Warm flat roof with EPDM. Insulation options?

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I have seen at least three options for building up a warm flat roof with EPDM.

The composite roof deck type boards have to have additional ply/osb on top if using EPDM.

I have also seen diagrams showing an additional overlay board between the PIR foam board and the EPDM.

So here are the options:

Joists-Firings-18mm OSB3-VCB-120mm foam board-EPDM

or

Joists-Firings-126mm Temp Deck (foam, VCB, 5.5mm ply)-12mm OSB (specified only if using EPDM)-EPDM

or

Joists-Firings-18mm OSB3-VCB-120mm foam board-12mm OSB (overlay board)-EPDM


I think the temp deck boards work out a lot more expensive especially if the extra 12mm OSB/ply is required...

Any thoughts on this?
 
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You do need board on top of the insulation, under the EPDM. The EPDM follows the contours of whatever is underneath it, so the finished roof will show every ding that you've made in the insulation by walking around on it. Plus I think the usual water-based EPDM adhesive is designd to stick it to OSB or ply.

I'm not familiar with some of the boards you mention, but there needs to be a board between the insulation and the rafters/firrings, or the insulation will crush and deform.

I'm speaking as a non-roofer who has done one 25 m2 EPDM flat roof.

Cheers
Richard
 
You do need board on top of the insulation, under the EPDM. The EPDM follows the contours of whatever is underneath it, so the finished roof will show every ding that you've made in the insulation by walking around on it. Plus I think the usual water-based EPDM adhesive is designd to stick it to OSB or ply.

I'm not familiar with some of the boards you mention, but there needs to be a board between the insulation and the rafters/firrings, or the insulation will crush and deform.

I'm speaking as a non-roofer who has done one 25 m2 EPDM flat roof.

Cheers
Richard

Hi Richard, that makes a lot of sense, thanks.

So what issues do you run into when you did your roof?

I'm just weighing up the pros and cons of diy epdm vs getting a contractor in to lay a GRP roof.
 
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So what issues do you run into when you did your roof?

I'm just weighing up the pros and cons of diy epdm vs getting a contractor in to lay a GRP roof.

I didn't run into any issues. Possibly my flashing wasn't as tidy as it might have been, but the roof is all flat and watertight. I hadn't done anything like it before and found it really straightforward. It's helpful to have a second pair of hands but even that isn't essential. I think including all the flashing and adhesives and so on, it came in at about £18 a square metre.

Cheers
Richard
 

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