New warm roof with some insulation within joists - what to do about firring strips?

[lookseehear]

We're speccing a new warm roof for our ground floor extension/roof terrace. Builder wanted to just insulate above the firring strips, but ideally I'd like to do 150mm PIR above and 80mm PIR between the joists. The builder pointed out that there would be a void between the 150mm and the deck because of the firring strips which isn't desirable.

 

Should I just put the 150mm on top and leave between the joists? Seems a waste given we have 200mm joists and don't need much of that for services.

 

Buildup would be:

 

Tiles on pedestals

EPDM membrane

150mm PIR

18mm OSB

Firring strips

18mm OSB

200mm joists with 80mm PIR pushed up against the deck

VCL

Plasterboard

[TommoUK]

Put your firrings on top your joists before you lay the deck.

[goodbyegti]

Some ideas:

 

- Get rid of OSB between joists and firring strips. As above, you can screw the firings at right angles directly into the joists.

- Put the VCL on the top side under the PIR. It's much easier to do a good job. Something like Alutrix will immediately give you a waterproof covering. I think there is a rule of thumb that you can then put 1/3rd of the top deck insulation below the VCL. You could use 100mm sheepswool/rockwool or something.

- Will the PIR compress under the force from the pedestals? Perhaps you want another layer of OSB under the membrane?

- I think it is costs more, but you can get tapered PIR which could replace your firrings and give you some extra thickness.

Edited October 4 by goodbyegti

[lookseehear]

5 hours ago, goodbyegti said:

Some ideas:

 

- Get rid of OSB between joists and firring strips. As above, you can screw the firings at right angles directly into the joists.

- Put the VCL on the top side under the PIR. It's much easier to do a good job. Something like Alutrix will immediately give you a waterproof covering. I think there is a rule of thumb that you can then put 1/3rd of the top deck insulation below the VCL. You could use 100mm sheepswool/rockwool or something.

- Will the PIR compress under the force from the pedestals? Perhaps you want another layer of OSB under the membrane?

- I think it is costs more, but you can get tapered PIR which could replace your firrings and give you some extra thickness.

 

This makes sense but the problem is that the joists will be going the wrong direction relative to the slope of the roof (see crappy drawing below).

(Grey is an internal block wall, blue is structural steels and yellow is joists). I was expecting the OSB to go above the joists, firring strips to go on top creating the slope away from the house, then another deck for PIR to sit on.

 

Re your point about VCL, I assumed I would be connecting this into a VCL behind the plasterboard inside the inner skin of the block wall. If it's above the OSB (under the PIR) how am I creating a barrier to moisture at the sides of the roof? Is there a detail somewhere that would help? On this point the builder wasn't planning on having a parapet. I'm indifferent but they seem quite popular with warm rooves from what I've seen. Are there merits or disadvantages of parapets?

 

Good point re the compression of PIR and pedestals. I've emailed a supplier to get their feedback, but I assume that OSB over PIR will distribute load enough to allow us to use the pedestals.