After a wet spring, a lot of London chimneys and garden walls grow white, chalky streaks that look like someone dusted them with flour. That's efflorescence: mineral salts that were dissolved inside the masonry, carried to the surface by water, and left behind when the water evaporated. The powder itself is harmless. What it tells you is the interesting part, because efflorescence is physical proof that water is moving through your brick, and the only real question is whether that movement is finished or ongoing.
Brush it dry, and leave the pressure washer in the garage
Pick a dry day, take a stiff nylon brush, and brush the powder off dry. Most of it will come straight off. If a light haze remains, a rag with plain water and a second pass on another dry day usually finishes the job. That's it. No product needed for a first attempt.
Two things not to do, because we see the aftermath of both. Don't pressure wash it: you'll blast the powder off in seconds and drive litres of water into the wall, which dissolves more salt and delivers a bigger bloom two weeks later. And don't reach for muriatic acid because a forum told you to. Acid on the wrong brick etches the surface, burns mortar joints, and can trade a white stain for a permanent dull patch. If a dry brush plus water won't shift it, the stain has likely mineralized into something harder, and that's worth a professional opinion before chemicals.
The real question is where the water came from
A single bloom on newer masonry is usually construction moisture working its way out, and it fades for good once the wall has fully dried. Brick a year or two old shedding one round of white powder is a non-event.
Efflorescence that comes back in the same spot every season is different. Recurring powder means a recurring water path, and the powder is drawing you a map of it. The two places we see constantly around London: chimneys above the roofline, where a cracked crown or a missing cap lets rain soak straight down into the brick, and the stretch of wall below a gutter that overflows or a downspout that's come loose. The wall isn't the problem in either case. The water delivery is.
So before spending anything on the brick itself, walk the water: look up from the stain for an overflowing gutter, a disconnected downspout, a failed cap or crown, or a windowsill shedding onto the wall. Fix the delivery and most recurring efflorescence quietly stops on its own.
One more mistake worth naming because it's the most common one we undo: sealing the wall first. A water repellent applied over brick that's still taking on water from above doesn't stop the problem, it traps the moisture inside, and trapped moisture does far more damage than the powder ever did. Sealer is sometimes the right last step. It is never the right first step.
When powder turns into popping brick faces
Efflorescence on the surface costs you nothing but looks. The trouble starts when the same salts crystallize inside the brick instead of on it. Salt crystals growing in the pores push outward, freeze-thaw cycles join in over a London winter, and eventually the face of the brick flakes or pops off entirely. At that point you've moved from a cosmetic issue to spalling brick, which is a genuinely different problem with its own repair path.
The tells that you're heading that way: brick faces flaking or crumbling near the white staining, mortar joints going soft and sandy when you scratch them with a key, or powder appearing alongside damp patches that never quite dry. Once the mortar or the brick faces are failing, brushing is over. Open joints let more water in, more water carries more salt, and the cycle feeds itself until the joints are repacked. That's repointing work, and the honest advice is to price it when the first joints go soft rather than after a winter or two of freeze-thaw has widened the bill.
Frequently asked questions
Does efflorescence mean my wall is failing?
No. It means water is moving through the masonry. If the powder brushes off and doesn't return, the water event is over and the wall is fine. Recurring blooms in the same spot mean an active water source worth finding.
How do I remove efflorescence without damaging the brick?
Dry stiff-bristle brushing on a dry day, then plain water and a rag if a haze remains. Skip pressure washing, which pushes water into the wall, and skip acid unless a professional has confirmed the brick can take it.
Will sealing my brick stop efflorescence?
Not if water is still getting in from above. Sealer over a wet wall traps moisture and makes the damage worse. Find and fix the water source first; seal only once the wall is dry and the source is dealt with.
Why does the white powder come back every spring?
Because the water path is still open. Melting snow and spring rain re-soak the same route through the masonry, dissolve more salt, and leave it on the surface as things dry. Check gutters, downspouts, and the chimney crown above the stain.