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Property Surveying Blog
February 5, 2018

Flat Roof Water Ponding

Written by Karim El Shenawi-Ali

In this blog, I will be discussing a topic and issue that I came across recently when undertaking a Building Survey of a traditional Victorian property in North London.

When inspecting the flat roof at the rear of the property, I came across areas where water had settled to part of the flat roof. This is commonly referred to as Water Pooling or Ponding.

Water Ponding is evidence that a roof could be coming close to the end of its useful life or needs additional works or repairs to avoid further issue.

Water Ponding naturally occurs 48 hours after a downpour of rain.  Rain water starts to collect in flatter areas of the roof and ultimately settles and causes further issues in the future. Some of the most common issues that can occur from water ponding are:

  • Leaks
  • Moss growth or vegetation
  • Roof materials deteriorating
  • Roof sagging

Why does Roof Ponding/Pooling occur?

There are many factors that could influence the event of roof ponding on a flat roof structure:

Incorrect Pitch to the Roof

A roof being too flat can cause water that lands onto the roof structure to settle and fail to transfer down to areas of drainage. With water starting to accumulate over time, even during periods of no rain, it may then slowly work its way through the most minuscule cracks and lead to further issues such as leaks.

To help prevent water ponding, many builders or contractors will have the flat roof built with a 10-degree pitch, ensuring that water will continuously make its way down the roof and towards the areas of drainage.


Blocked Drainage

Drainage is designed to gather, move and dispose wastewater and rainwater from interior appliances and the exterior roof. However, in many cases, drainage systems and gutters can tend to be blocked with leaves, debris and other items.

Therefore, if the gutters or drainage systems become blocked over a long period of time, rainwater will tend to pool around the low point where the roofs end of pitch meets the drainage system. This can then result in roof ponding.

Overtime, if the ponding or blocked drainage has not been noticed or remedied and further rainwater gathers on the flat roof, the pressure caused by the weight of the water will cause areas of the roof to deflect and alter the structural system of the roof. Deflecting of the roof structure is where the strength of the roof beam becomes overworked by the additional weight of the rainwater, therefore resulting with the beam bowing downward thus forming a slight decline in the roof.  This will then result in bigger puddles of water to be formed and causing further damage and issues to the flat roof.

Heavy items placed on the Roof

This cause is hand in hand with roof deflection. Roof deflection doesn’t only occur due to excessive rainwater adding further weight to the flat roof. The deflection naturally occurs due to additional loads being placed on the roof structure. It is important to be weary of the type of items placed on top of the roof structure and most importantly where on the roof they are placed.

Even if the roof is perfectly equipped to handle the additional weight, the actual placement of the items could be the downfall and the cause of the deflection of the flat roof, especially with the additional weight of the rainwater placed on the roof structure.

Solutions to prevent Water Ponding

As with many issues, there are short term and long-term solutions to every problem.

Long term Solutions:

Slight Roof Pitch

If you are considering constructing a flat roof structure, whether it is an extension or a brand new building, having a pitch to your roof can work wonders and prevent further issues from occurring in the future.

Traditionally, when avoiding water ponding or pooling, architects and designers will recommend that the flat roof should be designed with a 10 degree pitch to allow rain water to travel in certain directions towards the drainage system. To give you a better idea as to how this can be designed, please take a look at the below image:


Install a Pump on the Roof

Installing a pump on your flat roof can avert future issues of water ponding. There are different models of pumps that can start to drive away any water that start to mount up on the roof over time. This is considered to be a long-term solution if the pump is well maintained.


Short-term Solution:

Unblocking Drainage System or Gutters

If you notice that your gutters or drainage system is blocked and water is starting to pool, I would suggest that you employ someone to clear out the gutters. This may be short term as gutters can easily become blocked again but what’s important is that it is maintained and reviewed frequently enough to avoid the blockage to result in water ponding.

If you have noticed water starting to accumulate on your roof and you want to figure out the cause of it and how to prevent it, then please get in touch and we will be happy to provide you with that information.


If you think your property may be suffering from roof ponding, I’d recommend speaking to a roofer at the first instance to ensure the issue is dealt with promptly. Ignoring this problem could see the roof deteriorate at a far quicker rate than it should and could ultimately mean it will need to be replaced sooner than later.