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Ponding Water: The Downside Of Low-Slope Roofs

Many commercial buildings have flat or low-slope roofs, which are often easier to build and cheaper to install. But more and more homeowners are moving toward low-slope roofs as well, both for the convenience and cost, and because they offer benefits for easy installation of solar panels and other heating and cooling functionality.

One issue with low-slope roofs is that water can more easily accumulate on them. Called "ponding," this standing water can destroy roofing materials and lead to growth of mold and mildew. Most roofing contractors recommend at least some slope, a minimum of 1/4-inch per foot, to let water drain from the roof.

Why Ponding is a Problem

Ponding, or excess water accumulation, causes more than surface damage. Here's why ponding is such an issue on low-slope roofs:

  • Increased weight. Water weighs a lot, and when it stands over the surface of the roof, it adds to the load that the roof needs to bear. That can lead to failure of the entire roof more quickly than need be. For example, a 1-inch puddle of water over a 20x20 foot area can put more than one ton of extra weight on your roof!
  • Changes in weather. Sitting water can get colder to the point of freezing or warmer, and that fluctuation in temperature can make roofing materials get brittle and break.
  • Leaks. The roof membrane, or part that keeps water from getting into the interior of the building, is more easily damaged when it is wet, so leaks are more likely even after the standing water is removed.
  • Dirt and debris. More dirt can accumulate and clog drains or other openings on the roof.

In addition, ponding for more than 48 hours will likely void warranties on your roofing materials, so you cannot get them replaced if they fail over time.

What to Do to Eliminate Ponding

If you have noticed a problem with standing water on your roof, the first thing to do is to have a reputable roofing contractor perform a complete inspection of the roof. You want to make sure that there is not underlying damage to the roofing membrane or deck. There's no use in making changes and repairs to eliminate ponding if the issue has already caused damage that need to be repaired.

Once you've ruled out any structural damage to the roof, you can take steps to repair it. Your roofing contractor may have solutions for your specific type of roof. Some options include:

  • Application of a sloping material. This is ideal for areas where the flat roof slopes slightly down to a drain or opening. Instead of a downslope that accumulates water, the sloping product creates an upslope to shed water more easily.
  • Installation of roof flashing. Thin metal flashing can create a slope around openings.
  • Installation of a roof pump. Solar-powered pumps on the market can be used any time water begins to accumulate, but they should be used as a back-up system, not a replacement for adequate drainage.

Proper drainage begins with the initial roof installation. Make sure your roofing contractor, like JT Roofing, is familiar with how to install low-slope roofs and can prevent ponding issues all together. If you already have a problem, your contractor can help you come up with the most cost-effective solutions to keep your roof functioning as long as possible.


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