What rising damp actually is

Rising damp needs three things at once

Rising damp is not one process, it is three conditions holding hands. All three have to be present at the same time and in the same place. Remove any one of them and the movement stops, which is the entire logic behind every genuine remedy in this field.

The pores in masonry behave as an irregular network of capillaries. The finer the pore, the stronger the suction. Water is pulled into that network at the base of the wall, travels upward through the connected path, and leaves the wall as vapour from the face. The wall is not soaking like a sponge sitting in a bucket. It is a circuit, and the circuit has to be complete.

• Liquid water in contact with the base of the wall. Not humidity, not rain on the face — liquid water at the bottom. Groundwater, a garden bed watered daily, a path shedding water back against the brickwork, a leaking service.
• A continuous capillary path upward. Mortar joints and porous units connected all the way through. A functioning damp-proof course breaks this path, which is why the path is usually restored by something bridging the course rather than by the course failing.
• Evaporation from the wall face. Water has to be able to leave, or the wall simply saturates to a standstill. This is the condition almost every homeowner assumes is helpful, and it is the one that sets the height.

Why it stops at about a metre

It does not stop. It reaches equilibrium. At steady state the water arriving from below exactly equals the water leaving the face as vapour, and the top of the wetted zone sits at that balance point. That height is predictable from three measurable properties, and the relationship is the single most useful thing a homeowner can understand about this subject.

Height rises in direct proportion to how absorbent the material is. It rises with the square root of wall thickness, which means doubling the thickness does not double the height. And it rises inversely with the square root of the evaporation rate, which means anything that slows evaporation makes the damp climb. Notice what is not in that list: the depth or pressure of the groundwater below barely matters.

Gravity is a minor term, which surprises people who have decided the whole idea is impossible. Running the published model with and without gravitational drainage, cases with a no-gravity steady height of 500 mm and 1,000 mm come back at about 488 mm and 951 mm — a correction of 2 to 5 per cent. Capillary suction in fine masonry pores massively outweighs the hydrostatic head at these heights. Evaporation is the limit, not gravity.

In Australia the normal external limit is 1.0 to 1.5 metres above ground, with the active evaporative zone typically 0.5 to 1.2 metres and very little evaporation below 0.3 metres, because the air near the ground is more humid and moves more slowly. If the worst of the damp is well above that band, the water is arriving from somewhere other than the ground.

Your wall is a pump, and the water is only the delivery van

At equilibrium the wall is not a static wet object. It is moving water continuously. In the standard 150 mm example, that is roughly 0.88 litres per day per metre of wall length — about 320 litres per year for every metre. A 10 metre run of affected wall is therefore transporting on the order of 3,000 litres a year through the masonry. Mean residence time of a water molecule in the wall is around 21 days.

Every one of those litres arrives carrying dissolved salt and leaves as pure vapour. The salt stays. That flux, not the wetness itself, is what destroys masonry. Rising damp is best understood as a salt delivery mechanism that happens to use water as the courier.

It also gets worse at itself over time. Once enough salt has accumulated that the concentration in the wall exceeds that in the soil below, osmotic suction adds to capillary suction and draws more water toward the higher concentration. At the same time, dissolved salt depresses the vapour pressure at the evaporating surface, which reduces the evaporation rate — and reducing evaporation raises the height. Salt pulls harder and evaporates slower. An old salty wall is a fundamentally worse case than a young clean one with identical geometry.

Nothing about this happens quickly. Time to reach 95 per cent of the steady height from a dry start is about 31 days for the standard 150 mm example, and it is inversely proportional to evaporation rate — so behind a coating or in a poorly ventilated space it can be years. That cuts both ways. A newly created problem, such as a raised garden bed or a new path, may take one to three years to become visible. And a treatment declared a success at three months has proven nothing at all.

The salt is the part that breaks the building

Water alone does very little to a brick. Salt crystallising inside one does a great deal. This is the distinction that decides whether a wall is unsightly or structurally deteriorating, and almost nobody explains it before quoting.

When evaporation happens at the wall surface, salt crystallises in the open air as a white bloom. That is efflorescence, and on its own it is cosmetic. When evaporation is faster, the drying front sits inside the wall and salt crystallises within the pores instead. That is sub-florescence, also called crypto-florescence, and the pressure crystallising salt exerts is enough to disrupt the strongest masonry. It is what makes bricks fret, pops the harder fireskin off a fired brick in flakes, makes sandstone shed its case-hardened surface, and turns mortar into sand you can rub out with a thumb. The visible white bloom is the harmless version. The invisible one does the damage, and a wall with no visible salt can be in worse condition than one covered in it, because rain washes the evidence away.

