New ceiling was taped, but now cracks have appeared

14 Oct 2008
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United Kingdom
I can't believe new ceiling (plasterboard screwed over the existing lathe n plaster in lounge in 1930's house) is starting to crack where the boards join, I taped all the joins, I screwed up the boards and the plasterer did a fantastic job. How could I have cracks???
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When you walk in the bedroom above is there any movement in the floor? If you plaster boarded over the existing ceiling then there will extra weight that the old joists will be carrying. Check what depth (size ) the joists are and if there is any rot or woodworm in the ends that go into the walls. What scrim tape did you use?
80 year-old ceiling joists will have changed in various ways over the years, with warping, shrinkage and creep. The plaster and lath will expand and contract with temperature and relative humidity changes affecting over-boarding directly to the old plaster. Even opening and closing doors to rooms above and below will cause the ceiling to act like a diaphragm creating bounce and vibrations.

One way to present a more even, stable substrate for new boarding is to cross-batten the ceiling at right angles screwed into the joists, tying the whole together. Not only can this avoid messy and unpleasant removal of old, worn-out plaster, but insulation may be included or wiring for new lighting fitments, and the battens present an easily levelled and visible background for fixing the new sheets, with batten spacing chosen to be suitable for the room layout and fitting of the boards. Bounce can be absorbed somewhat by the space between the original ceiling and the board.
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Perhaps try repairing using dry lining joint cement next time over an old celiing. Much lighter and no plasterering or water based products required either. With the scrim over the cracks will flex too with the old ceiling.
The tape I used was the self adhesive plastic grid type stuff, the joists are 8x2,no rot or worm, the bedroom above has not had any heavy furniture moved or anyone jumping up n down.

Could I actually have differential expansion because the boards were all screwed THROUGH the lathnplaster thereby locking together n into the joists.

I have not had this problem before in previous houses. Does this mean that when I do my single storey extension across the back of the house, I am likely to get cracks?.............I thought if mesh tape was used you would never get cracks
How wide were the gaps between your plaster board? If they were quite wide it might have been better to fill the joints with skim first them either double tape it after or use 4" fibre tape , then let it go off and then skim over the lot..
I think the gaps were up to 5 mm, does a wide gap encourage cracking even though it has been taped?

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