Does damp proof injection actually work?

What actually happens inside the wall

A chemical damp-proof course does not plug the wall. That is the most common misunderstanding and it matters, because a plugged wall would be a worse outcome than a damp one. A silane or siloxane active — delivered as a high-solids water-based cream from a cartridge gun, or as a siloxane dissolved in an organic solvent at roughly 5–7% by weight — diffuses through the pore structure of the mortar, reacts with the substrate, and lines the pores with a hydrophobic silicone resin. The pores stay open. The wall still breathes, and it still dries. What changes is that the pore walls no longer pull water upward.

The holes go into the mortar bed rather than the bricks, because mortar diffuses the product far better than a dense fired brick does, and because a mortar joint is a continuous horizontal path across the whole wall. Typical setting-out is 10–15 mm diameter holes at roughly 120 mm centres, drilled to within about 30 mm of the far face. Those three numbers are the ones to ask about, and we come back to why below.

There is a formal procedural standard for this work — BS 6576, British rather than Australian, but cited in the Australian heritage guide's own standards list — and there is independent third-party product certification available through BBA Agrément certificates. Both matter, and neither is generic. Active-ingredient content varies enormously between certified products, from around 15–20% in weak formulations up to around 60–80% in the strongest, and low-strength products have still obtained certification. The certificate for the specific product matters far more than the word "silane" on the label.

Failure mode one: the diagnosis was wrong

This is the big one, and it is not close. Most injection that fails did not fail — it was installed into a wall that was never rising in the first place, and it performed exactly as designed while the actual water source carried on doing what it was already doing.

Injection is a barrier to capillary rise from the ground. That is the entire job description. Run down this list before anyone drills anything, because every item on it produces damp at the base of a wall and not one of them is affected by a chemical damp-proof course.

• A bridged damp course. The barrier exists and is intact, but render, plaster or hard pointing runs continuously across it, giving moisture a path around it. The fix is to cut the render back and form a clean drip. Installing a second damp course above a perfectly good existing one changes nothing.
• Ground built up over the damp course. Paving, a concrete path, a driveway, or a garden bed whose level has crept up as mulch was added. This is the single most common cause of genuine damp in Australian houses, and excavating a small pit usually finds the damp course simply buried.
• A bridged cavity. Mortar droppings on wall ties or at the cavity base, blocked weepholes, missing cavity flashing. The signature is damp patches that are higher than the damp course, sometimes in isolated spots corresponding to individual ties. Ten minutes with a borescope settles it.
• Falling damp from gutters, downpipes, rainwater heads or flashings. Traceable upward, and often worst high on the wall. Salt appears where the water evaporates, which can be well away from where it entered.
• Condensation. Worst in winter, worst overnight, on cold surfaces, behind wardrobes, at the tops of walls as often as the bottoms, showing black spot mould rather than salt bloom. Around 70% of mould problems trace to condensation. A data logger settles it; a meter does not.
• Failed shower or wet-area waterproofing, which characteristically presents as damp at the base of the wall in the room next door rather than in the bathroom. This is licensed waterproofing work under AS 3740:2021 and it is not work we carry out.
• Lateral damp from higher ground — retaining walls, cut-and-fill sites, garages, basements and split levels. The diagnostic signature is a wall that is wetter above the damp course than below it. Chemical impregnation prevents capillary rise but cannot resist water under a hydrostatic head.
• Slab moisture. A slab is a single monolithic element sitting on the ground: there is no mortar joint to inject and no course to cut. In-situ relative humidity probe testing to ASTM F2170, referenced by AS 1884:2021, is the right test. Anyone offering to inject your slab against rising damp is selling you something.
• Residual hygroscopic salt from a problem that was already cured. Gravimetric analysis shows negligible free water and high hygroscopic moisture content. There is nothing rising, so there is nothing to stop.

