How do I repair a cracked lath and plaster ceiling?

A lath and plaster ceiling hangs entirely on the nibs of plaster squeezed between the laths. When those break, the ceiling stays where it is only through friction, and any repair has to address the fixing rather than the surface.

Safety

What you'll need

• Goggles and a P3 dust mask
• A joist or stud detector
• Plaster washers and long screws
• Self-adhesive scrim tape
• Bonding coat and finish plaster, or fine filler
• Plasterboard for larger patches
• A filling knife and a plastering trowel

Step by step

1. Tap the whole ceiling.Knuckles or a broom handle. A solid, dull sound means the keys are intact. A hollow drumming sound means the plaster has separated from the laths in that area.
2. Press very gently on hollow areas.If the plaster flexes visibly, it is loose. Do not push hard — a large loose section can drop as one piece with all the accumulated dust above it.
3. Decide the extent of the work.Isolated cracks in a firm ceiling are a decorating repair. Extensive hollow areas, a sagging bulge, or plaster you can flex are a replacement job.
4. Find the joists.A detector, or the nail lines from the laths. Every mechanical repair has to reach a joist, because the laths themselves hold almost nothing.
5. Refix loose but intact plaster with plaster washers.Perforated steel washers and long screws driven into the joists pull the plaster back up against the laths and hold it. They are then skimmed over and disappear.
6. Rake out and fill a stable crack.Open it out, undercut, dust, damp, and fill with a flexible filler in thin passes.
7. Scrim a recurring crack.Self-adhesive scrim tape over the crack and a skim of finishing plaster or fine filler feathered well out either side.
8. Cut out a failed patch to the joists.Cut back to the centre of the nearest joist on each side with a padsaw or a multi-tool, so the new material has something to fix to.
9. Patch with plasterboard of the right thickness.Old lath and plaster is often 20–25mm thick. Build up with board plus a bonding coat so the patch finishes flush rather than sunken.
10. Scrim every edge of the patch.Tape all round the join between new board and old plaster before skimming, or a crack will appear at the perimeter within a year.
11. Skim in two thin coats.Feathered well out onto the surrounding ceiling. A patch that stops abruptly always shows in raking light.
12. Consider overboarding for a whole ceiling.Where most of a ceiling has failed, screwing new plasterboard through to the joists over the old ceiling is faster and far less messy than taking it down — provided the joists can take the extra weight.

Tips

• Plaster washers are cheap, quick and dramatically underused. A ceiling that is loose but not broken can often be saved with a few dozen of them for the price of a takeaway.
• Sheet the room completely and expect a century of dust. Lath and plaster ceilings contain more of it than anyone expects.
• Take a photograph of the joist positions before you close the ceiling up. It saves an hour next time.

Common mistakes

• Screwing repairs into the laths — Laths are thin riven or sawn timber about 25mm wide. A screw splits one easily and it holds almost nothing anyway. Every fixing has to reach a joist, which is why finding them first matters more here than on any other surface.
• Skimming over a hollow, drumming area — The new surface is only as sound as what is under it. Skimming a ceiling whose keys have failed adds weight to a section that is already barely attached, and the whole patch comes down together later.

If it doesn't work

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