Salt damp and rising damp — same thing, different state

One process, two names, different emphasis

Here is the mechanism both words are pointing at. Water in contact with the base of a wall is drawn upward through the connected pore network of brick, stone and mortar by capillary suction — the same effect that pulls a spill up a paper towel. It rises until the water arriving from below exactly balances the water evaporating from the wall face, which in Australia is normally somewhere between 1.0 and 1.5 m above ground, with the most active evaporation between about 0.5 and 1.2 m and very little below 0.3 m.

Water evaporates. Dissolved salt cannot. So the salt concentrates and deposits in a band at and just below the top of the wetted zone, and it accumulates there for as long as the water keeps arriving. On a 150 mm wall the flux is around 0.88 litres per day per metre of wall — roughly 320 litres a year per metre — so a 10 m run of affected wall is delivering on the order of 3,000 litres a year, every litre of which leaves its salt behind. That is the difference in emphasis in one number. The wetness is a transport mechanism. The salt is the payload.

And the salt is what does the destroying. Where evaporation happens at the surface, salt crystallises harmlessly in the open air as a white bloom — efflorescence, ugly and essentially cosmetic. Where evaporation is faster and the drying front sits inside the wall, salt crystallises within the pores instead, and the crystallisation pressure is enormous enough to disrupt the strongest masonry. That is sub-florescence, and it is what makes bricks fret and lose their fired outer skin, sandstone shed its hardened surface, and mortar turn to sand you can rub out with a thumb. A wall with no visible salt on it can be in worse condition than one covered in it, because rain washes the evidence away.

So there are four things a salt-damp problem needs at once: permeable masonry, available moisture, available soluble salts, and evaporation. Remove any one and decay stops. In practice you cannot fully remove any of them — sealing the masonry traps moisture behind the coating, you cannot eliminate atmospheric moisture, salts are everywhere, and preventing evaporation entirely means keeping the wall permanently saturated. Remediation is always about reducing several factors at once, which is why single-product answers to this problem tend to disappoint.

If you searched "salt damp" in Queensland, you are probably reading South Australian advice

This is worth knowing before you act on anything you have read this week, including this. The search term "salt damp" is heavily South Australian: the autocomplete around it runs to salt damp treatment Adelaide, salt damp repairs Adelaide and salt damp treatment Mount Gambier, with no Queensland modifiers appearing at all. The best available estimate puts something like 70–80% of Australian search volume for the term in South Australia, and that estimate is directional rather than measured.

Which means the pages ranking for it were written for a different climate, different soils and a different housing stock — commonly pre-1930 solid stone or brick with no damp course at all, in a city where hot drying summers and saline soils make this the national worst case. The advice on those pages is not wrong. It is calibrated for Adelaide.

Applied to a South East Queensland house it produces two predictable errors in opposite directions. It over-calls rising damp on houses whose real problem is condensation, a bridged damp course, a downpipe or a slab edge. And it under-calls the height at which genuine rising damp sits here, because our humidity suppresses evaporation and a suppressed evaporation rate raises the steady height. This is the only page on this site built on the words "salt damp", and that is deliberate.

Why Adelaide is the national worst case

Two things stack in South Australia. The climate drives hard evaporation from the wall face, and the soils are genuinely saline. Because the wall is a pump rather than a puddle, a high evaporation rate means a high throughput — more litres through the same wall every year — and because the groundwater is salty, each of those litres delivers more salt. The national heritage guide puts the outcome plainly: the result is much higher rates of decay in this country than in the UK, to the point where younger Australian buildings can be in worse condition than far older northern European ones.

It has been serious enough for long enough to leave a paper trail most of this industry does not mention. In South Australia in the 1960s and 1970s there were so many cases of failed damp treatments, and so many complaints to consumer affairs, that the state government established a Salt Damp Research Committee, which operated from 1974 to 1982. Adelaide City Council later co-published, with the Heritage Council of NSW, Heritage Victoria and the South Australian Department for Environment and Heritage, the technical guide that most of the numbers on this website come from.

There is one more piece of hard evidence for how seriously South Australia takes it, and it is in the building code rather than in a guide. Both South Australia and New South Wales vary the NCC to require an under-slab membrane with high impact resistance — a damp-proofing membrane rather than the medium-impact-resistance vapour barrier accepted elsewhere. That variation exists specifically because of salt damp risk in saline soils. AS 2870 also recommends damp-proofing membranes under slabs in areas prone to rising damp and salt attack.

What changes when the same physics runs in South East Queensland

The physics does not change. The inputs do, and they move in an interesting direction: humidity makes the damage slower and the damp higher at the same time.

