Guide
Skim or replaster: which one your wall needs
A skim is a finish coat over something sound. Replastering means floating a bare wall first, and it is roughly twice the work. Tapping the wall tells you which you have.
Tap the wall
Almost every decision on this page comes from one test that takes ten seconds and costs nothing. Rap the wall with your knuckles across its whole area, high and low.
A sharp, solid note means the plaster is still stuck to what is behind it. A dull hollow note means it has lost its key and is being held on by the paper and friction. Hollow over a small patch is a repair. Hollow over a wide area means the plaster is coming off, whatever anybody is about to quote you for skimming over it.
Skim, float and set, or board
Skimming is two or three millimetres of finish plaster over a surface that is already flat and sound. It is one day for a small room, it costs £450 to £750, and it is by far the commonest job in this trade.
Floating and setting is what a bare or blown wall needs: a backing coat ruled out to beads, scratched, left to firm, then skimmed. Roughly double a skim at £40 to £60 per square metre, and it is a two visit job on anything substantial.
Boarding is the third route, and on a wall that is in poor condition it is often cheaper than floating. Dot and dab over the old surface, then skim the board. It costs you around forty millimetres of room width, which is the trade-off nobody mentions until the boards are up.
When skimming over the old surface is a bad idea
The new coat is only ever as sound as what it is stuck to. So skimming fails where the surface underneath is failing, and the list is short: anything hollow, anything damp, anything salty, and wallpaper.
Wallpaper deserves its own sentence, because it is the commonest request and the commonest disaster. The water in the wet plaster soaks through into the paste behind the paper, the paste lets go, and the skim comes off with the paper it was stuck to. Most plasterers will ask you to strip it first. The ones who will not are pricing the risk into the job.
What it costs either way
For an average bedroom, walls and ceiling: skimming £450 to £750, floating and setting £900 to £1,500, boarding and skimming £700 to £1,100.
The gap between those numbers is why the state of the wall is the first thing to establish and why a quote given over the phone is worth what you paid for it.
Common questions
Can you skim over old plaster?
Yes, where it is sound and well stuck. Tap it first: a hollow note over a wide area means the old plaster has lost its key and anything skimmed over it will fail along the same lines.
Can you plaster over wallpaper?
It is done and it is a gamble. The wet plaster soaks into the paste behind the paper, the paste releases, and the skim comes off with it. Strip it first.
Is boarding cheaper than replastering?
Often, on a wall in poor condition, because dot and dab plus a skim is quicker than hacking off and floating. It costs you about forty millimetres of room width on every wall you do it to.
How do I know if my plaster has blown?
Tap it. A dull, hollow, drum-like note means the plaster has separated from the wall behind. Compare it against a wall you know is sound and the difference is obvious once you have heard both.