That damp line across the bottom of your wall

The line is salt, not water

The wet front inside a wall is comparatively sharp rather than a smooth gradient, because capillary conductivity collapses as water content falls and the moisture profile self-steepens. That is real. But the thing you can see is not the wet front. It is the salt front.

Water carries dissolved salt up through the masonry. The water evaporates at the face; the salt cannot. It concentrates and deposits in a band at and just below the top of the wetted zone, which is why the stain, the bloom, the blistered paint and the crumbling mortar all cluster along the same level. The mark is an accumulated record of where evaporation has been happening, sometimes for decades.

Two consequences matter when you are looking at your own wall. The visible mark often sits slightly below the true damp front, so the wall is wet a little higher than it looks. And the mark survives long after the wall has dried, because the salt does not leave when the water does. A tide mark on its own proves that water once evaporated there. It does not prove water is evaporating there now.

Why a level top edge carries so much weight

Pattern and geometry are more diagnostic than any instrument. Genuine rising damp is continuous along the base of a wall, has a defined upper limit, is worst where evaporation is easiest, and correlates with external ground level, damp-proof course position and site drainage. It is present all year rather than tracking the weather.

Almost every lookalike breaks at least one of those. Falling damp from a gutter or downpipe is worst high and traces upward. Condensation appears on cold surfaces, affects the tops of walls and ceilings as often as the bottoms, and tracks how the house is used. A bridged cavity produces isolated patches, often above the damp-proof course, matching individual wall ties. Lateral damp from higher ground on the other side of a wall is frequently wetter above the damp-proof course than below it, which is close to a signature.

How high it actually climbs, and why

The steady height of rise was derived from measurable material properties by Hall and Hoff in Proceedings of the Royal Society A in 2007. In plain English: the height goes up in direct proportion to how absorbent the wall material is, in proportion to the square root of the wall thickness, and inversely as the square root of the evaporation rate.

Notice what is not in that list. The depth of groundwater below barely matters. Gravity barely matters either: for cases with no-gravity steady heights of 500 mm and 1000 mm, including gravitational drainage brings them to about 488 mm and 951 mm, a correction of 2 to 5 per cent. Evaporation is the limit, not gravity. That is why a thicker wall rises higher rather than lower, and it is the part of this subject that surprises people most.

The square-root scaling is worth sitting with, because it is what makes the Rome case sane rather than absurd. Doubling a wall's thickness does not double the height of rise. It multiplies it by about 1.4. To get 5.3 metres of rise you need a wall four metres thick and very little evaporation, which is precisely what the Church of San Bernardo has. Your 230 mm brick wall in Logan does not.

Six things that move the line on your wall

• Wall thickness. More thickness means more height, but only by the square root, so a 350 mm wall is not dramatically worse than a 230 mm one.
• How absorbent the masonry and mortar are. Old lime mortar becomes progressively more sorptive after decades of water passing through it, which is why a 120-year-old wall behaves nothing like a freshly built test pillar.
• Anything that slows evaporation off the face. Render, paint, a sealed coating, a built-in wardrobe, a deck built against the wall, blocked subfloor vents. Cut the evaporation rate and the front climbs to find more surface area.
• Salt loading. Dissolved salt depresses the vapour pressure at the evaporating surface, which reduces the evaporation rate, which raises the height. Salt also adds solute suction once the concentration in the wall exceeds that in the soil, drawing more water up. An old salty wall is a worse case than a young clean one with identical geometry.
• Ambient humidity. South East Queensland's high relative humidity suppresses evaporation. That lowers the rate of decay compared with Adelaide, but it raises the height of rise, and it makes hygroscopic salt and condensation far more prominent in the differential diagnosis here than they are in a drier state.
• Time. Reaching 95 per cent of the steady height from a dry start takes around 31 days on the standard 150 mm example, but the time lag is inversely proportional to evaporation rate. Behind a coating or in a poorly ventilated space it can run to years. A garden bed built up in 2023 may only be showing now.

The two patterns that almost always mean something else

First rule. Damp above about 1.5 metres is almost never rising damp. To get a level front that high on ordinary Australian housing you would need either an unusually thick wall or an evaporation rate so suppressed that the wall is effectively sealed. Neither describes a standard 230 mm cavity wall in an occupied house. Damp at head height is not rising. It is arriving, and it is arriving from above or from the side.

Second rule. Blotchy damp with no level top edge is almost never rising damp. Rising damp produces a front because the physics produces a front. Patchiness points at something localised: condensation on cold spots, mortar droppings bridging a cavity at individual wall ties, a concealed leak, or a failed wet-area membrane on the other side of the wall showing up in the next room.

