Tackling Ponding Water On The Roof

If not addressed, ponding water can do significant damage to your roof and building.

By George Kubas, Ph. D.
From the August 2022 Issue

Ponding water is defined as any water remaining on a roof 48 hours after a rain. Any building with a flat roof has a potential for ponding water, but it usually occurs on low-slope commercial roofs. It is typically the result of poor building or roof design and/or poor drainage, and it can be widespread across the roof or in small areas of a roof section. Pockets of ponding water can also form after a building or roof surface settles.

There are two ways water can leave a roof—positive drainage or evaporation. When neither can occur quickly enough, the result is ponding water. In addition, penetrations on a roof’s surface can lead to ponding water because they make it more difficult to maintain positive drainage after a rain or snow event. In other words, the more penetrations a roof has, the more likely it is to have ponding water.

ponding water
Any building with a flat roof has the potential for ponding water, which can freeze in colder temperatures. (Photo: Simon Roofing)

The Dangers Of Ponding Water

Ponding water may not seem like a big deal from the human eye, but it is likely causing significant damage to your roof and building. First and foremost, it will rapidly increase deterioration of your roof membrane. In normal conditions, membranes can deteriorate at a rate of 1 to 1.5 mils per year, but those under ponding water conditions deteriorate even faster. This accelerated deterioration leads to earlier exposure of the reinforcement scrim which equates to less useful life of the roof. Another result of the ponding water is that some systems can lose their elasticity, causing them to become brittle. This can then cause the roof system to weaken and lose waterproofing capabilities. For those climates with higher UV rays, ponding water is extra troublesome because it essentially acts as a magnifying glass for the sun.  In addition, ponding water sitting on a roof can sneak under the membrane and cause wet insulation. This can negatively affect a building by leading to structural damage, compromised indoor air quality, mold, and more. Other issues that can result from ponding water include:

  • Vegetation. Weeds, grasses, algae, and other vegetation can begin to form and grow from excess moisture on the roof. This can lead to debris on the roof, clogs in the drains, and long-term damage.
  • Insects. Water attracts insects like mosquitos, cockroaches, carpenter ants, and more. When these insects get inside the building, your issues with them and costs to exterminate them both increase.
  • Birds. Water also attracts birds and with them comes unwanted noise, excrement, and debris.
  • Discoloration. Ponding water can discolor a roof over time, particularly a white roof. This can decrease a roof’s performance and make visible roof areas less attractive.
  • Ice. Ice formations from ponding water may develop in winter weather and cause further physical damage to the roof membrane.

Resolving Ponding Water

Depending on your roof type and building design, there are numerous ways to resolve ponding water.

  • Drainage Systems. Some drainage systems aren’t designed to handle additional loads of water at a given rate, which creates backups. You can improve drainage by adding more drains, lowering areas around the drains or gutter areas, and/or installing saddles between the drains.
  • Tapering. Tapering involves adding 1/8” or 1/4” per foot of tapered roof insulation to create slope and allow water to move toward new or existing drains. When tapering to existing drains, it’s important to ensure the drains can handle the water flow. Most buildings are built with very little slope in the roofing deck. If your facility has multiple HVAC or other rooftop units, tapering may not be practical because penetrations can make it expensive.
  • Timing. Address ponding water areas as a first step when you’re considering a re-roof or restoration. Adding reinforced layers of polyester or coating in ponding water areas will help to protect the original roof membrane underneath.

ponding water
An excess of water on the roof can sneak into the building’s membrane and may result in wet insulation.

roof maintenance
Facility managers need to address ponding water as soon as it becomes apparent to prevent any damage or issues it may cause. (Photos: Simon Roofing)

Combatting Ponding Water

Ponding water can be greatly reduced with proper roof design based upon your building design, roof slope, and climate. There are several ways roof products and design can help avoid ponding water and reduce deterioration of the roof.

  • Thickness. A roof membrane’s thickness plays a role in ponding water because the polymer is going to deteriorate over time—and even quicker with ponding water on it. A thicker membrane in areas where ponding water is prevalent can lengthen the overall roof’s expected life. • Reinforcement. Adding a layer of polyester reinforcement to the roof membrane, wherever possible, can strengthen the membrane in the presence of ponding water. It also lessens the ability for water to penetrate the membrane.
  • Specific Gravity. The heavier the material is compared to water, the better it will be able to shed water away. For ponding water areas, a product should have high specific gravity, low porosity, superior adhesion, and high-tensile reinforcement. Products like these may be cost-prohibitive for the entire roof, but they can make fiscal sense in ponding water areas.
  • Color. The color of the material can make a difference. White roofs aren’t better for ponding areas because they don’t hold the heat. Retaining heat with a darker colored roof, like gray, will help the ponding water evaporate quicker. Even in warmer climate, it can make sense to spot color the ponding water areas of a roof gray.
  • Seams and Penetrations. When ponding water sits above seams or around penetrations, which are the weakest points in a roof system, it can result in deterioration and make it more likely for water penetration. Look for materials that are seamless or be mindful to reinforce areas where sheets of material overlap around penetrations.

The next time it rains, wait a few days, and then take a look at your roof. If you notice any spots of ponding water, make note of where they are and how much water there is—even take photos, if possible. This will help your roofing provider understand the extent of the problem and offer the smartest solution. Remember that ignoring ponding water won’t make it go away—it needs to be addressed quickly because once the damage spreads below the roof surface, the cost and disruption can be magnified.

Simon RoofingKubas, Ph. D., is the Director of Technology at Simon Roofing. His background includes managing manufacturing plant operations, process optimization, and new product/process development.

Do you have a comment? Share your thoughts in the Comments section below, or send an e-mail to the Editor at jen@groupc.com.

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