No roof should be entirely flat but rather should slope either toward the perimeter or toward an interior drainage system. After 48 hours following a rain, any roof which still has standing water is known as a ponding roof. Telltale signs of past instances of water ponding include particularly muddy circles or areas displaying algae or vegetation growth.
Common causes of ponding water include:
- Poor roof design with no slope
- Inward sagging of the roof deck
- Inadequate number of drains and/or lack of drains
- Drains that are blocked or at high points
Pooling or ponding water on flat roofs reduces the lifespan of the roof as it can cause the following problems:
- Leaks at overlapping seams where water is causing the membrane lose it’s seal
- Clogged drains from algae and loose debris
- Plant growth
- The material used to seal flashings, mostly tar-based product will deteriorate over time
- Caulking will lose it’s holding properties
- Mosquito and other bugs will inhabit these areas
- Ice will form in the winter and cause unnecessary stress on the roof structure — leading to further sagging and more ponding
- TPO or EPDM membranes will not be affected as much in terms of the longevity of the material, however Modified Bitumen will deteriorate under standing water
- Corners of skylights, chimneys and vent pipes are highly susceptible
- Areas around heavy equipment (HVAC units etc) are also prime targets
There are a number of drainage options for flat roofs to combat ponding.
- Internal drains
- Gutters and downspouts
- Roof leveling compounds
Inner drains are located in central areas of the roof and are connected to pipes underneath the roof that drain the water to a sewer. These can be used on large flat roofs as well as residential. They’re good because you can place them wherever you need to and a custom filter can be used to prevent clogging. You don’t need gutters or downspouts remove the water and the interior piping is rarely ever an issue. On the downside, the strainers tend to get clogged easily, so the roof needs to be periodically checked and cleaned of debris. You’ll want a strainer with large enough slots where small debris can pass through so the drain doesn’t clog as easily. When inner drains become clogged with leaves, debris or otherwise, it leads to ponding. The pooling water adds a lot of stress to the roof. Think of a 3 ton elephant standing on your roof. When you go to clean the roof, remove the strainer so the so you can wash the debris down easily. If one of the drain pipes within the building does get blocked, it would require a professional to fix, which can be expensive. Inner drains are also a bit more costly than outside drainage systems like gutters and scuppers.
When you suspect a leak might be coming from an inner drain, first make sure the roof material is securely and fully attached to the drain. Look for any punctures or tears that might have been caused by someone with sharp equipment up on the roof – HVAC contractors, past roofers etc. If there’s a ton of tar and caulking around the drain, it means someone’s already tried to repair the area. It might be best to re-do the flashing.
Gutters and Downspouts
Gutters and downspouts transport water off a roof, defending the cladding system as well as the foundation. These work on flat roofs the same way they do for sloped roofs. Downspouts and gutters can be created using numerous materials, including aluminum, vinyl, galvanized steel, and copper. The main problem with gutters is that they get clogged easily with debris and leaves. If yours gets clogged easily, try installing a bigger downspout to let some of the smaller debris flow through. In the winter, ice buildup can cause damming. In both cases, whether leaves or ice, it can lead to ponding and leaks, which is why they need to be cleaned periodically.
Buildings with railed rooftops can use scuppers to let rainwater drain off. Scuppers are openings through perimeter curbs or parapet walls that facilitate drainage of excess water by helping move water to into a gutter or downspout through a conductor head. If neither a gutter or downspout is present, you can extend the scupper spout beyond the exterior surface of the building to prevent the building from getting wet. When the roof surface sinks below the level of the scupper opening, water pools within the railing of the roof. Roof leveling compounds can help level these areas to their original install slope to facilitate water drainage.
Roof leveling compounds
Roof leveling compounds can help bring the roof surface up in low areas to help water flow out a scupper opening, inner drain, or downspout. For deeper ponding areas, tapered insulation can be used to filled the area and then the filler material can be applied atop to fine-tune it. Topcoating these material is recommended to achieve full waterproofing. The GreenSlope Roof Ponding Repair Kit is available for this purpose.