Issue 4: what are the causes of water ponding on flat roofs and what are the solutions?

There are many reasons for water ponding on a flat roof, from the incorrect design of roof falls and insufficient slope, degrading materials causing roof sagging to drainage issues and build-up of silt on the roof surface.

The solution needs to eliminate the cause of the ponding to prevent the issue from returning in the future and causing damage to the roof or the structure that may be costly to repair.

Correct design of drainage and drainage falls to prevent water pooling on flat roofs

As a requirement of Building Regulations, Part H, adequate provisions must be made for rainwater to be carried away from a roof. Designers can follow guidelines to design a roof with a drainage fall (gradient) of 1:40.

At this design fall, normal construction tolerances can be accommodated resulting in a typical finished level of 1:80, in accordance with BS 6229 and BS 8217.

Flat roof areas should always be designed to freely drain water either directly to rainwater outlets or a gutter section, either off the roof elevation or to an internal box gutter section. Ideally, internal box gutters should also be constructed with an adequate fall.

Water ponding caused by lap build-ups

Lap build ups, especially around water outlets can be the cause of water pooling. It is good practice to ensure these are recessed or if possible, fitted in sumps.

Roofing material performance and other causes of water ponding

Damage to the roofing system or structure that results in uneven areas on the roof surface can often be the cause of water ponding.

Deterioration of the roofing material, damage to the insulation beneath the waterproofing layer and build-up of silt can all be causes of water ponding.

Correct design and use of appropriate materials and high-quality roofing systems with long design life can help prevent material failures that can cause water ponding.

Why should water ponding be avoided and remedied?

Water ponding can cause a variety of issues and when left unattended it can result in water ingress that can result in costly damage to the building structure.

Other issues include:

  • If the roof surface is suddenly damaged, there could be a significant water ingress into the building.
  • The dead load on the roof is increased causing extra stress on the structure.
  • Stagnant water causes moss, algae, and debris to collect creating a slip hazard and encouraging insects.
  • In sub-zero temperatures, the water can freeze and cause damage to the roof through expansion.
  • Degradation of materials that requires more frequent maintenance and reduced roof system design life.

What are the solutions for water ponding on flat roofs?

The majority of high-quality single ply roofing membranes, such as the IKO range, will be able to withstand water ponding for extended periods.

However, water ponding can eventually affect the design life of all roofing solutions and should be addressed as early as possible.

When upgrading or refurbishing a flat roof area, steps should be taken to eliminate existing areas of ponding. There is a variety of solutions to addressing zero or negative falls:

  • Tapered insulation.
  • Adjusting the height of supporting beams or purlins.
  • Addition of firring pieces.
  • Use of screed.

Whilst remedial action will depend on the type of project, budget, time frames and many other variables the pros and cons of each should be carefully considered. For example, the use of screed to fall systems involves lengthy drying time and additional weight loading.

Firring installation can be complex and result in condensation in the voids created. Installation of tapered insulation boards is an effective and economical solution for both new roofs and refurbishment projects.

If you have any queries regarding water ponding or require assistance with drainage fall design or remedial solutions, contact our Technical Team.

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