Repairs to lath and plaster walls and ceilings
When making any repair to lath and plaster, the same type of materials, or similar, to the original construction should be used.
Different materials (i.e. lime or cement) have different hardnesses etc which, if mixed, will lead to a repair failing. Refer to our article lath and plaster construction and techniques for details of basic techniques.
The technique for repairing lath and plaster is basically the same whether dealing with lath and plaster walls or ceilings.
Small repairs
Start by cleaning out all the dust and loose plaster from the damaged area, if the laths are not exposed, cut back the plaster to form a deeper hole - try to undercut the plaster around the sides of the hole so that the new plaster will have something to stick to - try to avoid having tapered edges where the new plaster will have to taper down to almost no thickness on top of the original plaster.
Mix up some new plaster filler and before applying, dampen the original plaster at the sides and back of the hole using a wet brush or water spray - take care not to over soak the plaster.
Use a filling knife to fill the hole with plaster filler, take care to push the new plaster into the edges of the hole and any cracks, add enough filler so that it is above the level of the surrounding plaster.
Use a wet wide bladed filling knife to level off the new filler to the surrounding plaster work - if the filler knife is not wide enough to bridge the repair, use the knife around the edges of it and then work into the middle.
The plaster filler used in the repair may shrink as it hardens, and the surface may pull back from the original plaster level - the deeper the repair, the greater the risk that this will occur. To overcome this shrinkage, a number of layers of filler may need to be applied to build up the surface. Each layer must be allowed to harden before applying the next. Between each layer, the surface of the repair and the surrounding area should be lightly rubbed down using a fine sandpaper - this needs care as lime plaster is extremely soft and easily rubbed away. The surface of the repair filler should be dampened before covering with another layer.
Large repairs
When a large area of lath and plaster needs to be repaired, a more satisfactory finish will be achieved if proper plaster and plastering techniques are used - i.e. apply three coatings, the render, the floating and the setting layers (see this other pages for the mixes etc).
When the laths are exposed and intact, remove all dust and loose plaster and make sure that the spaces between the laths are clear of old plaster.
Dampen the laths and the surrounding plaster using a wet brush or water spray and then apply a coat of render onto the laths making sure that render goes through between the laths to provide a grip. Keep the level of the render well below the surface level of the surrounding plaster to allow space for the floating and setting layers.
After the render layer has hardened, dampen the surface and apply the floating layer and bring the level up to about 3mm (1/8 inch) below the surrounding plaster.
After the floating layer has hardened, dampen the surface and apply the setting layer of plaster and bring the level up to the surrounding plaster.
If the laths are exposed but broken/damaged, the method of repair will depend upon the extent of the damage to the laths.
- With a large hole or area of damage, it may be worth thinking about
removing a complete section of the lath and plaster (i.e. between frames), or even a complete area, and
using plasterboard as a replacement - the final decision may depend upon the building itself -
it will probably not be allowed in listed buildings - and
how much you wish to return the building to its original state.
- When the lath and plaster is to be repaired, the plaster can be cut back to the next frame on each side of the damage, the damaged laths cut away and replacement laths nailed in their place.
Dampen the laths and the surrounding plaster using a wet brush or water spray and then apply a coat of render onto the laths making sure that render goes through between the laths to provide a grip. Keep the level of the render well below the surface level of the surrounding plaster to allow space for the floating and setting layers.
After the render layer has hardened, dampen the surface and apply the floating layer and bring the level up to about 3mm (1/8 inch) below the surrounding plaster.
After the floating layer has hardened, dampen the surface and apply the setting layer of plaster and bring the level up to the surrounding plaster.
- Where the damage to the laths is over a small area, expanded metal can be used to bridge any gaps:
- Place the expanded metal over the hole and mark it covering the damaged laths and at least two sound laths above and below. If the hole is not large enough to show enough laths, mark the expanded metal to to cover the exposed laths. Cut the expanded metal to size.
- Dampen the laths and the surrounding plaster work, apply a thin coating of the render to the laths making sure that render goes through between the laths to provide a grip, press the expanded metal into the render to cover the damaged laths and apply some more render to finish off the render coating.
- After the render layer has hardened, dampen the surface and apply the floating layer and bring the level up to about 3mm (1/8 inch) below the surrounding plaster.
- After the floating layer has hardened, dampen the surface and apply the setting layer of plaster and bring the level up to the surrounding plaster.