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

P onding 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 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:

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

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

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.

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.

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

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