Question on Venting a Cold Roof

Venting a Cold Roof

Question on Venting a Cold Roof

Further to my call to you yesterday, thank you so much for offering to look at a couple of photos and speak to me about this. Here is one, and I will follow with a couple more.

As I mentioned, we have been through one thing after another with this roof, and are being told different things by different people, so we were really happy to find your excellent videos on YouTube.

We set out to re-felt, having taken a chimney out of the room, but discovered the rafters were completely rotten. We wanted to take the opportunity to build a warm roof but unfortunately there wasn’t space above due to the main roof, so we had to make a quick decision to have it rebuilt as a cold deck, and agreed with our roofer that we would fit the insulation.

We are now looking into this, planning to put 100mm celotex or similar between the 150mm rafters, plus 50mm beneath, however we now discover that we need cross ventilation to the 50mm airspace above, which doesn’t look easy to retrofit as the roof is against our neighbours’ wall on one side, and we have been told that the flashing ventilators are not particularly good, so it probably wouldn’t be worth having these put in at this stage.

So, we can add ventilation (we have looked at the circular vents) to the soffit on one side, and we are wondering whether these will be sufficient if we invest in a really good air barrier/ vapour control layer between the lower insulation and plasterboard, or if we need to have something on the other side? An alternative a surveyor suggested to us was to put some holes through the rafters on the other side to the soffit vents, but we are not sure whether this will suffice without some sort of vent out of the roof on this side?

I would really appreciate any quick advice you could give on this. The roof is small (1.85 x 3m) but it is above a bathroom (we are fitting an extractor fan). I can give you a call anytime after 1 today if that would be convenient for you, or tomorrow.

Many thanks and very best,


If you make holes in the rafters you still will not get the through venting you will need,  personally I would look and try to turn this into a warm roof as its a much better way forwards if you cannot then try venting using mushroom vents. I think you already have an EPDM rubber roof so this is the kind of vent you will need but you will have to place one between each joist.

Ill give you a call and talk you through this.

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Comments (3)

  • James Reply

    Hi Steve, could you please advise me as to how I can vent a flat roof with a skylight in the centre of the flat roof, thank you

    14th June 2019 at 3:39 pm
  • Cecilia kwan Reply

    I have a cold flat roof ventilated by soffit vents on one side and no ventilation on the other side. On the unventilated side, there is no overhang, just a solid facia board flush with the wall. How do I add ventilation on this side to create through air movement. Thank you

    16th August 2019 at 6:16 am
  • Anita Reply

    Hi Steve
    I have cold flat roof with 8 degree pitch. Initially was plan to do pitch roof but not enough pitch for it. So left with this
    Planning to do fibreglass roof
    What is the best option for an abutment ventilation as my joist is cut short to facia means the joist is not overhanging
    Thank you

    3rd September 2019 at 9:56 am

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