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Commercial Flat Roof with Ponding Water

Flat Roofs: Is Ponding A Problem?

“Is ponding a problem on flat roofs?” Is a common question we are asked at design stage on a project, so we wanted to share our knowledge and experience on the subject.

Most roofs are designed to remove water as quickly as possible into rainwater gutters, through downpipes, into the drainage system and into rivers. While this process is extremely quick and effective, it can have a negative impact on flooding, as we have investigated in the past.

Alternative solutions can be designing a roof that stores rainwater, or releases it significantly slower, such as blue roofs and green roofs.

It’s always been the understanding that getting the water off a roof is essential, but how detrimental is sitting water on a roof (also known as ponding water)?

Many of the materials used in roofing today are also re-packaged and used for waterproofing swimming pools, boats and other aqua-resistant necessities. So is ponding water really an issue on flat roofs anymore?

Firstly, it’s important to note that unless the structure of the roof, either by structural re-design or incorporation of a tapered insulation scheme, is changed the renewal of the roof coverings will seldom result in any change to the existing water flow issues but in many cases a new, clean, moss and chipping free system will make any water ponding issues more evident.

Roofing membranes today are designed with the ability to hold a degree of standing water for decades, and unless the problem is significant and has been overlooked at the design stage, it should not have any impact on the guarantee. As mentioned earlier, the same materials are used in swimming pools, so their ability to remain resistant to water is arguably as robust as a fishes.

However, while a small amount of water in liquid form should not cause significant problems, its solid form of ice can cause many more. In winter months ice on a roof can not only cause serious health & safety risks to anyone undertaking works at roof level but, the expansion, due to its solidification, can add additional stress to a roof membrane. Which as years go by will cause larger issues and failure.

With any material, its exposure to the elements will, overtime, cause it to wear away and wear thin. It will become more brittle and start to fail.

These time frames have been hugely increased with advances in roofing technologies over the years, and now materials can last in excess of you and me. But, as this process occurs it’s inevitable that should ponding water be prevalent in these areas then the water will find its way into the system or internals.

And then it all comes down to the true life expectancy of roof, and the cost per year investment. A small amount of ponding water shouldn’t cause the roof to fail before the guarantee expires but, it will add additional strain to a roof and therefore be likely to cause it to fail sooner than had no water ever been prevalent. It may outlive its warrantee but, not necessarily its full life expectancy.

Larger amounts of ponding water can speed that process up significantly and cause additional health and safety issues. Therefore it should be eradicated at the design stage.

One must bear in mind that eradication at the design stage is going to add additional cost to the project, and that it comes down to taking the long term view on your investment. By taking that view however, you are also taking the more sustainable approach and maybe without even thinking about it, looking out for the environment.

There are a number of different products available today to ensure that the amount of water sitting on a roof is minimised, if not eradicated, and we’re always happy to advise of ways of achieving this across any commercial roofs.