A roof that is nearly flat or slightly pitched is called a flat roof or low slope roof. No roof should be dead flat because it must have at least a slight slope to drain.

The terms low-slope and steep-slope describe roof slope. Slope is how much a roof slants. A low-slope roof is one that has a slope of less than 3-in-12. This means that for every horizontal foot, the roof level goes up less than 3 inches vertically.

A steep-slope roof (typically a shingle roof) depends upon gravity to cause water to flow in one general direction so it can shed the water over the breaks and fasteners in the shingles until it flows to the edge. A low-sloped roof or flat roof, cant depend upon the water to flow in any particular direction so it must form a watertight, monolithic membrane that stays watertight all the way to the drains or edge.

Modern low-slope roof or flat roofs tend to use a continuous membrane covering which can better resist pools of standing water. These membranes are applied as continuous sheets that are bonded together with heat-welding or adhesives. Far more expensive low-slope or flat roof options (mostly for smaller traditional residential applications) include sealed metal roofs using copper or tin. These are soldered interlocking systems of metal panels.

Traditionally low-slope or flat roofs would use a built-up (tar and gravel) roof, which used to be good enough. Today, this traditional type of roofing suffers from performance, cost, and environmental issues that require other solutions to be considered a better value.

So besides having to stand up to wind, freeze-thaw cycles, and UV radiation from the sun like shingles must, a low-slope system must also withstand expansion and contraction of the building and all of the other dynamics going on above a building while staying 100% watertight. This requires well-engineered attachment, seaming, and weathering characteristics to meet these performance demands.

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The most common traditional low-slope systems are called built-up roofs. This is where multiple layers of roofing felt (often called tar paper) are mopped in place with hot asphalt or coal-tar pitch (bitumen) to build up a strong watertight membrane. The membrane is then surfaced with a coating or gravel imbedded in a pour-coat of more hot bitumen. While this type of on-site construction has a long history of performance, it is subject to adverse jobsite conditions, is not a very flexible membrane, has some environmental concerns, and is becoming increasingly cost-prohibitive with rises in crude oil and labor prices. Built-up roofing is therefore accounting for a shrinking share of the low-slope roofing market


Buildings today are better insulated than in the past while their structural components are becoming more and more lightweight. This requires the roof membrane to be able to move and stretch more than most traditional built-up systems. One response to this need is a modified bitumen, or modified system. This is where asphalt is chemically modified t o help it stay more flexible and then constructed over a heavy polyester or fiberglass mat to give it strength. This produces a quality-controlled membrane that is mopped or adhered over a base sheet to produce a system that stays flexible and is not as susceptible to installation variables as a built-up roof. Simon Constructions GAF line is an excellent example of a modified bitumen system that has decades of performance history. However, while modifieds address some of the flexibility and labor issues, they are still based on crude oil and are therefore still subject to environmental and cost concerns.


A different strategy for solving many of the problems with traditional low-slope roofing is to manufacture the entire membrane in factory-controlled conditions that can then be installed as a single ply. EPDM, or rubber, was the first popular type of single-ply system Rubber is known as a thermoset membrane, meaning it doesnt melt or dissolve. This causes it to weather well, but it also means that the seams are held together with contact adhesive, and this makes it a potentially weak point in the system. Rubber is also primarily black so heat build-up is an issue.

Another type of single-ply membrane is represented by thermoplastic membranes. These are heat-welded membranes, primarily PVC or TPO They are usually white which makes them very heat-reflective and highly-scored Energy Star products. Their seams are very tough since they are welded together, and they are generally more environmentally friendly in their manufacture, transportation, installation, performance, and disposal. Thermoplastics are the fastest growing type of low-sloped roofing.


Sprayed Polyurethane Foam (SPF)
Sprayed polyurethane foam is a relatively new product that is sprayed onto the roof as a liquid in order to form a single, continuous, seamless structure. Because foam is flexible and lightweight, its perfect for irregularly shaped roofs and protrusions. For re-roofing, SPF also eliminates the need to remove the previous roof, since it can be applied directly over existing roof installations, including asphalt shingles, built-up roofs, clay tile, concrete, metal, and wood. Foam roofing systems are easy to maintain, and with a high insulation value, they can combat some of the heat issues common with flat roofing.


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