I am about to start redecorating a room in a 1930s semi-detached with original lath and plaster ceilings. I want to drill holes and fit LV halogen lights and if possible, do not want to get into the nitty-gritty of tearing down the ceiling and replacing it with plasterboard.
I had the idea of punching a screwdriver through the ceiling in each light location, then going up into the loft and pouring a water/PVA mix around each hole (to bind the lath and plaster together) and leaving it to set for a few days before attempting to drill a hole from below.
This is a dodgey job Mike. You need to remove the plaster in the size and shape you need first, then cut the laths carefully with a saw, but remembering not to push them up and away from the plaster itself. A good way is to slip a piece of wire or string over the lath before you begin the first cut, so you can hold on to it and stop is pushing away from you. But you can't put to much strain on the downward pull of the string either, in case you pull the lath through the plaster, so it is a dodgey job and takes a bit time to do properly, so try to rush it. After you've made the first cut through the lath, then you can simply hold on to the loosened timber and make the second cut.
Use your plaster hole you made as the template and only cut out what timber has been exposed by its removal. If you've ended up on half and half of the laths, then you can slide a piece of metal, or big bent nail, through the gap and then make your cut with the saw around the shape of the hole.
Sorry, should have said at the beginning, that you'll need a big hacksaw blade, just the blade, and wrap a cloth tightly around one end, making sure the teeth are pointing away from your cloth handle.
Really good luck will be required based on experience of friend who tried to fit this type of light in Vic flat. He ended up replacing the ceiling because it was in such a poor state that working on it caused major damage. You could assess state of ceiling by removing existing rose and judging if its sound enough to proceed.
On The Other Hand, I had no particular problem in our previous place, a
1920s semi with lath-n-plaster ceiling; rather than a power tool I just used a padsaw-type blade (a sticky-out-saw-blade some 4-5 inchces long) sold to go with yer typical Stanley knife. Treating the blade as disposable, i.e. to be used up on the one job of putting in 5 ceiling lamps, taking the job neither ever-so-slowly nor at a rush, and concentrating the effort on the pulling-down stroke rather than the pushing-up, I had no problems. The same blade cut through the plaster and the laths, though I did drill a little ring of holes into the plaster to mark the cut and to reduce the amount of blunting plaster the padsaw blade would encounter. I did put the lights quite deliberately close to the joists above (we had the floor in the bedroom above exposed at the time, and quite a few boards got lifted and replaced with screws) to reduce the amount of flexing going on.
I think I'd go with the first poster to reply. You could get away with an ordinary jig-saw if you go careful and it's sound. You could then put some wet plaster up in the cavity if you needed to to bind the spoiled laths. Squirt some water up first. If the ceiling is dogy then plasterboard it under first. Then you'll know you can use a jig-saw with no fears.
I would have concerns not only about cutting neat holes in this type of ceiling, but also about ceiling thickness and irregular top surface.
Downlighters are held in place by spring clips which are designed to press down on the top surface of plasterboard. If the ceiling is significantly thicker than plasterboard - which lath and plaster probably is - the clips may not be able to spring into their proper positions.
That's true, but even if they don't open out above the plaster I think the pressure from the springs on the sides of the hole will hold the fitting in position, if it's possible to cut the hole to the correct size.
I did this recently in an old house with lath and plaster ceiling...
First thing I did was drill a very small pilot hole, go up in to the loft and make sure I wasn't near any joists.. The I used a pencil and a pepper mill (seemed to be the right size?) and drew a circle. I then used a jigsaw with a thin blade and went slowly... it seemed to work fine on 2 of the 3 holes, the third was a bit more "jagged", however the light seemed to cover the jagged bits, and the springs held it in place perfectly...
Seemed to work fine and my ceiling is in a pretty bad state, one reason for the LV down lights, everyone is blinded when they look at the ceiling so they can't see how bad it is!!
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