White stuff on your bricks

Seven things a white mark on a brick can be

People arrive at this page with one photograph and one word. There are at least seven different things that photograph could be showing, and they do not share a fix.

• Efflorescence — a loose white bloom of soluble salt left on the face when water evaporated out of the masonry. Lifts off dry as powder. Cosmetic in itself.
• Sub-florescence, also called crypto-florescence — the same salt crystallising just under the surface instead of on it. Often shows as barely any white at all, next to a brick face that is flaking, powdering or shedding its outer skin.
• New-build bloom — soluble alkalis out of the Portland cement in fresh mortar and render, mobilised by construction water. Normal, and it fades.
• Lime run — a hard white crust, sometimes with a glazed or drip-formed look, where lime has leached out of fresh concrete or mortar above and re-deposited. That is a water-path clue, not salt attack, and the fix is different.
• Paint chalking — an old acrylic or cement paint breaking down. Comes off evenly on a wiped finger across the whole surface, rather than concentrating in a band near the ground.
• Mortar dust — grey-white grit collecting at the base of the wall from joints that are fretting. That is a different page.
• Salt introduced by earlier work — hydrochloric acid used for a builders clean, or contaminated bedding sand under paving laid against the wall.

Efflorescence and sub-florescence: same salt, opposite outcomes

Both start identically. Water moves through the connected pore network of the brick and mortar carrying dissolved salt with it. The water evaporates. The salt cannot evaporate, so it stays, and it accumulates year after year.

The only variable that decides whether that is a cosmetic problem or a structural one is where the evaporation front sits. If drying is slow enough that liquid water keeps reaching the face, the salt crystallises out in open air as a bloom. Nothing is confined, so nothing is stressed.

If evaporation is faster than the wall can deliver water to the face, the drying front retreats into the material and the salt crystallises inside the pores instead. Now the crystal is growing in a confined space, and the pressure that generates is enough to disrupt even sound, strong masonry. That is what lifts the fireskin off a brick in flakes, sheds the case-hardened surface off sandstone, and turns a mortar joint into sand you can rub out with a thumb.

One salt is worse than the rest at this. Sodium sulfate exists as anhydrous thenardite and as the ten-water hydrate mirabilite, and it swaps between them with changes in humidity and temperature. The hydrate occupies far more volume. Sound bricks splitting and delaminating, rather than merely fretting at the surface, points at sulfate — and a sulfate result is a reason to look for a cement source, because Portland cement carries gypsum and alkali sulfates of its own.

The brush test, and what each result means

This is not a diagnosis. It is a triage that costs nothing and rules out the most common harmless case in about five minutes.

1. Wait for a dry spellTest a dry wall. A wall that has just been rained on has had its surface salt dissolved and washed away, and will under-read.
2. Brush a patch dryStiff bristles, no water, no chemicals, no pressure washer. Work a patch about 300 mm square and catch the residue on a dustpan or a sheet of paper so you can see what came off.
3. Read the residueA fluffy deposit that lifts as loose powder and leaves sound brick behind is efflorescence. A hard crust that resists the brush is either a lime run or a long-established deposit, and it is a longer story that should not be attacked mechanically until it has been identified. A patch that takes brick with it — grain, flakes, or the fired outer skin — is not a surface deposit at all. That is sub-florescence, and the brick itself is already failing.
4. Note whether it returnsRebloom within weeks, or after every heavy rain, means water is still arriving at that spot and evaporating there. A single bloom that never comes back is history rather than a live problem.

Where the salt came from

Salt gets into an Australian wall from saline soils and groundwater, sea spray on coastal sites, airborne salt blown inland from dry salt lakes, air pollutants, inorganic garden fertilisers, biological sources including bird droppings and leaking sewers, the brick clay or the mortar sand itself, salty water used to mix mortar, and cleaning compounds that contain or react to produce salts. Accumulation is slow and cumulative, and even low deposition rates matter across decades — particularly where they concentrate at one point, such as beneath a blocked rainwater head.

Why the species is worth knowing

Salt type is a forensic tool, which is the argument for paying for laboratory speciation on a wall that matters rather than accepting a generic total-salt figure. A nitrate-heavy result sends you looking for a leaking sewer, a failed septic system, an over-fertilised bed against the wall, or an old stable. A chloride-heavy result points at groundwater, sea spray or an acid clean. Predominantly magnesium sulfate points at a bag of sand. Treat the wall without finding the source and the clock simply restarts.

Two figures are worth carrying into any conversation about this. The working threshold is that more than about 0.5% salt by weight of the sample is cause for concern and reason to consider salt removal. And mixtures go wet at a lower humidity than either constituent alone — a wall carrying both sodium chloride at 75% RH and magnesium chloride at around 33% RH does not behave like either. Real walls always contain mixtures, so single-salt figures understate how often a real wall is actively cycling.

