Fifteen times we'll tell you not to inject

The three questions that decide it, before anyone drills

Every one of the fifteen cases below is caught by the same three questions, asked on site before a drill comes out of the ute. They take about twenty minutes and they are the highest-value part of the job.

The first question is where the water actually enters, not where it appears. Water tracks laterally between a membrane and the concrete, and between a structural slab and its topping, then exits at the weakest point — frequently metres from where it got in. In a basement or an undercroft garage, the visible wall crack is often innocent and the real path is the kicker joint where the wall meets the floor slab.

The second is whether there is a defined path at all. A crack or a joint is a line, and a line can be injected. A wall that is damp across two square metres is not a line, and there is nothing for resin to grip.

The third is where the water will go instead. If the honest answer is nowhere, sealing simply converts a leak into pressure, and pressure finds another way out.

• Has it rained in the last 72 hours, and does the damp correlate with rain at all?
• Where does the ag line discharge, and is it flowing?
• Are the weep holes clear, or have they been rendered over?
• Is there rust staining or a hollow, drummy sound beside the crack?
• Is the crack a uniform-width straight gap in a regular pattern — a designed joint — or an irregular fracture?
• Is the slab post-tensioned, and can anyone actually confirm that?

Cases 1 and 2: the structure has not finished moving

1. Case 1 — The element is still movingThe giveaways are diagonal cracking, a crack that widens toward the top, a crack that visibly opens and closes with the weather, or any measurable lean or bulge. Epoxy has effectively zero movement tolerance: it welds the two crack faces into one element, so a structure that is still moving simply cracks again a few millimetres away, and to the client that looks like your failure. Polyurethane foam is semi-rigid rather than elastomeric, so continued cyclic movement tears it open. The correct sequence is tell-tales or crack monitors read over 4 to 12 weeks so the crack is measured across a full temperature and moisture cycle, then an engineer to establish the cause, then stabilise or accommodate the movement, and only then seal. Who does it: a structural engineer first, us afterwards. Two months of monitoring costs a fraction of injecting the same crack twice.
2. Case 2 — The footing is still settlingCracking driven by a footing that is still moving will reopen regardless of what goes into it. In South East Queensland the usual causes are reactive clay working through wet and dry cycles, poorly compacted fill, a leaking sewer or stormwater line washing subgrade out from under the slab, and mature trees drawing moisture out of the ground within a root radius of the footing. Sealing the crack does nothing about the movement. The cause has to be corrected first — the service repaired, the site drainage fixed, the ground re-supported — and only then does a crack repair mean anything. Who does it: a geotechnical or structural engineer to diagnose, a licensed plumber if a service is the cause, and a ground-engineering specialist if the ground itself needs improving. That last one is a genuinely different trade from ours, and we have written the whole distinction out separately.

Cases 3 to 5: the water management system has failed, not the concrete

1. Case 3 — The membrane has reached the end of its lifeInjection seals a defined path through concrete. It cannot restore a membrane that has debonded, blistered, split or been UV-degraded across an entire deck, balcony or planter box. Where the membrane is at end of life the correct answer is removal and replacement — to AS 4654.1 and AS 4654.2 for external above-ground work, or AS 3740:2021 for internal wet areas. Note the edition: a contractor still citing AS 3740-1994 in 2026 is quoting a standard that has been superseded for years. Injecting instead typically buys 6 to 18 months. The scale difference is the honest part of the conversation: injecting a leaking planter box is half a day to two days on site, while a correct rebuild — strip the soil, remove and replace the membrane, reinstate the drainage cell, agi line and outlet — is one to three weeks. We will quote the interim measure if you need the time, but we will put in writing exactly what it is. Who does it: a remedial waterproofing contractor. Balcony and deck membrane replacement is our parent business's scope, not ours.
2. Case 4 — The drainage is blocked, undersized or was never thereBlocked or absent ag lines, weep holes rendered over during a repaint, no drainage cell behind a retaining wall, no fall, undersized or root-blocked stormwater, and downpipes discharging straight at the footing. This is the actual cause behind a very large share of enquiries that arrive asking for injection by name, and clearing a silted ag line is sometimes the entire fix for a few hundred dollars. It matters structurally as well as commercially: sealing the face of a retaining wall without restoring drainage raises the retained water head and increases the load on a wall that may never have been designed for saturated backfill. On core-filled block that is how mortar joints get blown out. Who does it: a drainage contractor, a landscaper or a plumber, depending on what is blocked. We check drainage before we quote, and the check appears in the quote, because it is the single most credible thing a specialist can put in front of you.
3. Case 5 — The source can simply be redirectedBefore any resin is priced, the question is whether something upstream is putting water against that wall unnecessarily. A downpipe discharging at the base of it. A path or driveway graded back toward the building. A leaking gutter, a neighbour's overflow relief, an air-conditioning condensate line, an irrigation head pointed at a retaining wall. A downpipe extension, a re-grade or a diverter is cheaper, more reliable and more permanent than anything we can sell you, and most of it is an afternoon's work. Who does it: a roof plumber, a landscaper, or you with a spade and a length of ag pipe. Telling a client this costs us one job and earns us the next five, which is the only honest reason it survives on a page written by a contractor.

