How to avoid a leaky flat roof

Some of you will have undoubtedly received that call to say, “my flat roof is leaking and needs to be repaired!” Flat roofs offer a simple, cheaper solution compared to pitched roofs. However, they are notorious for leaking if not properly designed, installed or maintained. Learn more about flat roofs and how you can avoid any issues.

There are two main types of flat roof:

  • Warm deck - insulation is placed above the roof deck but below the weatherproofing; and
  • Cold deck - insulation is placed between the flat roof joists at ceiling level.

However, there are many types of covering including felt, single ply, rubber and GRP, all of which require specific installation.

Common issues found with flat roofs

  • The most common issue is water leakage due to standing water on the flat surface.
  • Thermal movement due to thawing and cooling.
  • Roof membrane detachment due to trapped moisture and flashing defects caused by poor installation or damage.

How to avoid flat roof issues

  • All roofs should be insulated - the type of insulant used and whether you are constructing a cold or warm roof will determine the thickness of insulation.
  • In both cold and warm deck roofs, the external wall insulation needs to be extended up to the underside of the roof insulation to prevent cold bridging.
  • In cold deck roofs, a minimum of 50mm gap should be provided for ventilation above insulation to prevent condensation forming.
  • A vapour barrier should be placed on the warm side of the insulation, i.e. between the insulation and the ceiling finish.
  • Ensure the roof has ventilation openings of at least equal to a 25mm continuous gap running the full length of the eaves on two opposite sides
  • Don’t make it too flat - ensure there’s a small slope for effective drainage to prevent standing water, which will lead to water leakage. BS 6229 recommends that a flat roof should have a minimum design fall of 1:40.
  • Anchor the flat roof to the wall to prevent uplift and ensure all flashing over joints is robustly fitted to prevent water seeping in and causing damage
  • All materials should be suitable for use in flat roofs - they should be able to handle repeated cycles of heating and cooling, which cause expansion and contraction, leading to cracking and water penetration. The roofing product should have appropriate third party certification or CE marking that meets Regulation 7.
  • Ensure that upstands and flashings are suitable for the roof covering; a minimum 150mm upstand is usually required. Drainage outlet positions are also critical to avoid ponding.
  • Get the right tradesman to install your flat roof, many flat roofs fail prematurely due to poor workmanship.

LABC Warranty’s approach towards flat roofs

Flat roofing can be problematic due to poor design and workmanship. Chapter 7 of The LABC Warranty Technical Manual has substantial guidance on continuous membrane roofing, green roofing and balconies, all of which can be found here

Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.

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