Repairing lath and plaster walls
It is always best practice to use the same type of materials, or similar, when doing a repair to lath and plaster. Have a look at this page for details of basic lath and plaster construction and techniques.
Repair small holes in plaster
Clean all the loose plaster and dust from the hole, if the hole is shallow, cut back the render to give a deeper depression in the plaster- if possible, undercut the plaster at the edges of the hole to provide a good key for the filler.
When the plaster filler has been mixed and ready to use, dampen (but don't soak) the render at the back and sides of the hole by using a moistened brush or water spray.
Press the plaster filler into the hole, making sure that it is pushed into the edges, so that the filler is proud of the surrounding plaster.
Use a wet filling knife across the hole to level off the filler to the surrounding plaster - if the hole is too wide to be bridged by the filler knife, use the knife around the edges and then work on the middle part.
The filler used in holes may shrink as it hardens, and the surface may need to be built up using repeated coatings of filler. Between each coat, lightly sand the surrounding plaster surface (but be gentle as lime plaster is extremely soft) and moisten the previous filler before applying the next coat of plaster filler.
To repair large holes in plaster
For repairing larger holes in plaster it is better to use proper plastering techniques rather than just filler.
If the laths are exposed and intact, clean the laths and plaster to remove any loose plaster and dust, and ensure that the gaps between the laths are clear. Moisten the laths and the surrounding plaster and apply a coat of render making sure that render is forced between the laths. Keep the level of the render well below the level of the surrounding plaster to allow room for the floating and setting layers.
When the render layer has set, dampen the surface and apply a floating layer of render to bring the level up to about 3mm (1/8 inch) below the surrounding plaster.
When the floating layer has set, dampen the surface and apply the finishing floating layer of plaster to bring the level up to the surrounding plaster.
If the laths are exposed but damaged or broken the repair technique will depend upon the size of hole and the extent of damage to the laths:
- In the worse case, the size of hole and damage may make it worth considering removing a whole section of laths (between uprights), or a complete wall, and replacing using plasterboard - this will depend upon the type of building (i.e. it is not suitable for historic or listed buildings without consultation) and how much you wish to 'restore' the building to its original state.
- Some damage to the laths can be overcome by cutting back the plaster to the next vertical frame on both sides of the damage, replacing the original laths and replastering as above.
- Some small damage to the laths can be overcome by using expanded metal reinforcement:
- Offer up the expanded metal over the hole and mark an area covering the damaged laths plus at least two sound laths both above and below the damage ones. If not enough laths are exposed to do this, mark the metal to just within the hole.
- Dampen the laths and surrounding plaster, apply a thin coating of the first layer of render, as above, and press the expanded metal into the render to cover the damaged laths. Finish off the first coat to cover the expanded metal and give a suitable surface for the floating coat.
- Apply the floating and setting layers as above.