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Roofing

Single storey extension

Kevin 02/11/2026 - 9.12 AM

Ok here goes I am doing a self build . Going well roof joist and firrings are on flat warm deck roof blockwork and insulation taken right up to underside of bottom deck this will be sealed with foam to fill any tiny gaps when fitting deck over my question is I have a 100 mm cavity that has 90 mm k106 insulation fitted hard against the inner blockwork what happens to the 10 mm cavity gap that’s left ? Leave open to vent under the deck ? Will this not be damp moist air rising ? To sit under deck behind fascia’s and soffits ? I can’t find any info on what to do with this gap many thanks

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3 Answers

Cotswold property roofing

Rating: 5 out of 5
Banbury
Keep the 10 mm gap clear within the wall, but don’t leave it open into the space beneath the roof deck. The cavity should be closed correctly at the wall head.
Answered2 November 2026
0

D R Property Maintenance

Rating: 5 out of 5
Castleford
Good evening, what you are coming across is the part where engineering, designing and real life meet. This gaps living in a grey area as properties should be aloud to breathe. The amount of warm deck roofs I have taken off due to moisture getting trapped and starting to sweat is unbearable. If you bridge the gap you run the risk of sweating and mould developing from the moisture. If you leave the gap for air to flow your house can breathe a little. What's bad about that?
Answered2 November 2026
0

LH Timber Frame and Carpentry

No reviews yet

Llanelli
The 10 mm gap is meant to remain clear within the wall cavity, but it shouldn’t be left open into the space beneath the roof deck. It needs closing at the top with a suitable cavity closer/fire stop, keeping the wall and roof insulation continuous. Don’t fill the whole gap with foam, and ordinary expanding foam alone isn’t a suitable fire stop. Kingspan’s E14 detail shows this exact arrangement of 90 mm K106 in a 100 mm cavity meeting a flat warm roof. Get your designer or building control to confirm the detail before covering it over.
Answered2 November 2026
0