Failure mode two: the installation was wrong

Success depends entirely on achieving a continuous water-repellent zone through the full thickness of the wall. There is no gauge that reads this out at the end. It is judged by the operator, which makes skill and honesty the controlling variables — and the Australian heritage guide, a government publication, says so in print rather than leaving it to be implied.

Its wording is worth quoting because no contractor will volunteer it: dampcoursing fluid and creams are expensive, so there is cost pressure on contractors to use less; unscrupulous contractors might dilute the fluid with additional solvent, leading to insufficient water repellency, or space the drill holes at wider intervals than recommended, leading to incomplete coverage. Good operators deliberately over-apply, because the cost of certainty is much lower than the cost of coming back.

Then there is the geometry. On a 230 mm one-brick wall, the header course should be drilled by preference, because a header runs the full thickness. Where a stretcher course is drilled instead, the stretchers on the other side must also be treated — either from the far face, or by re-drilling through the same holes in a second phase. A wall like that treated from one side only has a treated zone with an untreated half behind it, and it will read as a successful job on the invoice and a failure on the wall.

The other installation faults are shorter to list and just as costly. Drilling the bricks instead of the mortar bed, on a wall where the mortar was the correct target. Injecting into a wall that is saturated, where the product cannot diffuse. Failing to grout voids in thick or rubble walls first, so the void swallows the product. High-pressure injection above about 1000 kPa, which causes viscous fingering. Drilling and impregnating directly into an old tar-and-sand damp course, which may be underperforming but will not be improved by perforating it. And treating the wall without a desalination poultice, when injection can displace saline moisture upward into masonry that was not previously damaged.

Failure mode three: expecting it to do things it cannot do

The third failure mode is the one where the work was correct, the diagnosis was correct, and the customer is still unhappy — because nobody explained the boundaries of what was purchased. A chemical damp-proof course stops water rising past a line. That is all it does, and each of the following is outside that line.

It does not remove salt. This is the most consequential omission in the trade. The salts delivered over decades stay in the wall above the new barrier, cycling in and out of solution with the humidity and continuing to break down the masonry — the Australian guide illustrates brickwork continuing to decay above a chemically injected damp course for exactly this reason. Where analysis shows more than about 0.5% salt by weight, desalination is required as well: dry vacuuming, captive-head washing, and clay or paper-pulp poultices applied wet so the salt crystallises in the poultice rather than in your wall. That is iterative masonry restoration work, typically $80–$250 per square metre, and it is not work we carry out.

It does not dry the wall. Drying is a separate, slow process that happens afterwards, and the Australian guide puts it at 3–6 months after replastering, up to 12 months for wet thick walls in cooler damp climates. The old BRE rule of thumb of about one month per 25 mm of thickness puts a 230 mm solid wall near nine months, and that rule was framed around construction moisture rather than a salt-loaded wall, so treat it as an order of magnitude. Do not repaint early; bubbling paint is close to guaranteed. Do not wallpaper for at least twelve months.

It does not resist water under pressure. Impregnation prevents capillary rise; it cannot hold back a hydrostatic head. A cellar, a basement, a below-ground garage or a retaining wall with ground higher on one side is a tanking and drainage problem, with different engineering, a drained cavity design and floor treatment. It is not our work and injection is not a substitute for it.

It does not touch condensation, and no guarantee in this industry could sensibly cover it. If the wall surface repeatedly falls below the room's dew point, moisture will condense on it whatever is happening in the mortar bed 900 mm below.

DIY, honestly

We are not going to sneer at anyone standing in a hardware aisle. Injection cream and rods sold retail are, in a lot of cases, a competent product; the chemistry in the tube may be perfectly good. What you cannot buy in that aisle is the diagnosis or the workmanship, and those are the two variables that decide the outcome. If you are within a metre of a wall you have not drilled, sampled, or checked the ground levels against, the cream is not the thing standing between you and a dry wall.

