Flat Roof Cross Ventilation

10 Jul 2020
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United Kingdom

I am currently renewing a large cold roof which has rotted from the inside, no ventilation or vapour barrier with a GRP top. The roof is on a dormer where the roof joists joined directly onto to the ridge beam with no ventilation and no scope to convert to warm roof due to ridge height. The roof is 9.5m wide by 3.5m span. I am going to fit PIR between the joists leaving the 50 mm ventilation gap, and add a vapour barrier under the ceilings (all the plaster board is down).

However to get the necessary ventilation my options appear to be to cross batten with 38x25 slate batten and fit a 25mm continuous vent along the short ends which would mean a 9.5m ventilation span.

But I wondered if I ventilated at the fascia as well giving ventilation on 3 sides would this help or cause a problem with ventilation paths?

Another option would be to cross batten and then add mushroom vents as well as belt and braces but I would rather not have unnecessary penetrations in the roof if it can be avoided.

Any thoughts or advice would be much appreciated.


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Another option would be to full fill the void with insulation, foam all gaps, tape and fit vapour barrier.
Another option would be to full fill the void with insulation, foam all gaps, tape and fit vapour barrier.

whilst that is a compromise with thermal bridging and it would be difficult to create a 100% continuous vapour barrier, I would say it offers a safer option than trying to vent a cold roof successfully as you cant vent the ridge side and a 9.5m ventilation path wont be great.

Are you redoing the deck and the GRP?
renewing from the inside doesnt sound like a good idea. is that what your doing? why not strip and do it as a warm roof? you can pack out the rafters if depth is a problem. maybe i dont understand?
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I'm looking at EDPM or or liquid system such as permaroof 500 all boards have to go as there is a mix of OSB and ply with some completely rotten.
im not a roofer and ive never fixed EPDM on a pitched roof only flat roofs. Not Gospel but i'd think you should roof the dormer with whatever kind of cover you have on your main roof.
The whole roof will be coming off and re boarded in OSB, Between the current finished roof height and the top of the ridge tiles is only 50mm with ridge tiles bedded directly on the GRP. I don't have the ridge height to convert to a warm roof - the roof is constructed roughly like this -

The roof was previously unventilated with full full rockwool, but with no vapour barrier. I am not 100% sure but I think the GRP has cracked (hairline) and then started to seep into the roof causing the boards to rot, so not necessarily because it was full filled?

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