Three problems that look identical and aren't

Six questions that separate the three

Work down the left column and answer honestly. You are looking for a pattern across all six rows rather than a single match — any one of these problems can borrow one feature from another, but none of them can borrow all six.

One caution before you start. Two of these three can be present in the same house at the same time, and on this coast they very often are, because a wall that is genuinely damp low down and a room that runs cold and humid overnight are not mutually exclusive. The table tells you which one to spend money on first.

Condensation: most of the mould, and almost none of the invoice

Air holds water, and how much it can hold depends on temperature. When a surface is colder than the dew point of the air touching it, water comes out of the air onto that surface. That is not a theory about your house, it is the definition of dew point, and it is why the bathroom mirror fogs.

In a house, the coldest surfaces are external corners, uninsulated masonry, window reveals, and any wall with a wardrobe or a lounge pushed hard against it — because furniture blocks both the warmth and the air movement. Floor level is also colder than head height. That is exactly why condensation so often presents as damp along the bottom of a wall, with blistered paint, bulging plasterboard, decaying skirting boards and black speckling, which is also, more or less word for word, how rising damp is usually described.

The distinguishing feature is that condensation tracks people, not ground. It is worse in the rooms that are shut up, worse in winter and overnight, worse after a shower without extraction, worse where washing is dried indoors, and worse where an unflued gas heater is running, because unflued combustion puts its water straight into the room. Open the house up on a dry mid-morning and the pattern changes. Rising damp does not care whether you dried the washing inside.

There is a version of this that happens inside the masonry rather than on it. As a wall warms, it exhales air from its pores; as it cools, it inhales. If the incoming air is humid and the masonry drops below dew point, water condenses within the pores. Where hygroscopic salts are present in the wall, that inhaled moisture is captured and retained. This is how a wall with a perfectly good damp course and no liquid water source whatsoever can read wet on any instrument, stay wet, and continue to decay. Any coating that stops the wall breathing shortens its life.

South East Queensland loads the dice, and it loads them in both directions at once. High ambient relative humidity suppresses evaporation from wall faces, which lowers the rate of salt decay but raises the height of rise — so where there is a live source here, the damp climbs further than the textbooks from drier states would predict. The same humidity makes hygroscopic salt effects and condensation far more prominent in the differential diagnosis than they are in Adelaide. Queensland copy that reads like Adelaide copy is describing somebody else's climate, and the coastal strip is not even the same problem as the hinterland.

On health, one number and no adjectives. Persistently damp surfaces support mould growth, and the accepted control threshold is keeping surface relative humidity below about 80 per cent over month-long periods. That is the target. Everything past that number is somebody's adjective.

The test you can run this week

The definitive instrument for condensation is a data logger recording air temperature, relative humidity and wall surface temperature over at least a week including cold nights — a fortnight to a month is more realistic, and seasonal behaviour may need a full year. You do not need to hire one to get a usable answer. You need two readings and a table.

What you are testing is a single proposition: is the wall surface colder than the dew point of the air in the room. If it repeatedly is, water is condensing on it, whatever else is also going on in the wall.

1. Get two numbers, not oneA cheap hygrometer that reads room air temperature and relative humidity, and an infrared thermometer to read the surface temperature of the wall. Both are inexpensive. A moisture meter cannot answer this question at all, because dew point is a comparison between two temperatures and a meter measures neither of them.
2. Read at the worst timeFirst thing in the morning, in the affected room, before opening any windows or doors. Overnight is when surfaces are coldest and when the room air has had all night to load up with moisture from breathing, cooking and drying.
3. Read the damp spot and a reference spotTake the surface temperature at the damp patch and again on an internal wall in the same room, away from the corner. The difference between the two is the cold-bridge effect that is doing the damage.
4. Look up the dew pointUse the table below. Find your room air temperature down the left and your relative humidity across the top. If the wall surface temperature is at or below the figure in that box, water is condensing on that wall. This is arithmetic, not an opinion.
5. Repeat for at least a weekOne morning proves nothing. Seven to fourteen consecutive mornings, including the coldest nights available, gives you a pattern. Write the numbers down with the date — an undated reading tells you about one morning and nothing about a building.
6. Then change exactly one thingRun the extract fan for twenty minutes after every shower. Stop drying washing indoors. Pull the wardrobe 50 mm off the external wall. Open the house up mid-morning when the outside air is drier than the inside air. Change one variable at a time and keep reading. If the numbers move, you have your answer for the price of a hygrometer.
7. Know what would not changeGenuine rising damp is indifferent to all of the above. If the band along the base of the wall is still there in February, at the same height, with the same level upper edge and the same salt bloom, after a fortnight of aggressive ventilation, condensation is not the thing you are looking at — or at least not the only thing.