The gap is also mechanical. Hitting ~120 mm centres consistently, drilling to within 30 mm of the far face of a wall whose thickness you have to establish first, identifying whether you are looking at a header or a stretcher course, and treating both leaves of a cavity wall are all things that go wrong quietly and only announce themselves eighteen months later. And there is a licensing dimension: in Queensland, building work above a threshold commonly stated as $3,300 including materials and GST requires a licensed contractor and triggers Home Warranty Scheme insurance. Confirm the current figure and the correct licence class with the QBCC directly for your specific scope — published QBCC material does not clearly settle which class covers remedial chemical damp-course injection, and you should get that answer in writing.

Here is the split we would give a friend. There is real, effective, free-to-cheap work you can do yourself, and it fixes a meaningful share of what gets sold as rising damp. There is also a category of DIY that reliably makes things worse.

• Worth doing yourself: clear the gutters, downpipes and rainwater heads, and check where the stormwater actually discharges in heavy rain.
• Worth doing yourself: pull the garden bed back to leave a strip at least 300 mm wide, preferably over 500 mm, surfaced in coarse gravel; move sprinklers to drippers kept at least 500 mm off the wall.
• Worth doing yourself: dig a small pit and measure the ground level against the damp course. It should be at least 150 mm below it, or 75 mm below where paving slopes away.
• Worth doing yourself: clear blocked, buried or rendered-over subfloor vents, and remove debris and vegetation from the subfloor. This is often the cheapest genuinely effective intervention available. Older subfloor soils may have been treated with organochlorine termiticides, so take appropriate precautions.
• Worth doing yourself: repipe an air-conditioner condensate line that discharges at the base of a wall, and move the outdoor unit if it is blowing warm air onto old masonry.
• Worth doing yourself: dry-brush or vacuum efflorescence off. Never acid-clean it. Hydrochloric acid used for a builders clean introduces chloride into the masonry, and chloride does not come back out.
• Do not: apply damp-proof paint or a water-repellent sealer to the wall face. Reducing evaporation raises the height of rise. Modelled on a 215 mm wall rendered to 1.25 m with evaporation cut tenfold, the damp front re-stabilises above the render — taking around 425 days to reappear, which is precisely why the treatment looks like a success first.
• Do not: render the base of the wall in hard cement, or repoint failed lime joints in cement mortar. Both drive the damp higher and the second can damage the bricks.
• Do not: replaster a wall that may stay damp in gypsum plaster. Gypsum is calcium sulfate — itself a slightly soluble salt — so residual moisture triggers salt attack inside the new plaster. Lime or lime-cement is the correct specification.

Treatments we will not install, and the evidence for that position

Passive electro-osmosis — an earthed copper strip looped into drilled holes and laid in a raked mortar joint about 300 mm above ground — has no scientific basis. The direction of causation is the wrong way round: it is the movement of water through the porous medium that creates the electrical potential, not the potential that drives the water, so earthing the resulting charge cannot prevent damp from rising because capillary suction is unaffected. Some analyses go further and suggest earthing the potential might increase upward flow rather than reduce it. It was widely installed across Australia in the 1960s and 70s and acquired a notorious record; most claimed successes are attributable to the gutter repairs, ventilation and drainage work done at the same time. The Australian heritage guide lists it among old treatments that should no longer be considered, and of the related Knapen tube it notes that tests have shown an empty hole is just as effective in drying the surrounding wall.

Active electro-osmosis — an applied DC current between platinum-coated titanium anodes and an earth-stake cathode — is a genuinely different case, and it deserves to be described accurately rather than dismissed. The mechanism is real and is used industrially to dewater silts and clays. But it has never been awarded a BBA Agrément certificate. BRE inspections of installations found them coupled with a replastering system that itself provided a good moisture barrier, so claimed successes may rest on the render rather than the current. Independent analysis suggests the flow produced by fields of that magnitude is small against capillary flux, and that electro-osmosis is further suppressed in the presence of salts. It must remain switched on permanently — it is an appliance with an indefinite power and maintenance obligation, not a repair — later building work can sever the cables, and stray currents can corrode reinforcing steel, pipes and buried metal. Its performance in very salty walls is explicitly described as unclear. We do not install it. If you are considering it, ask for independent evidence and exhaust the well-evidenced options first.

