Hollow (blistered) plaster

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. stays up because it is bonded to the brick or concrete. When that bond is lost, a gap forms between them. The plaster may stay in place for a long time, but it sounds hollow when you tap it, it blisters, cracksCrackA 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. and, at some point, falls.

The most common cause is damp, from a leak, from rising dampRising dampMoisture from the ground that climbs up inside walls through the pores of the materials. It shows as stains, blown render and salts at the foot of walls. or from water getting in from outside. Other causes are age, vibration and, on ceilings and balconies, rebarRebar corrosionRust on the steel bars inside concrete. Rusting steel swells, cracks the concrete around it and makes it fall away in pieces, leaving the bars exposed. corrosion pushing the material outwards.

Hollow plaster isn't filled over. It is removed until a sound base is reached, the gap is filled with new plaster, often with 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. where new meets old, and then come 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. and paint. On a ceiling, if it sounds hollow over a large area, keep the space underneath clear until someone has looked at it.