Efflorescence is a symptom, not a diagnosis

The salt coming out of your bricks arrived there in solution. Water carried it into the masonry, the water evaporated at the surface, and the salt could not follow it into the air, so it crystallised there in the open. That is efflorescence. It is not a health hazard and on its own it is cosmetic. The question worth asking is never what it is. The question is whether it keeps coming back.

On new brickwork it is normal. Portland cement contains soluble sodium and potassium compounds, and construction water mobilises them to the evaporating face. The bloom appears in the first year or two, washes off with rain and progressively diminishes. Left alone, it fades. Dry-brush it if it bothers you.

Bloom that returns season after season is different. Recurring efflorescence means water is still arriving and still evaporating at that point, which means there is a live source to find. Cleaning it without finding the source guarantees it comes back, which is why the cleaning aisle of the hardware store is not, in the end, a treatment.

There is a version of this that does the actual damage, and it is invisible. When evaporation is faster, the drying front sits inside the wall rather than on it, and salt crystallises within the pores. That is sub-florescence, or crypto-florescence, and the crystallisation pressure is enough to disrupt the strongest masonry. It is what makes bricks fret and lose their fired outer skin, sandstone shed its hardened surface, and mortar turn to sand. So the white bloom you can see is the harmless one, the one you cannot see is the destructive one, and a wall with no visible salt at all can be in worse condition than a wall covered in it — rain simply washes the evidence away.

Two hard rules. Never acid-clean efflorescence: acid cleaning introduces chloride into the masonry, chloride does not come out, and a cosmetic problem becomes a permanent one. That rule matters more on this coast than most places, because marine aerosol and saline groundwater have already contributed chloride to a lot of coastal brickwork and there is no sense adding to it. And where salt is measured at more than about 0.5 per cent by weight, desalination belongs in the scope of works rather than the optional extras.

When it really is the first one

Often, here. Rising damp gets written about in Australia as a rare misdiagnosis, and on a dry inland block with a deep water table that is fair enough. The Gold Coast and the low-lying coastal strip are a different building stock on different ground. Water tables sit shallow through the canal and reclaimed estates, that groundwater is frequently saline, marine aerosol drives chloride into masonry from the other direction, rainfall arrives subtropically rather than politely, and a large stock of older brickwork has had paths, slabs, driveways and garden beds built up against it over forty years until the damp course is buried. Every one of those is a mechanism for water to get into the base of a wall and stay there. Rising damp here is common, not exotic, and a page that told you otherwise would be lying to you in the other direction.

The signature set is specific. A continuous band along the base of the wall rather than isolated patches. A fairly level upper edge, typically between 1.0 and 1.5 metres. Present through the year rather than after cold nights. Correlating with external ground level and with where the damp course sits. Salt bloom and mortar deterioration within that band, with the mortar going before the bricks. Inside and outside on the same wall, on the lowest floor only.

That pattern is readable, and reading it properly is a real diagnosis. An experienced inspector looks at where the ground sits relative to the damp course, where the water goes when it rains, whether the gutters and downpipes discharge away from the wall or into it, whether the garden bed is above the course, whether the subfloor vents are buried or rendered over, how high the damp climbs and whether its upper edge is level, where the salt band sits and what the mortar is doing. A moisture meter comes out where it helps to compare one part of the wall with another. That combination — ground, drainage, height, shape, salt — is how this is actually diagnosed on a normal job, and it is what a licensed waterproofer do on the site visit. The other half of the same skill is being willing to say it is not rising damp at all, and to point at the downpipe instead.

Sampling is a different exercise, and it is worth knowing what it involves so you can tell when it is warranted. A full laboratory investigation drills masonry samples at intervals up the wall, oven-dries and reweighs each one to get moisture content by weight, then re-equilibrates the samples at 75 per cent relative humidity to measure hygroscopic moisture content separately, and identifies which salts are present and in what proportion. Subtracting the hygroscopic component from the total is what separates free capillary water from moisture that salt is simply holding out of the air. You do not need that on a wall where the ground is 200 mm over the damp course and the downpipe discharges into the garden bed. You do need it where the answer has to survive somebody else's scrutiny — a building dispute, an insurance claim, expert evidence, a contested pre-purchase decision, or a wall where competent people have looked at it and disagreed. That is investigation work rather than repair work, and it is what a forensic waterproofing consultancy such as an investigation-only consultancy exists to do; they investigate and report, and they do not carry out the rectification, which is exactly why their findings carry weight in a dispute.