Damp-proof paints, waterproof coatings and water-repellent sealers on the wall face we will not apply to a rising damp problem at all. The physics is not ambiguous: the steady height of rise varies inversely with the square root of the evaporation rate, so sealing the face makes the front climb. Worse, moisture and salt are trapped behind the coating and crystallise at the interface, delaminating the coating and, on case-hardened sandstone, taking the stone's protective surface with it. The Australian guide's instruction is not hedged: don't even think about sealing walls with water-repellent coatings. Where a coating is unavoidable, breathability governs — limewash first, then cement-based paints, then acrylics, and never oil-based alkyd paint on a wall that is damp or still drying.

When injection is the right answer

Set against all of the above, there is a real case for this work, and it looks like this. Sound, reasonably regular masonry with continuous mortar joints — brickwork rather than random rubble — where the mortar is not crumbling and the wall is not saturated. A building with no damp course at all, or one whose damp course has genuinely failed rather than merely been bridged. Source control already carried out and its effectiveness assessed over a period of time, at least a year, as the Australian guide requires. And drilled-sample evidence: a vertical gravimetric profile showing free water present and decreasing with height, with hygroscopic moisture content measured separately at 75% relative humidity so salt-held moisture is not being counted as rising damp.

On that wall, a correctly installed chemical damp-proof course forms a continuous water-repellent zone that capillary moisture cannot climb past. It is the best-evidenced chemical option available, it is what we install, and where salt is also present above 0.5% by weight we will tell you that desalination and lime replastering by a masonry restoration contractor needs to be scoped alongside it. We will not tell you the wall is fixed when only half the problem has been.

One boundary worth restating because the search results blur it constantly: this is work on external brickwork. Slabs are a different problem with a different answer. Where an internal concrete slab is passing moisture into new flooring, our treatment is from the top side — grind back, prepare the surface, apply a Hydropoxy epoxy moisture barrier before the floor covering goes down — and no injection is involved anywhere in it.

What it costs, and why a cheap rate should worry you

Chemical damp-proof course injection in Australia runs roughly $120–$400 per linear metre, most commonly $180–$300. A whole-house injection component typically lands at $3,500–$15,000; an 18 m terrace perimeter commonly sits around $4,000–$8,000 for the injection alone, before any replastering. Published Australian ranges disagree markedly with each other — some cost guides quote as low as $70–$120 per linear metre and others $200–$400 — so treat any single published figure with suspicion, including ours, and compare itemised scopes rather than headline rates.

The reason a very cheap rate should worry you is arithmetic, not snobbery. The chemical is the expensive input. There are exactly three ways to make the number smaller without changing the job: use less product, drill fewer holes, or treat one side of a wall that needs both. All three are invisible on the day, all three are named in a government technical guide as what happens under cost pressure, and all three announce themselves in year two or three when the damp comes back.

Two of the larger operators in this market advertise that they will beat any written quote. The same guide that describes contractors diluting fluid and widening drill spacing in response to price pressure is, in effect, describing the mechanism by which that promise gets kept. We would rather publish the six numbers in the callout above and let you compare scopes.

What it costs

Questions we actually get asked

Does injection cream work as well as injected fluid?

Both work when correctly specified and installed, and they suit different walls. Cream is a high-solids water-based emulsion delivered by cartridge gun, now the default remedial damp-proof course in Australia and the UK, and it is preferred where solvent VOCs, fire risk or polystyrene cavity insulation are concerns. Solvent-borne siloxane fluid has the longer track record — around thirty years in Australia and fifty in the UK at the time of the Australian guide — with gravity feed suiting porous mortars and soft bricks, and low-pressure injection suiting standard brickwork where an operator can watch fluid appear at inspection holes above the drilled line. The bigger variable is not cream versus fluid, it is active-ingredient content, which ranges from about 15% to 80% between certified products.

