How far can plasterboard project unsupported beyond joist?

Draughtsman · 16 Mar 2020

New ground floor ceiling job, will probably use standard 12mm sq edge (voids have 100mm soundproofing Rockwool).

It wasn't possible to put put some outer joists immediately adjacent the wall but I got them fairly close. I could always add some 38x38 CLS brothers along joist btm edges to take them closer to the wall. The wall will be wet plastered.

Is there a max that a ceiling plasterboard should float out unsupported?

Thank you.

bobasd · 17 Mar 2020

all edges must be supported as tight to abutments as possible.
pin the 38mm stuff to the last joist, & almost touch the wall with the board then bring the solid plaster up to support the edge(s).
what dimension do you want to cantilever out to - the unsupported distance?

Draughtsman · 17 Mar 2020

bobasd said:
all edges must be supported as tight to abutments as possible.
pin the 38mm stuff to the last joist, & almost touch the wall with the board then bring the solid plaster up to support the edge(s).
what dimension do you want to cantilever out to - the unsupported distance?

Wall to Joist is currently 85mm.

bobasd · 17 Mar 2020

surely you can get a slight angle on your drill to drill the wall for a few plugs, and then pin a one x board to the wall?
if you can easily remove the last joist out of the way it would give you plenty of room.

Draughtsman · 17 Mar 2020

bobasd said:
surely you can get a slight angle on your drill to drill the wall for a few plugs, and then pin a one x board to the wall?
if you can easily remove the last joist out of the way it would give you plenty of room.

No way to get the joist out.

Maybe a 63x38 CLS screwed and glued horiz. to side of joist along btm, leaving only 20mm to wall?

bobasd · 17 Mar 2020

try it.

fenny666 · 21 Mar 2020

I wouldn't worry about the 85mm overhang. I'd board tight(ish) to the external, run a bead of no nails or blob at 200mm centres along the joint. Technically it's 'fixed'. Tied in with scrim to the plastered wall it's not going anywhere.

Draughtsman · 21 Mar 2020

fenny666 said:
I wouldn't worry about the 85mm overhang. I'd board tight(ish) to the external, run a bead of no nails or blob at 200mm centres along the joint. Technically it's 'fixed'. Tied in with scrim to the plastered wall it's not going anywhere.

I put 63x38CLS along it, there's only a 20mm gap now.