Damp-proof course installation: chemical injection into the mortar bed

What the chemistry actually does

The active ingredient is a silane, a siloxane, or a blend. Delivered into the pore network of the masonry, it reacts with the substrate and deposits a hydrophobic silicone resin that lines the pore walls. Water can no longer wet those surfaces, so capillary suction has nothing to grip and the upward path is broken.

The important word is lines, not blocks. The pores stay open. The wall still breathes, still exchanges vapour with the air, and still dries out above the treated zone once the supply from below has been cut. This is the opposite of a surface sealer, which closes the face, reduces evaporation and drives the damp front higher.

The procedural reference for this work is BS 6576, the British code of practice for diagnosis of rising damp and installation of chemical damp-proof courses, which is cited in the Australian heritage guide's own standards list. Its central requirement is worth repeating: masonry samples must be taken, because rising damp cannot be diagnosed from surface measurements. AS/NZS 2904 governs sheet damp-proof course and flashing materials — the physical products built into new masonry — and explicitly does not cover mortar-type damp courses, so it is not the standard a remedial chemical DPC is installed to.

Cream or fluid

Two delivery systems dominate. Both use silicone chemistry; they differ in carrier, in how they get into the wall, and in what they are suited to.

Silane and siloxane cream is a high-solids water-based emulsion delivered by cartridge gun through a tube to the back of each hole. It stays where it is put and diffuses outward through the pore structure rather than running out of the hole, which is why it has become the default remedial damp course in Australia and Britain. It also avoids solvents, so it is the sensible choice where volatile organic compounds, fire risk, or polystyrene cavity insulation are in play.

Solvent-borne siloxane fluid is the older method, with roughly thirty years of Australian and fifty years of British use behind it. The active sits at around 5 to 7% by weight in an organic solvent and is either gravity-fed from feeder bottles, refilled three or four times over about 24 hours, or injected at low pressure. Gravity feed suits porous mortars and soft bricks. Low-pressure injection suits ordinary brickwork. One practical advantage is that an operator can watch for fluid appearing at inspection holes above the drilled line, which is direct evidence of penetration rather than an assumption about it.

Solvent-borne fluid carries two hazards worth naming. It dissolves polystyrene cavity insulation, and it can dissolve tar or bitumen out of an existing damp course and spread brown staining through the masonry. Neither is a reason to avoid it categorically; both are reasons the choice should be made after somebody has looked inside the wall.

Why the mortar bed and not the brick

The holes go into a horizontal mortar joint, not into the face of the bricks, and there are four separate reasons for that.

Mortar generally diffuses better than fired brick. Lime mortar in particular becomes more sorptive over decades of water passing through it, which is why old walls transmit water so readily in the first place and why the joint is the best route for getting a treatment to travel sideways and meet its neighbours.

A continuous bed joint is a continuous horizontal plane. A damp course only works if the treated zones from adjacent holes overlap into an unbroken band across the full wall thickness. Drilling a course of bricks would put the treatment in a series of separate blocks with mortar joints between them, which is the shape of the problem rather than the shape of the solution.

Mortar is repairable and replaceable. Face brick is not. A plugged mortar joint can be repointed and colour-matched; a row of 12 mm holes drilled through the fireskin of a face brick is permanent damage to the most weather-resistant part of the unit. The heritage guide makes the same point about sampling for testing: take samples from mortar in preference to face brick or stone, because it is less disfiguring.

And in one situation the rule flips. Where the mortar is markedly less permeable than the units — a hard cement pointing job over soft bricks, for instance — the salts and the moisture concentrate in the units instead, and the treatment strategy has to follow the water rather than the convention. Dense rubble stone can need enveloping patterns of holes above, below and beside each stone rather than a single course. That is not brickwork and it is not the job described on this page.

Drill diameter, spacing, depth and pressure

These are the numbers that decide whether the job works, and they are the numbers that quietly disappear from a cheap quote. We put them on ours.

Holes are 10 to 15 mm in diameter, set out at approximately 120 mm centres along the joint. That spacing exists so that the treated zone around each hole overlaps its neighbours. Widening it is the single cheapest way to reduce the amount of expensive product a job consumes, and it leaves untreated corridors between the holes that nobody can see afterwards.

Depth runs to within about 30 mm of the far face. Not through it — you want the chemistry to reach the far side of the wall without punching out the other face. On a rendered or lined internal face, breaking through is both a mess and a repair.

Injection pressure, where fluid is injected rather than gravity fed, sits in the range of about 150 to 500 kPa. Above roughly 1000 kPa the process changes character: the fluid stops advancing as a front and begins advancing as fingers, racing ahead through the paths of least resistance and leaving untreated gaps between them. That is viscous fingering, and it is a defect rather than a productivity gain. High pressure also blows out weak mortar in old walls.

