What causes plaster to bulge? (UK guide to bulging walls)

The short answer

Plaster bulges when it has lost its bond with the wall behind and is being pushed or pulled away from it. The plaster has separated from its background but is still held — by the surrounding plaster, by paint, or by its own rigidity — so instead of falling it forms a visible bulge. The usual causes are: a failed bond (blown plaster, from age, poor adhesion or vibration); damp and salts that soften the bond and physically force the plaster off as they crystallise; and on older homes, failing lath-and-plaster, where the plaster "nibs" gripping behind the timber laths have crumbled so whole panels sag. On ceilings, gravity adds to this and a bulge can become a sudden collapse. A bulging wall is usually a repair; a bulging ceiling is a safety issue that should be treated with caution and assessed promptly.

A bulge in a wall or ceiling is more alarming than a crack because the surface is visibly moving. It almost always means the plaster has come away from what is behind it. Here is what causes that, and how seriously to take it.

Bulging plaster — key facts

What it meansPlaster has separated from the wall
Common causeFailed bond (blown plaster)
Frequent driverDamp and salt crystallisation
Old-house causeFailing lath-and-plaster nibs
Ceiling bulgeSafety risk — assess promptly

A bulge means a lost bond

Plaster is meant to be firmly stuck — keyed — to the surface behind it, whether that is brick, block or timber laths. A bulge appears when that bond fails over an area but the plaster does not immediately fall off. Held by the surrounding sound plaster, by the paint film, or simply by its own stiffness, the detached plaster bows outward into a visible swelling. Tapping a bulge usually produces a hollow, drummy sound, confirming the plaster behind it is no longer attached.

This is the same underlying phenomenon as "blown" plaster — loss of adhesion — but seen as a three-dimensional swelling rather than just a hollow patch. The bulge is significant because it shows the detachment has progressed far enough for the plaster to physically move. The questions that matter are why it has come away, and how much risk the bulge poses (which depends heavily on whether it is a wall or a ceiling).

The main causes of bulging

Several mechanisms push or pull plaster away from its background:

Damp and salt crystallisation. A very common cause. Moisture moving through a wall softens the plaster's bond and carries soluble salts. As the salts reach the surface and crystallise, they expand, and repeated wetting and drying mechanically jacks the plaster off the wall — forming a bulge that often also shows staining, efflorescence and blistering.
Failed or poor original bond. Plaster applied over a dusty, oily, painted or wrongly prepared surface, or onto a background with the wrong suction, may never have keyed properly and can bulge as it ages or is disturbed.
Failing lath-and-plaster. In older homes, the plaster squeezed through the laths forms nibs that hook behind them. With age, movement and vibration these nibs crumble; once enough have broken, whole panels of plaster lose their anchorage and sag away from the laths — a classic cause of bulging ceilings and walls in period properties.
Trapped moisture behind impermeable surfaces. On old breathable walls sealed with dense gypsum, cement render or tanking, moisture that cannot escape builds up behind the impermeable layer and forces it outward.
Movement and vibration. Structural movement, heavy traffic and building works can progressively break a marginal bond, especially in combination with the causes above.

Find the cause before repairing: if damp drove the bulge, the moisture source must be fixed and the wall dried first. Re-plaster a damp wall and the new plaster bulges and blows in turn.

Why a bulging ceiling is different

A bulging wall and a bulging ceiling are not equally urgent. On a wall, the detached plaster is held against gravity and a bulge is usually a repair job that can be planned. On a ceiling, gravity is pulling the loosened plaster straight down, so a bulge is a warning that a section could come away suddenly.

This is particularly true of old lath-and-plaster ceilings, which are heavy. When the nibs gripping the laths fail, a sagging ceiling can hold for a while and then collapse without much warning, often dropping a heavy slab of old plaster. That is a genuine injury risk to anyone beneath it. Signs that warrant prompt caution include a visible sag or dome in the ceiling, cracking around the edges of a bulge, a hollow drummy sound when tapped, and bits of plaster or dust falling.

If a ceiling is visibly bulging or sagging, the sensible response is to keep people and furniture out from underneath it and have it assessed promptly by a competent person, rather than ignoring it or prodding at it. A heavy bulging ceiling is one of the few plaster problems that crosses clearly from cosmetic into safety.

Repairing bulging plaster

Once the cause has been diagnosed and any damp or moisture source dealt with, bulging plaster is repaired by removing the detached material and rebuilding the surface:

Remove the bulged, hollow plaster back to a sound, firmly bonded edge and down to the background (brick, block or laths). This is done carefully, especially overhead, where the loosened material can be heavy.
Repair the background. On lath-and-plaster, damaged laths are repaired or a suitable backing introduced so the new plaster has something to key to. On masonry, the surface is cleaned, suction controlled and a bonding agent applied to low-suction areas.
Re-plaster in coats. A backing/bonding undercoat builds the depth, keyed while soft, followed by a finish skim flush with the surrounding sound plaster. Where salts or residual moisture remain, a salt-tolerant breathable system may be specified.
On old breathable walls, use lime rather than dense gypsum or cement, so the wall can manage moisture and you do not recreate the trapped-moisture conditions that caused the bulge.

For a small, dry bulge on a wall this is a modest repair. For a sagging old ceiling or a damp-driven bulge over a large area, it is a bigger job that is safer and more durable in the hands of someone experienced with the material and the cause.

Frequently asked questions

Is a bulge in my wall plaster dangerous?

A bulge in a wall is usually a repair issue rather than a danger — the plaster has lost its bond and is held against gravity. It should still be investigated, especially if damp is involved, because the cause needs fixing before repair. A bulge in a ceiling is more serious, because gravity can pull the loosened plaster down suddenly, particularly with heavy old lath-and-plaster.

Why is the plaster on my old ceiling sagging?

In older homes the ceiling is often lath-and-plaster: wet plaster was pushed through timber laths to form nibs that hook behind them and hold it up. Over time those nibs crumble with age, movement and vibration, and once enough fail, panels of heavy plaster sag away from the laths. A visibly sagging old ceiling can collapse without much warning and should be assessed promptly.

Can bulging plaster be fixed without removing it?

No — bulged plaster has separated from the wall, so it has to be removed back to a sound, bonded edge and the surface rebuilt; you cannot push it back and make it stick. Before repairing, the cause must be addressed: if damp drove the bulge, the moisture source needs fixing and the wall drying, or the fresh plaster will bulge and blow as well.

Sources & further reading

Figures on this page are typical UK ranges drawn from published sources and depend on your specific room. They are guidance, not a quotation.