Weak lime mortar goes first, and that is by design rather than by failure. It sacrifices itself to protect the bricks around it. Mortar loss of around 50 mm across five to ten courses puts the brickwork at risk of local collapse, which is the point at which the timeline stops being leisurely.

The decay curve is exponential, not linear. Salt accumulates for decades with almost no visible damage while it slowly fills the pore space, and once the pores are loaded, decay accelerates sharply. The Australian heritage guide's notional curve has roughly 80 years of near-nothing followed by rapid acceleration, such that a 100-year-old building may be twice as damaged after only another 10. The optimistic corollary is real: reverse the decay and remove the salt and you reset the clock back down the curve. More than about 0.5 per cent salt by weight of sample is the working threshold at which desalination should be on the table.

Four things have to be present for salt attack: permeable masonry, available moisture, available soluble salts, and evaporation. Remove any one and decay stops. In practice none can be fully removed, which is why real remediation reduces several factors rather than eliminating one.

What a damp-proof course is, and when Australian houses started getting them

A damp-proof course is a waterproof layer built into the base of a wall, above ground level, to break the capillary path. Modern ones are commonly 0.5 mm embossed black polyethylene. The ABCB Housing Provisions list the acceptable materials — material complying with AS/NZS 2904, embossed polyethylene film, polyethylene-coated metal with an aluminium core of at least 0.1 mm, bitumen-impregnated materials not less than 2.5 mm thick, and termite sheet materials.

Position is codified. The course must sit not less than 150 mm above adjacent ground level; 75 mm above finished paving, concrete or landscaping that slopes away from the wall; 50 mm where protected by a carport, verandah or similar; and in low rainfall intensity areas 15 mm, or 0 mm where protected from the weather. Where a remedial course is installed, the heritage guidance recommends setting it 150 to 250 mm above finished ground, with 200 mm as the ideal, and maintaining that clearance afterwards.

Now the honest answer to the question people actually ask. There is no single date on which Australian houses started getting them. Nineteenth-century masonry frequently has none at all. A great deal of early twentieth-century work has a tar-and-sand course that is still doing something. The changeover was gradual and varied by state, by era and by builder, and any page that gives you a clean national year is guessing. The only reliable way to know what your building has is to excavate a small inspection pit at the base of the wall and look, then record the type, position and condition. Do not drill into and impregnate an old tar-and-sand course — it may be underperforming, but perforating it will not help.

Far more often than a course fails, something bridges it. Render or plaster carried down across it. Hard cement pointing over it. A garden bed that has crept up as mulch was added. A path, driveway or new concrete floor abutting it. A bell-cast that has been filled in. The tell for a bridged course is damp that is continuous along the rendered run and stops where the render stops, and it is one of the most common genuine causes of damp in houses that already have a perfectly good course.

Why Australia is not Britain, and why Brisbane is not Adelaide

Our building tradition is British. Our climate is not, and that changes the arithmetic in a way most of the search results for this question never mention.

Hotter, drier conditions drive much higher evaporation, and higher evaporation means much higher moisture throughput through the same wall. More litres per year, more salt delivered per year, faster decay. Combine that with genuinely saline Australian soils and younger Australian buildings can be in worse condition than far older northern European ones. The national heritage guide's phrasing is blunt: much higher rates of decay in this country than in the UK.

That is also why importing the British argument wholesale is a diagnostic error in its own direction. The British debate has spent two decades correctly attacking over-diagnosis, and it is right about the instruments and right about the commercial structure. It is not a reason to assume the physics stops at the equator.

Within Australia the picture varies just as much. Adelaide is the national worst case, with hot drying summers and very salty soils. Sydney's more humid climate and lower salt levels give slower decay. Humid subtropical South East Queensland behaves differently again: high ambient relative humidity suppresses evaporation, which lowers the rate of decay but raises the height of rise, and makes hygroscopic salt effects and condensation far more prominent in the differential diagnosis.

There is a housing-stock consequence to that, and it is the reason so much Queensland damp copy reads like it was written in Adelaide. A large share of South East Queensland is high-set timber on stumps, post-war brick veneer, and post-1980 slab-on-ground. In that stock the dominant moisture problems are subfloor humidity and ventilation, condensation, slab edge dampness and leaks — not capillary rise into living-area masonry. The genuine rising damp cases here are concentrated in older solid and cavity brickwork, and in buildings where the damp course has been buried or bridged.

What the real thing actually looks like

Pattern and geometry are more diagnostic than any instrument. Before anybody drills anything, the shape of the problem has already narrowed the field considerably.

