Ponding water on a flat roof

Ponding is the phenomenon of water pooling on flat roofs after storms, snow melts, or heavy rains. Pools should drain away without causing any damage to the structure – but building owners should know when to take action, how to identify the cause of the ponding, and how to resolve it.

A few reasons why the water may not be draining are:

  • The drains or gutters are blocked with debris
  • The pitching of the roof was not done correctly (weak membranes, caulking issues, etc)
  • There is too much roofing material on the edge of the roof
  • The flashing was not installed correctly
ponding on a roof

If your commercial building has a flat roof, sometimes there is a problem with ponding water. This is because the flat roof can hinder drainage from rainwater, which results in standing water on the roof. This is especially prevalent after a lot of rainstorms. So you need to know what to do to ensure that you take care of the problem early before it becomes a bigger issue.

Ponding water on a flat roof may cause long-term negative effects. So here is what to know to protect your roof, and your investment, in the care of your building.

It may seem as if a little water on the roof isn’t a big deal but there are a few effects that affect the longevity and value of your roof such as:

  • There are more leaks
  • A roof that sags
  • There is deterioration in the roofing matters that are in the ponding water
  • Moss, algae, and plant growth may be inevitable
  • There is damage or weakening of the roof membrane

Ponding water isn’t an immediate threat.  In fact, roof membranes are made with the same material as pond liners, so water penetration is unlikely. Over time, however, ponding water can lead to accelerated aging of the roof membrane.

Ponding water accelerates roof system aging by:

  • Degrading the roofing material itself: This is a particular concern for older built-up and modified roofs.
  • Intensifying the sunlight: On modern white roofs, the water focuses the sunlight that passes through it (like a prism) and intensifies UV degradation.
  • Collecting dark colored debris: Water draws dirt and debris into small puddles and these dark masses sit on the membrane after the water’s evaporated and heat up under direct sun, putting unnecessary strain on the membrane.

All roof systems should have positive drainage, which means that water should flow off the roof within two days. If water remains on the roof system after 48 hours, consider it an issue that could lead to larger problems requiring costly repairs. To resolve ponding before it damages the roof, determine the cause of the ponding and select the appropriate solution.



When insulation becomes wet and saturated or gets crushed from extensive foot traffic, it compresses, creating an indentation in the roof system. Rainwater collects in these depressions and, if it does not evaporate quickly, can cause the aforementioned damage.

Solution: Have a qualified contractor replace the crushed insulation below the ponding water as a quick, economical fix. However, owners and managers should monitor the roof. If the water build-up simply moves to another location, this indicates a larger problem – most likely with the drainage on the roof, which may require a more complicated solution.


Ponding problems appear on old and new buildings alike if there are structural issues, such as irregularities created by deformed joists or beams.

Solution: Structural problems vary greatly in terms of severity. Sometimes, adding tapered insulation can resolve ponding water issues, but structural irregularities tend to signal the advent of unpredictable, challenging, and costly repairs. The only way to determine the scope of a structural problem is to seek out an experienced professional roof contractor.


Certain elements on the roof are especially susceptible to ponding. Large Roof Top Units (RTUs) like HVAC units or skylights that are unprotected by “crickets” tend to be places where water collects because it cannot flow past the curb of the unit or skylight.

Solution: Installing crickets around RTUs and skylights is an easy and economical method to resolve ponding. Crickets are triangles of tapered insulation that direct water to flow around a skylight or other protrusions.


Positive drainage moves water toward drains and when installed correctly, the water drains away. However, when drains are installed incorrectly, they can actually block water from exiting, creating large pools around them. Even a slight lip can block a significant amount of water.  Unfortunately, owners and property managers must proceed with caution before attempting to resolve the issue, because in some cases, attempts to “fix” the problem exacerbate it.

Solution: When fixing ponding near drains, one solution is to install tapered insulation to redirect the water. Another option is to lower the insulation leading to the drains, by cutting away some of the material to encourage water to move to the exit. Sometimes, the issue can be plumbing-related if the drains are not set at the correct height in the first place.

Some of these solutions are troubleshooting methods; owners may have to apply more than one at a time. When ponding water occurs near drains, consult an experienced contractor who can evaluate the situation, identify the real cause of the ponding, and determine the best solution.

Ponding water will not void a warranty or destroy a roof; treat it as a pressing – but not immediate – concern. All water should drain off within 48 hours, but building owners and facilities managers should monitor and analyze any water build-up that remains after that time frame, as it can degrade the roof’s materials and lead to long-term issues. Before taking action, however, it’s crucial to determine the root cause of the ponding – because the cause directly influences which solution is the most appropriate choice, both in terms of cost and effectiveness.