Causes for flat roof pooling and when it can become a problem

During our recent downpours, you may have had concerned clients calling about their flat roof collecting pools or puddles of water, worried about the damage it may be causing.

Its a very common concern, particularly after an excessive amount of rain, but not all cases may require a repair and could be very easily explained without the requiring a closer look.

Here we aim to explore what causing water to pool on a flat roof, when it can become a problem and when it can actually be beneficial.

Why does water puddle on a flat roof?

It is common for puddles to appear on a flat roof as unlike a pitched roof, a flat roof struggles to drain rain water as it doesn’t have the benefit of a pitched slope, which can often cause water to sit until it is eventually evaporated away when the weather dries up.

Small puddles are often nothing to worry about, however large pools of water could cause damage if left untreated.

What causes a pool of water to appear on a flat roof is a process called “ponding”. Ponding occurs when excess waters that is left on a rooftop after a downpour is unable to drain away and remains for 24-48 hours.

What can cause ponding?
  • Insufficient drainage – A flat roof can pond if there is insufficient drainage, such as a blocked outlet or downpipe.
  • Weight damage to the flat roof –  A flat roof is not designed to withstand a large amount of weight. If a heavy object is left on a flat roof, or if someone was to regularly walk on a flat roof (i.e. to use as a balcony) not designed for this purpose, the roof can begin to concave inward, trapping water and causing the roof to pond.
  • A roof fixture – New roof fixtures placed into a flat roof such as a lantern or skylight can create gaps causing water to get trapped on a flat roof.
  • Moss and waste – Moss and other waste such as dead leaves can hold water causing the rood to pond. It is always important to remove waste and any excess moss. Read our blog on removing moss from roof top tiles and slates.
  • A change in materials –  Originally a flat roof would be lay with a material called limestone grit which gave a gravel appearance on the top of the flat roof. As water would pool, the pool would lay invisible as the water would sit inside the gravel and remain undetected. Since then this material is no longer used and more modern materials such as EPDM rubber and GRP are becoming more popular. These modern coverings tend to be smooth faced meaning pooling water that would previously be hidden by grit becomes much more obvious.
  • Structural fatigue and settlement – Over time, roof joists and timber can settle and fatigue. As this happens, the substrate sinks, creating a concave in the roof covering for water to collect and pool. As time goes on, the pooling and settlement go hand in hand and becomes a never ending cycle which often results in full roof replacement.
What is the risk of puddles on a roof?

There is no actual risk of puddles on a roof top, these are very normal, however an extreme amount of water causing ponding on a roof top could be a risk.

Extreme ponding will only ever get worse until its too late. Ongoing ponding will eventually damage the structure of a flat roof, eventually causing it to leak, by which point any repair would be too late, and a complete re-roof is required.

When can puddles on a flat roof be beneficial?

Excess water on a flat roof can be risky, however it can be beneficial to have a little extra water as a form of protection against extreme heat in the summer months. This is because certain materials such as bitumen based felts used on a flat roof can crack when exposed to direct sunlight, damaging the roof. In these cases, it is actually better to have some excess water on the roof top to help keep the roof cool on hot days.


If you have a client in need of a flat roof repair after ongoing ponding that is causing a leak, we can help! Here at Kidderminster Roofing Supplies ltd, we can provide a free no obligation quotation for roofing materials and tools you need. Contact us today to see how we can help you!

Tel: 01562 748270


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