Genuine rising damp is continuous along the base of a wall rather than appearing in isolated patches. It has a defined upper limit. The upper edge is fairly level and runs across the wall regardless of where the furniture, the fittings or the cold corners are. It correlates with ground level, damp-course position and site drainage. It is present all year rather than tracking last night's weather, though a long dry spell can mask it. And the mortar deteriorates before the bricks do.

The visible line is usually a salt line, not a water line. Water carries dissolved salt up, the water evaporates, the salt cannot, so it concentrates and deposits in a band at and just below the top of the wetted zone. That band is where you see the stain, the blistered paint, the bloom and the crumbling. It is a record of where evaporation has been happening, accumulated over years, which is why it often sits slightly below the true damp front and why it survives long after the wall has dried out.

• Continuous along the base, not patchy — patchy with no top edge points elsewhere.
• A fairly level upper edge, typically 1.0 to 1.5 metres, with the busiest zone between 0.5 and 1.2 metres.
• Worst where evaporation is easiest, and very little activity below about 0.3 metres.
• Correlates with external ground level and with where the damp course sits, or should sit.
• Salt bloom, blistered coatings, drummy render and mortar going to sand within that band.
• Present through the year rather than appearing after cold nights or after last week's storm.
• Worse high on the wall than low means water is arriving from above, not rising from below.

The things that get called rising damp and are not

Every item below presents at the base of a wall, in a band, sometimes with salt. Several of them are cheaper to fix by an order of magnitude, and most of them are somebody else's trade. Any competent investigation considers and excludes each one explicitly, in writing.

• Bridged damp course. The course is intact but render, plaster or hard pointing runs continuously across it. The damp is continuous along the rendered run and stops where the render stops. Cut the render back and form a clean drip — do not inject a second course above a perfectly good one.
• Raised external ground. Paving, concrete paths, driveways, garden beds and mulch sitting at or above the course. This is the single most common cause of genuine rising damp in Australian houses, and garden beds are the worst case because the soil creeps up, the fertiliser supplies nitrate, and the watering washes both into the wall.
• Bridged cavity. Mortar droppings on wall ties or at the cavity base, blocked weepholes, missing flashing. Damp patches that are often higher than the damp course, sometimes at individual tie positions. Ten minutes with a borescope settles it, and injection does nothing for it.
• Falling damp from gutters, downpipes, rainwater heads and flashings. A vertical stripe or localised patch, traceable upward, and often worst high on the wall. The salt appears where the water evaporates, which may be well away from where it entered.
• Air-conditioning condensate lines and irrigation. Localised and often seasonal. A condensate line discharging at the base of a wall, or a sprinkler wetting brickwork, is a Saturday's work to fix.
• Concealed plumbing and sewer leaks. Damp that does not correlate with ground level, weather or season, and a water meter that creeps with everything turned off. High nitrate on a salt analysis is a strong pointer to a leaking sewer.
• Condensation. Worst in winter, worst overnight, on cold surfaces, affecting the tops of walls and ceilings as often as the bottoms, with black spot mould rather than salt bloom. Tracks how the house is used.
• Failed shower and wet-area waterproofing. Presents as damp at the base of the wall in the room next to the bathroom, with blistered paint, lifting skirting boards and drummy tiles. Licensed waterproofing work in Queensland, to AS 3740:2021.
• Lateral damp from retaining walls, cut-and-fill sites and basements. Ground higher on one side than the floor on the other. The diagnostic signature is a wall that is wetter above the damp course than below it. Chemical injection cannot resist a hydrostatic head.
• Slab edge dampness in slab-on-ground construction. Efflorescence and drummy render below the damp course, damp carpet edges, tile bond failure and rust staining near the slab edge, very often following post-construction landscaping.
• Residual hygroscopic salt from a problem that was fixed years ago. The wall reads high on a meter, feels damp in humid weather and dry in dry weather, and blooms again after every repaint — but gravimetric analysis shows negligible free water. The source is gone. The salt stayed.
• Construction moisture in a building less than one to two years old, with readings that decline steadily over successive tests and a harmless bloom that washes off and does not return.

How it is diagnosed properly

Every credible authority — BS 6576, BRE Digest 245, the Australian heritage guide, and the 2022 joint position statement signed by the RICS, Historic England and the damp industry's own trade body — agrees on one thing. You cannot diagnose rising damp from the surface. If nobody drilled the wall, nobody diagnosed anything.

