How to Fix Water Ponding on a Flat Roof

Water that is collecting on a flat roof can cause damage to the building’s structure because of the excess weight. Increased weight can create stress on the roof trusses and other parts of the building. Pools of water on a flat roof can cause problems such as:

• Erosion of roofing materials
• Growth of algae, moss, and lichen
• Animals (vermin, squirrels) or insects (mosquitoes)

These issues can cause leaks in the roof and can become costly for repairs both outside and inside.

Drainage Issues

Clogged or slow-moving roof drains can create ponding. Some large flat roofs need siphonic drains to move water promptly because of the roof size. On residential roofs, the connection point between flat and sloped roofs is a common issue. When this connection has been installed improperly, the result can be poor drainage and ponding.

Improper Installation

A flat roof isn’t perfectly flat. It needs to be installed on a slight pitch to promote positive drainage. The slope should guide water towards the drains. If it slopes away from the drains or has an inconsistent slope, you may get ponding at the lowest points on the roof.

Dips Near Roof Features

When there are features such as skylights or plumbing vents on the roof, there could be small dips that allow ponding, which grows worse over time. An experienced roofing professional should install these features and the flashing to ensure there are no dips.

Structural Deterioration

Over time, buildings settle which can create problems with the slope of the roof and cause ponding.

HVAC Leaks

Sometimes HVAC equipment that is installed on the top of a flat roof can cause ponding because it is too heavy, resulting in a dip. The ponding water may be from normal condensation or an abnormal leak from the air conditioner. Even if the water is slowly draining, it is continuously supplied by the equipment. When this occurs, you will need an HVAC professional to fix the equipment’s leak or reroute the normal condensation.

Compressed Insulation

Not all flat roofs are built to withstand foot traffic or the storage of heavy items. When these roofs are subject to that purpose, the insulation can compress. The compression creates a low spot that can collect water. Don’t store objects or place heavy decorations on a flat roof unless it is specifically designed to accommodate the load. For more information about waterproofing materials, refer to our article “Best Flat Roof Waterproofing Methods.”

Have the Roof Inspected

The first thing a professional roofing company should do is inspect the roof to find out what is causing the problem. The roofers can explain what the problem is and provide you with a quote before they begin the work.

Fix Structural Issues

If ponding water has caused structural damage, this should be addressed first. To fix structural issues, the roofing company may need the assistance of an architect or structural engineer if the water has caused structural damage to the building.

Fix Slope Issues

If the roofers find that the flat roof was installed without the proper slope, the entire roof may need to be replaced. This is expensive and time-consuming. However, if the roof is not sloped properly, replacing it is the recommended long-term solution.

Fix Drainage Issues

If the drains are the problem, there are several solutions for ponding water. Perhaps a new drain needs to be added or the drainage system needs to be changed to accommodate more water. If the drain continues to get clogged, a better drain screen will work as well as getting the roof cleaned regularly. In a cold climate, meltwater from snow can freeze in the drain. In this situation, your roofer may recommend an electric cable to re-melt ice in the drain.

Fix Low Points on the Roof

If the roof has developed low points from improper installation, misuse, or damage, the roofer can correct them. They may suggest installing a roof cricket that directs water away from the low point. They can also use a flat roof puddle filler to raise the low points back up to the appropriate level. If the low spot is near a roof feature or a drain, they can reinstall the flashing.

Fix Other Roof Damage

After the roofers have corrected the underlying cause of the ponding, they will replace any materials that were damaged by the ponding such as the insulation that has been compressed under the weight of the water.