Guide
Damp and plaster: what has to happen in which order
Plastering over a damp wall hides it for a winter. The order is find the cause, fix the cause, let it dry, then replaster, and the last step is the shortest of the four.
The order, which is the whole of it
Find the cause. Fix the cause. Let the wall dry. Then replaster.
Every expensive damp job that goes wrong has done those in a different order, and almost always by putting the plastering first because it is the part you can see. A replastered wall looks finished immediately and takes months to prove that nothing underneath was solved.
What is actually causing it
Four things, and they are not equally likely.
Condensation is by far the commonest, and it is not a wall defect at all. Black spotted mould in corners, behind furniture and around windows, worse in winter, worse in bedrooms and bathrooms. The answer is ventilation and heating, not plaster.
Penetrating damp comes through from outside at a specific point: a cracked render, a failed sill, a blocked gutter, a bridged cavity, pointing that has gone. It makes a patch that maps to something on the outside wall.
Leaking pipework, which is common, underdiagnosed, and indifferent to what the wall is made of.
Rising damp is real and is much rarer than the industry selling treatments for it suggests. It shows as a tide line up to about a metre with salt staining, and on a solid pre-1919 wall it is frequently misdiagnosed condensation or a bridged damp course instead.
Why the plaster has to come off at all
Water moving through masonry carries salts out of it: nitrates and chlorides, left behind as the water evaporates at the surface.
Those salts are hygroscopic, meaning they pull moisture out of the air. A wall can be structurally dry and still show damp patches on the decoration, because the salts in the old plaster are collecting humidity out of the room.
That is why replastering is part of the treatment rather than making good afterwards, and it is why the old plaster comes off to a metre above the highest staining rather than to the damp line. Salts travel further than the water does.
The specification
On a cavity or modern wall: hack off, render in sand and cement with a salt retardant additive, let it dry, skim. £40 to £70 per square metre.
On a solid pre-1919 wall: lime, for all the reasons in the lime guide. A cement render with a waterproofer on a wall that has no cavity seals the inside face and moves the problem rather than solving it. £60 to £110 per square metre.
Either way there is a drying interval between the render and the skim that is not optional and that lengthens the job by weeks.
How long before you can decorate
Weeks between the render and the skim, then the skim's own drying time, then a mist coat. Two to three months from hack-off to a finish coat is realistic, and longer in winter.
Painting a replastered damp wall early is how the whole job gets repeated.
Common questions
Do I need 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 the wall keeps showing damp patches even once it is dry.
Is my damp rising damp?
Probably not. Condensation is far commoner, and penetrating damp and leaking pipework account for much of the rest. Rising damp is real but is diagnosed far more often than it occurs, particularly on solid walls.
How high does the plaster 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.
Can I just replaster and see?
You can, and the wall will look perfect for a winter. If the cause is still there the salts and the moisture come back through, and you pay for the plastering twice plus the decoration.