Sign of damp
Efflorescence
A white or off-white powder, fluffy crust or crystalline film on plaster, often along the bottom of a wall, around a window or on a ceiling patch. It brushes off as dust and comes back. Paint over it usually bubbles or flakes because the salt pushes the film off from behind.
What it might mean in a flat
White salt deposits on plaster.
Efflorescence tells you water has moved through that wall at some point, not that it is wet today. In a new flat it can be construction moisture drying out. In a resale flat with fresh paint, salt pushing through the new coat is a strong hint that the old damp was painted over rather than fixed.
Common causes
- Water moving through brick, block or cement plaster dissolves salts and leaves them on the surface as it evaporates
- Rising damp at ground-floor skirting, carrying salts up from the soil and the masonry
- A slow leak from the bathroom floor above or a concealed pipe, drying out through the plaster below
- Rain driving through cracks in the external plaster on a monsoon-facing wall
- New construction drying out, where the salts come from the cement and bricks themselves in the first year or two
What an inspection checks
How we confirm it.
- Pin and pinless meter readings on the salt patch and on the clean plaster around it, both compared with a dry reference wall in the same flat
- A pinless reading taken alongside every pin reading, because salts conduct electricity and can push pin readings up on a wall that is dry
- Thermal image of the wall to see whether the patch is cooler than its surroundings, which suggests active evaporation, then a meter check to confirm
- The likely path of the water: the bathroom above, the window head, the skirting, or the outside face of the wall
- Whether the salt line is growing, judged from paint condition, earlier repair marks and the seller's account
When to worry
Signals that it's active.
- The salt comes back within weeks of being brushed off or painted over
- Both pin and pinless readings are high on and around the patch compared with the dry reference
- Plaster under the salt sounds hollow when tapped or crumbles at the edges
- The patch sits directly under a bathroom, kitchen sink or terrace and is spreading
Where it usually comes from
Questions buyers ask
Can I just brush off the white powder and repaint?
You can, but if water is still moving through the wall the salt will come back and lift the new paint. Find out first whether the wall is wet today and where the water comes from.
Why does my moisture meter show high readings on a wall that feels dry?
Pin meters read electrical resistance, and salts conduct electricity. A wall full of old salts can read high with very little water in it. That is why an inspector compares pin and pinless readings, checks the thermal image, and compares everything against a dry wall in the same flat.
Is efflorescence normal in a new flat?
Some salting in the first year or two can come from construction moisture drying out. It should fade over time. If it keeps growing, or sits under a bathroom or on an external wall, have it checked within the defect liability period.
Not ready to book? Walk the flat with our 10-minute self-check and get a snag list to take to the seller.
Start the Damp CheckFind the damp before you pay for it.
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