Which brings up the fourth possibility that belongs on a page about lookalikes: the wall where the source is already gone. High hygroscopic moisture content, negligible free water, a meter that screams, bloom that returns after every repaint, damp on humid days and dry on dry days. The leak was fixed in 2019. The salt stayed, and the salt is hygroscopic — common salt goes wet at around 75 per cent relative humidity, sodium nitrate in the low-to-mid 70s, calcium nitrate at 47 to 55 per cent, magnesium chloride around 33 per cent and calcium chloride at 29 to 32 per cent. Below about 33 per cent relative humidity is genuinely rare in an occupied Australian building, and on this coast it essentially never happens, so a wall carrying chlorides is permanently in the wet phase. It will read damp on any instrument, forever, with no liquid source at all. That wall needs salt removed and replastering in lime, not a damp course. It is also the wall where sampling earns its money, because that particular mistake is expensive in both directions.

The fourth answer: a leak with an alibi

The three-way comparison is a useful frame, but there is a whole fourth family — water arriving from a specific defect — and two of them impersonate rising damp so well that they are routinely treated as it.

Failed wet-area waterproofing presents as damp at the base of the wall in the room next to the bathroom, not in the bathroom itself. Blistered paint and lifting skirting boards in the hallway, drummy or loose wall tiles, damp that has no relationship to the ground outside. It is remedied by removing and reinstating the membrane to AS 3740:2021, which is licensed waterproofing work in Queensland and runs $3,000 to $15,000 per wet area. A regrout and reseal usually buys a year.

Slab edge dampness is the other. Persistent damp on the exposed slab or footing edge, efflorescence and drummy render below the damp course, external paint blistering below the damp course, damp carpet edges, pungent odours, tile bond failure near external walls and rust staining on metal near the slab edge. It very often follows landscaping added after construction that buried the edge — which, on the Gold Coast, is most of the landscaping. It is a drainage, membrane-termination and concrete-quality problem, and injecting brickwork does not address it.

Then the cheap ones, which are worth ruling out before anything else. A blocked or leaking gutter, downpipe or rainwater head produces damp that is worst high on the wall and traceable upward, with the salt appearing wherever the water finally evaporates, which may be well away from where it entered — $400 to $6,000 to put right. An air-conditioning condensate line discharging at the base of a wall. An irrigation dripper 100 mm from the brickwork. A concealed supply or sewer leak, which shows up as damp with no relationship to weather or season and a water meter that creeps with every tap shut.

Causation is established by exclusion, and exclusion takes time rather than instruments. Isolate the water meter and watch it. Flood-test the shower. Run the downpipes and hose the ground to check the falls. Dye or CCTV the waste lines. Put a borescope into the cavity to look for mortar droppings on the wall ties and blocked weepholes. Dig a small pit and find out whether the damp course exists and whether the ground is sitting over it.

Why the cheap answer gets mistaken for the dear one

No company is named here. This is about why two unrelated problems produce a wall that looks the same, and why the mistake tends to run in one particular direction.

Start with the instrument, because most damp conversations start with one. A two-pin electrical resistance moisture meter measures conductivity between two pins and is calibrated for timber. Soluble salts are electrolytes, so on masonry the reading responds to salt as much as to water, and a dry but salty wall can read as high as a genuinely wet clean one. On salt-loaded walls it is common to get readings above 100 per cent moisture content, a figure that leaves no room for the masonry itself. The Australian heritage guide, jointly published by three state governments, puts it plainly: moisture meters should not be used as the sole basis for diagnosing a damp problem, or on their own to prove that a wall is unacceptably damp. That is not a reason to throw the meter away. It is the reason it belongs alongside ground levels, drainage and the shape of the damp band rather than in front of them.

Then the weather, which is the real trap in a subtropical climate. Inspect after a week of heavy rain and every wall in the street reads worse. Declare success after a dry spell and nothing has been demonstrated, because a long dry period can produce an incorrect assessment that the damp has been controlled. This cuts both ways and it is why the date and the recent weather belong on every measurement, along with a second look at a different time of year when the answer matters.

