Partial Garage Conversion

31 Aug 2016
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United Kingdom
Good evening all, could some of you assist me with the following please? We are in the process of converting the rear half of our garage to a small office for the good lady due to work relocation.
Roof - double skinned insulated cladding (warm roof?) fixed to 150 x 47 joists, I have installed noggins level with the outside of the wall plate either side leaving 50mm of the plate viewable internally which I intend to fill behind the noggins with say 100mm thick rock wool insulation. Will it be ok to just plasterboard (and plaster) the underside of the joists?

Floor - DPM (turned up all walls by approx 300mm with say 50mm battens/bearers infilled with 40mm celotex (all ok?)

Wall 1 - Newly formed internal stud work wall - can I build this directly onto the DPM turning the DPM up the outer side 300mm before plasterboarding/plastering both sides (100 x 47mm studwork I filled again with 100mm rock wool)

Walls 2 & 3 - existing single skinned brick walls - 47mm battens infilled with 40mm celotex (taping joints) before plasterbaording/plastering, this will obviously only leave a 7mm air gap between the rear of the celotex and the outer brick wall, is this asking for trouble? I have spare DPM so would it be better to have this against the brick and then rather than fix the battens to the wall actually build it as a studwork frame?

Wall 4 - existing single brick wall in very poor condition (looks like a car once went through it!), cut brickwork just inside the centre and rear two piers and replace with a 150 x 47mm stud wall (this wall will be supporting the roof) infill with rockwool, plasterboard/plaster internally. I was going to build this wall 35mm proud of the external brickwork as we intend to clad the outer wall at a later date, the front half of this garage wall is fine so was just going to fit 35mm battens for the cladding. This wall is probably the one I need the most help with as I believe we may need a vapour barrier etc?

I have read several garage conversion related posts on here but none are quite the same as ours and there are also a lot of conflicting replies, as always, any help would be gratefully received.

Thanks in advance.
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Looks fine, 2&3 make sure your walls render is in good condition.

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