How do I repair a hole in a lath and plaster wall?

Lath and plaster is typically 20 to 25mm thick including the nibs behind the laths, so a plasterboard patch is thinner than what it replaces and will finish sunken unless the depth is made up. That, and the fact that the laths themselves hold almost nothing, are the two things that make this different from patching a modern wall.

What you'll need

• Plasterboard offcut
• Timber battens
• Drywall screws and woodscrews
• Scrim tape
• Bonding coat and finish plaster, or a deep-fill filler
• Padsaw, multi-tool or a sharp knife
• P3 mask and goggles
• Stud detector

Step by step

1. Establish the extent of the damage before cutting.Tap outward from the hole. Plaster that has lost its keys around the hole will come away later, so the repair has to reach sound, well-keyed material. Cut back to that, not to the visible edge.
2. Find the studs either side.In an old partition the studs may be at irregular centres. A detector plus a probe through the damaged area tells you where the timber is, and the patch wants its edges on it.
3. Cut the hole square.A clean rectangle, cut carefully so the laths are cut rather than torn — a multi-tool or a fine padsaw. Ripping laths out loosens the plaster keys well beyond the hole and makes the job much bigger.
4. Get timber behind every edge.Where an edge lands on a stud, fine. Where it does not, slide a batten in behind the laths and screw through the sound plaster into it at each end, so the patch has something to fix to on all four sides.
5. Cut a plasterboard patch to fit.A snug fit rather than a forced one — a couple of millimetres of gap is easier to fill than a patch that has to be hammered in and breaks the edge.
6. Pack the patch out to reduce the depth difference.Twelve and a half millimetre board in a 22mm hole leaves a 10mm step. Either pack behind the board with battens or an offcut so it sits closer to the surface, or accept the step and make it up in bonding coat. Packing is easier than plastering 10mm.
7. Screw the patch home.Into the studs and battens, screw heads just below the surface, not tearing the paper.
8. Scrim every joint.All four sides. Old plaster and new board move differently and this is the only thing that stops the perimeter cracking.
9. Build the depth in bonding coat.Applied over the board and the scrim, in layers no thicker than about 10mm, ruled off with a straightedge riding on the sound plaster either side, and left 2 to 3mm below the surface.
10. Skim across, feathering well out.Two thin coats of finish plaster, taken 150mm or more past the patch on every side and thinned to nothing, so there is no visible edge in raking light.

Tips

• Where the hole is small — a fist or less — plaster washers and a deep-fill filler over scrim can work without a board patch, provided the surrounding plaster is well keyed.
• Old lath and plaster is often not flat. Rule the backing coat to the wall as it exists rather than to a theoretical plane, or the patch will stand proud in the middle of a wall that dips.
• Wet the exposed laths and old plaster edges lightly before plastering, or they draw the water out of the mix and the patch goes powdery.
• If the wall is a partition you may find no insulation and a clear void through to the next room, which is worth knowing before you close it up — it is a good moment to add sound insulation.

Common mistakes

• Screwing the patch to the laths — Laths are thin strips of timber that split under a screw and are only nailed to the studs themselves. The patch has to reach a stud or a batten.
• Filling the hole without a backing — A void behind filler means it slumps, cracks and eventually falls out. Something rigid has to bridge the hole first.
• Skipping the scrim — The join between an old, moving, hair-reinforced plaster and a new rigid board will crack. Tape is not optional here.
• Tearing the laths out rather than cutting them — Every lath you pull loosens the plaster keys along its whole length, and the repair grows across the room.

Questions people ask