White powdery or fluffy deposits on walls are salts left behind as water evaporates from the masonry or plaster, known as efflorescence. On new brickwork it's usually harmless, but on internal walls it shows moisture is moving through the wall, and some salts attract moisture from the air and keep the wall damp.
Salts are one of the most useful clues on a damp survey. They tell you water has been moving through the wall and evaporating at the surface, and sometimes they tell you where that water came from.
What efflorescence is
Bricks, stone, mortar, soil and plaster all contain soluble salts. When water travels through a wall, it dissolves those salts and carries them along. When the water evaporates at the surface, the salts are left behind as white powder or fluffy crystals. That’s efflorescence.
Common situations
- New brickwork or new plaster. As a new wall dries out, salts often appear on the surface. This is usually harmless and stops once the wall has dried.
- Walls drying out after a leak or flood. Salts appear as the moisture leaves.
- Rising damp. Ground water carries salts, typically chlorides and nitrates, up the wall. They concentrate in a band around the height where the damp evaporates, which is the tide mark people talk about.
- Chimney breasts. Burning fuel leaves salts in the flue, which come through the plaster when the chimney gets damp.
- Penetrating damp, where rainwater carries salts out of the masonry.
Why some salts matter more
Certain salts are hygroscopic: they pull moisture out of the air. Plaster contaminated with them can stay damp, or go damp again on humid days, even when the original source has stopped. They also make damp meters read high. This is why, after rising damp or chimney damp has been dealt with, contaminated plaster usually needs to be removed and replaced with a salt-resistant system.
What to do
- Brush salts off dry with a stiff brush, and vacuum them up. Don’t wash them off; water just dissolves them back into the wall.
- Don’t seal them in with paint. They’ll push through or blow the paint off.
- Work out where the moisture is coming from. The pattern and position of the salts are a big clue.
- Where rising damp is suspected, salt analysis of plaster samples can help confirm it.
Efflorescence on a new wall that stops after a few months, I wouldn’t worry about. Salts on an internal wall in an older house deserve a proper look.
When to call a specialist
If salts keep appearing on internal walls or chimney breasts, get a survey to find the moisture source and check whether the plaster needs replacing. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Is efflorescence a sign of rising damp?
Not on its own; it shows water has moved through the wall, which could have several causes.
How do I remove efflorescence?
Brush it off dry and vacuum it up; don't wash it, as water dissolves the salts back in.
What are hygroscopic salts?
Salts that attract moisture from the air, keeping contaminated plaster damp.