Warm roof deck design - is this modification OK?

4 Nov 2013
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United Kingdom
I have a roof which is being totally redone from scratch, new rafters, everything.

The top is flat, pitched at the sides,l so nothing too complex.

Traditionally, a warm deck roof design has insulation sitting on top of the roof, as per image below.

My question is this: in between the plasterboard ceiling and 1st layer of OSB3, is there any reason I cannot fill that void with Rockwool insulation, marked with a red X, below?


  • Warm Flat Roof Design 2.jpg
    Warm Flat Roof Design 2.jpg
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Watch it does not interfere with any proposed lighting (downlights) or cables/pipes etc.
I will be using Rockwool RW3 (60kg/m3 weight), I have used it in walls before, it is a semi-rigid slab that can be cut with ease. I am using LED downlights, so no heat displacement, plan is to cut out circles in the plasterboard and also in the Rockwool. The layer of plasterboard in my pic will house the downlights. :)
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I have a question about sarking boards, do I post a different thread or should I stick it in here?

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