Because the steady height of rise varies inversely with the square root of the evaporation rate, our high ambient humidity — which suppresses evaporation — raises the height the damp reaches. And because throughput is set by evaporation, the same suppression lowers the number of litres passing through the wall each year, which lowers the rate of salt delivery and therefore the rate of decay. So a South East Queensland wall with genuine rising damp typically decays more slowly than an equivalent Adelaide wall and may show its tide mark higher up.

The bigger change is in the differential diagnosis. Suppressed evaporation and high humidity push two other conditions much further up the list of what your damp probably is. Hygroscopic salt effects become more prominent, because salts pull moisture directly out of humid air — sodium chloride goes wet at about 75% relative humidity, which is an ordinary day here — so a wall with no live water source can stay damp indefinitely. And condensation becomes far more prominent, which matters because around 70% of mould problems trace to condensation rather than rising damp.

Then there is the housing stock, which works against rising damp more here than in Adelaide. High-set timber on stumps has no continuous capillary path from soil to wall at all. Brick veneer and slab-on-ground from the 1960s onward generally has an intact polyethylene damp course. Where we do find genuine rising damp in South East Queensland it is usually pre-1930 solid brick in the older inner suburbs, or a house of any age where the damp course has been buried or bridged by later paving, landscaping or render.

Which salt you have, and why anyone would pay to find out

"Salt damp" is a single term for what is in practice a mixture, and the mixture is diagnostically useful. Salt species points at source, and the humidity at which each salt goes wet tells you how often your wall is cycling — which is what actually drives decay, because each wetting and drying cycle enlarges the pores slightly and lets the next one do a little more damage.

• Sodium chloride, ordinary salt, goes wet at about 75% relative humidity and re-crystallises when the air dries. In South East Queensland that means a wall can cycle several times a week, which is a very large number of crystallisation cycles a year. It is also an excellent electrolyte, which is why it is the salt most responsible for impossible readings on a two-pin moisture meter.
• Calcium nitrate goes wet at roughly 47–55% relative humidity. There is effectively nowhere in an occupied Australian building that is drier than that, so a wall carrying it is wet more or less permanently.
• Magnesium chloride goes wet at about 33% and calcium chloride at about 29–32%. Below 33% is genuinely rare indoors in Australia, so these walls read damp on any instrument, forever, with no liquid water source whatsoever. If a wall defies every treatment and stays wet year-round, chloride speciation is the test to run.
• Mixtures go wet at a lower humidity than either constituent alone, and real walls always contain mixtures — whatever groundwater, cement, fertiliser, sea spray, sewage and cleaning chemicals have delivered over a century. Do not reason from a single textbook figure to a prediction about your wall.
• Sodium sulfate does its damage by swapping between an anhydrous form and a ten-water hydrate as humidity and temperature change, with a large volume difference between them. It is the prime suspect where sound bricks are splitting and delaminating rather than merely fretting at the surface, and its presence is a reason to check for a cement source.
• The species is a forensic tool. Heavy nitrate points at a leaking sewer, an over-fertilised garden bed, or a building that was once a stable. Heavy chloride points at groundwater, sea spray or a past acid clean. Magnesium sulfate points at contaminated bedding sand or dolomitic aggregate rather than at groundwater at all — the Australian guide documents a case where pyrite in quarry crusher fines used as paving sand oxidised, attacked the dolomite, and produced severe decay in the adjacent wall.

What the word changes about the fix, and what it does not

It changes nothing about the physics and quite a lot about the scope. If you have been told you have salt damp, the person telling you has usually put the emphasis in the right place — because salt is the part a damp-proof course does not touch, and it is the part that keeps costing money after the invoice is paid.

A correctly installed chemical damp-proof course forms a continuous water-repellent zone that capillary moisture cannot climb past. It does nothing about salt already in the wall above it. The Australian guide illustrates brickwork continuing to decay above a chemically injected damp course for exactly that reason, and it makes desalination its seventh key step rather than an optional extra: when salts abound, do not just insert a damp course — also remove excessive salts from above it. Injection can even make the short term worse, by displacing saline moisture upward into masonry that was not previously damaged, which is why a desalination poultice should be applied at the same time.

There is also a case where the word "salt damp" is the whole answer and "rising damp" is not. A wall reads high on a meter, feels damp in humid weather and dry in dry weather, and keeps blooming after every repaint — but gravimetric analysis shows negligible free water and high hygroscopic moisture content. The source was cured years ago and the salt stayed. That wall needs desalination and replastering in lime, and it does not need a damp course at all, because there is nothing rising. The test that separates the two is hygroscopic moisture content measured at 75% relative humidity against total moisture content, and it is the difference between a two thousand dollar job and a twenty-five thousand dollar one.

