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I'm looking to do a loft dormer soon, but can't decide on a warm or cold roof. I would much rather a warm roof as it seems more reliable and generally better performance (and easier to install without all the venting), but I can't get the height to work with the building regs insulation requirement. The house is a semi detached Edwardian, with the loft dormer being an L shape.
The height I have to work with, from min. finished ceiling height to the peak of the ridge is 337mm. The current buildup i'm assuming is 15mm skim/ pb, 125mm joists, 50mm firrings, 18mm OSB, 150mm PIR, 18mm OSB, GRP. Total for this is around 376mm, so 40mm over the ridge.
The only thought i had for tying in at or below the ridge, was to run the firrings from back to front (instead of front to back as in the above), and have the water for the front section of dormer drain over the ridge and down the pitched roof. This would mean the height would be the same as the current ridge at the ridge, and a few cm higher further back, which I think in practice you wouldn't be able to tell unless up on the roof as there is a parapet between houses. I have a feeling this is a no no however, and drainage like this is likely to cause issue and/ or won't be allowed by BC? I have a feeling it would be asking for trouble draining this way, but unsure if it's more common than I think.
Edit: This is what the ridge looks like currently (left hand side of the image being the main ridge). The parapet wall on the right will be built up when the dormer is built.
The height I have to work with, from min. finished ceiling height to the peak of the ridge is 337mm. The current buildup i'm assuming is 15mm skim/ pb, 125mm joists, 50mm firrings, 18mm OSB, 150mm PIR, 18mm OSB, GRP. Total for this is around 376mm, so 40mm over the ridge.
The only thought i had for tying in at or below the ridge, was to run the firrings from back to front (instead of front to back as in the above), and have the water for the front section of dormer drain over the ridge and down the pitched roof. This would mean the height would be the same as the current ridge at the ridge, and a few cm higher further back, which I think in practice you wouldn't be able to tell unless up on the roof as there is a parapet between houses. I have a feeling this is a no no however, and drainage like this is likely to cause issue and/ or won't be allowed by BC? I have a feeling it would be asking for trouble draining this way, but unsure if it's more common than I think.
Edit: This is what the ridge looks like currently (left hand side of the image being the main ridge). The parapet wall on the right will be built up when the dormer is built.
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