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:
• Why is my roof sheathing sagging between the trusses?
• Why is granule loss a problem for an asphalt shingle roof?
• What are the mistakes to avoid when doing attic improvements?
• What causes roof shingles to curl up at corners?
• What causes shingles to buckle along a line on the roof?
• What causes leaks at a fake roof dormer?
• What causes a sagging roof ridge line?
• What causes bubble-like blisters in a built-up and gravel roof?
• Why does it cost so much more to replace a steep roof than a low slope roof?
• What is "ponding" on a flat roof?
• Is an attic required to have a light by the building code?
• How can I inspect my roof for hurricane damage?
• Why is premature curl of roof shingles a problem?
• How can I tell if a roof has more than one layer of shingles?
• What are the common problems with attic insulation?
• What is the life expectancy of an asbestos cement shingle roof?
• What's the average lifespan of a roof?
• Why is it a mistake to replace a roof and not replace its flashings?
• Why is there no attic access hatch in the house?
• What is the building code requirement for an attic access hatch, scuttle, or door?
• Does a roof with multiple layers of shingles last longer?
• What can I do to prevent roof leaks?
• Are roof trusses better than roof rafters (stick framing)?
• Why is a popped nail in a shingle roof a problem? How do I fix it?
• What are the most common problems with wood roof trusses?
• What causes a lump or dip in the roof?
• If my roof is not leaking, why does it need to be replaced?
• How can I be sure my roofing contractor got a permit?
• How many layers of roofing are allowed on a home?
• What are the dark lines running parallel to shingles on my roof?
• Can metal roofing be used on a low slope/pitch roof?
• How can I make my roof last longer?
• What are the warning signs of a dangerous attic pull-down ladder?
• How can I find out the age of a roof?
• Should I buy a house that needs a new roof?
• Should I buy a house with an old roof?
• What are those metal boxes on the roof?
• What does "lack of tab adhesion" in an asphalt shingle roof mean?
• Why do roof edges start leaking?
• Why do my dormer windows leak?
• Do home inspectors go on the roof? Do they get in the attic?
• Should I put gutters on the house?
• How much of a roof truss can I cut out to make a storage platform in the attic?
• What's the difference between an "architectural" and a regular shingle roof?
• What does a home inspector look for when examining a roof?
• Do stains on the ceiling mean the roof is leaking?
• How can I tell if the house needs a new roof?
• Why does my homeowner's insurance want a roof inspection?
• What are the hazards to avoid when going into an attic?
Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.