Blown Plaster: What It Is, and 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.

What blown plaster actually means

Blown plaster is plaster that has separated from the wall behind it. The coat itself may look intact — no crack, no hole — but it is no longer bonded to the brick, block or lath underneath, and the hollow drum sound when you tap it is the air gap you are hearing. The term covers everything from a palm-sized patch to a whole wall that has let go.

On internal walls the usual causes are age and movement in period properties, persistent damp from any source, and salts crystallising behind the coat. On external walls, water trapped behind a hard, impermeable render or plaster coat freezes in winter and physically pushes the finish off the wall — which is why blown areas often appear or grow after a cold snap.

Blown plaster on internal walls: the common patterns

Chimney breasts blow where old soot salts draw moisture through the plaster. Walls below bathrooms and around shower trays blow from slow plumbing leaks that may have run for years. Ground-floor walls on solid-floored period houses blow along the base where the damp-proof course is missing, bridged or overwhelmed.

Each pattern has a different fix, and the plaster is the last thing repaired, not the first. The leak, the salts or the damp route is dealt with and the wall allowed to dry before any new plaster goes on — otherwise the new coat fails on exactly the same line.

How blown plaster is repaired, step by step

The repair sequence is consistent whatever the cause. The blown area is cut back to a firm, well-bonded edge — never feathered over. The exposed background is checked for damp and salts and treated where needed. The patch is then built up in backing coats to match the thickness of the surrounding plaster, and the whole wall, or the patch feathered wide, is skimmed so the repair disappears under paint.

Where the background is doubtful — friable brick, old lime, a wall that has been damp — a mesh or a bonding agent is used so the new coat is mechanically tied in, not just stuck to a dusty surface. That preparation is most of the job, and it is what separates a repair that lasts from one that rings hollow again within a year.