Flat Roof Light Upstand

[SBMS]

Just detailing my fixed flat roof light upstand and following on from a conversation with @JohnMo (sorry for my persistent questioning John!). In that other thread John suggested a timber frame with compacfoam on top for the upstand. 

 

It's going to be made out of compacfoam for the upstand - the rooflight will sit on this.  On a warm roof with 150mm PIR.  Can I run the PIR right to the edge of the aperture, and then form a 150mm upstand on top of the PIR? Roof light will weigh about 250kg - Detail 1. 

 

Or do I need to do two layers of compacfoam as per detail 2?

 

Edited September 2 by SBMS

[JohnMo]

I would do neither of those, definitely not detail 1, its not structural. You must have a structural opening that is designed to take the weight and wind loads, this requires input from your structural engineer.

 

You have no need to rebate compafoam, it needs a flat top for the flat base of the roof light to screw down on too.

 

Your typical install details are this, which you need to modify as you are a warm roof (insulation on top)

 

Notice how they (Roof Maker) say to use a 40 to 75mm up-stand, this is what you are replacing with compafoam, the roof maker sky light already has part of the upstand built in to the unit. So build a wooden structure first, this is the structural element of the roof and opening. As previously discussed, don't waste a small fortune on making the whole up-stand arrangement from Compafoam as it isn't cheap and doesn't add to the thermal values. You only need to start as you come out of the PIR layer.

 

So the above gets modified to something like this

 

 

[SBMS]

Thanks @JohnMo - got it.  My challenge is that I need 150mm clear above the roof deck so will need to do a 150mm + 20mm compacfoam above, but that should be fine.  The detail for my roof light needs a rebate in the compacfoam as below:

 

This is where I am up to with my detailing:

 

 

  Two questions:

 

1. Does insulated plasterboard on the internal face of the aperture deal with my VCL as well, or would it be better to run the VCL up the face as well and then put regular plasterboard on?

2. Would GRP adhere to the outside face of the compacfoam, or would it need to have OSB screwed to it?

 

Thanks

Matt