Plasterboard over celotex.....

[r6deano]

Morning all
Looking to mechanically fix foil backed plasterboard to the underside of rafters on an outbuilding's flat roof, and sandwich 50-70mm celotex in between the rafter and plasterboard (insulate under the rafter). In addition to 100mm friction fit Omni slab between rafters.
I've done this before with 25mm pir, and tacked the celotex up first using clout nails, but I wondered if anyone's had experience using thicker pir insulation.....didn't want extra movement and cause plaster skim to crack 6m down the line.
Any advice very much appreciated.

Thanks all
Deano

 

[domdee]

You could screw to rafters using penny washers under the head of 100m screws. and stagger the joints of the boards and insulation. could even lay in opposite direction too. You'd need some long plaster board screw though and i don't know what lengths they come in???

 

[domdee]

150mm apparently

https://www.ukflatroofing.com/insul...MIkMLd087f6gIVzOvtCh1jUgf1EAQYAyABEgLWkPD_BwE

 

[23vc]

50mm is doable by tacking the PIR up first then using 100mm drywall screws.
70mm could get tricky, 120mm drywall screws? They tend to be hard work and flex

 

[Mr Chibs]

Long screws and insulation washers would be suggestion.

 

[oldbutnotdead]

You could use slating battens & long screws to fix the insulation up, then normal length screws to fix the pb to the battens. Gives you a handy wiring void as well

 

[^woody^]

Just use a screw that gives you 40-50mm of fixing into the joist, and once the boards are fixed tight there wont be any movement. Make sure the board joints are bonded relative to the insulation joints, and max. 200mm spacings for the fixings.

 

[r6deano]

Thank you all. In theory I thought it would be fine going upto 50mm, your responses have helped reassure me.