In short
Blown plaster has lost its bond to the wall behind it, which is why it sounds hollow when tapped and bulges or falls away. The usual causes are damp coming through the wall, salts crystallising behind the plaster, or old plaster that was weak to begin with. The damp has to be fixed first; new plaster over a wet wall will blow again.
This is a job for our Plastering service.
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How to tell if plaster has blown
Tap the wall with your knuckle. Sound plaster gives a dull, solid note; blown plaster sounds hollow and papery. You may also see a bulge, a crack around the edge of a patch, paint lifting in sheets, or a white powdery bloom. On our jobs around Norwich, Wymondham and Attleborough the hollow patch is usually bigger than the bit that has fallen off, so we map the whole area before quoting.
What causes it
- Damp from outside. Leaking gutters, cracked render or a bridged cavity keep the wall wet and the plaster loses its grip. We find rain getting in far more often than rising damp.
- Salts. Moisture carries salts to the surface and they crystallise behind the plaster, pushing it off. SPAB describe the horizontal tide-mark with staining below it, and warn that moisture meters read high on salty walls even when the wall is nearly dry.
- Hard cement plaster or render on an old solid wall. Cement traps moisture in the wall instead of letting it dry out. SPAB's advice is to replace hard cement finishes with a more suitable lime-based material.
- Poor preparation. Plaster skimmed over dust, distemper or a shiny painted surface with no bonding agent will let go in time.
- Age. Lime plaster on lath in older Suffolk cottages reaches the end of its life, especially near chimneys.
What we do
- Find the water. Before we plaster anything we look outside: gutters, downpipes, ground levels, render, pointing. If the wall is being wetted from outside we deal with that first, and we do the roofline work ourselves.
- Cut back to sound. Every hollow area comes off, not just the bit that fell. We take it back until the tap sounds solid.
- Let it dry. A wall that has been wet for years needs time. Plastering too soon means doing the job twice.
- Choose the right plaster. On a modern blockwork wall a standard undercoat and skim is fine. On a solid brick, flint or clay-lump wall we use a breathable system so the wall can keep drying through it.
- Blend the repair. The patch is brought flush and skimmed into the surrounding wall so it disappears under paint.
Repair or re-plaster the whole wall?
A single blown patch on an otherwise solid wall is a repair. If several patches sound hollow, or the whole lower part of the wall is soft and salty, re-plastering the wall is cheaper than chasing patches for years. Checkatrade make the same point about ceilings: cracks do not always mean a new ceiling, but the cause has to be dealt with. We will tell you which it is on the survey, and we will not skim over a wall that is still wet.