Where this starts
Plaster works by gripping what it was applied to: mechanically into a keyed background, into the gaps between laths, or chemically onto a prepared surface. Blown plaster is that grip failing, and the surface appearance is the last thing to change.
TAP EVERYTHING. Knuckle or a light tap across the whole wall, listening for the change from a dull solid note to a hollow drummy one. Mark the boundary. That map is the real extent of the problem, and it routinely runs far beyond the crack or the bulge that prompted the question — which is why a small patch repair so often turns into a wall.
THEN ASK WHY, because the answer decides whether replastering is the whole job. THREE FAMILIES. A bond failure: the background was dusty, too smooth, too absorbent or unprimed, and the plaster never had a grip. Moisture: the wall behind is wet, and the water and the salts it carries break the bond from the inside. Movement: something structural has moved and taken the plaster with it.
SALT IS THE SIGNAL PEOPLE MISS. A wall carrying groundwater or combustion salts — a chimney breast is the classic — brings soluble salts to the surface, and those salts are hygroscopic: they draw moisture out of the air and stay damp even after the original source is stopped. Plaster on a salted background fails again within a year or two whatever it is made of, which is why a chimney breast that has been replastered three times has a contamination problem, not a plastering problem.
WHAT NOT TO DO: skim over a hollow area. It adds weight to something already not holding, it hides the boundary, and the failure continues underneath. The same applies to filling a crack over a drummy patch.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
The plaster never bonded to the background
Most commonA large hollow area with the plaster itself sound and the back of a removed piece clean and smooth, often on a painted, dense or unprimed surface, and failing within a few years of application.
How to check: Look at the back of a loose piece. Clean and smooth means it never gripped; carrying background with it means the background failed.
Moisture in the wall behind
Most commonHollow areas at low level, around a chimney, under a window or below a defect outside, often with staining, a tide line or a musty smell.
How to check: Whether the affected area has a water path above or behind it. A wet background breaks the bond from the inside.
Salt contamination
CommonRepeated failure in the same place after correct replastering, a white bloom or a damp feel that comes and goes with the weather, classically on a chimney breast or at the base of a wall.
How to check: Whether it has failed before. Salts draw moisture from the air, so they keep a wall damp long after the original source has stopped.
Lath-and-plaster keys breaking
CommonA bulging or sagging area on a wall or ceiling in an older building, solid rather than hollow when tapped in places, with fine cracking and often a gap opening at the edges.
How to check: The age of the building. Lath and plaster is held by keys hooked over the laths, and keys break with vibration and with time.
Movement in the wall behind
Less commonA crack running through the plaster in a straight line, continuing past the plaster's own boundaries, sometimes with a door binding or a crack outside in the same position.
How to check: Whether the crack continues onto another surface. Plaster is reporting the wall rather than failing itself.
Frost or heat during application
Less commonWidespread failure of a recent job, sometimes powdery or friable plaster rather than a clean debond, and traceable to work done in a cold or very hot spell.
How to check: The dates and the conditions. Plaster that froze before it set, or dried before it cured, never reached its strength.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
The hollow area is the real problem and it is almost always larger than the visible damage. Everything about the cost and the method follows from that boundary.
- 2
Clean and smooth means a bond failure; background stuck to it means the background failed; damp or salty means the wall, and those are three different jobs.
- 3
Repeated failure in one place after correct work is contamination or a live water source, and replastering it a fourth time will not change that.
- 4
A defective sill, a leaking gutter, a chimney with no cap and a raised external ground level all produce plaster failure in a specific and predictable place.
- 5
A crack that carries onto another surface or shows outside is structural in origin, and repairing the plaster leaves what moved unexamined.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the hollow area covers a large part of a wall or ceiling, since the weight of plaster involved makes a partial collapse a real risk rather than a theoretical one.
- Where the wall is persistently damp, because the moisture source has to be found and stopped before any replastering is worth paying for.
- Where the same area has failed repeatedly after correct work, which indicates salt contamination and needs a specific specification rather than more plaster.
- Where the building is old and lath-and-plaster, particularly if it is listed, because the plaster may need repairing in place rather than replacing.
- Where a crack runs through the plaster and continues onto other surfaces or shows outside, which is a movement question rather than a finish one.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.