The wall is still damp six months after injection. Did the treatment fail?

Not necessarily, and there are three quite different explanations worth separating before anyone argues. First, drying takes months, not weeks: 3–6 months is normal after replastering and up to 12 months on wet thick walls, so six months on a 230 mm wall is inside the expected window. Second, if the wall is salt-loaded it may never read dry on a meter at all, because hygroscopic salts hold atmospheric moisture indefinitely — which is why acceptance should be judged on drilled-sample free water rather than on an instrument. Third, if the damp was never rising, the barrier is working perfectly on a problem you did not have. Retesting with a gravimetric profile plus hygroscopic moisture content will tell you which of the three it is.

Can I install a damp-proof course myself with a kit from the hardware store?

Physically, on a single-skin external wall you can access, yes. The chemistry sold retail is often sound. What is hard to get right is everything around it: establishing the wall thickness and construction, identifying header versus stretcher courses, hitting roughly 120 mm centres consistently, drilling to within about 30 mm of the far face, treating both leaves where both need it, and grouting voids first. It is also worth checking the licensing position before you start — in Queensland, building work above a threshold commonly stated as $3,300 including materials and GST requires a licensed contractor and triggers Home Warranty Scheme insurance, and you should confirm the current figure and the applicable licence class with the QBCC. Before any of that, spend the weekend on the ground levels and the downpipes instead. That work is genuinely DIY and it resolves a much larger share of cases.

Will injection stop water coming through my garage or basement wall?

No, and this is a hard technical boundary rather than a scope preference. Chemical impregnation prevents capillary rise; it cannot resist water under a hydrostatic head. Where ground is higher on one side than the floor on the other, the wall is often wetter above the damp course than below it, which is the diagnostic signature. That is a drainage and tanking problem: external agricultural drainage in geofabric with free-draining backfill, or a drained cavity system on the retained face, taken to a legal discharge point. It is different engineering, and injecting brickwork will not substitute for it.

Does injection work on a rendered wall?

The injection itself can be carried out, but render changes the job in two ways that need addressing first. If the render runs down across the damp course, it is bridging it, and cutting the render back to expose the course and form a clean drip may be the entire fix — installing a second damp course above a perfectly good bridged one achieves nothing. And where the render is salt-contaminated or drummy, it is both a moisture trap and a salt reservoir, so it typically needs removing and replacing in a lime or lime-cement system rather than being left in place over new work. Removing salt-contaminated render commonly runs $40–$120 per square metre and replastering $80–$180 per square metre in a standard system, and that removal and replastering is not work we carry out.

How do I know the injection was actually done properly?

You cannot see a continuous water-repellent zone, so you check the inputs and the record instead. Ask for hole diameter, hole spacing centre to centre, hole depth relative to wall thickness, which face and which course was drilled, litres or cartridges used per linear metre, and the product name and certificate number — before the work, in the scope, not afterwards. Ask how the contractor determined that sufficient fluid had been impregnated. Ask whether desalination is in scope and, if not, why not. And ask what drying period is specified before replastering and repainting. A scope that answers all of those is a scope you can hold someone to.

Is there any treatment that removes the salt at the same time?

Not a chemical damp-proof course, no — and this is where most disappointment in this trade originates. Desalination is a separate, staged, labour-intensive process: dry-vacuuming loose surface salt, captive-head washing to remove near-surface salts, then absorbent clay or paper-pulp poultices applied wet so that salt crystallises in the sacrificial poultice as it dries rather than in the masonry, followed by sacrificial lime mortars and renders continuing the job over the long term. It is iterative — you re-sample nearby and re-treat until the salt level drops — and it should ideally begin before injection, because injection can drive saline moisture upward into masonry that was not previously damaged. It is masonry restoration work at roughly $80–$250 per square metre and we do not carry it out. Where your wall needs it, we will say so before quoting and point you to who does.