of these roofs are unknown, though some manufacturers offer a 10 year warranty and claim a life expectancy of 30 years.
Metal Roofs: These can be made of copper, galvanized steel, pre-painted or coated steel, or tin. They are the most expensive type of roofing system (especially copper). Metal roofs are very difficult to repair. Once they have begun to rust and leak, so replacement is usually required. Metal roofs should never be covered with tar as moisture trapped below the tar causes accelerated rusting.
Corrugated Plastic Roofs: These are single ply, translucent roofing systems which are generally placed over patios and light structures. They should not be used over living spaces, and are not considered water tight. Corrugated roofs are low quality, easily damaged, discolor with sunlight, and leak at the joints. They are extremely weak and should never be walked on.