How do I know if plaster has blown or is just cracked?

Tap it. Cracked plaster still sounds solid because it is still keyed to the wall. Blown plaster sounds hollow and drummy, and the hollow note usually stops at a clear edge. Cracks in sound plaster are a filling job; blown plaster has to come off.

Can it be skimmed over instead?

No. A skim is a few millimetres of finish coat and it cannot bridge a void or add key to plaster that has let go. It looks right for a season and then cracks and blows along the same edges, because nothing underneath it changed.

Does blown plaster always mean damp?

Not always — poor preparation, an over-thick coat or movement can do it on a perfectly dry wall. But on an external wall, in a bathroom, or low down near the floor, moisture is the first thing to rule out, because replastering over a wet wall fails.

How long should I wait before decorating a replastered wall?

Until it is dry, not until it looks dry. Fresh plaster changes from dark to a uniform pale pink as it dries, and a vinyl emulsion put on too early traps the remaining moisture and peels. A thinned mist coat goes on first.

Caution

Updated 30 August 2026

Blown Or Hollow Plaster On A Wall

A patch of wall sounds hollow when tapped, bulges slightly, or crumbles away when you press it.

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What's happening?

Plaster on a masonry wall is held on by its key — the grip the backing coat takes on the brick, block or laths behind it. Blown plaster is plaster that has let go of that background and is now bridging a void. It stays up because it is stiff and because the paint or paper is holding it, which is why a wall can sound hollow for years before anything falls. Tapping is the whole diagnosis: sound plaster gives a dull, solid note, blown plaster gives a drummy hollow one, and the hollow area usually has a definable edge you can map with a knuckle. What blown plaster is not is a plaster defect on its own. Nearly always the key was destroyed by moisture, salts or movement behind it, and that is the thing that decides the repair.

What usually causes it?

Damp in the wall behind it, from penetrating damp, a bridged damp proof course or a leak, breaking the bond
Salts brought through the wall by moisture, crystallising behind the plaster and pushing it off
Plaster applied over a smooth, dusty or painted background that gave it nothing to key into
A backing coat mixed too weak, or applied too thick in one pass, so it never gained strength
Movement in the background — a settlement crack, a lintel deflecting, a chimney breast drying and shrinking
Frost damage where an unheated wall got wet and froze with the plaster still green
Heavy tiles or a mirror fixed to a skimmed wall, pulling the finish coat away from what is behind it

Is it dangerous?

A hollow patch on a wall is a low risk day to day, and it will not bring anything structural with it. The risks are second-hand. Large blown areas on a stairwell or above a bath do eventually shed in sheets, and a sheet of old lime plaster is heavier than people expect. More importantly, blown plaster on an external wall is usually a damp signal, so the real question is what is wetting the wall — the Property Care Association's guidance on penetrating damp is where that starts. Hacking off is the genuinely hazardous part of the job: HSE names silicosis, chronic obstructive pulmonary disease and lung cancer among the diseases caused by construction dust, so this is water suppression and respiratory protection work, not a Sunday with a bolster.

Can I fix it myself?

Mapping it, yes. Tap the wall systematically with a knuckle or the handle of a screwdriver and pencil round every hollow area, then check whether it corresponds to something outside — a downpipe, a cill, ground level, a chimney. A small blown patch on an internal wall in a dry room can be hacked back to sound plaster and made good, and that is reasonable DIY. What does not work is skimming over it, filling it, or lining it with paper: the void is still there, the cause is still there, and the new finish blows in the same shape within a year or two. Never hack off dry with a grinder or a breaker and no dust control.

When should you call a plasterer?

Call a plasterer where the hollow area is large, where it is on an external wall, where salts or tide marks are showing, or where a previous repair has already blown. If the wall is wet, the water source is fixed first and the wall allowed to dry — plastering a wet wall simply buys a repeat. Call sooner if the plaster is bulging away from the wall or shedding, and get the cause looked at rather than the stain covered.

What will the plasterer actually do?

Tap out and mark the full extent of the blown area, which is normally larger than the visible damage
Establish why the key failed — damp, salts, movement or background preparation — before mixing anything
Hack off to sound plaster with dust suppression, cutting back square rather than feathering into loose material
Prepare the background properly: dubbing out, keying, and a bonding or backing coat suited to that wall
Rebuild the plaster in correct coats and thicknesses, using a salt-resisting or renovating system where salts are present
Refer the water source to the trade that owns it, and leave the wall to dry before the finish coat where required

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Hollow Or Cracked Render OutsideCaution New Plaster Will Not Dry OutCaution Crumbling Lime Plaster In An Old HouseCaution Damp Patch On An Internal WallCaution

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