Condensation in a Flat Roof

Condensation in a Flat Roof

This customer in Ealing had a new flat roof  and  thought all his problems were over until he removed some of the pasteboard from the ceiling and saw this condensation on the underside of the new roof decking.

He contacted the roofer and they came back and placed a mushroom vent in the roof but still the problem persists.

He contacted us at London Flat Roofing and sent over these photos and asked our opinion. After some discussion we worked out that the new roof is around 40m2 and was replaced with new decking and three layers of mineral felt.

When the original roofer came back they only placed one or two vents in the roof and after some months the problem persists.

After some questioning it was established that the top floor flat was built-in 1960’s of solid wall construction and the condensation is mostly in the kitchen and bathroom area. It was also established that the flat always feels cold and that he gets lots of condensation on the windows.

Warm Roof Vapour Barrier

London Flat Roofing’s Opinion

Part L 1b of the building regulation states that if you strip a roof you should update the thermal quality to the new specifications so realistically the roofer sound have informed the customer of this so its questionable if they have a responsibility to come back and do the roof again.

However the problem was probably there before they changed the roof realistically they haven’t changed any part of the thermal element of the roof just exchanged old roofing materials for new. To solve the problem now we need to work out our options.

To add mushroom vents to the roof would mean adding a vent at both ends of each cavity between the joists so through ventilation could be achieved. This only address the ventilation and not the thermal element of the roof also this is going to be mess and expensive as probably 40 or so vents would have to be used.

Another way would be to introduce strip vents to the sides of the roof, probably cheaper but again doesn’t address the thermal properties of the roof.

We can upgrade the insulation of the roof in one of two ways:

  • Form a cold roof and place the insertion under and between the joists.
  • Form a warm roof and place the insertion over the new roof using the old roof as the vapour barrier.

The warm flat roof construction above will be much more cost-effective and won’t impact the living space below as the ceiling won’t need dropping to add the roof insulation.

The warm flat roof will solve the problem of condensation on the bottom of the roof decking however it will still be worth reducing the amount of humidity in the rooms below as this will improve the living conditions.

social position

Share this post

Comments (3)

  • Vitesh Reply

    Hi Steve,
    thanks for the video which is simple yet very helpful.

    Do you have more videos to show if you have to install vents from the extractor fans or presence of soil pipe running down from first floor, skylight etc?

    How would you address the thermal bridge etc.

    Kind Regards,


    4th December 2017 at 12:23 pm
    • Steve @ London Flat Roofing
      Steve @ London Flat Roofing Reply

      No, I haven’t got any videos on this subject, you have to make sure that any vent or protrusion is insulated so that it doesn’t sweat.

      11th December 2017 at 8:45 am
  • Mrs Ann Martin Reply

    Hello, having a newly constructed single story extension with a timber/lantern (cold) flat roof to be completed soon. The building inspector has recommended a wall abutment and one mushroom vent. Can’t understand just one mushroom ventilator if an abutment is being inserted too? Also will the rubber roof which has been laid be compromised with the cutting out for the mushroom vent? Hope you can help? Thank you.

    20th September 2018 at 5:52 am

Leave a Reply

Your e-mail address will not be published. Required fields are marked *