What is ponding on a flat roof?
Monday, June 18, 2018
To understand what ponding is, and why it is a problem, you need to know that a flat roof is not supposed to be perfectly flat. It should have a slight slope toward the edges and even a fraction of an inch per foot can be sufficient. When a roof is built without a tiny slope, or if settlement or sagging of the roof structure negates the built-in slope, or if roof drains are inadequate or blocked with debris, you can end up with a shallow lake on top of the roof.
A very thin layer of standing water is normal immediately after a heavy rain on a low-slope roof but, if the water is still there a couple of days later, that’s ponding and it will shorten the life of the roof surface. It is even possible to observe that a roof has a ponding problem if there is no standing water when you examine it: when there are darkened areas with a defined outline of fine debris, you are seeing a dried-up ponding problem.
The National Association of Home Builders (NAHB) publishes a book of residential construction performance guidelines, and the standard regarding standing water on a flat roof is “water shall drain from a flat roof except for minor ponding within 24 hours of rainfall.” Determining what is “minor ponding” leaves a little room for disagreement, but definitely a large puddle of water on the roof a day after rain would be ponding.
Sometimes the accumulation of leaf debris will create a ponding issue on a correctly sloped roof all by itself, and the acidity from the decaying leaves in the pooled water will further speed up the deterioration of the roof surface, like in the photo shown below.
Correcting ponding by removing leaf debris accumulation or clearing blocked roof drains is relatively inexpensive. But the problem we often see that causes ponding is roof rafters that have sagged over time at the center of the span. One fix for that is to have your roofer build up the surface under the roofing when it is time to reroof, starting at the center and using progressively thinner rigid insulation panels towards the edge.
Also, see our blog post Why are most house roofs slanted instead of flat?
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To learn more about roofs and attics, see these other blog posts:
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