White bloom on a new build is usually nothing

Portland cement contains gypsum plus soluble sodium and potassium compounds. Construction water mobilises them, they migrate to evaporating surfaces, and they deposit there. New brickwork and new render bloom white in the first year or two, wash off with rain, and progressively diminish. It is cosmetic and it is self-limiting. Time and ventilation are the treatment; there is nothing to buy.

The test is direction, not quantity. A bloom that is fading year on year is construction moisture doing exactly what it does. A bloom that is growing year on year means something is supplying water, and that is a different investigation.

If moisture-sensitive flooring is going down anywhere in the same building, the bloom on the brickwork is beside the point but the slab is not. Test the slab's internal relative humidity with in-situ probes to ASTM F2170, drilled to 40% of slab depth for a slab drying from one side. AS 1884:2021 makes that the normative test. Laying an impermeable floor covering over a slab that has not finished drying is the single most common cause of a new floor failing.

When it keeps coming back, there is a live source

Efflorescence is not self-generating. Each bloom is a delivery receipt: this much water arrived here, and evaporated here. Recurrence means the supply has not stopped.

Trace it in this order, which is roughly the order of frequency and very nearly the inverse order of cost.

• Roof plumbing. Go outside during heavy rain and watch the wall. A blocked gutter, a split downpipe or a downpipe discharging at the footing is the most commonly missed cause of all, and the salt often appears well away from the entry point, because it deposits where evaporation happens rather than where the water got in.
• Ground level. Measure it. NCC Housing Provisions put a damp-proof course not less than 150 mm above adjacent ground, 75 mm above finished paving that slopes away, and 50 mm where protected by a carport or verandah. Garden beds creep upward every time mulch is added, and the fertiliser in them supplies nitrate.
• Irrigation and condensate. Sprinklers throwing at the wall, drippers sitting against it, a split-system condensate line discharging at the base. Replace sprinklers with drippers kept at least 500 mm off the wall.
• Paving hard against the wall with no fall. AS 2870 and the NCC require a minimum fall of 50 mm over the first metre away from a slab; the remedial target for regrading is about 25 mm of fall across the first metre with the low point 1.5 to 2.0 m out.
• The wall cavity. Mortar droppings on wall ties, blocked weepholes, missing cavity flashing. Ten minutes with a borescope settles it, and no damp course will do anything for it.
• Concealed plumbing. Damp that ignores the weather and the season. Isolate the water meter with everything off and watch whether it creeps.
• And, genuinely, rising damp — groundwater climbing the wall by capillary suction. Real, well characterised, and comparatively common in pre-DPC masonry and where a damp course has been buried or bridged. It is not the first thing on this list for a reason.

How to take it off without making it permanent

• Never acid-clean masonry with a damp history. Hydrochloric acid leaves chloride behind in the wall, and chloride is the most hygroscopic of the common salts.
• Never wash it off with water. Wetting dissolves the salt and drives it straight back into the pore structure, where it will crystallise again out of sight.
• Never seal or render over it. Water-repellent coatings and damp-proof paints reduce evaporation from the face, and height of rise varies inversely with the square root of the evaporation rate — so cutting evaporation makes the damp front climb. Modelling of a 215 mm wall rendered to 1.25 m has the front re-stabilising above the top of the render in roughly 425 days, which is precisely why this treatment appears to succeed before it fails.

1. Dry-brush, then vacuumStiff bristle and no water. Vacuum the residue rather than sweeping it into the garden bed at the base of the wall, which is where a good deal of it came from in the first place.
2. Leave a hard crust alone until it has been identifiedA deposit that will not brush off is either a lime run or a mature salt crust, and mechanical attack on either takes brick face with it.
3. Fix the water, then waitRemoval is cosmetic and the bloom returns until the source stops. Once it has, expect months rather than weeks before the wall is dry enough to judge or repaint — thorough drying is commonly 3 to 6 months, and up to 12 months on thick walls.

When white salt does mean rising damp, and when it does not

Pattern beats deposit. Genuine rising damp is continuous along the base of a wall, has a defined upper limit, and correlates with ground level, drainage and damp-course position. The normal Australian exterior limit is 1.0 to 1.5 m above ground, with the active evaporative zone typically between 0.5 and 1.2 m and very little evaporation below 0.3 m, because the air near the ground is more humid and slower moving.

Against that pattern: a vertical stripe traceable upward is falling damp from roof plumbing. Damp that is worst high on the wall is not rising anything. Blotchy patches with no line, worst in winter and overnight, on cold surfaces and in unheated rooms, showing black speckled mould rather than white bloom, is condensation — Archicentre's published figure has about 70% of mould problems coming from condensation and 30% from rising damp.

And then there is the case almost nobody tests for. A wall whose damp problem was genuinely cured years ago keeps every gram of salt it ever collected. Chlorides and nitrates pull water vapour straight out of the air, so the wall really does go damp on humid days and dry on dry days, blisters paint, reblooms after every repaint, and reads sopping wet on a resistance meter — with no liquid water source whatsoever. That wall needs salt removal, not a damp course, and the difference between those two scopes is the difference between a small job and a very large one.

