warm flat roof details

10 Sep 2012
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United Kingdom

I am rebuilding the rear extension on my Victorian terrace, and would like some advice on dealing with cold bridging and detailing where the roof meets the back wall (which will have 3m wide folding doors in it.)

Apologies for the drawings, I suddenly got obsessed with this on a Sunday night!

Image A shows how I'd like to do it. the joists are notched into the 152x152 UC section, and fascia boards etc are outside this. it gives me a smaller downstand above the doors, and allows the doors to be taller by 152mm. Any problem with this?

Image B shows how my builder would like to do it. Here the joists sail over the UC section to give an eaves overhang.

Can anyone see why option A would be a problem?

secondly, what is the best way to deal with the cold bridge where the uninsulated area between the joists meets the outside world? do i just fill in between them with insulation? This is where option A seems preferable as the joists are within the building so should be easier to keep warm...

Any advice and ideas on detailing would be much appreciated!



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Ceilings at frame head height never look right - and you wont be having any curtains. But it's up to you.

The builder's plan is more typical, but it gives a crap looking deep fascia. I'd project the roof out as is, but step the fascia back where the beam is

Insulation would go in the web of the beam and between the joists
Those details show the chief drawback of the warm roof - the deep fascia.
Your sketches seem to show the insulation proportionately thinner than it needs to be. Doing it the builder's way you are looking at 125 Kingspan; decking; (say) 150 joists and 152 beam which gives a fascia about 450 deep - rather ungainly.
I'd much prefer to use a warm deck - could you not use a catnic type lintel with a course of blocks/bricks to avoid the deep facia. (essentially the joists can run into the back of the outer skin - also the lintel has some basic insulation which would aid the cold bridge).
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Thanks for your replies.

tony you're right, the proportions are all wrong. The joists are actually specced at 195mm so the projecting fascia would be pretty thick.

If I were to step the fascia back to the beam like you say woody, the projecting bit would be around 350-400 thick which doesn't seem too bad... What would you say was the minimum I should have above the glass doors for things to look right, and is there an advantage to having an overhang, or should I be trying to get the fascia as close to the outer wall of the building as possible?

limerock, I'll have a look at using a lintel instead, will it span 3m? would you hang the joists off this? btw the outer wall is solid block which will be insulated internally, not cavity.

thanks again for your replies.

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