The laths were nailed horizontally to the ceiling joists, the lime plaster mix was worked into the gaps between the laths, this acted as a bonding agent. Once set it was covered with a further coat of plaster before painting.
Most houses built before 1930s in the UK have ceilings made of lath and plaster. Most modern properties now, in most cases, use plasterboard and are then skimmed and are considerably more stable. Lath and plaster ceilings do not cope with water ingress and the modern world very well. When they were installed there would not have been the constant vibration that is caused by the amount of traffic on todays roads.
Traditional plaster is brittle by nature and can crack under vibration, which will also cause the nails holding the laths, which also have a tendency to rust, to weaken and potentially snap. If enough nails loosen or snap the weight of the ceiling can not be supported and the ceiling will collapse.
Similarly, if water soaks into the porous plaster from above, either by roof damage or faulty pipework over a period of time, onto a ceiling that is already weakened by vibration the combination could also cause a collapse.
Ceiling collapses are very common in London and other inner city residential properties, and they can cause extensive damage to anything below and could also cause quite a serious injury if it fell on to somebody.
REMEDIES FOR LATH AND PLASTER CEILINGS
If you have a lath and plaster ceiling you should look for evidence the the ceiling has become unstable, look for cracks, movement and hollowness, a gentle tap should tell you if the ceiling plaster is hollow.
If you have any concerns you should seek professional advice immediately.
There are a few ways that you can repair a lath and plaster ceiling, they can be overboard, taken down completely or possibly patch repaired.
Overboarding is exactly that. You find the joists in the ceiling, either by poking a bradle from above, if you have a loft space, or using a small hammer to make holes from below to discover where the joists are. Joists were generally installed with 14 inch centres, once you have discovered the joists you can screw fix plaster board to the ceiling, using 75mm x 100mm drywall screws. This method is not without its difficulties, you will need at least 2 people to do this, as plasterboard sheets are heavy and awkward to manoeuvre. Also you must take care to locate any electric cables and pipework so as not to screw through them.
You could also patch repair if the damage is localised, and the remainder of the ceiling is sound. To do this you would need to remove the damage, then cut and fix plasterboard to the hole.
The last and most recommended method would be to remove the whole ceiling, although this can be expensive, time consuming, and very…very messy. Make sure you cover the flooring, ventilate the room, then seal off the remainder of the house, the dust will go everywhere. Always wear protective clothing, masks and a hard hat. Pull the ceiling down using hammers and crow bars, best to do this in small sections to make it easier to bag up. Remove all the debris from the room and clear the working area in preparation to boarding the ceiling. The next step is the same a overbearing but you only need to use 40/50mm drywall screws.
Once the ceiling is boarded, you will need to use scrim tase on all joints, now you can apply the finishing plaster, before painting.
This is just a quick reference to lath and plaster ceilings, you should always seek professional help if you have any concerns that your ceiling could be unsafe.