Water damaged ceilings can cause a lot of mess and disruption and prior to plastering it is essential that the water coming through the roof or from a leaking pipe is properly repaired. 
 
A slow water leak over a period of time can cause wooden timber joists to rot and sections of the joist may have to be replaced. 
 
The damaged plaster section should be removed and the joists located to screw-fix into the new plasterboard. If the original surface is lath and plaster, it can be an inch thick and sometimes two layers of plasterboard are required to level with the ceiling. 
 
Bonding material can be used to fill any gaps and acts to join the solid existing ceiling with the new plasterboard. Once this has dried, the normal preparation for plaster skimming over the ceiling can be carried out.