Then the shape of the cure, which is where the money is. A chemical damp course stops water rising through the treated zone. It does not remove the salt already in the wall above it, it does not raise a buried garden bed, it does not redirect a downpipe, and it has no effect whatsoever on condensation. So the mistake worth being genuinely careful about is not that somebody misses rising damp — around here it tends to be there — but that a wall is injected while the actual water is coming from a bathroom membrane, a rainwater head or the air in the room, and nothing changes.

One point about position, because it decides how the rest of this page should be read. This site does no damp work of any kind, holds no building licence and sells nothing, so no answer on this page is worth more to it than any other. Of the causes set out below, two are work for a licensed waterproofer. Most are not, and the visit that ends with somebody being told to fix the downpipe is the visit that did its job.

What would change these answers

No comparison table survives contact with every building. These are the observations that should make you re-open the question, whichever answer you had arrived at.

• The damp band has no level upper edge. Whatever the meter says, that is not the signature of capillary rise.
• The worst of it is above about 1.5 metres. Look up — gutters, flashings, a rainwater head, a balcony above.
• It appeared within one to three years of landscaping, paving, a new path or a renovation. Suspect a bridged damp course first; the time lag between creating the problem and seeing it is measured in years.
• The building is less than two years old. Construction moisture, with readings that decline steadily across successive tests, and a bloom that washes off and does not return.
• Sampling has been done and shows negligible free water with high hygroscopic moisture content. The source has already gone. Salt is the remaining problem, and a damp course would be treating nothing.
• It is worse in humid summer weather and there is a clear 150 mm or more of bare brick above the ground. That combination points at hygroscopic salt rather than a live source.
• The wall is wetter above the damp course than below it. That is the diagnostic signature of lateral damp from higher ground on the other side, and chemical injection cannot resist a hydrostatic head.
• It only shows up in the room adjoining a bathroom, shower or laundry. Flood-test the wet area before anyone drills brickwork.
• It shows up on a slab-on-ground floor rather than in masonry. There is nothing to inject and no course to cut; the reliable measurement is an in-situ relative humidity probe to ASTM F2170 at 40 per cent of slab depth.
• The house is on a canal, a filled block or a low-lying reclaimed estate and the damp is at the base, level-edged and year-round. That is the local pattern, and it deserves a proper look at ground levels and drainage rather than a shrug about humidity.

Who fixes which one, including when it isn't a licensed waterproofer

Nine likely answers, and the honest allocation of each. Two rows on this table are a waterproofer's work. The other seven are not, and where the evidence points at them, this table names the trade that owns them.

Where it does turn out to be one of the two, the work belongs to a licensed waterproofer, who attends, diagnoses what is actually causing it, and carries out the remedial work. This site researches and publishes; the attending, pricing and doing sit with the trade.

• Chemical damp-proof course to external brickwork. The installer drills holes roughly every 120 mm along a mortar joint, 10 to 15 mm in diameter, to within about 30 mm of the far face, and introduces a silane or siloxane cream or fluid at low pressure or by gravity feed. It forms a continuous water-repellent zone that capillary moisture cannot climb past, without blocking the pores, so the wall still breathes and dries. It does nothing about salt already above it, and where a wall needs treating from both faces the internal side is outside a licensed waterproofer's scope.
• Hydropoxy negative treatment to internal concrete slabs. Their crew grind back, prepare the surface, and apply an epoxy moisture barrier to the top of the slab before new flooring goes down. It manages moisture at the flooring interface. It is not a damp-proof course, and it is not a retrofitted under-slab membrane — a membrane cannot be installed beneath a slab that has already been poured.

What it costs

Questions that actually get asked

Does efflorescence mean I have damp?

It means water has been in the masonry and evaporated at that spot, which is not the same thing as a problem. On new brickwork it is normal, comes from soluble compounds in Portland cement mobilised by construction water, and fades over the first year or two. What matters is recurrence: bloom that returns season after season means water is still arriving and still evaporating there, and something is supplying it. Efflorescence is a symptom that tells you to go looking. On its own it is not a diagnosis and it is certainly not a reason to inject a wall.

Can condensation cause efflorescence?

