Repairing a sagging lath and plaster ceiling

Small areas of sagging can be repaired in the following fashion:

• Nail a square piece of chipboard or plywood to the end of a timber prop that reaches from floor to ceiling (this is known as a deadman). Wedge the prop between the floor and ceiling with the plywood or chipboard supporting the sagging area of ceiling.
• To repair the plaster you need to gain access to the ceiling from above. An upper floor ceiling can be reached from the loft, while a lower floor ceiling will require you to lift the floorboards in the room above.
• Thoroughly clear away the dust and debris between the joists in the area above the sagging ceiling.
• Mix up a quantity of fairly runny bonding plaster and pour it over the sagging area. When the plaster dries it should bond the ceiling plaster to the laths.
• The supporting prop must be left in place until the plaster dries. 
• Alternatively, you can cut away the sagging area and patch it with a piece of plasterboard the same size as the hole you have created. But it is important to remember when removing the area of sagging ceiling that you will need to expose the ceiling joists to provide fixing points for the plasterboard.

Having nailed the plasterboard in position, mix a small amount of finishing plaster and apply it to the repair ensuring you tie it in with the original ceiling plaster.