cracked plaster/ ceiling movement

[sawm24]

my flat roofed extension has a plaster board ceiling, but where it joins the original house wall the paint and plaster cracks along the joint every six months.

normal filler and flexible caulk have both cracked over time and i have noticed that if i press the ceiling with my finger tips near the joint there is a lot of movement.

from reading on here it seems that the ceiling might need to be secured to the joists (if this is the right term!) above.

What sort of screws or nails would be best for this job?

What is the best way to locate the joists, i think you can use a machine

 

[D.S.D]

not a plasterer but my scant knowledge of it is this. You board the ceiling first and then the wall so that the wall holds the ceiling up as well along the edge that way no cracks Seems they may not have done this in your case. You need to fix down along the edge onto the joists. You can get a joist locator for this for a few quid at one of the sheds. If memory serves they should run at 18 inch centers but not to sure about that fact so someone else may put me right on that. The best screw IMO would be the black plasterboard screw and again you can pick these up in the sheds. When fixing ensure that the screw is sunk below the surface of the board, fill it (easyfill) then flexible caulk along the wall to ceiling join. Paint away after that. Hopefully the end of your craking problem let us know the result.