9mm boards for vaulted ceiling?

[SuperPav]

So we have vaulted ceilings upstairs up to an exposed ridge beam. Eaves approx 2.1m, ridge approx 3.6m, so approx 2.6m long on board from eaves to ridge

 

Currently I've fitted the 50mm celotex under rafters and taped. So it's ready for plasterboard.

 

Due to tricky access and ability to lift the boards into place etc, it will need to be "patched" rather than fitting 3m boards in a single length.

 

I'll be glueing the plasterboard to the celotex with foam, and then a few 100mm PB screws through the celotex into the rafters - trying to limit the number of these due to cold spots. 

 

Any reason not to use 9mm here? as it would make my life installing them a hell of a lot easier (even considering using 1800x900 , it'l lbe worth the £100 extra cost, although I suppose I could just cut full boards down if cheaper before I take them upstairs)

[Mr Punter]

I am not keen on 9mm boards.  12 or 15 is better.  The cold spots from screws will be non-existent.  Use a plasterboard lifter to get the boards in place.

[ProDave]

What i found made the job of a vaulted ceiling MUCH easier, is screw a temporary batten to the wall with JUST enough gap to slot the bottom edge of the PB in than that's the bottom edge secured.  You only then have to hold the top edge while you get a couple of screws in.

 

For a long (as in >1 board from eaves to ridge) screw another temporary batten over the top edge of the bottom board to similarly slot the bottom edge of the next board in.

 

Start with full size boards, if you must cut, cut them from that, but persevere to get full boards up.

 

9mm boards won't be any cheaper than 12.5mm and you may have to order them.

[JohnMo]

1 hour ago, SuperPav said:

I'll be glueing the plasterboard to the celotex with foam, and then a few 100mm PB screws through the celotex into the rafters - trying to limit the number of these due to cold spots. 

They end up on your head. Do it correctly with the correct number of fixings. Cold spots from screws - I'm sure it would risk loosing a watt or 2 on the coldest day, rather than killing someone when your ceiling plasterboard falls down.

[Iceverge]

Noise reduction would be better with thicker plasterboard. I would be happier with mechanical fixings too, safer long term than foam I feel and less messy to install.

[Conor]

I'd batten out first using either 25mm or 50mm, depending on what services you need to get up there. Then you can fit the boards easily with the correct screws and spacing. How did you fix the ceoltex to the rafters? 

[nod]

9 boards are rubbish Definitely wouldn’t use them on a ceiling I definitely wouldn’t foam them to the celotex You need to cover the rafters with multi-Foil or use minimum of 70 mil insulated plasterboard 

[eandg]

We had a similar build up but ended up battening out ceiling (screwing through insulation) and then put 15mm on the ceiling. Would expect that to be a much neater and cleaner job, a solid finish and with less risk than what you propose and the difference in cost was not a lot in the scheme of things.  

[SuperPav]

Thanks all! 

 

The celotex is not the only insulation, there's 200mm of rockwool between the rafters, the 50mm celotex is underside (held with insulation washers with screws into rafters)
Also to be clear I wasn't just going to rely on the adhesive for the plasterboard, it would get screwed in, the rationale for the adhesive was that I could use a thinner board bonded to the celotex for stiffness between the 400 c/c rafters.

 

Consensus seems to be 12mm boards so will just get some of that and some more weetabix 👍 and will just chop it down if I find it too heavy to maneouvre.

 

The batten approach as a ledge to rest on works well for the first board at the eaves, but that's the one that's easy to get up with the PB lifter anyway.

Where it doesn't work well is the top board which needs to be tight into the corner between the ceiling and the ridge beam (which is exposed), so it needs to be slid up, rather than tipped into place, all at a height which make it all a bit awkward.