Rising damp treatment for South East Queensland

The two things we do, and the long list we don't

S&S Remedial are waterproofing and concrete repair specialists. We are not builders, and we do not present ourselves as a one-stop remedial contractor. Two services, described exactly.

Chemical damp-proof course, external brickwork. Holes are drilled into a mortar bed on the external face at roughly 120 mm centres, 10 to 15 mm in diameter, to within about 30 mm of the far face, and a silane or siloxane cream or fluid is introduced so that a continuous water-repellent zone forms through the full thickness of the wall. The chemistry lines the pore walls rather than plugging them, so the masonry still breathes and still dries. The mechanics of that are set out in detail on the damp-proof course page.

Hydropoxy negative treatment, internal concrete slabs. The slab is ground back, the surface prepared, and an epoxy moisture barrier applied to the top of the slab before a new floor covering goes down. It is a negative-side treatment, which means it sits on the opposite face to the water. It manages the consequence — a floor covering that would otherwise debond, cup or smell — for a slab that is going to keep receiving moisture from underneath. It does not stop the moisture arriving, and you cannot retrofit a membrane under an existing slab no matter who tells you otherwise.

That is the list. It is short on purpose. Below is the work this problem usually needs, and the trade it belongs to.

• Lowering ground levels, cutting back paving and removing garden beds — landscaping or excavation contractor.
• Site drainage: regrading, spoon drains, agricultural drains, stormwater to a legal discharge point — drainage contractor.
• Gutters, downpipes, rainwater heads and flashings — roof plumber.
• Concealed supply, waste and sewer leaks, air-conditioning condensate lines and irrigation — plumber.
• Subfloor ventilation to NCC Housing Provisions Part 6.2 — subfloor ventilation contractor.
• Wet-area waterproofing rectification to AS 3740:2021 — licensed waterproofer.
• Removal of salt-contaminated render and plaster, desalination poulticing, replastering in lime or lime-cement — plasterer or masonry conservator.
• Physical damp courses by slot sawing or undersetting, and undercutting at a slab or path junction — specialist masonry contractor; undersetting also engages builder licensing and may need engineering certification.
• Repointing failed lime joints in a weak lime mix — repointing contractor.

The order this has to happen in

The sequence below is not ours. It is adapted from Salt attack and rising damp, written by David Young OAM and published jointly by the Heritage Council of NSW, Heritage Victoria, the South Australian Department for Environment and Heritage and Adelaide City Council. It is free, it is downloadable, and it puts damp course insertion at step six of seven. The industry sells step six to people who needed step two.

The guide's own instruction is unambiguous: 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. We work that way because a treatment judged too early proves nothing in either direction.

1. Establish what is actually wet, and where the water is coming fromInside, outside, subfloor, cavity, roof and drainage — ideally on more than one visit and not only after rain, because rain washes surface salt off and hides the evidence. External ground and paving levels are measured relative to the damp course, not estimated. The location, type and condition of any existing DPC is recorded. Borescope access into a cavity converts a presumed rising damp case into a diagnosed bridged cavity in about ten minutes.
2. Prove it with drilled samples, not with a meterA vertical profile of six to eight samples per line, taken past the last visible sign of damp, at depth intervals of roughly 0–10, 10–20 and 20–40 mm. Samples are sealed immediately, weighed, oven-dried and reweighed for true moisture content, then re-equilibrated at 75% relative humidity to establish hygroscopic moisture content. Total moisture minus hygroscopic moisture is free capillary water. It is the shape of that gradient — decreasing with height — that is the actual evidence for rising damp.
3. Remove the causeGround levels, paving, garden beds, falls, drainage, gutters, downpipes, flashings, plumbing leaks, condensate lines, wet-area membranes and subfloor ventilation. In a substantial share of Australian cases this step alone resolves the problem, and it is the cheapest money on the whole job. None of it is our work.
4. Deal with the saltWhere laboratory analysis returns more than about 0.5% salt by weight, the salt has to come out: dry vacuuming, captive-head washing, poultices of absorbent clay or paper pulp, and sacrificial lime mortars as ongoing maintenance. Desalination should start before any injection, because injecting a wall can displace saline moisture upward into masonry that was previously undamaged.
5. Monitor for at least twelve monthsStorms, floods, drought and seasonal humidity all move the readings. A long dry spell can look exactly like a successful treatment. Twelve months of monitoring is the guide's own review stage and it is the single highest-return option available, because a meaningful share of cases never need step six at all.
6. Then, and only then, a chemical damp courseIf drilled samples still show free water rising, the prerequisites have been corrected and held for a year, and the masonry is sound enough to take the treatment, this is where a chemical DPC belongs in the sequence. This is our work, and it is the only masonry step on this list that is.
7. Dry the wall, then make goodMonths, not weeks. Replastering, repainting and re-fitting skirting boards happen after the wall has dried, not on the same trip. Doing it in the wrong order is why so many freshly finished walls blister within the year.

