Plasterboard re-fixing

More likely caused by movement in the timber it's attached to.

Have you tried 'jacking up' the plasterboard to meet the underside of the joists?

When the board cracks its often because youre putting the screw in too close to the board edge. The other point is not to countersink them at all, make them dead flush, again less chance of cracking.

NT

Possibly for most, but some is actually down a bit (mm or so) relative to the adjacent board.

Yes, but if the p/b is only a cm or so over the joist there's not much choice. I'm trying to get them flush but that in itself is cracking the board. I might have a go at putting in a flush countersink and propping the board, so that the only starin is the weight.

If you mean countersinking and predrilling each hole, in extremis that does help, and sounds like it may be worth it for you.

NT

My joists are very uneven so I screwed thin battens under the joists then fixed plasterboard to the battens many of which i managed to attach at the exact width of the plassterboard so I didnt have to trim the edges.

Yes. A bit slow, but I'd need fewer screws anyway, as they should be better at holding. Doing it this way, I can push the board upwards with one hand whilst running in the screw.

Good move for new boards - not on for existing ones. It might be worth me putting some extra timber where the overlap is really poor, otherwise the screw would have to be at an angle!

It sounds like youre out of other faster options. Any fixing that breaks the board up will make things worse rather than better.

There is another way, but probably not warranted in your case. Prop the ceiling up using timber uprights and sheets of whatever, then lift some floorboards above and pour a thin plaster mix on.

NT

Hadn't thought of that - thanks. It's actually a case of lifting the insulation, as this is to the loft.