Replacement flat roof - warm or cold details?

30 Dec 2006
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United Kingdom
I am looking to replace an existing flat roof that has reached the end of its life and is leaking. Its on an existing kitchen extension and currently has no insulation.

I'm looking at both warm and cold roof replacements but neither seems a straight forward solution. I marginally favour a warm roof simply because I believe them to be less prone to condensation and the complication of cabling and lights between the existing joists. But both warm and cold seem to come with complications.

Putting a warm roof on top of the existing joists would I think lead to very tall fascia's (ie to cover the height of both the existing joists and the insulation above). I've not seen any pictures or designs that show this. Is there a standard way of tackling this? The other complication is I think the flashing where the extension joins the main house will need to be raised.

For a cold roof I think the solution would need to be (working top down): waterproof layer, board, vent space, insulation within existing joists, extra insulation below joists, vapour barrier, void for cables, plasterboard. This inevitably means the ceiling would need to be lowered.

Any comments or advice on these options and the issue tall fascia board very welcome.
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