Why a meter reading is not a diagnosis

A two-pin electrical resistance moisture meter measures electrical conductivity between two pins. It is calibrated for timber, where the wood-moisture relationship is well characterised. On masonry it responds to whatever conducts, and soluble salts are excellent electrolytes. A dry but salty wall therefore reads as high as, or higher than, a genuinely wet clean one, and the instrument cannot tell you which you have.

The Australian heritage guide records that it is common on salty walls to get readings above 100% moisture content — a physically impossible figure, because it leaves no room for the masonry itself. Foil-backed plasterboard, buried cables, pipes, nails, conductive paints and carbon in some finishes all produce false highs as well. The guide's caution is direct: moisture meters should never be used as the sole basis for diagnosing a damp problem, and never on their own to prove a wall is unacceptably damp.

This matters because of one specific fork in the road. If a drilled sample comes back with high hygroscopic moisture content and negligible free water, the wall is salt-contaminated with no live source. There is nothing rising. That wall needs desalination and replastering, and a damp course would be money spent on the wrong problem entirely. Chlorides go wet at around 75% relative humidity, calcium nitrate at roughly 47%, and magnesium chloride at around 33% — and mixtures go wet at a lower humidity than either constituent alone. A wall carrying those salts is permanently in the wet phase and no damp course on earth will make it read dry.

A meter is useful for two things: mapping relative highs quickly to decide where to sample, and monitoring the same wall over time with the same instrument. Those are legitimate uses. Neither of them is a diagnosis.

Removing the cause, which is usually not our work

Rising damp needs three things at once: liquid water in contact with the base of the wall, a continuous capillary path upward, and evaporation from the wall face. Break any one of them and it stops. The first is nearly always the cheapest one to break, and it is nearly always somebody else's trade.

Ground level is the biggest single offender in Australian housing. The NCC requires a damp course not less than 150 mm above adjacent ground level — 75 mm above finished paving that slopes away, 50 mm where protected by a carport or verandah, and as little as 15 mm in low rainfall intensity areas with protection. The heritage guide recommends a remedial damp course be set 150 to 250 mm above finished ground, with 200 mm as the ideal, and that the clearance then be maintained. Excavate a small inspection pit and measure it. The damp course is very often simply buried.

Garden beds are the worst version of this, and for three reasons simultaneously: the soil level creeps up every time mulch is added until it buries the course, fertiliser delivers nitrate salts directly into the masonry, and enthusiastic watering washes both into the wall. Pull the bed back, leave a sterile strip at least 300 mm wide and preferably over 500 mm, surface it in coarse gravel so rain cannot splash up and soil moisture can still evaporate to the sky rather than through your bricks, and swap sprinklers for drippers kept at least 500 mm off the wall. Grade the first metre to fall about 50 mm away from the building, with the low point 1.5 to 2.0 m out.

One caution on that. On highly and extremely reactive clay sites — Class H and E under AS 2870 — aggressively drying the soil near footings can cause shrinkage and structural cracking. That is a genuine trade-off the guide names explicitly, and it needs geotechnical or structural input before large changes are made to the soil moisture regime around a house.

When a chemical damp course is the right answer

A chemical DPC is a good, well-evidenced intervention with a British Standard behind it and independent third-party product certification available. It is the right answer under a specific set of conditions, all of which have to hold at once.

• Drilled samples show free capillary water — total moisture content minus hygroscopic moisture content — and that free water decreases as you go up the wall.
• There is no continuous existing damp course, or the existing one has genuinely failed rather than simply been bridged by render, pointing, paving or soil.
• The prerequisites have already been corrected: ground levels, drainage, gutters, plumbing, wet areas and subfloor ventilation — and they have held long enough to be assessed, ideally through a full year.
• The damp is continuous along the base of the wall with a defined upper limit, typically 1.0 to 1.5 m in Australia, rather than blotchy, patchy or worst high on the wall.
• The masonry is sound, reasonably regular brickwork with continuous mortar joints. Crumbling mortar will not hold the treatment and needs repointing first.
• The wall is not saturated — very wet masonry limits diffusion — and any voids have been grouted so they do not simply swallow the product.
• The external face is accessible, so the course can be set out at the correct height and the treated zone sits below all floor timbers.
• Desalination is in the scope, not left out of it, because a damp course does nothing whatever about salt already in the wall above it.

