Why does my plaster sound hollow?

Blown plaster is a symptom. The plaster itself rarely fails on its own — something usually moved, got wet, or was applied to a surface it could never stick to.

Safety

Step by step

1. Map it by tapping.Knuckle or the handle of a screwdriver across the whole wall in a grid. Solid gives a dull tap, blown gives a drummy hollow note. Pencil the boundary as you go — it is nearly always larger than the visible damage.
2. Look for the cause before the repair.Damp from a leak or rising moisture, salt crystallising behind the surface, movement, or plaster applied over paint or dust. Repairing without fixing the cause means doing it again.
3. Check for damp specifically.Staining, a tide mark, salty white deposits, or a musty smell. Blown plaster low on an external wall is a damp question until proved otherwise.
4. Understand old lime plaster.In a pre-1919 house the plaster is likely lime on timber lath. Hollow there often means the nibs that grip the lath have broken off — a different repair, and cement or gypsum over it will fail.
5. Judge the size honestly.Up to about a dinner plate is a patch. Half a wall is a reskim. Repairing a large blown area in patches gives a visibly uneven wall and it will keep spreading.
6. Cut back to sound plaster.Score the boundary with a knife so the edge breaks cleanly, then remove everything drummy. Leaving loose material at the edge is why patches fail around their perimeter.
7. Let it dry fully.If damp was involved, the wall behind has to dry before anything goes on it — weeks, not days, and with ventilation. Plastering onto a damp wall traps the moisture.
8. Control the suction.Bare brick and old plaster drink water out of new plaster, which then cracks and fails to bond. PVA or a proprietary primer, applied as the product directs, is what prevents that.
9. Build up in the right layers.A deep patch needs a bonding or undercoat first, scratched to key, then a finish skim. One thick coat of finish plaster in a deep hole will crack.
10. Feather the edges.Work the new plaster out onto the sound surface and trowel it flat. A hard edge shows through paint from every angle.
11. Mist-coat before painting.New plaster is porous. A thinned first coat soaks in and gives the topcoat something to hold, or the paint peels later.
12. Get help for the big cases.Whole walls, lath and plaster ceilings, anything with a damp or movement cause. Plastering is a skill and a badly skimmed wall is visible forever.

Tips

• Tap the whole wall, not just around the damage. Blown areas are almost always bigger than they look.
• Blown plaster low on an outside wall is a damp investigation before it is a plastering job.
• Score the edge before hacking off. It stops the removal running into sound plaster.

Common mistakes

• Skimming over hollow plaster — The new surface is only as attached as the layer beneath it. It sounds hollow immediately and cracks within months.
• Using gypsum plaster on old lime walls — It is harder and less permeable than what is there, so it traps moisture and pushes the failure into the surrounding original plaster.

If it doesn't work