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Low Slope Flat Roof

Roof Types

  • Membrane Roof
  • Metal Roof
  • Low Slope/ Flat Roof
  • Steep Slope Roof
  • Shingle Roof
  • Wood Shingle / Shake Roof
  • Slate Roof
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  • Green Roof

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.

Trust Us with your next project

Commonwealth Roofing is a KRCA Certified Contractor and a BBB Accredited Business with an A+ rating. Our commitment is to say what we mean and do what we say.

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Louisville, KY 40213


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