Warm roofing and GRP Fibreglassing Single Storey Flat Roof

[Harry_W]

Looking for advice please. I've got a flat roof single storey bathroom that needs a new roof and I've looked at GRP Fibreglass as the route to go down. I also wanted to insulate the flat roof and have read all the advice stating that a warm roof is superior to a cold roof. If the Celotex for the warm roof sits on top of the OSB3 board then do I need another layer of OSB3 board above the Celotex for the fibreglass to go on?

Fibreglass
-----------------
OSB3
-----------------
Celotex
-----------------
Vapour Barrier
-----------------
OSB3
-----------------
Rafters
-----------------
Celotex between
-----------------
Plasterboard

 

[geraldthehamster]

Yes you do. Personally I'd use EPDM rather than fibreglass. Easy to DIY, less messy, quicker and more durable.

 

[Notch7]

Yes you do need the outer osb3

silver tape on the celetex joints as an extra vapour barrier if you want

I dont know if the vapour barrier is needed if you tape the joints, I never bother -if your celetex that is in between is set flush to the top of the joists then taping across each joist will act as a vapour barrier.

If you have firrings make sure you dont have voids on the cold side anywhere.

 

[Harry_W]

Thanks for the responses. What thickness of OSB3 board would you suggest? I was going to use 18mm OSB3 tongue and groove directly above the rafters.

 

[Harry_W]

Would I use 18mm OSB3 tongue and groove above the rafters and above the celotex insulation? Also, as I have downlights in the bathroom, can I use 200mm knauf mineral wall insulation between the rafters instead of celotex and just use the 120mm celotex above the vapour barrier?

 

[Notch7]

we use 11mm osb above rafters -its only to spread the load on the insulation.

18mm T&G is ideal on the deck, although I never bother -how many joints would you need above a bathroom?

 

[Harry_W]

Thanks for the advice on using 11mm OSB3 on the Celotex. How do I fix the Celotex in position? Can I use a grab adhesive? Also, how is the 11mm OSB3 fixed in position on top of the Celotex.

Is it OK to use Knauf loft insulation between the rafters because I've got LED downlights in the bathroom that I'll be putting insulation hoods over?

 

[Alastairreid]

you dont need any insulation between the rafters if you are creating a warm roof.

 

[geraldthehamster]

How do I fix the Celotex in position? Can I use a grab adhesive? Also, how is the 11mm OSB3 fixed in position on top of the Celotex.

You don't need to fix the Celotex, just screw down through the top layer of OSB, through everything, into the joists. You can get proper fixings that supposedly avoid cold bridging. I've done two roofs like this and just used 6 inch screws.

 

[Harry_W]

you dont need any insulation between the rafters if you are creating a warm roof.

The video I watched on Youtube states that you do need to insulated between the rafters otherwise you can get thermal bridging at the fascia:

 

[Harry_W]

You don't need to fix the Celotex, just screw down through the top layer of OSB, through everything, into the joists. You can get proper fixings that supposedly avoid cold bridging. I've done two roofs like this and just used 6 inch screws.

Something like this?

https://www.twistfix.co.uk/warm-roo...FozipPZdSHwZs_d3QIYyv_pH7_ijrXtEaAt_1EALw_wcB

 

[datarebal]

I dont know if the vapour barrier is needed if you tape the joints, I never bother -if your celetex that is in between is set flush to the top of the joists then taping across each joist will act as a vapour barrier.

Wrong side of the insulation though.....

 

[bobasd]

thats a good video but the part about cold bridgin is ref to projecting eaves.

would fillin the joist bays with insulation and settin celotex aabove them cause any problems - i dont know?

 

[Alastairreid]

The cavity insulation meets up with the roof insulation.

 

[bobasd]

what do you mean?