1. Look at the whole building, and look twiceInside, outside, subfloor, roof, stormwater, external levels, paving, garden beds, wall cavities. Ideally on two visits with different weather, because rain washes surface salts away and a single wet-weather visit under-reads the problem. Rendered and painted walls conceal the pattern. The investigation may appropriately be commissioned under AS 4349.0 as an inspection of particular technical aspects.
2. Measure the ground against the damp courseNot eyeballed — measured, and recorded. Excavate a small inspection pit where necessary to establish whether a course exists, what type it is, what condition it is in, and how far above or below the adjacent ground it sits.
3. Drill a vertical profile, not a spotSix to eight samples per line, at known heights, taken past the last visible sign of damp, and at known depths — commonly 0 to 10, 10 to 20 and 20 to 40 mm. Sealed immediately in airtight containers. Slow-speed drilling, so the heat does not drive the moisture off before it is weighed. Sample mortar in preference to face brick or stone, unless the mortar is much less permeable than the units, in which case the salts concentrate in the units and those must be sampled instead.
4. Oven-dry them and plot the gradientWeigh wet, oven-dry, reweigh. The loss of mass is the true moisture content by dry weight. It is the shape of the gradient — moisture decreasing with height — that is the actual evidence for rising damp, not any single number.
5. Then run the test almost nobody runsEquilibrate the oven-dried samples at 75 per cent relative humidity and reweigh. The mass gained is the moisture the salt content will hold from the air alone. Subtract that hygroscopic moisture content from the total, and what remains is free capillary water. This is the only method that distinguishes moisture due to rising damp from moisture due to hygroscopic salts, and it is the difference between a wall that needs a damp course and a wall that needs salt removed.
6. Speciate the salt and quantify itIon chromatography and ICP-AES, or at minimum total dissolved solids by conductivity plus sulfate and nitrate test strips. More than about 0.5 per cent by weight is cause for concern and reason to consider desalination. The species points at the source: nitrate at a leaking sewer, fertiliser or a former animal use; chloride at groundwater, sea spray or acid cleaning; magnesium sulfate at contaminated sand or aggregate.
7. Exclude the plumbing and the drainage on paperIsolate the water meter and watch for movement. Flood-test showers and balconies. Run the downpipes and hose to check falls and stormwater discharge. Dye or CCTV the waste lines where indicated. Inspect the subfloor and the cavity with a borescope. Negative results are only as good as the test duration, and that should be stated.
8. Say what is proven and what is inferredA defensible report separates the two, records weather and site conditions at inspection, names every instrument and its limitations, sets out a staged remediation sequence with the prerequisites first, includes a monitoring plan, and photographs sample locations precisely enough that somebody can re-test nearby later and prove whether anything changed.

What we do about it, and what we hand to someone else

We are waterproofing and concrete repair specialists. We are not builders, and we perform exactly two services relevant to this subject. Everything else on this page that needs doing is done by somebody else, and we will tell you who.

The first is a chemical damp-proof course injected into the mortar bed of external brickwork. Holes are drilled roughly every 120 mm along a mortar joint, typically 10 to 15 mm in diameter, to within about 30 mm of the far face. A silane or siloxane cream or fluid is introduced at low pressure or by gravity feed. The active reacts with the substrate and lines the pores with a water-repellent silicone resin, forming a continuous zone that capillary moisture cannot climb past. It does not block the pores, so the wall still breathes and dries. Injection pressure above about 1,000 kPa is a defect rather than a feature, because the fluid advances as fingers and leaves untreated gaps between them.

Two limits on that work, stated up front. A chemical course provides a barrier to rising damp and nothing else — it does not remove the salt already in the wall above it, which will keep cycling in and out of solution and keep causing damage until it is removed by somebody who does desalination and replastering. And on a 230 mm wall where a stretcher course is drilled, the masonry on the other face has to be treated too. Where that means internal work, it is outside our scope and we will say so before anyone signs anything.

The second is a hydropoxy negative treatment to internal concrete slabs: grinding back, surface preparation, and an epoxy moisture barrier applied to the top of the slab before new flooring goes down. That controls moisture at the flooring interface, which is where slab moisture usually announces itself. It is not a damp-proof course, and it is not a retrofitted under-slab membrane, because there is no such thing — a membrane cannot be installed beneath a slab that has already been poured.

We do not do salt-retardant or lime replastering, physical damp courses by slot saw or undersetting, undercutting, landscaping, drainage works, plumbing, subfloor ventilation installation, or wet-area waterproofing. Those are the right answer more often than our two services are, and when they are, that is what the report will say.

What it costs

Questions we actually get asked

Does rising damp go away on its own?

The movement stops as soon as any one of the three conditions is broken, and that happens more often than people expect — a gutter gets fixed, a garden bed gets pulled back, a path gets re-graded, and the supply of liquid water at the base is gone. The wall then has to dry, which takes months rather than weeks: typically 3 to 6 months after replastering, and up to 12 months for wet thick walls in cooler damp conditions. What does not go away on its own is the salt already deposited in the masonry, which is why a wall can stop rising and keep deteriorating.