When it isn't rising damp, and how often that is

This is the list a homeowner deserves before anybody quotes a five-figure number. Each of these presents as damp low on a wall and each has a different fix, a different trade and a different price.

Condensation. Worst in winter, worst overnight, on cold external corners, behind wardrobes, in unheated rooms and poorly ventilated wet areas. It shows black spot mould rather than salt bloom, and it affects the tops of walls and ceilings as often as the bottoms. Archicentre Australia's technical sheet attributes about 70% of mould problems to condensation and about 30% to rising damp. The definitive test is not a meter but a data logger: record room temperature, room humidity and wall surface temperature for at least a week including cold nights. If the surface repeatedly drops below the room's dew point, no damp course will change anything. Around 80% surface relative humidity sustained over month-long periods is the accepted control threshold for mould growth.

A bridged damp course. The DPC exists, it is intact, and render, hard cement pointing, a path or a garden bed is carrying moisture straight around it. The damp is continuous along the rendered or paved run and stops where that run stops. The fix is to cut the render back and form a clean drip, or to lower the ground — not to inject a second damp course above a perfectly good existing one.

A downpipe, gutter or flashing. Falling damp reads as a vertical stripe or a localised patch, traceable upward, and is often worst high on the wall. That is the giveaway. Salt appears where the water evaporates, which may be well away from where it got in, so the trace has to be deliberate. Go outside in heavy rain and watch.

A sprinkler, a hose bib or a split-system condensate line. Localised, often seasonal, right next to a fixture. Repipe the condensate to a proper drain and move the dripper. This is a Saturday.

Lateral damp. Retaining walls, cut-and-fill sites, garages, basements and split levels, where the ground on one side sits higher than the floor on the other. The signature is a wall that is wetter above the damp course than below it. Chemical injection cannot resist water under a hydrostatic head — that is drainage and tanking, a different engineering problem with a different price.

A bridged cavity. Mortar droppings on wall ties, mortar at the cavity base, blocked weepholes, missing cavity flashing. Damp patches are often higher than the damp course and sometimes isolated to individual ties. A borescope settles it quickly and injection does nothing for it.

A concrete slab. Nothing to inject and no course to cut. Slab moisture comes from a missing, torn or badly terminated under-slab membrane, permeable or under-cured concrete, ground water at the slab edge, or landscaping added later that buried it. The correct measurement is in-situ relative humidity probe testing to ASTM F2170 at 40% of slab depth, referenced by AS 1884:2021.

Salt with no live source. High hygroscopic moisture content, negligible free water, a wall that feels damp in humid weather and dry in dry weather, and a bloom that keeps coming back after every repaint. The source was fixed years ago and the salt stayed. This wall needs desalination and replastering. It is the most expensive misdiagnosis in the book, and hygroscopic moisture testing at 75% relative humidity is the only thing that catches it.

Then you wait, and this is the part nobody quotes

Cutting the source does not dry the wall. It stops the wall being resupplied. Everything already in the masonry then has to leave by evaporation, and that takes as long as it takes.

The working rule of thumb, which traces to BRE Digest 163, is roughly one month of drying per 25 mm of wall thickness. A 230 mm solid or double-brick wall therefore takes the better part of a year. The Australian heritage guide reaches the same broad answer from a different direction: after replastering, thorough drying of construction water and residual deep dampness may take 3 to 6 months, or up to 12 months for wet thick walls in cooler damp climates. Both figures agree on the important word — months.

Treat the 25 mm rule as an order-of-magnitude expectation rather than a specification. It was originally framed around drying construction moisture out of masonry under reasonable conditions, not around a salt-loaded wall recovering from decades of rising damp. Real drying is often slower in thick walls, poorly ventilated rooms and humid weather, which describes a good deal of South East Queensland.

Two things follow from this that are worth knowing before you plan a renovation around it. First, a salt-contaminated wall may never read dry on a meter, because hygroscopic salts hold atmospheric moisture indefinitely — which is why acceptance has to be judged on gravimetric free-water content from drilled samples rather than on an instrument reading. Second, do not try to force it. Increasing the drying rate increases the rate of salt crystallisation inside the masonry, and each crystallisation cycle does a little more damage. Steady, moderate ventilation beats aggressive dehumidification, and an air-conditioning outdoor unit blowing warm air at an old wall concentrates salt damage directly behind it.

Do not repaint early — blistering is close to guaranteed. Do not hang wallpaper for at least twelve months, longer on thick walls. And be sceptical of any assessment made at three months: the time to reach 95% of steady height in the standard worked example is about 31 days, but it runs inversely with evaporation rate, so behind a coating or in a poorly ventilated space it can be years. A treatment that looks successful at three months has proved nothing at all.

