Does my wall need re-plastering, or can it just be filled?
The four tests that separate a wall needing filler, a wall needing a skim, and a wall needing taking back to the brick — with an honest account of what each costs you in time and mess.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 4 min
Short answer
Tap the whole wall. Anything that drums has lost its bond and cannot be filled. If the drumming areas are small and isolated, cut them out and patch. If more than roughly a quarter of the wall drums, or the surface is more than about 5mm out of flat, skim the whole wall. If the plaster is failing through its depth or is wet, it comes off back to the masonry.
This decision gets made backwards constantly. People start filling, discover the filler will not stick to what is around it, fill more, and end up with a wall that has been repaired six times and still looks wrong in daylight. Ten minutes of survey up front tells you which of three jobs you are actually doing, and the three jobs are very different in cost, mess and skill.
Step by step
- Tap the entire wall with a knuckle, in a grid.Solid and dead means the plaster is bonded to what is behind it. A hollow drumming means it has separated, and that is not a filling problem — filler bonded to a piece of plaster that is not attached to anything achieves nothing.
- Mark every hollow area in pencil and step back.The pattern matters more than the total. Two hollow patches the size of a hand are a patching job. A wall that drums in a dozen places, or drums along a whole edge, is a re-plastering job.
- Check flatness with a long straightedge.A two-metre straightedge or a clean length of timber held against the wall in several directions. Gaps of a millimetre or two are normal and will disappear under paint. Gaps of 5mm and more will show in any light and cannot sensibly be filled out.
- Check for movement rather than damage.A crack that has been filled and has come back is not a filling failure, it is movement. Re-plastering a wall that is moving produces a re-plastered wall that cracks. Diagnose the crack before choosing the repair.
- Check for moisture.Damp plaster, salts, staining, a musty smell. All of it changes the decision, because plaster applied over a wet wall fails regardless of how well it is applied.
- Pick the job from the evidence.Sound, flat, a few holes and hairlines: fill and sand. Sound but tired, uneven, lots of small damage, previous poor repairs: skim the whole wall. Hollow across large areas, soft, wet, or salt-contaminated: hack off and re-plaster properly.
- Consider lining paper as the middle option.Heavy lining paper hides an uneven but sound wall and costs a fraction of a skim. It is a legitimate answer, not a bodge, and it is reversible. It will not hide movement or drumming.
- Consider overboarding as the other middle option.Plasterboard fixed over the existing surface gives a fresh flat face without the mess of hacking off, and it is often the practical answer on a wall that is sound but hopeless, or where the existing surface should not be disturbed.
Common mistakes
- Filling a hollow area because it looks fine on the surface — The plaster is not attached to the wall. Anything applied to it is riding on a piece that will eventually come away, and the repair goes with it.
- Skimming a wall to hide a crack that keeps returning — Skim is 2mm of brittle material. If something behind it is moving, the crack reappears in the new surface, usually within a heating season.
- Deciding from one corner of the room — Walls fail unevenly. Survey all of it before committing, because the cost difference between patching and re-plastering is large and it is discovered halfway through otherwise.
Alternatives
- Fill and sand: For sound, flat walls with local damage. Cheap, low-skill, low-mess, and completely adequate where the wall underneath is genuinely sound.
- Skim the whole wall: Two coats of finish plaster over the existing surface, suitably prepared. Gives a new surface without demolition. Needs a plasterer for a good result and adds only 2 to 3mm.
- Lining paper: Heavy grade lining hung horizontally, then painted. Disguises a rough but stable surface for very little money. Does not fix hollow plaster or movement.
- Overboard with plasterboard: Board over the top, fixed through to something solid, then taped or skimmed. No hacking off, no drying time, and it buries a surface that is best left undisturbed. Costs you 12 to 15mm of room dimension.
- Hack off and re-plaster: The full job: back to the masonry, backing coat, finish coat. The only right answer where the plaster has failed through its depth, and by far the most disruptive.
Questions people ask
How much hollow plaster is too much?
There is no rule, but a practical threshold is whether the patches can be cut out and made good without the repairs joining up. Once the patches start meeting each other you are re-plastering the wall in an inefficient order.
Can a plasterer skim over anything?
No. Skim needs a sound, suitably absorbent, well-keyed background. Over gloss paint, flaking emulsion, distemper, unstable surfaces or hollow plaster it will fail, which is why plasterers charge separately for preparation.
Is it cheaper to overboard than to re-plaster?
Usually, once the cost of hacking off, disposal and the days of drying time are counted — and it is much less disruptive. The trade-offs are the lost room dimension and the need to move sockets, switches and skirtings forward.