After a chemical DPC, the old salt-contaminated plaster normally needs removing and replacing with a salt-resistant replastering system, as set out in BS 6576. Ordinary gypsum plaster shouldn't go straight onto salt-contaminated masonry, because the salts will draw moisture and spoil it.
I’ve been plastering since 1983, and if there’s one thing I want people to understand about damp proofing, it’s this: the replastering isn’t the finishing bit. It’s half the treatment.
Why the old plaster has to come off
Rising damp carries salts up from the ground. As the water evaporates, those salts stay in the plaster and the surface of the masonry. Many are hygroscopic: they pull moisture out of the air. So even when the new DPC has stopped moisture rising, the old plaster can stay damp, and the decorations keep failing. People then think the DPC hasn’t worked.
BS 6576, the British Standard covering diagnosis of rising damp and installing chemical DPCs, expects contaminated plaster to be removed and replaced with a suitable salt-resistant system.
How far should it be removed?
A commonly used approach is to remove the plaster to at least a metre high, or 300mm above the highest signs of damp or salts, whichever is higher. On our Royston job we took it off to 300mm beyond the last signs of damp. Stopping at the edge of the stain is a common shortcut and leaves contaminated plaster behind.
What goes back on
- Sand and cement render with a salt-retardant or waterproofing additive, then a finish skim. The traditional approach, and still widely used.
- Pre-bagged renovating or damp-resistant plasters designed for use after damp proofing. Follow the manufacturer’s guidance and use one with proper third-party certification.
- Membrane systems in some situations, such as walls with very heavy salt contamination or where speed matters, with plaster or board over the top.
- On older solid walls and listed buildings, the system needs choosing carefully. Lime plasters have their place in old buildings, but lime on its own won’t stop rising damp. The whole wall needs thinking about.
What not to do
- Don’t put ordinary gypsum plaster directly on salt-contaminated masonry.
- Don’t carry the new plaster down onto the floor. Stop it just above, so it doesn’t bridge the new DPC.
- Don’t rush the decorating. Let the plaster and wall dry, and use a breathable finish first.
If a quote for damp proofing doesn’t include replastering, ask why.
When to call a specialist
If you're having a DPC installed or have had one fitted and the walls are still showing damp, have the replastering specification checked. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Can I use gypsum plaster after a DPC injection?
Not directly on salt-contaminated masonry; a salt-resistant system should be used.
How high should plaster be removed after rising damp?
Commonly to at least a metre, or 300mm above the highest signs of damp or salts.
Why is my wall still damp after a new DPC?
Often because salt-contaminated plaster was left on, or the wall hasn't finished drying.