Repair a plaster ceiling using plaster buttons, drywall patches, fiberglass mesh, plaster patches and new plaster. Plaster buttons are designed to screw into existing plaster on the ceiling in order to push it up and secure it to the lath, or layer of material that the plaster is secured to.
Screw the plaster buttons in only until they are snug and flush with the plaster in order to avoid damaging it further. It's a good idea to first screw the plaster buttons through the original plaster and into the beams that support the laths. Next, insert the buttons into any small cracks or defects in the plaster ceiling.
Patch large holes and cracks in the plaster with drywall or plaster patches. Drywall patches are best used for larger holes where the old plaster has crumbled away and exposed the laths behind it. Secure the drywall patch using drywall nails, and fill in any open space around the patch with new plaster.
If the entire plaster ceiling is in need of repair, then use a piece of fiberglass mesh to add a new layer of plaster to the ceiling after the buttons and patches have been applied. Otherwise, simply apply new plaster over the buttons and patches to hide them. The fiberglass mesh should be secured to the old plaster using new plaster. Make sure to work from the center of the mesh outwards towards the walls in order to avoid air bubbles forming between the mesh and plaster.