Jimmy_k

Thanks for that. I had no idea. We’re still a good way from electrics but I guess knowing about modern lighting at the early stages could influence the way we construct the roof! And, they’re cheap too!

I’ve just worked it out on the back wall and with 125mm timbers, 60mm insulation on top, 38mm counter batten, 19mm slate batten and then the slates (20mm), I can get a pitch of 20.5°. I’ll have to check but I think this is ok for slate. I’d much rather keep it a warm roof if possible as it’s much easier to add spotlights wherever needed. I’ll weigh up my options for a cold roof though and see what pitch I can get away with

Based on what I’ve read, Dave, I’d say yours is a hybrid. Although, from what info I can find on the kingsman website. The only roof listed as a warm roof is a flat roof with 120mm insulation above and above only. It says in the pdf for the kooltherm k7 pitched roof board that the thermal resistance of your above rafter insulation should be greater than that of the insulation between them. This picture is taken from the following document https://www.kingspan.com/gb/en-gb/products/insulation-boards/resources/product-literature/kooltherm-k7-pitched-roof-board

Thanks Russell. https://www.uvalue-calculator.co.uk/calculator/pitched roof/insulation between and over rafters/unventilated/no sarking board/125mm/400mm/120/ I came across the above calculator on Kingspan’s website and it calculates the U-value depending on insulation thickness and a few other factors. If I select… - Pitched roof - Insulation between and over rafters (for some reason above rafters alone isn’t an option) - Unventilated - No sarking board - 125mm rafter depth - 400mm centres 120mm insulation (60mm between and 60mm on top) I get a U-value of 0.16W/m2.K According to the document below, that’s good enough to comply with building regs. https://www.kingspan.com/gb/en-gb/products/insulation-boards/insulation-technical-hub/approved-document-l-england

Basically that the ceiling follows the profile of the roof rather than being a standard, flat, horizontal ceiling. See ProDave’s picture. That’s the goal. Is it necessary to use 200mm timber for the rafters? The structural engineer has said we only require 125mm timber. In your warm roof scenario would this have insulation between them or purely 120mm on top? Thanks Dave, the structural engineer has Ok’d this for the steel. We’re just waiting for him to come back to us as to whether it needs to be 152 or 203mm. As you can see, there’s a strip of steel running horizontally, preventing the pitched steels from spreading. As for the roof covering, it’ll have to be slate to match the rest of the house.

Hi everyone, I’m looking for some advice for a single storey extension at the rear of my house. The build will extend 4.3m out the back and be 5.5m wide. The roof will have a ridge in the centre extending out to the back of the build. As this will have a vaulted ceiling, this will determine the way it’s constructed. I’m currently trying to figure out what the pitch of the roof will have to be to have the ridge below the upstairs window. This will be determined by the thickness of the roof. My question is, if I was to construct a warm roof, how much insulation would I need above and in between the rafters? And what size timber would I use? Please see the attached photo for reference. Also, rather than windows, it’ll be bi-folds at 3.8m across the back. Thanks in advance