Making good — render, plaster and paint

This is where a technically correct job gets undone, and it is not our scope. It is worth stating what a correct specification looks like so you can hold whoever does it to that standard.

Never use gypsum plaster on a wall that may stay damp. Gypsum is calcium sulfate, which is itself a slightly soluble salt, so any residual moisture triggers salt attack inside the new plaster. Lime or lime-cement is the correct choice, with low-alkali cements specified where salt needs to be minimised. Hard, brittle cement render on a soft, flexible old wall will fail regardless of what the damp is doing, because the stiffness is wrong.

Sacrificial mixes are a deliberate consumable, not a defect. A weak, porous lime-and-sand mortar or render with porous particulates in it acts as a poultice: salt crystallises in the cheap renewable layer rather than in your bricks. You renew it on a maintenance cycle. That is the point of it.

On coatings, breathability is the governing property and the ladder is fixed: limewash is the most vapour-permeable, then cement-based paints, then acrylics, with oil-based alkyd paints the worst and unsuitable for a wall that is damp or still drying. Do not seal the face. Reducing evaporation raises the height of rise, because the steady height varies inversely with the square root of the evaporation rate — render a 215 mm wall to 1.25 m and cut evaporation tenfold and the damp front re-stabilises above the render, taking roughly 425 days to reappear. That lag is exactly why the treatment looks like a success for a year or two and keeps getting sold. The heritage guide's instruction is not hedged: don't even think about sealing walls with water-repellent coatings.

Allow several weeks with good natural ventilation after replastering before decorating, and treat the twelve-month wallpaper rule as real.

Things we will not sell you

Some of these are actively harmful, some have no evidence behind them, and one is a technique that is legitimate in the right hands but a defect at the wrong pressure. We do not install any of them, and here is the reasoning rather than a slogan.

• Passive electro-osmosis — an earthed copper strip looped into the wall. It has no scientific basis. Water moving through the porous medium is what creates the electrical potential, not the reverse, so earthing the resulting charge cannot prevent capillary rise. The Australian heritage guide lists it among old treatments that should no longer be considered, and notes of the related Knapen tube that tests showed an empty hole is just as effective. Most historic successes are attributable to the gutter and drainage work done at the same time.
• Active electro-osmosis — an applied DC current through platinum-coated titanium electrodes. The mechanism is real and is used industrially to dewater soils, but the building application is contested: it has never been awarded a BBA Agrément certificate; BRE inspections of installed systems found them paired with a replastering system that may itself have been providing the moisture barrier; independent analysis suggests the flow produced is small relative to capillary flux and is further suppressed by salts. It must also stay switched on permanently, stray currents can corrode reinforcement and buried metal, and long-term performance in very salty walls is openly described as unclear. We do not install it.
• Damp-proof paints and water-repellent surface sealers. They reduce evaporation, which raises the damp front, and they trap salt at the coating interface where it crystallises and forces the coating off. On case-hardened sandstone they can take the stone's protective skin with them.
• Hard cement render applied over a damp wall as the fix. Same mechanism, larger bill, and it also traps salt behind it.
• High-pressure injection above about 1000 kPa. The fluid advances as fingers rather than a front, leaving untreated gaps between them, and it can blow out weak mortar. Correct chemical DPC injection is low-pressure or gravity-fed.
• A damp course sold on the strength of meter readings alone, with no drilled samples, no hygroscopic moisture content, no salt analysis and no drainage assessment. If that is the evidence base, we would rather you spent the money on the assessment.

What this costs, honestly

The published Australian range for chemical DPC injection is $120 to $400 per linear metre, commonly $180 to $300. That is a wide band and the reasons are real: wall thickness, whether both leaves or both faces need treating, the number of holes at correct spacing, brick versus rubble stone, access, whether voids need grouting first, and the active ingredient content of the product being used.

A quote well under about $100 per linear metre should prompt questions rather than relief. The heritage guide names the mechanism in print: dampcoursing fluid and cream are expensive, there is direct cost pressure on contractors to use less, and the responses are diluting the fluid with extra solvent or spacing the drill holes wider than recommended. Both produce incomplete coverage, and both are invisible, because the failure is sealed inside a wall for two or three years. The other cheap-quote mechanism is treating a wall from one side when it needs both.

The injection is rarely the largest line on a properly sequenced job. Desalination runs $80 to $250 per square metre and is the line item most often missing from cheap quotes — which is precisely why decay so often continues above a technically successful damp course. Removing salt-contaminated render runs $40 to $120 per square metre, replastering $80 to $180 per square metre in a standard system and $200 to $400 for heritage lime, and redecoration $40 to $90 per square metre on top.

