Flat warm roof... does the top deck require additional joists or could it screw through to the main structure?

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I'm looking to build a warm-roof on a cabin, employing rigid PIR insulation and an EPDM rubber top layer.
The basic order would be, top->down:

EPDM rubber sheet
OSB top deck
PIR boards (150mm most likely)
VPL membrane (PVC sheet)
boarded bottom deck (also serves as the ceiling)
roof super-structure


So my question is whether we need to build a 'raft' on top of the bottom deck - basically a stud wall with the insulation inserted into the gaps. Or, whether we could cover the bottom deck with PIR boards to cover the whole area, and screw the top deck through the insulation into the substructure (so it's basically like an ice-cream sandwich)? Obviously we would need a frame around the perimeter for fascias and so on.

This is not a roof anyone would be walking on except when working on it and it would actually have a shallow pitch so no standing water. The sub-deck would certainly be plenty strong to walk on, but is rigid insulation OK without additional support? It seems like it probably should be with any load spread over the area and it is nicer from a thermal efficiency perspective.
Is this a)feasible b)a common approach or a little unusual?

Thanks, and sorry if I got any terms wrong I hope it's clear what I meant.
 
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I just had my flat dormer roofs upgraded into warm roof by adding 150mm PIR.

So far as I can tell you can just lay the PIR without any additional support other than the boards around the edge. I went up on the roof when it was finished and it supported my weight just fine. Mine was felt, though.

Structure after retrofit:

- felt
- 150mm pir
- VCL (in my case we used the existing felt)
- OSB
- joists with insulation only around the perimeter
- plasterboard

Photos attached. You can see I had insulation inserted behind the fascia, but otherwise the insulation is just a layer above the joists and VCL.
 

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Yes. The standard make up is the insulation "sandwich", no additional support required.The hardest thing with that much insulation is keeping your 240mm screws straight so they hit the centre of the joists.
 
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I just had my flat dormer roofs upgraded into warm roof by adding 150mm PIR.

So far as I can tell you can just lay the PIR without any additional support other than the boards around the edge. I went up on the roof when it was finished and it supported my weight just fine. Mine was felt, though.

Photos attached
Felt straight on the insulation?
 
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@^woody^ I think so - it had a slight give to it. Can't say for sure though - I don't have photos of that bit.

If it's horribly wrong best don't tell me ;)
 
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So my question is whether we need to build a 'raft' on top of the bottom deck - basically a stud wall with the insulation inserted into the gaps
Absolutely not as that is creating thermal bridging.

if You need firring pieces to create a fall, that has to go under the insulation not above.

the best warm roof method, becoming more common, I using PIR that is glued in place and the deck face has a finish suitable to glue EPDM to. It avoids using long screws, which being metal create cold spots….I used screws on a garden office and on a cold morning, you can see where they are as the warmth from the room dries out the rubber enough to change colour.

have a look at Steve roofer channel on YouTube

 

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