New roof with skylight (should it be warm or cold construction)

8 Jun 2010
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United Kingdom
Hi all,

Building an extension on the back of our house atm. Part of it is single storey with a flat roof, and a large skylight (1m x 2m).

The roof was specified as a warm roof but my wife concerned by the thickness of it and that it will look out of place against the rest of the house (old cottage). The rafters are already on and a deck down ready for insulation above, T/G OSB and GRP roof. We could change it slightly and install the insulation below the deck (making it a cold roof) which would reduce the roof thickness by about 150mm.

My concerns with this is that we will need to introduce ventilation below the deck (which I guess is doable) but where the rafters go around the skylight there will be an area that can't be ventilated...

What are peoples thoughts on this conundrum ?!?

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You can introduce a flat roof vent above and below the rooflight but they look a bit pants, google ‘flat roof breather vents’.
You can introduce a flat roof vent above and below the rooflight but they look a bit pants, google ‘flat roof breather vents’.

Googled and agree - they look terrible! the flat roof will sit directly below a bedroom window so we can't be having them stuck up in the air!

Are there no other options?
I guess roof abutment vents could be utilised to get ventilation, assuming the roofligts have an upstand and the rooflight cowl was adequate/lent itself to having a vent behind it, the detail would be similar to a flat roof abutment vent detail where your flat roof meets the wall. But it would need careful forethought/design to ensure it worked OK.
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Years ago when we did loft conversions and the roof was cold and you introduced a rooflight you'd have the same issue, vents weren't added back in the day and I'm not aware of a particular problem now of timbers adjacent to such roofligts in such situations rotting per se .....
fair point, unfortunately though BC will have their opinion and I'd rather not get on the wrong side of our inspector!

I don't mind the warm roof... my wife will have to get used to it! :rolleyes::rolleyes: (y)
noseall posted an interesting thread or maybe added the photos to someones thread in the roofing section about achieving a thinner fascia for a warm room roof a few months back, might be worth looking.
thanks for the pics - tried to look through noseall's historic posts but there were too many!

Will show the wife but not sure she'll like that approach. I may be wrong...

I attach a poor quality image of the back of the house below (flat roof on the right is the one in question). The rafters are 175mm and you can currently see currently the rafter on the end above the wall, the roof drains to the side. I'm thinking that we should continue to build it as a warm roof and lap the GRP 100mm down from the top , this would then leave circa 20cm of white painted wood visible on the back (which shouldn't look too thick).

Also, we were going to add down lighters above the bifold doors... just had a thought that we could introduce a horizontal strip of LED's in a diffuser between the bottom the GRP and the white fascia to "hide" the lights and make them a bit funky, could even get the multicolored ones ready for a party or two!


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