In older properties, lathe and plaster ceilings can become damaged and the bond between the old lathes and the plaster gets broken. Small areas can be repaired but larger areas are best cut out and replaced with plasterboard and then skimmed with finish plaster.
If you look at the topside of a lathe and plaster ceiling you'll see the lathes spanning across the joists. The plaster will have been trowelled onto the lathes so that it squeezed through the gaps and formed nibs. As it dried this will have formed a bond holding the plaster in place. This can get broken - most commonly in attics when work like boarding is carried out. Once damaged, the plaster below no longer has as much support from the lathes and can easily drop a little causing a bulge in the ceiling
Small bulges in lathe and plaster
This can be repaired by firstly propping the sagged area back into place using boards and props. Using a wide board long enough to span the bulge, prop the ceiling back up using a length of sturdy timber spanning from the floor below.
Mark the line of the ceiling joists above and screw the plaster to these. Use dry lining screws with large washers on them. The washers will prevent the screw pulling right through the plaster and can be sunk a fraction into the surface. Dry lining screws will cut through easily and grip well in the joists.
Once the sagging area of lathe and plaster has been re-supported like this, the recesses for each screw can be filled and the ceiling redecorated
Larger areas of sagging ceiling
Place a large enough sheet of ply or similar material over the bulging area and wedge it back up with a sturdy length of timber from the floor below.
You now need to access the top side of the ceiling. For upstairs rooms this is usually easy enough as you can get to it from the loft. For other ceilings, you will have to lift the floor in the room above to get access to the old lathes.
Clean away all the broken plaster from the area above and remove any other debris. Vacuum the area as well to remove as much dust as possible. Wet the area with clean water and a brush then make up a slightly sloppy batch of bonding plaster. Trowel this out over the damaged area so that it bonds completely with the old plaster and completely covers the lathes.
Allow the plaster to dry overnight before gently removing the timber supports below. The ceiling can now be redecorated
Substantial damage repair
As mentioned, for larger areas or more substantial damage, cut out the area back to the mid point of the next joist and patch with plasterboard and blend in with a skim coat of finish plaster.