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New Plaster Will Not Dry Out
Weeks after plastering, a wall is still dark in patches instead of drying to an even pale finish.
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What's happening?
Fresh plaster dries from dark brown-pink to an even pale pink, and it does it from the surface inwards. Uneven drying is therefore a map: the pale areas have given up their water and the dark ones have not. Two very different things cause that. The plaster may simply be thicker there, or the room may have nowhere for the moisture to go, in which case it dries eventually once air is moving. Or the dark patch is being fed from behind — a leak, penetrating damp, a bridged damp proof course, or salts holding moisture in the wall — in which case it never dries, because the wall is supplying water as fast as the room takes it away. The test is time and shape: a patch that shrinks weekly is drying, a patch that holds its outline for a month is being fed.
What usually causes it?
- No ventilation in the room, so the air is already saturated and the plaster has nowhere to release moisture to
- Plaster applied thicker in that area, or a backing coat put on in one heavy pass, so more water has to leave
- A leak behind the wall — a pipe, a waste, a failed shower tray or a roof or gutter defect above
- Penetrating damp on an external wall, arriving faster in wet weather than the plaster can dry
- A bridged or failed damp proof course, or ground raised outside above it, keeping the base of the wall wet
- Hygroscopic salts left in the wall from an earlier damp problem, drawing moisture out of the room air
- Decorated too soon with a vinyl emulsion, which seals the face and stops the plaster releasing water at all
Is it dangerous?
The wet plaster itself is not hazardous, but leaving it is a false economy in two directions. Painting a wall that has not dried traps the moisture, and the paint peels while the plaster behind it stays wet — the finish is then wasted as well as the time. And where the source is a leak or damp rather than slow drying, the wall keeps wetting the plaster until the bond fails and the plaster blows, at which point the repair is hacking off rather than waiting. Persistent damp in a poorly ventilated room is also what grows mould on the new finish, which is why Approved Document F treats extract ventilation in kitchens, bathrooms and utility rooms as a requirement rather than an option.
Can I fix it myself?
The drying conditions, yes, and often that is all it needs. Open windows for part of the day, run the extract fan in a bathroom or kitchen, keep gentle background heat rather than blasting one wall, and move furniture off the wall so air can reach it. Photograph the dark patch weekly against something fixed such as a socket, so you can see whether it is shrinking. What you should not do is paint it to make it look finished, seal it with a waterproof coating, or aim a dehumidifier at one patch — a dehumidifier dries the room, but it will never win against a wall being fed water.
When should you call a plasterer?
Call a plasterer where the plaster is drying but the finish is patchy, blowing or salting as it goes, since that is the wrong system for a wall carrying moisture. Call a plumber, roofer or damp specialist first where the patch holds its shape for weeks, where it is worse after rain, or where it is at the base of an external wall — the plaster cannot be finished until the source stops. Get it resolved before decorating rather than after.
What will the plasterer actually do?
- Distinguish slow drying from an active source by the pattern, the timing and how the patch responds to weather
- Check the background and the wall behind the finish rather than treating the surface stain
- Hack off and rebuild in a salt-resisting or renovating plaster system where salts or residual damp are present
- Set out realistic drying conditions and ventilation for the room, including the extract that Approved Document F expects
- Return to finish or re-skim once the wall has dried, rather than closing the surface early
- Hand the leak, roof, gutter or damp proof course fault to the trade that owns it, with what was found
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