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Low Slope / Flat Roof
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Low Slope / Flat Roof

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.

Low-slope & steep-slope terms describe roof slope, the slant of a roof.
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 & fasteners in the shingles until it flows to the edge.

A low-sloped roof or flat roof, can’t 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, 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 & environmental concerns requiring better value solutions.

Besides performance in wind, freeze-thaw cycles & UV radiation from the sun, a low-slope/flat roof must also withstand expansion & contraction & remain 100% watertight. This requires well-engineered attachment, seaming & weathering characteristics to meet these performance demands.

Built-Up Roof

  • 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.
  • Membrane is then surfaced with a coating or gravel imbedded in a pour-coat of more hot bitumen.
  • This type of roof has a long history of performance.
  • This roof system is not known as a flexible membrane, may have some environmental concerns & at times cost-prohibitive due to crude oil prices.

Modified Bitumen – Tamko Awaplan

  • Asphalt is chemically modified to allow flexibility & constructed over a heavy polyester or fiberglass mat for strength.
  • This produces a quality-controlled membrane that is mopped or adhered over a base sheet creating a flexible system less susceptible to installation variables.

Single-Ply Membranes – Carlisle or Duro-Last

  • EPDM, or the rubber roof, was the first popular type of single-ply system (see our Carlisle page).
  • Rubber is known as a thermoset membrane, meaning it doesn’t melt or dissolve.
  • Entire membrane roofs are manufactured in factory-controlled conditions that can then be installed as a single ply.
  • Thermoplastic membrane roofs are heat welded single-ply membranes, our systems include Carlisle TPO or Duro-Last’s PVC.
  • Usually white which makes them very heat-reflective and highly-scored Energy Star products.
  • Roof seams are very durable since they are welded together.
  • These roofs are generally more environmentally friendly in their manufacture, transportation, installation, performance & disposal.
  • Thermoplastics are the fastest growing type of low-sloped roofing. To learn more, you may want to visit our Membrane Roof page.

No matter what the problem is with your low-slope roof or flat roof, Commonwealth Roofing has earned the title “The Roofers” and will give you good advice on the best solution for your roofing needs.