Indirectly, yes. Salts already present in masonry are mobilised by any water moving through it, and that includes moisture condensing inside the pores. As a wall warms it exhales air from its pores and as it cools it inhales; if the incoming air is humid and the masonry drops below dew point, water condenses within the wall, and where hygroscopic salts are present that moisture is captured and held. That is how a wall with a sound damp course and no liquid source can keep blooming. The classic recurring bloom low on external brickwork is more often a liquid source, but the two mechanisms genuinely overlap, which is why how much salt is in a wall and how much free water is in it are two different questions.

It's black speckles, not white powder. What does that tell me?

That you are almost certainly in the condensation family rather than the salt family. Black spot mould is a biological growth that establishes on surfaces which stay humid — the common culprits are Cladosporium and Aspergillus — and it appears on cold external corners, window reveals, behind furniture and in wardrobes on external walls. Salt does not go black. The accepted control threshold is keeping surface relative humidity below about 80 per cent over month-long periods, and the levers that move it are ventilation, extraction ducted outside, heating pattern, insulating cold spots and reducing the moisture being put into the air.

Is it rising damp or a leak?

Timing and traceability separate them. A leak correlates with an event — rainfall, a shower being used, a tap running, a sprinkler cycle — and it can be traced back to a defect. Rising damp correlates with ground level and stays broadly the same through the year. The way you settle it is exclusion testing rather than surface readings: isolate the water meter and watch it with all fixtures off, flood-test the shower, run the downpipes and hose the ground to check falls, dye or CCTV the waste lines, and borescope the cavity. Negative results are only as good as the duration of the test, so the tests that matter need to run long enough to mean something.

Is rising damp actually common on the Gold Coast, or is that a sales line?

It is common, and the reasons are checkable rather than rhetorical. Much of the coastal strip is low-lying canal, filled and reclaimed land where the water table sits close to the surface. That groundwater is often saline, and marine aerosol adds chloride to masonry from the air side as well. Subtropical rainfall arrives in volume and keeps the ground wet. And there is a large stock of brick housing from the 1960s onwards where paths, driveways, slabs and garden beds have been built up against the walls over the decades until the damp course is buried. Those are the exact conditions capillary rise needs. That does not mean every damp wall here is rising damp — plenty are condensation, a downpipe or a shower — but it does mean rising damp deserves to be tested rather than dismissed.

Why does the same wall feel damp on humid days and dry on dry days?

Because the wall contains hygroscopic salt and is responding to the air, not to the ground. Chlorides and nitrates pull water vapour straight out of the atmosphere: above a critical humidity they dissolve, below it they crystallise. Common salt goes wet around 75 per cent relative humidity, calcium nitrate at 47 to 55 per cent, magnesium chloride around 33 per cent and calcium chloride at 29 to 32 per cent — and mixtures, which is what real walls contain, go wet at a lower humidity than either constituent alone. On a coast where chloride is delivered by both groundwater and sea air, that phase is more or less permanent. The treatment for it is salt removal and replastering, not another damp course.

If I run a dehumidifier and it improves, does that prove it was condensation?

It is strong evidence, and it is a fair experiment provided you keep dated readings and change one variable at a time. Two cautions. First, drying is limited by how fast water can move through a wall to the surface, not only by how fast you strip it out of the air, so beyond a point extra dehumidification achieves little. Second, on a salt-loaded wall, increasing the drying rate increases the rate of salt crystallisation inside the masonry and therefore the rate of decay — the heritage guide notes that adding heating or air-conditioning increases drying and so increases the associated decay. Steady moderate ventilation beats aggressive forced drying, and a dehumidifier is a diagnostic aid rather than a repair.

I've been told I need a damp course. What should I ask before I sign anything?

Ask what they think is causing it and what they saw that led them there — ground levels against the existing damp course, where the downpipes discharge, the state of the subfloor vents, the height and shape of the damp band, where the salt sits. Ask what has been excluded, particularly the bathroom or laundry on the other side of that wall and the gutters above it. Ask whether the ground, paving or garden beds need cutting back first, and whether that is in the price or on you. Ask whether desalination and replastering are in the scope, what drying period is allowed before replastering, what spacing, depth and product will be used, and what the guarantee excludes. Ask for the QBCC licence number and check it. And if the wall is contested — a dispute, an insurer, a purchase you might walk away from — that is the case where it is worth commissioning an independent laboratory investigation from a consultancy that only investigates, before anybody prices repairs.