Why is my plaster crumbling and turning to dust?
Friable, sandy plaster that comes off on your hand has four distinct causes — moisture, salts, a weak or over-trowelled mix, and simple age — and only one of them is fixed by re-plastering alone.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 4 min
- Safety
- caution
Short answer
Press it and see how deep the softness goes. Surface dust only, on a sound wall, is usually a chalky old distemper or a weak finish coat and can be stabilised and skimmed. Softness through the full depth, especially at low level or with white crystalline deposits, is moisture and salts, and re-plastering without solving that will fail again.
Plaster that has lost its cohesion is telling you that its binder is no longer doing its job. Gypsum needs to stay dry to keep its strength; lime needs to have carbonated properly; both fail where salts have been carried into them and crystallised. Working out which of those you have takes ten minutes and decides whether you are looking at a decorating job or a building problem.
Safety
What you'll need
- P3 mask and goggles
- A blunt scraper or filling knife
- A torch for raking light
- Stabilising solution (optional)
Step by step
- Press and scrape a small area with a filling knife.Note how far in the softness goes. A millimetre of chalky dust with hard plaster under it is a surface problem. Softness that continues 5, 10 or 20mm in is a material failure.
- Map the extent with the back of a knuckle and a torch held flat against the wall.Raking light shows the boundary between sound and failed plaster far better than looking straight on. Mark the edge in pencil — you will need it later and it tells you whether the problem is spreading.
- Look at where it is, not just what it is.A band at low level around a room points at ground moisture. A patch under a window or in one corner points at a specific leak or a cold spot. Widespread and even points at age, mix or a failed finish coat.
- Look for white crystalline deposits.Fluffy white salts on or in the plaster mean water has moved through the wall and left hygroscopic salts behind. Those salts will keep drawing moisture out of the air even after the leak is fixed, and plaster applied straight over them fails again.
- Find and fix the water first if there is any.Nothing about the repair is worth doing until the source is dealt with — a leaking gutter, a raised external ground level, a failed seal, a plumbing leak, a bridged damp proof course. This is the step people skip and then repeat the whole job.
- Stabilise, if the failure is genuinely only at the surface.A stabilising solution soaks into a chalky but sound surface and binds it. Test a small area first: if it darkens and hardens and does not dust when rubbed the next day, it has worked. If the surface still moves, it needs to come off.
- Hack off failed plaster back to sound material.Not to the edge of the visible failure — a good margin past it, to a straight line, because a feathered edge of half-sound plaster is where the next failure starts.
- Re-plaster with the right material for the wall.Gypsum backing and finish on a dry wall. Lime, or a salt-resistant renovating plaster, where the wall is old and has been wet. Matching the material to the conditions is what makes the repair last.
Common mistakes
- Skimming straight over crumbling plaster — The new coat is only as sound as what it is stuck to. Two millimetres of finish over failed backing comes off as a sheet, usually with the paint still attached.
- Treating salt damage as a decorating problem — Salts keep pulling moisture from the air indefinitely. They have to be plastered out with a suitable specification, not painted over.
If it doesn't work
Fine white powder on the surface only, plaster hard beneath
Cause: Chalking distemper, limewash or a very weak old finish — Fix: Wash back, test a stabilising solution, then skim or line the wall. Painting over it without stabilising guarantees the paint peels in sheets.
Sandy plaster that scrapes away in a band near the floor
Cause: Rising or penetrating moisture and salt contamination — Fix: Diagnose the moisture source first, then hack off well above the tide mark and re-plaster in an appropriate specification.
Crumbling around a window or in one corner only
Cause: A local leak or persistent condensation on a cold spot — Fix: Trace the water. A repair without the cause returns in the same shape within a year.
Plaster is soft everywhere and the house is very old
Cause: Lime plaster that never fully carbonated, or a very lean original mix — Fix: Often stabilisable if it is sound. If not, re-plaster in lime rather than gypsum.
Powder appears again weeks after redecorating
Cause: Hygroscopic salts still in the wall drawing moisture from the air — Fix: Salt-resistant renovating plaster or a re-plaster with a salt barrier. Ordinary filler and paint will not hold it.
Questions people ask
Can I just paint over powdery plaster?
No. Emulsion applied over a chalky surface bonds to the loose layer, not to the wall, and it peels. Stabilise first if the surface is otherwise sound, or take it off.
How do I know whether to stabilise or hack off?
Depth. If sound plaster starts within a millimetre or two, stabilising usually works. If you can push a knife in and keep going, it has to come off.
Is crumbling plaster dangerous?
Not in itself, but the dust is an irritant and the underlying cause can matter a great deal. Persistent damp affects the building and the air quality in the room.