Warm Roof Layers

[i_am_fubar]

I've had conflicting information from a couple of different roofers on the layers needed for a warm deck roof to be used as a roof terrace. Ignoring the pavers and supports, I've had:

Top
EPDM membrane
18mm OSB3 top deck
110mm PIR insulation
6mm WBP ply sub deck
Joists
Bottom

Top
EPDM membrane
9mm OSB3 top deck
110mm PIR insulation
18mm OSB3 sub deck
Joists
Bottom

The obvious difference here is that one says the thinner deck needs to be on the bottom, the other says it needs to be on the top. Are both options sound, or is one preferable?

Neither mentioned that a vapor barrier was needed. I assume it still is?

If it is still needed, is there any particular reason it needs to be between the sub deck and insulation?

To reduce deflection of the deck, I was considering the use of contact adheasive to fix the PIR to deck(s) to give a single composite slab. Any thoughts on this?

Cheers,
Fubar.

 

[i_am_fubar]

Ok, so vapour barrier has now been confirmed as a requirement.

Is there a reason to fit it between the sub deck and the insulation, or could it be fitted between the sub-deck and joists (even though you lack some support then) or between plasterboard and joists?

If not, would it be possible to include it in the laminated stack of insulation and OSB3?

Fubar.

 

[tony1851]

The vapour barrier is supposed to go immediately under the insulation and then be turned up at the edges and sealed to the top of the insulation.

Doubt the thickness of the board would make that much difference, though one roofer I know who does EPDM says the adhesive works better with plywood rather than OSB?

 

[i_am_fubar]

One layer of OSB, not much, but 2 layers of OSB either side of 110mm of insulation makes a nice composite board. Makes a big difference to the calcs.

So, what is the purpose of the barrier? Prevent ingress of moisture into and between the insulation and subsequent condensation on the underside of the upper deck?

Ply would always be my structural preference, but cost is limiting to OSB. I imagine the improved adhesion comes from the flatter surface of ply. Did he indicate how much better? It is the difference between the roof being sound and getting huge bubbles?

Cheers Tony,
Fubar.

 

[i_am_fubar]

Also, was that the waterbased deck adheasive, or the contact adheasive?