Blown Plaster: Patch Repair or Full Replaster?

Map the failure before pricing anything

Tap the wall systematically with a knuckle or the handle of a scraper, working in a grid from skirting to ceiling. Sound plaster gives a dull, solid note; blown areas ring hollow. Mark the hollow zones with chalk and stand back — the shape tells you the cause.

An isolated hollow patch under a window usually points to an old leak or condensation run. A hollow band along the base of the wall points to rising or penetrating damp. Hollow plaster spread evenly across a whole wall in a period property is normally age and background movement, and patching it is false economy.

When a patch repair is the right call

Patching works when the sound plaster around the failure is thick, well-bonded and on the same plane. We cut back to a firm edge, re-bond the background, build up in coats to match the existing thickness, then skim the patch feathered into the surrounding wall.

Failure covers less than roughly a quarter of the wall · Surrounding plaster is solid when tapped and not bowing · The cause of the damage has been fixed and the area has dried · The wall is due for repaint anyway, so the patch line disappears

When to strip back and start again

Once several separate hollow zones appear on one wall, patching becomes a run of visible repairs that still need a full skim to unify. At that point stripping to the brick or block and re-floating is usually quicker, cheaper per square metre and gives a genuinely flat wall.

Lath and plaster ceilings and walls are a separate case: if the plaster keys behind the laths have snapped, no amount of surface repair holds. Those areas get overboarded or taken down and reboarded before skimming.

Fix the cause first, or the repair fails again

Plaster rarely blows for no reason. Before any trowel goes near the wall we look for the source: a failed gutter or downpipe soaking the outside face, a bridged damp-proof course from raised ground levels or render carried down past the DPC, a slow plumbing leak, or a cold, unventilated room where condensation soaks the surface every winter.

Replastering a wet wall traps moisture, and the new finish stains, bubbles and blows on the same line within a year. Allow drying time after the cause is fixed — a saturated masonry wall needs weeks, not days.

Frequently asked questions

Can you just skim over blown plaster?

No. A skim coat is 2–3mm of finish plaster and it bonds to whatever is beneath it. If that layer is already detached from the brickwork, the new skim is being carried by a failing background and it will crack or come away with it.

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

Cracks are a surface symptom and can exist on perfectly well-bonded plaster, especially around ceiling lines and old fixings. Blown plaster is identified by sound and movement — a hollow note when tapped, or slight flex under hand pressure.

Do you need to remove skirting boards to replaster a wall?

For a full strip-and-float, yes in most cases, because the new plaster needs to run behind the skirting line and old boards are usually damaged during removal of the old coat. Careful removal means they can often be refitted rather than replaced.

How long before a replastered wall can be painted?

New plaster needs to lose its moisture and go a uniform pale colour before decorating, and the first coat should be a thinned mist coat so it soaks in rather than sitting on the surface. Rushing this is the most common reason paint peels off fresh plaster.