repairs
Re-plastering after damp treatment
Re-plastering after damp treatment costs £40 to £70 per square metre including hacking off and rendering. The order matters more than the price: find the cause, fix the cause, let it dry, then plaster. Plastering first hides it for one winter.
What this covers
What it costs. Re-plastering after damp treatment costs £40 to £70 per square metre including hacking off, salt retardant render and a skim. Full cost breakdown.
What the job covers
- Hacking off to a metre above the highest salt staining
- Rendering in sand and cement with a salt retardant additive
- Skimming the render once it has dried out
- Plastering over a tanking system in a cellar or basement
- Reinstating skirting and making good after the render has cured
What is involved
Replastering is the last step of a damp job and the shortest. The old plaster comes off to a metre above the highest salt staining, because the salts travel further up the wall than the water does and any that are left will come through whatever goes on top. The wall is then rendered in sand and cement with a salt retardant additive, left to dry, and skimmed.
The reason the specification is so specific is that hygroscopic salts, mostly nitrates and chlorides carried out of the masonry, pull moisture out of the air. A wall can be structurally dry and still show damp patches on the decoration because of them, which is why replastering after a damp course is part of the treatment rather than making good afterwards.
On a pre-1919 solid wall the whole approach is different, and this is where a lot of money gets wasted. Those walls are meant to breathe. A cement render with a waterproofer, on a wall with no cavity, moves the problem up and along rather than solving it. That is a lime job.
Hacking off to a metre above the highest salt staining, one of the jobs a re-plastering after damp specialist is hired for.
Choosing someone for the job
Ask what caused the damp, and be unconvinced by an answer that is only the word rising. Ask how high they are hacking off and why that height. Ask how long the render has to dry before the skim goes on. And on anything solid-walled and pre-1919, ask specifically whether lime is the right answer here, because cement on that wall is the expensive mistake.
Common questions
Do I have to replaster after a damp proof course?
Usually yes, and most damp course guarantees require it. The old plaster holds hygroscopic salts that pull moisture from the air, so it will keep showing damp patches even once the wall itself is dry.
How high does the plaster have to come off?
To roughly a metre above the highest salt staining, not the highest damp. Salts wick further than water, and anything left behind comes through the new work.