People want to know how to get rid of the white stuff. The more useful question is where it is, because a salt bloom is a map of how water is moving through a wall, and once you have read the map, the cleaning takes twenty minutes.
What is actually depositing
Masonry contains soluble salts — from the clay the brick was made of, from the cement or lime in the mortar, from de-icing salt, from groundwater, sometimes from a sand that was never washed. Water dissolves them, carries them, and leaves them behind when it evaporates.
Where the water evaporates at the surface, you get efflorescence: a powdery bloom on the face, usually white, harmless in itself, and mostly washable.
Where the water evaporates just below the surface — because the face is less permeable than what is behind it, often thanks to a sealer or a cement render — you get subflorescence instead. The crystals grow inside the pores and break the material apart. That is not cosmetic, and it is why sealing a wall that is blooming makes things worse rather than better.
Reading the pattern
- A band across the bottom two or three feet. Rising damp or splash-back. Check the grade, the paving, and whether a downspout discharges nearby.
- A patch under a windowsill. Failed sill, missing end dam, or water tracking behind a sill that has no drip.
- A vertical streak. A leaking gutter joint or downspout above.
- Around a penetration. Unsealed pipe or vent, or a failed sleeve.
- Under a parapet or coping. Water entering the top of the wall — the most commonly missed source, because people look at the bottom.
- Uniformly across a whole elevation. Usually new construction drying out, and it should stop within a year.
The top edge of a bloom marks roughly how high the water is reaching. That single observation separates rising damp, which has a fairly level ceiling, from a leak, which is irregular and often has a source you can point at.
The tests worth doing
Taste is the traditional field test and it is more useful than it sounds: efflorescence from mortar and brick salts is largely insipid, while de-icing salt contamination is unmistakably salty. A more respectable version is a salt test strip on a damp swab.
Beyond that: hose test one suspected source at a time from the bottom up, with someone watching inside. Check the moisture content of any adjacent timber with a meter. And look at the wall after two dry weeks — efflorescence that keeps returning after cleaning means the water path is still live.
One caution on interpreting the map: a wall can have more than one water path, and fixing the obvious one often reveals a second bloom in a different place a season later. That is progress, not failure — work through them in order of severity rather than expecting a single cause.
Cleaning, last
Dry brush with a stiff bristle brush first, on a dry wall. Most efflorescence comes off. If it does not, a proprietary masonry cleaner at low dilution, applied to a pre-wetted wall and rinsed thoroughly, will handle the rest. Skip acid on limestone and marble entirely.
Do not clean until you have fixed the source, and do not clean before you have photographed the pattern. Most of the sources are outside the wall rather than in it — gutters, grade and splash zone — and those are covered in the piece on water at the base of a wall. The Brick Industry Association technical notes at gobrick.com have the chemistry if you want to identify a specific salt.
Frequently asked questions
Is efflorescence harmful? The bloom itself is not. What it indicates — water moving through the wall — can be, and the subsurface version of the same process is genuinely destructive. Treat the deposit as information rather than as the problem.
Why does it come back every spring? Because winter puts water into the wall and spring evaporation brings it back out with the salts dissolved in it. A seasonal cycle that gets weaker each year is a wall drying out. One that stays constant means an active water source.
Can I paint over it? No. Paint will not bond to a salt-contaminated surface and will peel within a season, and a film-forming paint on a damp wall drives the crystallization below the surface where it does real damage. Fix the water, let the wall dry for months, then decide.