Warm Deck flat roof - vapour barrier

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I am having an extension built, and my builder is constructing the flat warm deck roof in the following way:

GRP
Insulated decking board (Ecotherm insulation 120mm with 6mm ply attached)
Firrings
Joists
Plasterboard

I have read that I need a Vapour barrier. Where in the roof construction should this be installed?
Would the blue DPM used in solid concrete floors be ok as a vapour barrier in the ceiling?
 
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No decking? the vapour check goes on top of the decking.
firrings then 18mm deck followed by the build up.
 
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My builder said that the ply attached to the insulation counts as the decking. Should I insist on him doing the following:

GRP
Insulated decking board
DPM sheet
18mm decking board
Firrings
Joists
Plasterboard
 
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Yes it's something like that.

Ok so if I don't need the decking on the joists, then how would I install the vapour barrier? Would the vapour barrier go straight on to the firrings and the insulation boards on top of that?

Or can I staple the vapour under the joists before I put the plasterboard on?
 
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should go immediately below the insulation. don't forget to insulate the open ends too.
 
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My builder said that the ply attached to the insulation counts as the decking. Should I insist on him doing the following:

GRP
Insulated decking board
DPM sheet
18mm decking board
Firrings
Joists
Plasterboard

We had an outhouse done and the builder's workers put a cold roof on, wasn't done properly.

On our internal extension I took control and bought all materials myself and paid him labour...

The roof is same as yours with 120mm insulation and this is what I insisted on;

GRP
18MM OSB3 T&G, writing side up, check instructions, leave gaps at ends
120 celotex
VCL, I got this from Wicks
18mm OSB3 sheets, leave expansion gaps all around
Firrings
Joists
Plasterboard

Originally I went with 120mm with 6mm decking built in, the stuff builder was sent was utter cr*p, broken and 2 brands. So I opted for 120 celotex and 18mm is based on top.

You will need helical nails, I used 195mm from selco and told them where to hammer them. Only last week I noted they hadn't used them all... Wtf

Even after that they messed up with Grp and didn't put 25mm fillets on the ends to take upvc edgings, which should have been done first. So I have an argument coming with the builder to take the grp edges off and redo the roof.

Its simple stuff but getting good builders is an issue.

I may have some pics if you like.
 
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