A lath and plaster ceiling may need to be repaired over time. Lath tends to loose moisture and shrink, while plaster won't do this. When this happens the plaster will break away from the lath and start to fall. If you have a patch of ceiling with crumbling plaster, or you have a crack in it, here's what you can do to fix it.
Step 1 - Repairing Small or Medium Sized Holes
If you have areas that need to be repaired that are smaller than a few inches in size you can fix this with a simple plaster or paris mixture. Use a spray bottle to lightly moisten the area around the hole. This will help the compound form a bond and prevent it from falling away later. Just mix the plaster in a bucket according to the directions and stir it with an old paint stick. Apply the plaster to the holes. Using a trowel works well for this repair. Allow it to dry and apply another coat.
A medium sized hole will be repaired more easily using a joint compound. You can find this pre-mixed and ready to use at any hardware supply store. The repair itself will be the same as a small hole. Just use a trowel to apply, wait for it to dry, and apply additional coats as needed.
Step 2 - Large Fall Outs
If you have a large area that has fallen, this can be fixed, but is going to require more work. Use a mix of cornice adhesive and perlite for this. If you can't find perlite easily, look for vermiculite. These are both sold in most garden supply centers. Using this allows you to patch larger areas without worrying about it falling out again. Mix the substances using about 60% adhesive. Apply the mixture with a trowel and wait for it to dry.
Step 3 - Repairing Cracks
This is the more common repair that people need to make. It's easy, but does require some extra steps. You will want cornice adhesive for this. Use a damp paint brush in the crack to knock out any loose debris. The moisture also helps to form a more solid bond when you begin to make the repair. Use the cornice adhesive the fill in the crack. This won't be the final patch, so don't worry about how it looks. This is just to create bulk. You do need to scrape away any excess material around the crack though after it's dried. Once this is done you can apply the second coat. Use a trowel and apply pressure as you pull the compound over the crack. This will give you a finish that is even with the rest of the ceiling. Allow this to dry and apply a finishing coat so the difference in color is minimal.