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Avoiding dripping roofs

By Rupert Pool, Principal Consultant, BRE Building Technology Group. Please let us know if you have an issue where our investigations team can help.  Tel. 01923 664200 Email construction@bre.co.uk

Dripping from roofs into the rooms below is often caused by a build-up of condensation in the cold roof voids, rather than by leaks which may initially be suspected.

At BRE we regularly investigate roofs from which water is dripping into the rooms below.  Often a leak is suspected, particularly as the drips sometimes coincide with periods of bad weather.  But our investigations frequently reveal that they are actually caused by condensation build-up in the roof space, which leads to water running downwards and dripping out. To prevent this, the amount of water vapour entering a cold roof void must be controlled.

In typical pitched roofs and flat roofs, problems can occur when water vapour permeates the internal finishes and insulation, faster than it can be ventilated out through the roof covering.  This is particularly important when:

  • The roof covering is impermeable, such as a bituminous membrane or metal sheeting
  • The ventilation of the space beneath the roof covering is restricted, for example where eaves vents are obscured by insulation
  • The amount of water vapour coming through the internal finishes is particularly high, such as above kitchens, bathrooms or swimming pools, or where the internal finishes have been compromised by the installation of spotlights.

Problems can be avoided if the internal finishes include a vapour control layer on the ‘warm side’ of the insulation or if adequate ventilation is provided on the ‘cold side’ of the insulation.  Condensation risk calculations can be carried out to determine if there will be a build-up of condensation for a particular roof design in a particular geographic location. 

Ventilation of a flat roof  through the eaves

Ventilation of a pitched roof, preventing loft insulation blocking the ventilation path at the eaves