Cracks and Loose Plaster

Most small repairs on walls and ceilings follow the same order: whatever is loose comes off, whatever is missing is built back, the surface is smoothed, and then it is painted. What changes from job to job is how deep each step has to go.

Fine cracks

A hairline crackCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention. in the paint is opened up slightly with a scraper, so the fillerFillerA fine paste or powder that fills holes, fine cracks and dents so a wall, ceiling or piece of wood is smooth before it is painted. has something to grip. It is cleaned of dust, filled and, once dry, sandedSandingSmoothing a surface with sandpaper or a power sander to remove bumps, loose old paint or gloss, so the new paint or varnish grips well and comes out smooth. flush with the rest of the wall. Filled patches take up paint differently, so they get a coat of primerPrimerThe thin first coat that goes on before the main material. It binds dust, seals the pores and helps paint or waterproofing stick firmly. before painting.

If a crack opens again in the same place after it has been filled, it is moving, and filler alone won't hold it. Then it needs a different approach, such as a more flexible material or meshReinforcement meshA mesh or fabric, usually glass fibre or polyester, set into render, a coating or liquid waterproofing to hold it together and stop cracks from opening., and above all someone needs to find out why it is moving.

Where new meets old

Cracks open easily where two different materials, or two layers of plasterRender (plaster)A layer of mortar (sand, cement or lime, and water) spread over brick or concrete to give a wall a straight, sound surface ready for paint or tiles. of different ages, meet: a patch and the old plaster around it, a bricked-up opening and the wall beside it, brick next to a concrete column. That is why mesh is set into the plaster over these joints. In a living room where a wall opening was bricked up, mesh went over the joints with the old wall before the second coat of plaster.

Plaster that has blistered or fallen

Plaster that has blistered or sounds hollow is not filled over. It is removed until a sound base is reached, even if the hole ends up bigger than it first looked. Then the gap is filled with new plaster, in more than one coat if it is deep, with filler and sanding on top.

In the home where a piece of the ceiling had come down, the loose pieces were taken down, the hole was closed with new material, and the ceiling around it was filled before painting.

Painting after the repair

It is best to paint the whole surface: the whole ceiling, or the wall from corner to corner. The old paint has changed with time and light, so a patch painted on its own usually stands out.

What to watch out for

• Filler over hollow plasterHollow (blistered) plasterPlaster that has come away from the wall or ceiling behind it. It sounds dull when you tap it lightly, and often blisters or cracks before it falls. looks fine for a while, then comes down together with the paint.
• Repairing a ceiling makes a lot of dust. In the bedroom with the cracked ceiling, the furniture was covered in plastic before anything started.