Three preconditions govern whether any of this can proceed. Voids in thick or irregular walls have to be grouted first or they simply swallow the product. Very wet masonry limits diffusion, so a saturated wall may need the source cut and some drying time before the treatment is worth applying. And weak, crumbling mortar will not hold a chemical damp course at all — that wall needs repointing in a weak lime mix first, with the additional trap that extensive repointing can itself bridge a treated course, so the zone may need re-treating once the new mortar has cured.

One thing we do not do: drill and impregnate directly into an old tar-and-sand damp course. It may well be underperforming, but perforating it does not help it, and it is the route by which bitumen staining gets spread through a wall.

Where salt is present, a desalination poultice should go on at the same time as the injection, not afterwards. Introducing fluid into a wall can displace saline moisture upward into masonry that was previously undamaged.

Where the treated course is set out

Height matters as much as chemistry. A perfectly installed damp course in the wrong place is a perfectly installed nothing.

The NCC sets the positions for a damp course in new masonry, and they are the benchmark to set out against: not less than 150 mm above adjacent ground level; 75 mm above finished paved, concreted or landscaped areas that slope away from the wall; 50 mm where protected by a carport, verandah or similar; and in low rainfall intensity areas 15 mm, or 0 mm where protected from the weather. These sit in the ABCB Housing Provisions at Part 5.7, with acceptable materials listed at clause 5.7.3.

For a remedial course, the heritage guide recommends setting out 150 to 250 mm above finished ground with 200 mm as the ideal, and then maintaining that clearance as an ongoing maintenance item. The extra height above the code minimum is deliberate: it gives you room for the mulch, the new paving and the garden bed that will otherwise bury the course in ten years.

The treated zone must also sit below all floor timbers. Placing it above them leaves the bearers and joists on the wet side of the barrier, which converts a damp problem into a timber problem.

One currency note for specifiers. NCC 2025 was published in 2026 but adoption is fragmented: Victoria, the ACT and Western Australia adopted from 1 May 2026, while New South Wales and Queensland deferred to 1 May 2027. As at August 2026 the operative edition in Queensland is still NCC 2022. Confirm the adopted edition, the clause numbering and any state variations for your project date before quoting a clause in a specification.

Working from the external face, and the 230 mm problem

We work from outside. That is a deliberate limit and it has consequences worth understanding before you compare quotes.

On a 230 mm solid one-brick wall the header course should be drilled by preference, because a header runs the full thickness of the wall and a single line of holes can therefore treat the entire section. Where a stretcher course is drilled instead, the stretchers on the other side of the wall have not been reached, and they must also be treated — either from the far face, or by re-drilling deeper through the same holes in a second phase. A wall that needed both and got one is the most common form of the cheap quote, and it is undetectable from the finished surface.

A cavity wall is two separate walls with an air gap between them, and each leaf is its own capillary path. Treating the outer leaf does nothing for the inner one. Any quote for a cavity wall should state plainly which leaves are being treated and how the inner one is being reached.

Brick veneer is different again. The masonry skin is a rain screen; the timber or steel frame behind it is the structure. Damp appearing on the inside of a brick veneer house is far more often a bridged cavity — mortar droppings on wall ties, mortar at the cavity base, blocked weepholes, missing cavity flashing — than capillary rise. Ten minutes with a borescope answers that question, and injecting the masonry skin does nothing for it.

Where a wall genuinely can only be treated correctly from inside, that means plaster removal, the works, and reinstatement afterwards. It is legitimate work and it is not ours. We will say so.

Abutments, thresholds and internal walls

A chemical damp course is a horizontal line, and buildings are not made of horizontal lines. The junctions are where the line gets broken.

Where a concrete path, driveway, floor slab or hearth abuts the wall at or above damp course level, that element bridges the barrier — moisture travels through the concrete and into the masonry above the treated zone, and the new damp course is simply bypassed. The remedy is undercutting: cutting through the bridging element at the junction and introducing a damp-proofing element, paired with vertical damp-proofing at the junction so the bridge does not simply re-form. There are structural implications where the slab is load-bearing or edge-thickened. It is a different scope with a different trade and different licensing, and we do not perform it.

Door thresholds, steps and porches present the same problem in miniature. A treated course that stops at a threshold and resumes on the other side has a gap in it, and the damp will find the gap. Where the threshold detail cannot be resolved, that has to be recorded in the scope rather than glossed over.

Internal walls also carry rising damp, and they are frequently the part of a job that gets quietly dropped. Treating them means removing plaster to expose the masonry, drilling, treating, then reinstating the lining and the skirting boards after the wall has dried — months later, not the same week. That is plastering and carpentry work. We can tell you where the internal walls need treating and what the correct sequence is. We do not do the internal work.