And there is one more option that nobody quotes, because there is nothing in it for the contractor. Fix the gutters, the levels, the garden bed, the drainage and the subfloor vents; then monitor for twelve months. Cost of the masonry work: nothing. In a meaningful share of cases it removes the need for a five-figure treatment entirely. That is a service we offer, not a line we throw away at the end of a sales call.

What it costs

Questions we actually get asked

Can you just come and inject it? We already know what it is.

Not without drilled samples, and that is a commercial decision as much as a technical one. A chemical damp course installed against the wrong diagnosis does not fail visibly — the wall keeps doing whatever it was doing, and two years later everybody is arguing about a product that was never the answer. The test that settles it is oven-dry gravimetric moisture content compared against hygroscopic moisture content at 75% relative humidity, on a vertical profile of samples. If that comes back showing free water rising, we will quote the injection. If it comes back showing high salt and negligible free water, we will tell you the damp course is not your problem and hand you to the people who do desalination and replastering.

Why won't you do the drainage and the render as well? It would be simpler.

It would be simpler and it would also mean the party diagnosing the problem gets paid more the bigger the problem turns out to be. We are waterproofing and concrete repair specialists, not builders, and the two masonry things we do well are a chemical damp course into external brickwork and a Hydropoxy barrier on an internal slab. The drainage, the levels, the plumbing, the vents and the plaster are genuinely other trades doing work they do better than we would. We will name who does what, and we would rather lose the extra scope than own the conflict.

The report we were given is two pages of meter readings and a price. Is that enough to act on?

No. BS 6576 requires masonry samples and BRE Digest 245 requires a vertical profile with both total and hygroscopic moisture content, because a resistance meter cannot distinguish a dry salty wall from a wet clean one. A defensible report also records weather and site conditions on the day, external ground and paving levels measured against the damp course, the location and condition of any existing DPC, a drainage and plumbing review, a subfloor and cavity inspection, salt type and percentage where salt attack is suspected, explicit exclusion of each lookalike cause, and a clear separation of what is proven from what is inferred. If those are missing, what you are holding is a quotation.

How much of this sequence can we do ourselves before spending anything?

Quite a lot, and it costs nothing. Go outside during heavy rain and watch what the gutters, downpipes and ground actually do. Dig a small pit against the affected wall and find the damp course, then measure how far the soil, mulch or paving sits above or below it. Pull garden beds back from the wall and leave a gravel strip at least 300 mm wide. Move sprinklers or replace them with drippers at least 500 mm off the wall. Clear subfloor vent grilles if you have a suspended floor, and check none have been rendered over, buried or covered by a deck. Then leave it alone through a full wet season and see what changes. That sequence resolves a surprising share of cases for the price of an afternoon.

We had a damp course injected three years ago and the bricks are still crumbling. Was the work bad?

Quite possibly not. This is a predictable scoping outcome rather than a product failure. A chemical damp course stops water rising past it. It does nothing about salt already in the wall above it, and those salts keep dissolving in humid weather and re-crystallising in dry weather, and each cycle breaks the masonry down a little further. The heritage guide illustrates exactly this case: brickwork continuing to decay above a chemically injected damp course because the salts were left in place. Have the wall tested before spending anything more. If free water is now negligible and salt content is high, the damp course is doing its job and desalination is the outstanding work.

We settle in eight weeks. Can this be finished before then?

The injection can be done in that window on most single-storey brick jobs. The drying cannot. A 230 mm double-brick wall takes roughly nine months to dry after the source is cut, and replastering and repainting belong after the drying, not before it. What we can realistically produce inside eight weeks is a diagnosis with drilled samples, a staged scope with prerequisites first, and dated readings you can put in front of a buyer, a seller or a conveyancer. That is usually more useful in a transaction than a hurried repair that has to be judged on faith.

We are in a high-set Queenslander. Is rising damp likely?

Usually not, and this is where South East Queensland genuinely differs from the Adelaide copy most of this industry runs. A high-set timber house on stumps has no continuous masonry path from the ground into the living areas, so the classic capillary mechanism has nowhere to work. What it does have is a subfloor, and in this climate the damp under a Queenslander is far more often blocked or buried ventilation, ground clearance, grading, a plumbing leak or condensation. NCC Housing Provisions Part 6.2 sets aggregate ventilation openings by climate zone and a ground clearance of typically 150 mm, or 400 mm where termite inspection access is required. Clearing vent grilles is frequently the cheapest genuinely effective intervention available.