None of that is decidable from the surface. The test that settles it is a vertical profile of small drilled samples taken at known heights and depths, sealed immediately, weighed, oven-dried and reweighed for total moisture content, then re-equilibrated in a 75% relative humidity atmosphere for hygroscopic moisture content. Subtract the second from the first and what remains is free capillary water. The Australian heritage guide states plainly that this is the only method that will distinguish moisture due to rising damp from moisture due to hygroscopic salts. If nobody drilled the wall, nobody diagnosed anything.

What we do about it, and what we do not

We are waterproofing and concrete repair specialists. We are not builders, and we perform exactly two services.

The first is a chemical damp-proof course injected into the mortar bed of external brickwork. A silane or siloxane is delivered into holes drilled along a mortar joint at roughly 120 mm centres, 10 to 15 mm in diameter, to within about 30 mm of the far face. The active diffuses through the pore structure and lines the pores with a water-repellent resin. It does not block the pores, so the wall still breathes and still dries. The second is a Hydropoxy negative treatment to internal concrete slabs — grind back, prepare the surface, then an epoxy moisture barrier applied over the top of the slab before new flooring goes down.

What a damp course will not do is the reason this page exists. It will not remove one gram of the salt already in the wall above it, and those salts go on cycling in and out of solution with the humidity and go on working on the masonry — the Australian heritage guide illustrates brickwork continuing to decay above a chemically injected damp course for exactly this reason. It will not resist water under hydrostatic pressure, so it is the wrong tool for a retaining wall or a basement. It will not fix a bridged cavity, a downpipe, a sprinkler or a failed shower.

The work that a salt-loaded wall needs after that — desalination by poulticing and washing, removal of salt-contaminated render, replastering in lime or lime-cement, repointing in weak lime — is masonry and conservation work. We do not do it. Nor do we do landscaping, drainage, plumbing, subfloor ventilation or wet-area waterproofing. Where those are the right answer, and on a page about white salt they very often are, we will say so and point you at the trade that does it.

The most common honest outcome for a wall with white salt on it is no damp course at all. Move the garden bed, clear the gutter, shift the dripper, and monitor. The government guide is explicit on the sequence: do not undertake insertion of any form of damp-proof course until the basic housekeeping measures have been completed and their effectiveness assessed over a period of at least a year.

What it costs

Questions we actually get asked

Is the white powder on my bricks dangerous?

Efflorescence is soluble salt left on a masonry surface after water evaporated. On its own it is a cosmetic deposit, not a hazard, and it does not need to be treated urgently. What it does mean is that water reached that point and evaporated there. The question worth answering is not whether the deposit is dangerous but whether it keeps coming back, because recurrence means the supply of water has not stopped.

Why does it come back every time it rains?

Because rain is the delivery mechanism. Water enters at a defect — a blocked gutter, a downpipe discharging at the footing, paving with no fall, saturated soil built up above the damp course — travels through the masonry, and evaporates at the face, leaving its dissolved salt behind. The bloom appears where the evaporation happens, which can be metres from where the water got in. Trace it back deliberately rather than treating the spot where the white is.

Can I pressure-wash it off, or use an efflorescence remover?

Dry removal only. Water redissolves the salt and drives it back into the pore structure, where it crystallises again out of sight — and salt crystallising inside the pores is the destructive version. Acid-based cleaners are worse again: hydrochloric acid used for a builders clean leaves chloride behind permanently, and chloride goes wet at about 75% relative humidity, which in this climate means a wall that cycles wet and dry several times a week indefinitely. Brush it dry, vacuum the residue, and put the money into finding the water.

My house is two years old and the render is blooming white. Do I need treatment?

Almost certainly not. Portland cement carries gypsum plus soluble sodium and potassium compounds, construction water mobilises them, and they deposit at evaporating surfaces. New brickwork and render bloom in the first year or two, wash off with rain, and progressively diminish. The direction is the test: fading year on year is normal, growing year on year is not. If the building is still under builder's warranty, raise it with the builder before engaging anyone.

There is no white salt on my wall at all, but the bricks are crumbling. Is salt still the cause?

Very likely, and this is the counter-intuitive result that matters most. Salt only forms a visible bloom when it crystallises in open air at the surface. Where evaporation is faster, the drying front sits inside the material and the salt crystallises within the pores instead, generating pressures capable of disrupting the strongest masonry. On an exposed external face, rain then washes away whatever surface evidence there was. A clean-looking wall with fretting joints and flaking brick faces can be in considerably worse condition than a wall covered in bloom.

What does the 0.5% salt threshold actually mean?

It is the working figure from the Australian heritage guide: more than about 0.5% salt by weight of a drilled sample is cause for concern and reason to consider desalination. It is a decision threshold for scope, not a pass or fail on the building. The exact damaging concentration varies with the salt species, the pore structure of the masonry and the cohesive strength of the material — a soft underfired brick fails at a lower salt load than a hard well-fired one. It should appear as a number in any competent report on a salt-affected wall.