Carbonate Staining & Lime Run
Hard white or grey crusts running from mortar joints, cracks and soffits. Unlike efflorescence it doesn't brush off and doesn't vanish when wet — and it always means water is getting somewhere it shouldn't.
Lime run is the problem most often misdiagnosed as efflorescence, and the distinction matters because they mean completely different things. Efflorescence is a loose salt deposit and is frequently harmless. Lime run is a hard calcium carbonate crust, and it's telling you that water is moving through the structure and dissolving the lime out of the mortar or concrete as it goes.
In other words, efflorescence is often just a wall drying out. Lime run is a leak with a visible signature.
Telling them apart
The wet test settles it in seconds and it's worth doing before anyone quotes.
Efflorescence dissolves when the surface is wet and disappears, then reappears as it dries. It's loose and powdery and comes off on a dry hand.
Carbonate staining or lime run stays visible when wet. It's a hard crust bonded to the surface, it doesn't brush off, and in bad cases it forms genuine stalactites under a soffit or a deck.
The pattern is a giveaway too. Efflorescence drifts across an area. Lime run has a clear source — it emanates from a specific joint, crack, construction joint or drainage outlet and runs downwards from it. If you can trace the deposit back to a point, it's lime run.
Lime run is a water-ingress symptom first and a cleaning job second. If you have it under a car park deck, a balcony or a bridge soffit, the deposit is evidence that water is tracking through the structure — which is worth knowing regardless of how the staining looks.
What's happening chemically
Cement-based mortar and concrete contain free calcium hydroxide. Water passing through the material dissolves it and carries it to the surface; on contact with carbon dioxide in the air it converts to calcium carbonate and crystallises as a hard white deposit. It's the same process that forms limescale in a kettle and stalactites in a cave, running on a building.
That's why it's so tenacious compared to efflorescence. It isn't a soluble salt sitting on the surface — it's an insoluble mineral crust that has crystallised in place and bonded on.
It's also why it keeps growing. As long as water keeps tracking through the same path, more lime is delivered to the same spot, and the deposit thickens. That's a slow depletion of the binder within the structure as well as an appearance problem, which is another reason not to treat it as purely cosmetic.
White crusting on a structure or soffit?
We can remove it — and tell you where the water's coming from, which is the part that stops it returning.
Removing it
Removal is genuinely difficult, and it's worth being realistic about that. Water and steam alone won't shift an established crust — it's insoluble in water, which is precisely why it's still there after years of rain.
Light deposits can be worked off mechanically with careful scraping and steam. Heavier deposits need an acidic treatment to dissolve the carbonate, and that requires real care: acid attacks limestone directly, etches concrete, burns lime mortar and can stain some bricks. Applying a proprietary product to a soffit without testing is how a white streak becomes a permanently etched patch.
So we test, we choose the mildest effective product, we work in small areas with short contact times, and we neutralise thoroughly afterwards. On sensitive masonry a poultice holds the treatment in contact without letting it run across the surrounding surface.
Even done well, heavy deposits on a rough surface may leave a faint mark where the crust bonded. That's usually accepted as the reasonable outcome, given the alternative is aggressive removal that damages the substrate.
Where we see it most
The multi-storey car park case is the one we deal with most. White crusting on the soffit of a deck is a routine finding and it's routinely cleaned off without anyone asking why it's there — but it means water is passing through the deck, and water passing through a reinforced concrete deck is reaching the reinforcement. It's worth raising with whoever looks after the structure, not just cleaning off.
- Car park decks and soffits — running through construction joints and cracks from the deck above
- Bridge and subway structures, often with stalactite formations under the soffit
- Balconies and walkways where the waterproofing has failed or the drainage is blocked
- Retaining walls with saturated ground behind them and no functioning drainage
- New brickwork where mortar has been repeatedly rained on before curing
Doing it in the right order
The same rule as rust staining: deal with the water first, then the deposit. Cleaning lime run while the leak continues gives you a clean soffit and a returning crust, and it can come back surprisingly quickly where the flow is steady.
Tracing it is often straightforward — a blocked outlet, a failed movement joint, a cracked upstand, ground saturated against a wall. Sometimes it needs a proper investigation of a waterproofing system, which isn't our trade. Either way, we'll photograph the source and the path, and tell you what we think is feeding it.
Where a client wants the cleaning done regardless — a car park being marketed, a structure needing to look presentable for an inspection — that's a legitimate decision, and we'll do it. We'd just rather you were making it knowingly than assuming the crust was cosmetic.
How To Recognise It
What it looks like
- Hard white or grey crust that stays visible when the surface is wet
- Runs and streaks emanating from a joint, crack or soffit rather than drifting
- Stalactite-like deposits under bridge soffits, balconies and car park decks
- Doesn't brush off — it needs scraping or chemical treatment
What causes it
Dealing with the cause is what stops it coming back.
- Water tracking through cracks, joints or a failed waterproofing membrane
- Leaching of free lime out of mortar, concrete and grout
- Blocked or absent drainage above the affected area
- Persistent saturation of a structure — decks, retaining walls, soffits
Surfaces It Affects
Frequently Asked Questions
What's the difference between lime run and efflorescence?
Wet the surface. Efflorescence dissolves and disappears when wet, then returns as it dries — it's a loose, harmless salt deposit. Lime run stays visible when wet because it's a hard, insoluble calcium carbonate crust. Efflorescence usually means a wall drying out; lime run means water is actively tracking through the structure.
Can lime run be removed?
Yes, but it's difficult. Water and steam won't shift an established crust, so it needs careful mechanical removal or an acidic treatment — which has to be tested first, because acid etches limestone and concrete and burns lime mortar. Heavy deposits on rough surfaces can leave a faint mark, which is generally preferable to damaging the substrate to remove it completely.
Is lime run a sign of a serious problem?
It's a sign that water is moving through the structure, which is worth understanding. Under a car park deck or a balcony it means water is reaching the reinforcement, and it also represents a slow loss of binder from the concrete or mortar. The staining itself is cosmetic, but what it's telling you may not be.
Will it come back after cleaning?
Yes, if the water source is still there — sometimes quite quickly where the flow is steady. The right order is to trace and fix the ingress, then remove the deposit. If you need it cleaned in the meantime for an inspection or a letting, that's a perfectly reasonable decision, we'd just want you to know it's a temporary result.
Areas We Cover
We deal with carbonate staining & lime run across Manchester, Greater Manchester and Cheshire, including:
Other Contaminants We Remove
Need graffiti gone — fast?
Call 0161 399 5247 or request a free quote. We respond within one working hour.