Design of warm Flat roof (moved from Projects)

[DJDAY]

Please could someone answer some questions on installing a flat roof, I've been told by building control I need 120mm insulation on top furring strips then board 18mm OSB as I'm having GRP.
I also know I need a vapour barrier is this under the insulation and on to the furring strips ?
The other question is have I got to board out on top furring strips then insulation then board out on top then insulation then GRP roof covering?

 

[tomfe]

I would do it like this:
Internal to external
Firrings
18mm osb3
Vcl
Insulation, depending on what type you use will depend on depth.
9or11mm osb3 or ply. I think GRP likes ply more than OSB
Finish roof material

If you use felt/epdm/single ply you can go straight on top of some insulation.