For small areas of damage to lathe and plaster walls and ceilings, it is possible to carry out a simple localised repair. More substantial damage is best cut out and repaired using plasterboard and skimming with finish plaster
If you have areas of lathe and plaster which have fallen away or are 'live' you can cut away the loose material back to the sound or solid areas using a cold chisel and hammer. Make sure you use a cold chisel with built in hand protection to avoid the inevitable !
Inspect the old lathes for stability. Providing they are still firm they can be used as the base for the repair. If the lathes are also broken or loose, remove them and fill the gap with expanded metal lathe.
Preparation and undercoat plaster
Dust off the area and vacuum to remove as much loose dust and debris as possible. Use a brush and clean water to wet the area. Mix up an undercoat plaster and trowel onto the lathes making sure that some of the plaster is squeezed through the gaps to form nibs which will give the repair strength.
For deep areas, build up the undercoat in two layers. Scratch up the surface using a block of wood with protruding nails before it sets to provide a good key for the subsequent layer.
The undercoat plaster should be a couple of mm below the surface of the surrounding material.
When dry, finish with top coat plaster and trowel to a smooth finish feathering over the sound edges to blend cleanly.
Larger areas of damaged lathe and plaster
If the area is larger, it's better to cut back the damaged material to a sound area. To make life easier, trim back to a rectangle whose edges are mid point along the studwork. Cut a sheet of suitable thickness plasterboard to patch the area and finish as for skimming plasterboard
Once fully dried the area can be redecorated