How do I fix into a lath and plaster wall or ceiling?

Why plugs and cavity fixings both fail in lath and plaster, how to find the timber behind, and the fixings that actually work in it.

Difficulty: intermediate
Time: 30 min
Read: 4 min
Safety: caution

Short answer

Fix into the studs or joists behind, not into the plaster. Where you cannot, a fixing into the laths themselves will carry very little; a spreading toggle behind the laths is better, and for anything of weight the honest answer is to open up and fix to timber.

Lath and plaster is thin timber slats with plaster squeezed between them to form keys. It is a surface, not a structure. Its failure mode is that the plaster keys break off and a whole panel between the laths comes away.

Safety

Lath and plaster is brittle and old, and drilling it can bring down a much larger area than you intended, especially overhead. Wear eye protection, expect dust, and treat pre-1960s ceilings as potentially containing asbestos in later repair coats — if in doubt, do not sand or drill until it has been checked.

What you'll need

stud or joist detector
spring toggles (optional)
bradawl
long screws

Step by step

Confirm it is lath and plaster.Grey dust with coarse horse hair in it, thin timber splinters, and a total thickness of 20 to 30mm rather than 12mm. It sounds hollow but tapping gives a series of ridges.
Find the studs or joists.They are typically 350 to 450mm apart in older houses, and less regular than modern spacings. A deep-scan detector, careful tapping, or a fine bradawl probe will find them.
Fix into the timber whenever you possibly can.This is the whole answer for anything with weight. A long screw into a joist or stud through the plaster is as strong as any fixing in the house.
Allow for the plaster thickness in the screw length.You may lose 25 to 30mm before the screw even reaches timber. A 50mm screw may only have 20mm of grip.
Drill, never hammer.Hammer action shatters the plaster keys and can drop a panel. Rotation only, slow speed, light pressure.
Do not overtighten.Pulling a fitting hard against old plaster crushes it and the fixing loosens as the plaster crumbles. Snug is enough, and a wide washer helps spread the pressure.
Use a spring toggle where there is no timber.Drilled through the plaster and the laths, a spring toggle opens behind and spreads its load across the laths rather than into the plaster. It is the best of the poor options.
Avoid fixings that expand.Plugs, expanding anchors and self-drilling plasterboard fixings all press outwards and simply break the plaster away from its keys.
For heavy loads, open up and add a noggin.Cut a neat patch, fit a timber noggin between the studs, screw into that, and make good. It is more work but it is the only method that carries real weight reliably.

Tips

Old ceilings are the worst case. Never hang anything of consequence from lath and plaster overhead without fixing into a joist.
Drill a small pilot first with the drill on its slowest setting; the plaster is what cracks, not the lath.
A wide penny washer under a fitting spreads the clamping pressure and stops the plaster crushing.

Common mistakes

Using plasterboard cavity fixings — They are designed for a 12mm board with a firm face. Lath and plaster has no consistent thickness and the expanding action breaks the plaster away from the laths.
Using hammer action — The percussion breaks the plaster keys behind the laths, which is what holds the whole surface on. Damage extends well beyond the hole.

If it doesn't work

Plaster crumbled away as you drilled

Cause: Hammer action or a blunt bit on brittle plaster — Fix: Fill the void with a bonding filler, let it set, and refix into timber. Rotation only next time.

Fixing loose after a few weeks

Cause: Plaster crushing under the fitting — Fix: Refix into a stud or joist, or use a spring toggle with a wide washer and only nip it up.

Large area of plaster now hollow around the hole

Cause: Keys broken behind — Fix: This needs a plaster repair, not a fixing repair. Re-fix the panel with plasterboard screws and large washers into the laths, then skim.

Detector gives confusing readings

Cause: Laths themselves reading as timber — Fix: Look for a consistent regular spacing at stud or joist centres rather than the close, narrow readings the laths produce.

Questions people ask

How much can a spring toggle hold in lath and plaster?

Far less than in plasterboard, because the laths are narrow and brittle and the load has to reach several of them. Treat it as suitable for a picture or a small shelf, not a cabinet.

Can I just replace the lath and plaster with plasterboard?

It is a common renovation, and it makes fixings straightforward. It is dusty, heavy work, and in a listed or historic building it may not be appropriate — lath and plaster has value in older houses.

Why does my drill go in easily then hit something?

You have gone through plaster into a lath. Keep going gently and you will pass through into the void. Try not to drill through the middle of a lath if you can avoid it.