Neither rule is a diagnosis by itself. Both are strong enough to change who you ring first, and that is usually worth several thousand dollars.

Why the mark stays after the wall is dry

This is the most expensive misdiagnosis in the field. A wall reads high on a resistance meter, feels damp in humid weather and dry in dry weather, blooms again after every repaint, and has no live water source whatsoever. The source was fixed years ago. The salt stayed.

Hygroscopic salts pull water vapour straight out of the air above a critical humidity and re-crystallise below it. Sodium chloride goes wet at around 75 per cent relative humidity, which in this part of the country is most of the year. Sodium nitrate sits in the low-to-mid 70s. Calcium nitrate goes wet at roughly 47 to 55 per cent. Magnesium chloride at about 33 per cent and calcium chloride at about 29 to 32 per cent. Real walls contain mixtures, and mixtures go wet at a lower humidity than either constituent alone.

A wall carrying those chlorides is effectively in the wet phase permanently. It will read damp on any instrument, forever, with nothing rising. No damp-proof course will change that reading. Only removing the salt will. This is exactly the case where an injected course is money spent on the wrong problem, and it is the reason the hygroscopic moisture test is not optional.

What it takes to actually prove it

A two-pin electrical resistance meter measures conductivity, not water, and it is calibrated for timber. Salt is an electrolyte, so a dry salty wall reads as high as or higher than a wet clean one. It is common on salt-contaminated masonry to get readings above 100 per cent moisture content, a figure that leaves no room for the masonry itself. The Australian heritage guide is explicit that a moisture meter must never be the sole basis for diagnosing a damp problem. Useful for mapping and for monitoring the same wall over time with the same meter. Useless as proof.

This is the sequence that settles it.

1. Measure the ground against the damp-proof courseExternal ground and paving levels recorded relative to the damp-proof course, with a small inspection pit dug where the course cannot be seen. The Housing Provisions require not less than 150 mm above adjacent ground, 75 mm above paving that slopes away, and 50 mm where protected by a carport or verandah.
2. Exclude the lookalikes deliberatelyGutters and downpipes watched in heavy rain, stormwater discharge traced, wet areas flood tested, the water meter isolated, cavities and subfloor inspected with a borescope, and condensation excluded with logged surface temperature against room dew point over at least a week.
3. Drill a vertical profileSix to eight samples up a single line, taken past the last visible sign of damp, at depth intervals commonly 0 to 10, 10 to 20 and 20 to 40 mm. Slow-speed drilling so heat does not drive moisture off. Samples sealed immediately and locations recorded so the same line can be re-sampled later.
4. Oven-dry themWeigh wet, oven-dry, reweigh. The loss is true moisture content by dry weight. It is the shape of that profile up the wall, decreasing with height, that is the actual evidence for rising damp.
5. Re-equilibrate at 75 per cent relative humidityThis is the test that settles the argument and almost nobody runs. The mass a dried sample regains at 75 per cent RH is the moisture its salt content will hold from the air alone. Subtract that from total moisture and what remains is free capillary water. High hygroscopic content with negligible free water means salt, not rising damp.
6. Speciate and quantify the saltWhich salts and how much. More than about 0.5 per cent by weight is cause for concern and reason to consider desalination. The species points at the source: heavy nitrate suggests a leaking sewer, fertiliser or a former animal use; heavy chloride suggests groundwater, sea spray or acid cleaning; magnesium sulfate suggests contaminated sand or aggregate.

The cheap things to do before anyone quotes you

Raised external ground level is the single most common cause of genuine rising damp in Australian houses, and it is also the cheapest to fix. In a meaningful share of cases the damp-proof course is not failed. It is buried.

• Pull garden beds back and leave a clear strip at least 300 mm wide, preferably more than 500 mm, surfaced in coarse gravel. Gravel stops rain splashing up the wall while still letting soil moisture evaporate to the sky rather than through your bricks.
• Swap sprinklers for drippers and keep them at least 500 mm off the wall. Fertiliser washed into masonry is a nitrate delivery service.
• Grade the first metre to fall about 25 mm away from the wall, with the low point 1.5 to 2.0 metres out. For slabs, AS 2870 and the NCC require a minimum fall of 50 mm over the first metre.
• Where a remedial damp-proof course is eventually installed, the guidance is 150 to 250 mm above finished ground with 200 mm as the ideal, and that clearance has to be maintained afterwards.
• Go outside in heavy rain and watch what the gutters and downpipes actually do. Salt appears where water evaporates, which can be well away from where it got in.
• Clear subfloor vents that have been rendered over, buried, decked across or blocked by an outdoor unit.
• Then monitor for at least twelve months before anyone inserts a damp-proof course. The Australian heritage guide builds that review period in deliberately, to allow for storms, floods and drought.