The honest summary: on many houses a chemical damp course to the external brickwork is one component of a job, not the whole job. A quote that covers only what we do is not a quote for a dry house, and it should never be presented as one.

What a chemical damp course does not do

This list is not fine print. Every item on it is a case we see quoted as a damp course somewhere in this market.

• It does not remove existing salt. Salt already in the wall above the treated course keeps drawing moisture from the air, dissolving in humid weather and re-crystallising in dry weather, and each cycle keeps breaking down the masonry. The heritage guide shows brickwork continuing to decay above a chemically injected damp course for precisely this reason. Desalination is a separate step and it should start before the injection, not after.
• It does not dry the wall. It stops the wall being resupplied. Everything already in the masonry leaves by evaporation, at roughly a month per 25 mm of thickness — about nine months on a 230 mm double-brick wall, and often longer in humid weather.
• It does nothing for condensation. Condensation is warm moist air meeting a surface below its dew point, and it is diagnosed by logging surface temperature against room dew point over at least a week, not with a meter. Around 70% of mould problems are attributed to condensation rather than rising damp. No damp course affects it in the slightest.
• It does not resist water under pressure. Impregnation prevents capillary rise; it cannot hold back a hydrostatic head. A retaining wall, a basement or a cut-and-fill garage wall is a drainage and tanking problem with different engineering behind it.
• It does not fix a bridged damp course. If render, hard pointing, paving or soil is carrying moisture around an intact existing course, the fix is to cut the render back and form a drip, or to lower the ground. Injecting a second course above a perfectly good first one is money for nothing.
• It does not fix a bridged cavity. Mortar snots on wall ties and mortar at the cavity base are cleared and flashed, not injected.
• It does not apply to a concrete slab. There is nothing to inject and no course to cut. Slab moisture is measured with in-situ relative humidity probes to ASTM F2170 at 40% of slab depth, referenced by AS 1884:2021, and managed by drainage, edge detailing and — where a floor covering has to go down over a slab that will stay wet — a surface barrier such as a Hydropoxy treatment on top of the slab.

Does it work? The honest position

Yes, under two conditions, and the conditions are not decoration.

Condition one is that the diagnosis is right. A chemical damp course installed against condensation, a bridged course, a downpipe, lateral damp or hygroscopic salt with no live source will perform exactly as designed and change nothing you can see, because the water was never rising. This is the dominant failure mode in the category and it is a diagnostic failure, not a product failure.

Condition two is that the installation is disciplined. The known failure modes are specific and well documented: drill spacing widened beyond about 120 mm, fluid diluted with extra solvent, a wall needing both leaves or both faces treated from only one side, voids not grouted before injection, injection into masonry so saturated that diffusion cannot occur, and injection pressure high enough to cause viscous fingering. The heritage guide states the commercial mechanism in print — dampcoursing fluid and creams are expensive, so there is cost pressure on contractors to use less, and unscrupulous contractors may dilute the fluid or space the holes wider than recommended. Good operators deliberately over-apply to be certain of penetration.

The product matters more than the category name. Active ingredient content across certified products varies from roughly 15 to 20% at the weak end up to around 60 to 80% at the strong end, and low-strength formulations have still obtained certification. The word silane on a container tells you very little. The specific product's certificate tells you a great deal.

Which is why an unusually cheap per-linear-metre rate is a technical signal rather than a bargain. The published Australian range is $120 to $400 per linear metre, commonly $180 to $300. Below about $100 per linear metre, ask three questions and get the answers in writing: what is the drill hole spacing, what volume of product goes in per linear metre, and is the wall being treated from one side or both. The fraud, where there is one, is sealed inside a wall and nobody can ever see it — which is exactly why the numbers have to be on the paperwork before the drill comes out.

What happens on site

Access is external, along the base of the wall. Garden beds, plants and anything within about a metre of the wall need to be clear, and where the ground has to come down to reach the correct set-out height, that excavation is arranged before we arrive because it is not our work.

Drilling produces silica dust. It is controlled with on-tool extraction and water suppression, and the exclusion zone around the work is real rather than notional. Where the building predates 1990 and any render, plaster or flooring is being disturbed as part of the wider job, asbestos and lead paint have to be identified and managed by the party doing that work before it starts. These are legal obligations under the Work Health and Safety Regulation, not optional extras, and they materially affect the programme.

Duration on a typical single-storey brick perimeter is measured in days rather than weeks. The holes are plugged and colour-matched to the existing mortar as closely as the mortar allows — on a weathered nineteenth-century lime joint, honestly, they will be visible for a while and will weather in.

