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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Flat roof dormer vapour control layer https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1604130284 Message started by Derek555 on Oct 31^st, 2020, 7:44am

Title: Flat roof dormer vapour control layer
Post by Derek555 on Oct 31^st, 2020, 7:44am

Hi all

My roofer is about to stick a warm roof on a flat roof dormer and I need to buy the materials. I have some 100mm rigid board insulation already but need to get a vapour control layer. Can I use some of the breathable membrane that went under the tiles of the main house? if not, can you tell me what I need please?

It will be a rubber roof and located in north west uk.

Cheers

Derek

Title: Re: Flat roof dormer vapour control layer
Post by woodsmith on Nov 2^nd, 2020, 8:12am

It’s a bit of a worry that your roofer hasn’t specified exactly what he needs you to get for the job.

However, a breathable membrane won’t work as a vapour control layer as this needs to be completely impervious to moisture. It is possible to use foil faced insulation as the VCL but this would involve careful sealing of the joints. The easiest, and most foolproof method, is to use a thin plastic membrane. You can get this from a builders merchant, just ask for vapour control barrier.