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 things.

One: a chemical damp-proof course injected into the mortar bed of external brickwork. Holes at roughly 120 mm spacing, 10 to 15 mm diameter, drilled to within about 30 mm of the far face, with the header course drilled by preference on a 230 mm wall. It forms a continuous water-repellent zone that capillary moisture cannot climb past. It does not block the pores, so the wall still breathes and dries. It does nothing about salt already in the wall above it.

Two: Hydropoxy negative treatment to internal concrete slabs. The slab is ground back, the surface prepared, and an epoxy moisture barrier applied to the top of the slab before new flooring goes down.

That is the list. We do not replace salt-retardant render, cut physical damp courses, underset, undercut slabs, do landscaping or drainage works, do plumbing, install subfloor ventilation, or waterproof wet areas. Those are frequently the correct answer to a tide mark, and when they are, we will say so and tell you which trade to ring. Render and plaster work goes to a renderer or a heritage lime plasterer. Drainage and levels go to a landscaper or drainage contractor. Shower and bathroom membranes go to a QBCC-licensed waterproofer under AS 3740:2021. Gutters, downpipes and stormwater go to a roof plumber.

What it costs

Questions we actually get asked

How high does rising damp go?

In Australian conditions, usually 1.0 to 1.5 metres above ground on an external wall, with the most active evaporation between about 0.5 and 1.2 metres and very little below 0.3 metres. The height is set by an equilibrium between water arriving from below and water evaporating off the face, so it varies with how absorbent the masonry is, the square root of the wall thickness, and inversely with the square root of the evaporation rate. Thicker walls and sealed walls go higher. A 4 metre thick church wall in Rome shows 5.3 metres of rise, which is the model working correctly rather than an exception to it.

My damp line is at about 1.8 metres. Is that still rising damp?

Almost certainly not. To hold a level front that high you would need either an unusually thick wall or an evaporation rate suppressed close to zero, and a normal occupied house is neither. Damp at that height is usually falling damp from a gutter, downpipe, flashing or parapet, lateral damp from higher ground on the other side of the wall, a bridged cavity, or condensation. Trace it upward before anyone quotes on a damp course. The check that settles falling damp is free: stand outside during heavy rain and watch.

There is no line at all, just blotchy patches. What is that?

Blotchy damp with no level top edge is very rarely rising. Rising damp produces a front because unsaturated flow in masonry self-steepens into one. Patchiness points somewhere else: condensation on cold spots and external corners, mortar droppings bridging a cavity at individual wall ties, a concealed leak, or failed wet-area waterproofing showing up in the room next to the bathroom rather than in the bathroom. The definitive test for condensation is a data logger recording wall surface temperature against room dew point over a week or two, not a meter reading.

Why is the line higher on one wall than on the next?

Because evaporation differs. The equilibrium height rises as the evaporation rate falls, so a wall behind a built-in wardrobe, under a deck, in a shaded side passage, or coated in render or film paint will sit higher than an exposed, ventilated wall of identical construction. Wall thickness contributes too, but only by the square root, so a difference in thickness produces a much smaller difference in height than a difference in ventilation does. If one wall is dramatically higher than the rest, look at what is covering it or standing against it.

The tide mark is still there months after we fixed the drainage. Did the fix fail?

Not necessarily, and this is the normal case rather than the exception. The visible mark is a salt deposit, not water, and salt does not leave when the water does. Drying after the source is cut typically runs three to six months and up to twelve on wet thick walls. On top of that, a salt-contaminated wall may never read dry on a meter at all, because hygroscopic salts hold atmospheric moisture indefinitely. Acceptance should be judged on the free-water content of drilled samples, not on the mark and not on a meter.

My house is on a concrete slab and the line is level. Same thing?

Different mechanism, different fix. A slab is a single monolithic element sitting on the ground, so there is no mortar course to inject and nothing to cut. Persistent damp at the slab or footing edge, efflorescence and drummy render below the damp-proof course, damp carpet edges and tile bond failure near external walls are the signature of slab edge dampness, and it is a drainage, membrane-termination and concrete-quality problem. Injecting brickwork does not address it, and an under-slab membrane cannot be retrofitted once the slab is poured.

Can the mark come back after a damp course is installed?

Yes, and it is one of the most predictable outcomes in this trade. A chemical damp-proof course stops water rising. It does nothing about the salt already in the wall above it, which keeps dissolving in humid weather and re-crystallising in dry weather with every cycle doing a little more damage. That is why salt analysis, the 0.5 per cent threshold, and desalination belong in the scope. The Australian heritage guide documents brickwork continuing to decay above a technically successful injected course for exactly this reason.