What we hand over at completion is the set of numbers this trade usually keeps to itself: the drill hole spacing and depth used, the product and batch, the volume introduced per linear metre, the set-out height of the treated course above finished ground, and photographs with the locations recorded so the wall can be re-sampled nearby later. That last point matters more than it sounds. Acceptance of a rising damp job is judged on gravimetric free-water content from drilled samples over time, not on a meter reading on the day, and you cannot compare readings you cannot locate.

What it costs

Questions we actually get asked

Someone has quoted $85 per linear metre. Is that a bargain?

It is a reason to ask three specific questions. The published Australian range is $120 to $400 per linear metre and commonly $180 to $300, and the product itself is a real cost that does not vary much between contractors. So ask what the drill hole spacing is, what volume of product goes in per linear metre, and whether the wall is being treated from one side or both — and get those answers in writing on the quote. The heritage guide names the two responses to price pressure explicitly: diluting the fluid with extra solvent, and spacing the drill holes wider than recommended. Both are invisible once the holes are plugged, and both surface as damp two or three years later when the paperwork has been thrown out.

Do you drill from inside or outside?

Outside. We install chemical damp-proof courses to external brickwork and that is the scope we hold ourselves to. Internal walls also carry rising damp and often need treating, but doing it properly means removing plaster to expose the masonry, drilling, treating, and then reinstating linings and skirting boards after the wall has dried months later. That is plastering and carpentry work and we hand it across. We will tell you where the internal walls need treating and in what order it has to happen.

Our wall is rendered. Can you inject through it?

Two things have to be established first. If the render is carried down over an existing damp course, it is very likely the actual cause of your problem — the barrier is intact and the render is carrying moisture around it — and the answer is to cut the render back and form a clean drip rather than to inject anything. Where injection is genuinely warranted, render in the treatment zone generally has to come off so the mortar joint can be located and set out accurately, and it is a poor idea to reinstate hard cement render on a wall that is going to spend the next nine months drying. Render removal and replastering are not our trade; the specification to hold your plasterer to is lime or lime-cement, never gypsum on a wall that may stay damp.

We have cavity brick. Is one line of holes enough?

No. A cavity wall is two walls with an air gap between them, and each leaf is a separate capillary path with its own damp course requirement. Treating the outer leaf does nothing for the inner one. Any quote should state which leaves are being treated and how the inner leaf is reached. It is also worth ruling out a bridged cavity before treating anything — mortar droppings on wall ties, mortar at the cavity base, blocked weepholes or missing cavity flashing produce damp patches that are frequently higher than the damp course and sometimes isolated to individual ties. A borescope settles it quickly, and injection does nothing for it.

Will the drill holes be visible afterwards?

For a while, yes. The holes are 10 to 15 mm in diameter at roughly 120 mm centres along one mortar joint, and they are plugged and colour-matched as closely as the existing mortar allows. On modern grey cement mortar the match is usually good. On a weathered nineteenth-century lime joint the new material will read slightly different until it weathers in, and honest colour matching on old mortar is a genuine skill rather than a formality. Anyone promising an invisible result on a hundred-year-old joint is overpromising.

Do you inject brick veneer?

Rarely, and usually only after something else has been ruled out. In brick veneer the masonry skin is a rain screen and the frame behind it is the structure, so damp appearing on the inside is far more often a bridged cavity, a blocked weephole or a missing cavity flashing than capillary rise through the skin. Those are cleared and flashed, not injected. Where the veneer skin itself is genuinely wicking and the prerequisites have already been corrected, a chemical course into the skin can be appropriate — but the diagnosis has to come first and a borescope is cheaper than a drill.

How will I know enough product actually went in?

By what is written down. Where solvent-borne fluid is gravity-fed or injected, penetration can be observed directly as fluid appears at inspection holes above the drilled line. Where cream is used, the evidence is dosage against hole spacing, which is why the volume per linear metre and the spacing belong on the quote and on the completion paperwork rather than in somebody's head. Longer term, the only real verification is drilled samples: gravimetric free-water content at recorded locations, re-sampled nearby after the wall has had time to dry. Record the sample locations and photograph them, because readings you cannot locate cannot be compared.

Is this the same thing as waterproofing a basement or garage wall?

No, and confusing the two is expensive. A chemical damp course prevents capillary rise; it cannot resist water under a hydrostatic head. Where ground sits higher on one side than the floor on the other — a basement, a cellar, a cut-and-fill garage, a retaining wall — the water is being pushed sideways under pressure, and the diagnostic signature is a wall that is wetter above the damp course than below it. That needs external drainage, free-draining backfill, and tanking or a drained cavity system taken to a legal discharge point. It is a different engineering problem and injection is the wrong tool for it.