The preferred way to resolve this is not by pulling the ceiling down and replacing it with drywall but rather to reattach the plaster ceiling to the supporting lath.
Repairing plaster ceiling cornice.
Then the rings required for the installation of the cornice are fastened to the beam.
Start by taking down the lath and plaster going very carefully up to the cornice.
In this video i show how i repair damage to plaster coving.
You may like to leave your cornice stripped for a distressed look but if you re planning to paint it use a basic matt emulsion.
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Use paint scraper to remove any loose paint or plaster along crack.
The repair of cracks begins with removing any loose plaster from within the cracks and making sure that any plasterboard is reattached to the walls and ceilings with nail clouts plaster screws or glue.
Apply the first coat leave to dry and then paint over with a second coat.
Use scraper to fill gap between cornice and ceiling with cornice.
Use drill to screw plasterboard screws up through cornice into ceiling so cement oozes out.
On the base plate of the ceiling you need to fix the bar its length should be equal to the cornice.
The old way of doing this was by using plaster washers which are a larger wash the is screwed through the face of the plaster into the lath to mechanically support the plaster.
Once close to the cornice score the plaster at the front edge of the cornice a few times with a sturdy craft knife often the plaster breaks off cleanly and mostly flush with the cornice.
You will see how with this method it.
Mix cornice cement in bucket to form a paste.
Once this is done you can begin filling the cracks with a plaster mixture and paper tape to help with strengthening of the joint.