How Much Ponding Is Acceptable on a Flat Roof?

Flat roofs come with many benefits. However, they are susceptible to a unique problem – ponding. Here’s everything you need to know about ponding and when it should become a cause for concern.

What is ponding?

Ponding is when water collects in pools on a flat roof after a period of heavy rain and doesn’t drain off once the rain has stopped. Whilst brief and occasional ponding is acceptable on a flat roof, regular or long-lasting ponding could be an indication that there is an issue with the roof that needs to be addressed.

What causes ponding?

There are a number of reasons ponding can occur, such as an insufficient slope on the roof, blocked drains or scuppers, an inadequate drainage system, dips or depressions in the roofing material or a sagging roof deck.

How much ponding is acceptable on a flat roof?

The good news is that ponding is not an imminent threat if your roof is made with the appropriate materials and has been properly installed with a sufficient drainage system. Ponding that drains away or evaporates within 48 hours is acceptable and isn’t something to be concerned about, but the regular occurrence of ponding should still be investigated.

If the ponding remains after 48 hours, you should seek the help of a professional roofer, as this suggests that the roof drainage is not functioning properly, or that there is a problem with the roof slope, roofing material, or roof deck.

How does ponding affect the roof?

Recurring ponding can cause damage to the roof over time, as the added weight of the pooled water can put extra stress on the roof and can degrade the roofing material. Ponding can also allow water to ingress, especially if the pooling is occurring where seams overlap. Ponding also provides an opportunity for other moisture-related problems to occur, such as the growth of moss and algae and creating a breeding ground for insects.

What can be done about flat roof ponding?

The first step you can take if you notice ponding on your flat roof is to make sure your gutters are clean and free of debris. In some cases, this may be the extent of the problem and the pooled water will drain off.

Dips can occur if the roof is frequently walked on, compressing the insulation below the roofing material. If this is the case, the insulation in that area may be able to be replaced to remove the dip.

However, if the ponding is being caused by a structural issue such as an insufficient slope or a sagging roof deck caused by improperly installed or damaged joists, beams, or frames in the roof, then you will need to call a professional roofer who can advise you on the best course of action.

If you are at all concerned about ponding on your flat roof, calling a professional is always the best bet. They can assess exactly what is causing the problem and recommend the best solution to fix the problem and prevent it from happening again in the future. If you need to have your flat roof inspected, or are concerned about ponding, please contact us and see how we can help.


All roof work undertaken

Fully qualified for heritage tiling & repair

Fully qualified in traditional roofing works

Contact Us

Make an Enquiry