Laying Flat Roof Decking

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Hi all,

Nearly ready to install approx 6x6m timber deck roof (warm-deck), divided in the middle by an RSJ to keep joists shorter and costeffective. Deck will be 18mm OSB3 T&G. Firring on top of joists to create fall.

I plan to raise the inner skin side walls to same height as the firring strips (ie follow the firring slope) so that I can start laying decking from top inner skin.
Is there anything wrong with this?

This way I'm keeping the OSB from bridging the cavity and touching the external wall
(wet skin) and also allowing me to continue cavity insulation to meet the roof insulation without any thermal break. I then raise the external skin in the same way, sloping, to follow the internal skin. vcl/insulation/osb/epdm will then complete the roof.
Will this work?

Cavity insulation is Drytherm32 and roof insulation will be foild faced rigid poly. Is there a danger of vapour rising from the inside of cavity condencing on the underside of foil-faced roof insulation in the cavity part where they intersect, at roof height? Should I build air-brick (in external skin only) near tope of wall to allow cavity vapour to escape. Walls are Fibolite 3.6N Block both skins.

Whilst Im at it, another related question if I may please...

Should the facia boards and nailer battons around the entire roof line be touching the building or installed slightly away to allow top of wall to dry from outside. With insulations and VCL thrown around in this area Im worried that top of walls will sweat rising cavity vapour and will have no where to dry.

Will appreciate any expert advise.

Thanks.

mrafiq
 
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Why dont you google up some diagrams of flat roof framing (or carcassing as its sometimes known) & warm flat roof construction?
Reading the Similar Threads at the bottom of this page might also help you.
Usually, the simple, straight forward way of doing stuff is the best method.
 
vinn, thanks for prompt feedback. I think you're right, Im complicating something that should be simple.
As its my own job and I have time, I thought I should improve on areas that tend to trouble people long after builders are gone.
If the outer skins can be built up to roof level (epdm level) why people resort to capping off roof perimeter with rather wide fascia boards.
Thanks for looking.
 

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