How high can it actually get?

In Australia the usual external limit is 1.0 to 1.5 metres, with most of the evaporation happening between 0.5 and 1.2 metres. Thicker walls go higher, but only with the square root of thickness — doubling a 150 mm wall to 300 mm takes the modelled height from 0.61 m to 0.87 m. Suppressing evaporation raises it much faster: cut the evaporation rate to a quarter and the same wall goes to about 1.2 m. The extreme documented case is the Church of San Bernardo in Rome, where a 4 metre thick wall shows 5.3 metres of rise. If the worst of the damp in your house is well above head height, something is delivering water from above.

My house is on a slab and was built in 1998. Can I have rising damp?

You can have slab dampness, but the mechanism and the treatments are different. In masonry, water wicks up through mortar joints and brick pores and a barrier can be introduced partway up. A slab is a single monolithic element sitting on the ground, so there is nothing to inject and no course to cut. Slab moisture comes from a missing, torn or badly terminated under-slab membrane, unusually permeable or under-cured concrete, ground water sitting at the slab edge, or landscaping added later that buries the edge. The correct measurement is an in-situ relative humidity probe test to ASTM F2170, drilled to 40 per cent of slab depth, referenced by AS 1884:2021. Anyone offering to inject your slab against rising damp is selling you something.

Why does my wall read wet on a meter but feel dry to touch?

Almost certainly hygroscopic salt. Resistance meters measure electrical conductivity, and salt is an excellent electrolyte, so a dry but salt-loaded wall reads as high as or higher than a genuinely wet clean one. Readings above 100 per cent moisture content are common on salty masonry and are physically impossible. The way to settle it is to drill samples, oven-dry them for total moisture content, then equilibrate them at 75 per cent relative humidity to measure how much of that moisture the salt was holding from the air. High hygroscopic moisture with negligible free water means there is nothing rising and a damp course would achieve nothing.

Does it matter that the mortar is going before the bricks?

It matters, and mostly in a good way. Weak lime mortar is meant to be the sacrificial component — it decays instead of the bricks and poultices salt away from them. What changes the timeline is the amount lost. Mortar loss of around 50 mm across five to ten courses is a local collapse risk in the brickwork above and needs assessment now rather than next year. When it is repointed, it must not be repointed in hard cement: that drives the damp further up the wall and can damage the bricks. There is also a trap on the other side — extensive repointing after a chemical course has been installed can bridge the treated zone, so the work may need re-treating once the new mortar has cured.

Is there something cheap I should try before paying anyone for treatment?

Yes, and the national heritage guide sequences it for you. Go outside in heavy rain and watch what your gutters, downpipes and ground actually do, then fix the leaks and overflows. Check whether soil, mulch, paving or render is burying or bridging the damp course, and excavate a small pit if you cannot see. Pull garden beds back to leave a clear strip at least 300 mm wide, preferably over 500 mm, surfaced in coarse gravel, and move sprinklers to drippers kept at least 500 mm off the wall. Grade the first metre to fall away, with the low point 1.5 to 2.0 metres out — AS 2870 and the NCC require a minimum fall of 50 mm over the first metre for slabs. Clear subfloor vents. Then monitor for a year before anyone inserts a damp course, which is precisely what the guide instructs.

Is rising damp urgent?

Not in the way it is usually sold. Rising damp is a process measured in decades, and a wall that has been doing this since 1890 is not going to change its mind this month. The honest counterpoint is that the decay curve accelerates once the pore space is loaded with salt, so a building that has looked much the same for a century can deteriorate noticeably in the following ten years — and structural mortar loss is the specific thing that changes the timeline from monitoring to acting. Anyone using the words emergency, immediately or before the damage spreads about a masonry wall is describing a sales cycle, not a building.

Who should look at it, and what should the report contain?

Someone independent of whoever would perform the remedial work. The heritage guide recommends this explicitly, to avoid bias toward a particular commercial treatment, and it is the single most valuable thing a homeowner can do here. The report should record weather and site conditions, external ground and paving levels measured relative to the damp course, the location, type and condition of any existing course, a full drainage and plumbing review, subfloor and cavity inspection, moisture mapping with each instrument and its limitations named, a vertical profile of drilled samples with both total and hygroscopic moisture content, salt type and percentage where salt attack is suspected, explicit consideration and exclusion of each alternative cause, a clear separation of proven from inferred, a staged remediation plan with prerequisites first, and a monitoring plan. If it arrives as a one-page quote with meter readings and a price, it is a sales document.