What FLat Roof Specification is needed for extension?

[Don Clarke]

After researching and watching Celotex video guides I have started to build a flat roof to my extension. The roof is 7.2m long by 2.2m wide. The decking will be T&G 18mm OSB3, a VCL topped with 100mm of PUR insulation and a finishing layer of EPDM. There will be two roof lights on the roof. A contractor offering an estimate has said I must have another layer of 18mm OSB on top of the PUR. He also said I must only use OSB for the decking and that plywood was not 'allowed'. Celotex, and other guides, suggest that EPDM can be fixed directly to the PUR. Your comments on this issue would be welcome.

 

[chappers]

you can use crown bond celotex and fix direct to that. No reason really not to use ply, but the advantage with OSB is it doesn't de-laminate when wet and is structurally graded, my preference is also to use a second deck over the insulation but as you have said there are methods which mean you don't have to.

 

[Don Clarke]

Hi, thanks for your response. Why would you use a second deck and how would you fix it? Any sort of nail or screw through the decking and PUR would surely compromise the performance of the VCL?

 

[^woody^]

Celotex, and other guides, suggest that EPDM can be fixed directly to the PUR.

The question is, do the EPDM guides suggest that it can be fixed directly to the PUR?

 

[chappers]

Hi, thanks for your response. Why would you use a second deck and how would you fix it? Any sort of nail or screw through the decking and PUR would surely compromise the performance of the VCL?

indeed that is true but you can't just plonk your insulation up there and fix you're roof covering to that, it needs fixing too, a few small holes won't compromise the VCL too much. A second deck gives a better surface to fix too and Woody makes a good point. I have seen multi layer torch on felt direct to bond it and the thickness means you couldn't tell the difference, but I wonder if joints would show through single ply and whether EDPM alone would have the same effect of bonding the whole structure together quite as well.
EDPM needs a good solid surface to adhere to both after and during the installation.
But as you have pointed out it isn't necessary, but it's the way I am happy with as is your contractor by the sound of things. These specialist insulations are fairly new and on top of that not that widely available so probably end up being special order items

 

[geraldthehamster]

By the time you've walked around on the roof, spreading the adhesive and rolling out the EPDM, you'd have lots of little dents in the Celotex. I'd install an upper deck.