We should be plain about where our own scope stops. We inject chemical damp-proof courses into the mortar beds of external brickwork, and we treat internal concrete slabs from the top side with a Hydropoxy epoxy moisture barrier after grinding back and preparing the surface. We are waterproofing and concrete repair specialists, not builders. Desalination, poulticing, sacrificial lime renders, salt-retardant render replacement, saw-cut damp courses and undersetting are all real answers to real salt damp problems, and none of them are things we do. When your wall needs them we will say so before quoting, and point you at the masonry restoration and heritage trades that carry them out.

For interstate movers and anyone raised on the South Australian word

If you grew up in Adelaide, you know what salt damp looks like and you will recognise it faster than most Queensland homeowners will. Three adjustments are worth making before you apply that instinct to a house here.

The base rate is lower. A great deal of the South East Queensland stock is either raised on stumps with no capillary path at all, or built since the 1960s with an intact polyethylene damp course. The prior probability that a damp patch at the base of your wall is genuine capillary rise is meaningfully lower here than it was at home.

The height rule is different. Our humidity suppresses evaporation, which pushes the steady height up rather than down. A tide mark sitting higher than the Adelaide norm is not automatically evidence of something worse; equally, damp above about 1.5 m with no level top edge is still much more likely to be arriving from somewhere than rising from the ground.

And the shortlist of alternatives is different. In Adelaide the leading rival explanations are a buried damp course and hard cement render. Here, add condensation, a failed shower membrane in the next room, slab-edge moisture, and a subfloor whose vents have been rendered over or buried. Check those, check the ground level against the damp course with a shovel and a tape, and watch the downpipes in the next storm. In a surprising number of cases that is the entire investigation.

What it costs

Questions we actually get asked

Is salt damp the same as rising damp?

Effectively yes — the same physical process, described from a different end. Rising damp names the water and its direction of travel; salt damp names the salt that water leaves behind and the damage it causes. In technical writing the phrase "salt attack and rising damp" is often used to cover both, because in an old Australian wall you almost never get one without the other. The practical difference is which half of the problem the word makes you think about, and the salt half is the one most quotes leave out.

Do we get salt damp in Queensland?

Yes, but it is neither as common nor as severe as in South Australia, and the local vocabulary is "rising damp". Where we find it here it tends to be pre-1930 solid brick in the older suburbs, houses where later paving, landscaping or render has buried or bridged the damp course, and coastal or canal-side properties where sea-spray chloride adds to the load. Our humidity suppresses evaporation, which slows the rate of decay compared with Adelaide but raises the height the damp reaches, and makes hygroscopic salt and condensation much more prominent in working out what you actually have.

Is salt damp expensive to fix?

The salt is usually the expensive part, not the damp course. Indicative Australian ranges: chemical damp-proof course injection $120–$400 per linear metre, most commonly $180–$300; desalination by poulticing and captive-head washing $80–$250 per square metre, often $2,000–$10,000 per job; removing salt-contaminated render $40–$120 per square metre; replastering $80–$180 per square metre in a standard lime or lime-cement system and $200–$400 for heritage lime plaster. Correcting ground levels and drainage — which is frequently the highest-value money on the whole job — runs $500–$6,000 for levels and paving and $1,500–$12,000 for drainage works. These are indicative bands assembled from published cost guides, not quotes, and they vary widely by state and contractor.

What does "salt damp on a concrete floor" mean? Can it be injected?

It means moisture moving through or around a concrete slab and depositing salt at the surface, and no, there is nothing to inject. A slab is a single monolithic element sitting on the ground: no mortar joint to drill, no course to cut. The real causes are a missing, torn or badly terminated under-slab membrane, permeable or under-cured concrete, groundwater sitting against the slab edge because the site does not drain, landscaping added later that buries the slab edge, or simply construction moisture in a young slab. The correct test is in-situ relative humidity probe testing to ASTM F2170, referenced by AS 1884:2021, done at 40% of slab depth. You cannot retrofit an under-slab membrane, and anyone offering to inject your slab against rising damp is selling you something.

Is salt damp covered by insurance?

Generally not. Home building policies typically exclude gradual deterioration, and rising damp and salt attack are close to the textbook definition of gradual — this is a process measured in decades. That said, an insurer's wording is the only thing that governs your policy, so read your product disclosure statement and ask your insurer directly rather than relying on any contractor's summary, including this one. Where damp is the consequence of a sudden event, such as a burst pipe or a storm, the position can differ entirely.

How do I know whether my wall has enough salt to need desalination?

By testing a drilled sample rather than by looking at it, because the visible bloom is a poor guide — a wall covered in efflorescence may be in better condition than one where rain has washed the evidence off and the salt is crystallising inside the pores instead. The working threshold is about 0.5% salt by weight of the sample. A laboratory can do full speciation by ion chromatography and ICP-AES, which also tells you where the salt is coming from; a much cheaper conductivity-based total dissolved solids test plus sulfate and nitrate strips gives a usable total. Take samples at several heights and at several depth intervals, because salts are distributed unevenly and a single sample can mislead.