Many old homes in New Zealand have walls and ceilings that have been plastered using a method called Lath and Plaster. This technique involved nailing long thin strips of wood (laths) across wall and ceiling joists and then plastering over them. Traditionally the laths were about 1 1/2inches wide by 5 to 6 feet in length and set about ¼' apart, the thickness tends to vary depending on the original builder, so if you need to replace some carefully check the measurements of the original.
If you are renovating a home with this type of wall surface you will have a few choices, (1) If the surface is in good condition do nothing, (2) Repair any damaged areas or (3) Rip the whole lot out and replace with plasterboard sheets.
For the purpose of this exercise let's assume that you want to repair damaged areas of the walls and ceiling. Sagging ceilings are a common fault in older homes with lath and plaster finishes so we will start in this area. First what tools will you need?
One of the most important item(s) you will need for this job will be a prop or props to hold up the sagging part of the ceiling. These can be made simply from lengths of timber (100x150mm or thicker) plus a piece of board about 25mm thick and say 300 x 300mm square padded with an old piece of blanket or foam or any other material that will prevent damage to the ceiling surface. The length of the prop plus board and blanket must be exactly that of the height of the ceiling when it is in its original position. This length is simply calculated by measuring floor to ceiling in an area where the ceiling is in good condition.
Push your prop or props carefully under the sagging ceiling so that it is in its original position. If you can gain access to a loft or attic above the ceiling and get to the area under repair, do so. A quick sideline note here, when you are in areas like attics or ceiling spaces that may have been undisturbed for some years be prepared for very dusty and dirty conditions! It will pay to be clothed in old tight fitting clothes, coveralls are good if you have some, use a good dust mask, wear gloves and,if you can, take a vacuum cleaner with you. Most importantly be very careful where you tread or crawl. Use the ceiling joists, if possible spread your weight over a few of these by means of a strong board or plank do not step or put any pressure on the top side of your ceiling! Clean the area carefully where you will be working and inspect the condition of the wooden laths, hopefully they will be unbroken.
There are now a few different ways to progress from here, I prefer the traditional method to affect a repair. The ceiling will be sagging because the laths have become detached from the ceiling joists, so now they have to be fixed on again. The old way, from above the ceiling, is to use lengths of thick hemp or sisal string and tread this through the laths and tie them to the joists. Wire or plastic straps can also be used. Other methods involve using fibreglass patches fitted on top of the old plaster and secured to the joists or using chicken mesh and fresh plaster to hold the broken laths in place. Using either of these methods involves placing the mesh or fibreglass patch over the area turning up the edges so that they fit neatly along the joists nailing them in place and then pouring a plaster mix over the patch. Leave for least twenty four hours to dry and then remove your prop or props and repair any damage to the plaster from below the ceiling.
Repairs can be affected to correct a sagging portion from under the ceiling providing you can locate the ceiling joists. Carefully prop up the sagging part using props as previously explained. Use a fine drill, drill through the plaster and lath and using either galvanised or stainless steel screws fix the laths back to the joists. Remove your props and plaster over the small holes left by the screws
If you have a large hole where the plaster has crumbled away from the laths the best way to repair this would be to use a piece of plaster board of the same thickness as the plaster.
Clean away all loose and damaged plaster make the hole a regular shape, either square or rectangle is best. Nail any loose or broken laths to the joists using galvanised nails. Cut your piece of plasterboard to shape press it into the hole and nail in place using plasterboard nails. Now you will have to plaster over the repair to finish off the job. Starting as I'm sure you will remember with a bedding coat of plaster and working through to the top or finishing coat, sand back and paint.
Walls are much easier to repair. Carefully remove all damaged plaster from the wall reaffix any loose or damaged laths by once again using galvanised nails. If you are going to replaster the damaged area you may find it necessary to replace some of the damaged laths.
The easiest way is to use a piece of plasterboard in the same way as you did for the ceiling.
Finally if the job involves a very large area you may chose to remove the old lath and plaster finish entirely and replace with plasterboard.