Plaster Repair

"Blown" plaster has lost its bond with the wall behind it. The simplest test is to tap the wall lightly with your knuckles or the handle of a screwdriver: sound plaster gives a solid, dull note, and blown plaster sounds hollow. Other signs are:

• Bulging or bowing in the surface.
• Cracks around an area that moves slightly when pressed.
• Crumbling, powdery or salty patches, often near damp.
• Paint or wallpaper that keeps lifting in one place.

Common causes are damp, past leaks, age, vibration, and damage when tiles, fittings or wallpaper are removed. We tap out the whole wall and mark the hollow area before quoting, because it is often bigger than it looks.

1. 1

   Find and fix the cause: a patch over a damp wall or a leaking pipe will fail again. If water is the cause, our leak detection and pipe repairs service deals with it first.

2. 2

   Cut back: loose plaster is removed back to firm, sound edges, and the background is cleaned.

3. 3

   Prepare the background: the brick, block or board is sealed or dampened so it does not pull water out of the new plaster too quickly.

4. 4

   Backing coats: deeper areas are built up in layers. Bonding coat suits low-suction backgrounds such as concrete or painted surfaces; a backing plaster suits ordinary brick and block. Each layer is scratched to key the next.

5. 5

   Skim: a finishing plaster such as multi-finish is applied and trowelled smooth, flush with the existing wall.

6. 6

   Blend in: the edges are feathered so the patch does not show as a ridge, ready for painting or wallpaper hanging.

A patch is right when the damage is in one or two areas and the rest of the wall is sound. A full skim or replaster of the wall is usually better when:

• Large parts of the wall sound hollow when tapped.
• There are several patches close together, so the wall would look uneven.
• The surface is badly cracked all over, or has been patched many times before.
• Old wallpaper removal has torn the surface across the whole wall.

We tell you which makes more sense when we have tapped out the wall, and why.

Many Victorian and Edwardian homes have lime plaster on brick, or lath and plaster on timber stud walls. These need a different approach. Loose laths are re-fixed, and repairs are often made in lime plaster, which lets the wall breathe and moves with the old structure. Modern gypsum plaster on a lime wall can trap moisture and crack at the join. For larger works on a period home, see our period property restoration service.

Older walls may also carry lead-based paint (common in homes built before about 1960), so we control the dust when cutting back painted plaster and never dry-sand it.

It depends on how thick the repair is, the type of plaster, the background, the temperature and the ventilation. A thin skim dries much faster than a deep patch built up in backing coats, and lime plaster dries more slowly than gypsum. New gypsum plaster changes from dark to an even pale pink as it dries. Patches with dark areas still need time.

Do not force drying with heaters, as the plaster can crack. Once dry, the patch is sealed with a mist coat of diluted emulsion before full coats of paint go on.