Understanding damp · 26 Sept 2026 · 4 min read
Efflorescence: the white powder on your wall and what it means
White powder or crust on plaster is salt left behind by moving water. What causes efflorescence, what it tells you about the source, and when to worry.
You wipe the white powder off the wall and a few weeks later it is back. Sometimes it is a fine dust, sometimes a crust that lifts the paint, sometimes fluffy white crystals in a corner. This is efflorescence, and it is one of the most useful signs of damp in an Indian flat, because it shows you where water has been moving, even when the wall feels dry.
What it is
Cement, sand, bricks and blocks all contain soluble salts. When water moves through a wall, it dissolves some of those salts and carries them towards the surface. At the surface the water evaporates and the salts stay behind as a white deposit. No water movement, no new efflorescence. The efflorescence sign page shows the common forms.
That is why efflorescence keeps returning after you wipe it. Wiping removes the salt on the surface, not the water bringing more of it.
Where it shows up, and what that points to
| Where you see it | Likely source to check |
|---|---|
| A band along the skirting on a ground floor | Rising damp from the ground |
| Around a ceiling patch under a bathroom | Bathroom floor waterproofing above |
| On an external wall, inside the room | Rain through cracks in the external plaster |
| Below a window sill | Water entering at the frame or sill |
| In a line along a wall | A concealed pipe running inside it |
| On new plaster in a new flat, evenly spread | Construction water drying out |
On ground floors, a band that stops at roughly the same height along a wall is typical of rising damp. On upper floors, patches on the monsoon-facing wall usually point to the external wall.
New building, old building
In a new flat, some efflorescence in the first year can come from water used during construction drying out of the plaster and masonry. It tends to be spread fairly evenly and to reduce over time. Efflorescence that is concentrated in one spot, that keeps growing, or that sits next to a bathroom, sink or window is more likely to be fed by a leak.
In an older flat, fresh efflorescence means water is moving through that wall now or was recently. Old salt under fresh paint can push the new paint off within months, which is one way a recent repaint gives itself away.
Why it can confuse a moisture meter
Salts conduct electricity. A pin moisture meter works by measuring resistance between two pins, so a salty wall can read higher than it really is, even after it has dried out. This is one reason readings are taken against a dry reference wall in the same flat and checked against the thermal image and what is visible. A high reading on a salty patch, with no cool area on the thermal image and no fresh staining, may be old salt rather than active damp.
When to worry
- It comes back quickly after cleaning, within days or a few weeks.
- It is growing, or the paint and plaster around it are bubbling or crumbling.
- It sits next to a bathroom, kitchen sink, window or shaft wall.
- You see it on the ceiling, where construction water rarely explains it.
- It appears along with tide marks, a musty smell or rust stains.
What not to do
- Do not paint over it. The salt keeps coming and lifts the new paint.
- Do not wash it with lots of water. You add moisture to a wall that already has too much.
- Brush it off dry, note where it was, and check whether it returns.
- Find the source before any repair. Treating the surface does not stop the water feeding it.
A pre-purchase damp inspection reads every salty area against a dry wall and states the likely source for each one.
Find the damp before you pay for it.
Book a moisture and thermal inspection. Report on WhatsApp within 24 hours.