The slope of a roof encourages water to flow off of the structure rather than gathering in a low spot and causing rot in the structure. A roof with little to no slope uses other methods to encourage proper drainage. These low slope roofs offer special advantages, but only to buildings designed to work with this type of roof.
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Disadvantage: Limited Materials
Low slope roofs change how water flows off of the structure. High pitched roofs don't let water sit, so asphalt shingles and other materials that overlap work well. On a roof with a lower pitch, standing water soaks between these materials. Shingles can't be used on roofs with a slope below 8.5 degrees, says the Canadian Asphalt Shingle Manufacturers' Association. Only rolled sheet roofing works on flat or very low sloped roofs.
Advantage: Improved Heating and Cooling
A steeply sloped roof creates extra space in the structure. All of the space below the roof fills with air, changing how heating and cooling systems work. A low slope roof limits this extra air and makes a structure easier to keep comfortable throughout the seasons, according to RAM Buildings. Depending on the amount of slope, limited attic space still fits between the ceiling and the roof for storage or ductwork.
Advantage: Maintenance Requirements
Since water and ice sit on the surface of a low slope roof much longer than on one with a high pitch, the roofing materials receive more wear, which increases the maintenance requirements and associated costs over the lifespan of the structure, warns Schluter Roofing. If rolled roofing materials are used, they will need annual cleaning and resealing. However, low slope roofs make maintenance easier because walking around on them is easier. The risk of slipping and falling off is much lower than on a steep roof.
Advantage: Installation Costs
A low slope or flat roof requires less material to build, lowering the overall installation cost, says RAM Buildings. Even if you choose a roofing material more expensive than the shingles that steep roofs use, the lower labor costs for low slope roofs offset the cost. The structure is easier to build because supports are installed from the sides of the building without workers having to lift up large trusses. Rolling out and sealing the roofing materials also takes less time than installing individual shingles.