By BEM Expert | June 21, 2019

Common Roof Issues: Ponding Water

Now that we’re in the in rainy season a common issue you may deal with if you have a flat roof is ponding water. Ponding water is a common but extremely detrimental roof condition that can lead to structural problems with the roof deck, damage to the roof surface, unwanted vegetative growth, and accumulation of damaging contaminants. Even if your roof is in good shape, it’s important to know when water is a problem – and how to respond.

What is ponding water?

Ponding water is defined as an excessive accumulation of water, typically in a low-lying area, that remains on a roof for 48 hours or longer even when conditions are suitable for drying. Left unresolved, ponding water leads to accelerated erosion and deterioration of the membrane surface and results in premature failure of the roof system.

Even reasonably small amounts of moisture intrusion underneath the roof membrane may decrease the thermal resistance of the insulation. More importantly, moisture intrusion can cause serious damage to the deck, insulation, and building interior as well as the membrane.

Common effects of ponding water:

  1. Damage to the roof surface. Ponding water magnifies UV rays, acting as a magnifying glass on the roof under the pond, which increases the damaging ultraviolet exposure of that area of the roof, which prematurely age the membrane causing splits, cracking, and mineral loss. Ice can also build up and, with temperature changes, move across the roof surface “scrubbing” the membrane and potentially causing considerable physical damage.
  1. Deformation of the deck structure. Ponding water can substantially increase the load on roof decks. As water accumulates, deck deflections can increase and result in additional ponding water that could compromise the structural integrity of the deck.
  2. Growth of algae and vegetation. When water stands for long periods of time, algae and vegetation growth will likely occur and may cause damage to the roof membrane. Additionally, vegetation and other debris can clog drains and cause additional ponding.
  3. Accumulation of dirt, debris, and other contaminants. These elements can affect and damage the membrane surface over time.


Preventing ponding water, and avoiding these headaches, begins with proper roof design. If a deck does not provide the necessary slope to drain, a tapered insulation system can be used. A combination of different approaches—single slope, two-way slope, and four-way slope—is often used to achieve the necessary slope and to allow for moisture drainage. A roof designed with adequate slope built in can shed water quickly and effectively – a minimum positive slope of ¼:12 is required on asphalt roof systems and is a good rule of thumb. Adequate sloping should be considered during the design process. A roof’s structural frame or deck should be sloped, and drainage components like roof drains and scuppers should be included in the design.

In addition, secondary (or emergency) drains may be required by local plumbing codes to help reduce the risk of a structural failure due to clogged drainage systems.  Talk to your roof membrane manufacturer and/or roof system designer to determine the proper location of these components. Additionally, crickets installed upslope of rooftop equipment and saddles positioned along a low point between drains can help prevent localized ponding in conjunction with a tapered insulation system.

By following the proper drainage practices above, building owners can positively impact their low slope roofing system and help to ensure it will remain durable and reliable throughout its service life. Maintaining positive drainage is a sound maintenance practice for any roof. If you see ponding water on your roof, don’t ignore it!

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