Advice needed on building an internal timber partition wall

Hello,
I want to divide my large roof space (triangular cross-section 4m high at highest point) into 2 bedrooms by timber partition walls. Currently the roof insulation is exposed and I can see the roof joints (to which of course I will be fixing the head plate of my future partitions).
Should I build the partition walls before "closing the roof" (will be using timber cladding for the roof) or should I do the roof first meaning that the cladding will between the head-plate and the joist ?
I'm going to need to pass electricity cables through the studs of the partition wall so it seems to me more practical to build the walls before doing the roof cladding. The power cables are in position already behind the roof joists and thus currently easliy accessed.
Ideally I want to do the cladding now so that the roof space is usable in the short term but say in 2 years divide the space into 2 rooms. Any advice on the pros and cons of the two approaches would be gratefully received.
Thanks
Tom
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TomC wrote:

Tom
You would normally do the walls first and then clad everything afterwards.
You do realise you need Building Regulations approval for a roof conversion, don't you? (check out your Local Authority's website.) Should you decide to proceed with an unauthorised conversion (which isn't wise), then for safety's sake, at least take into account how anyone trapped in your roof space will manage to escape in the event of a fire.
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walls first. Make sure you have a secondary (fire escape) exit ... Building Regs will want one.
Think well about insulation, heat loss will be significant, and just plonking insulation in, is not good enough - need to consider water vapour, or you could end up with interstitial condensation. also importantly think about heat gain if a dark roof - or room will be baking in Summer.
I would use a foil faced Urethane foam board.

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All,
Thanks for the responses and helpful advice received so far.
Just to clarify : the "roof space" is actually the first floor of the house which can be described as open plan. It's a new house and the architect designed the first floor so that we could later put in partition walls to divide it into 2 bedrooms. There is a window in place for one bedroom and a velux in place for the other. The house is actually in France, but I would be interested to know if these count as secondary fire escapes under UK regulations?
Thanks again
Tom
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TomC wrote:

Yes they can, if they meet certain standards.
These are extracts from the UK Building Regulations.
'The window should have an unobstructed openable area that is at least 0.33m2 and at least 450mm high and 450mm wide (the route through the window may be at an angle rather than straight through).The bottom of the openable area should be not more than 1100mm above the floor' (for normal windows).
For Velux windows, in the plane of the roof slope, the Regulations refer to the bottom of the window being a max 1700mm up from the eaves (measured along the roof slope), and 600mm min, 1100mm max measured vertically from the floor to the bottom of the openable area.
You can down load a copy of the AD's here:
http://www.planningportal.gov.uk/england/professionals/en/1115313971412.html
Of course, the French might have their own rules.
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