Cases 6 to 8: the water is not coming through the concrete at all

1. Case 6 — It is rising damp, not a leakRising damp is capillary moisture drawn upward out of the ground through porous masonry. It is not water passing through a crack, and no crack repair will affect it. The signature is a tide mark to roughly a metre above ground level, white salt bloom along that line, plaster or render perishing, and no correlation with rainfall at all. The treatment is a chemical damp-proof course: a silane or siloxane fluid or cream injected into the mortar bed low in the wall, forming a continuous horizontal water-repellent band that capillary moisture cannot climb past, or a physical damp-proof course cut in. The two jobs get confused constantly because both involve the word injection and a man with a drill, and they share almost nothing else — different chemistry, different target, different failure mode. Who does it: a rising damp specialist. That is our parent business's work.
2. Case 7 — It is condensationDamp that appears on cold surfaces in still, humid, poorly ventilated spaces — basements, plant rooms, storerooms, sealed garages, lift lobbies — with no correlation to rainfall is very often condensation. South East Queensland's summer humidity makes it far more common here than the overseas literature suggests. There is a test that costs nothing: tape a 300 by 300 mm square of clear plastic sheet tightly to the damp surface on all four edges and leave it 48 hours. Moisture on the room side of the plastic is condensation. Moisture trapped between the plastic and the concrete is water coming through the wall. If it is condensation, injection achieves precisely nothing and the client is worse off by the cost of it. Who does it: ventilation, dehumidification, insulation and reducing internal moisture sources — a building services or HVAC contractor, and sometimes just a correctly sized extraction fan on a humidistat.
3. Case 8 — It is a leaking pipe or serviceA weeping wall above a corroded main, a cracked sewer, a blocked stormwater line surcharging under a slab, a failed irrigation solenoid inside a planter. This is quick to rule out and expensive to get wrong, so services get pressure-tested before anyone injects anything. The signature is damp that persists through dry weather with no seasonal pattern, sometimes accompanied by a water bill that has moved without an explanation. There is a second reason it matters: a leaking service under a slab washes fines out of the subgrade, so the same defect can be the cause of both the water and the cracking, and sealing the crack fixes neither. Who does it: a licensed plumber, with a camera and a pressure test.

Cases 9 to 11: sealing it would actively make things worse

1. Case 9 — The crack is corrosion driven (concrete cancer)Cracking that runs along the line of the reinforcement rather than across the element, with rust staining, spalled edges, or concrete that sounds hollow and drummy when tapped beside it, is caused by steel expanding as it corrodes. Rust occupies several times the volume of the steel it came from, and that expansion is what is opening the crack. Injecting seals the crack while the steel keeps expanding: the repair looks perfect and fails from the inside out, usually taking the surrounding cover with it. The correct scope is concrete repair — break out to sound concrete behind the bar, clean or replace the steel, apply a passivating primer, reinstate with a compatible repair mortar, and on larger structures consider cathodic protection or a migrating corrosion inhibitor. Who does it: a concrete repair contractor. That is our parent business. This is the most common misdiagnosis we see on car park soffits, where the drip and the rust stain arrive together and only one of them is the actual problem.
2. Case 10 — It is a designed movement jointExpansion joints, control joints and the vertical joints between precast or tilt panels exist to move. Filling one with resin defeats the joint and transfers that movement into the concrete either side of it, which then cracks — and the new crack is unambiguously your repair's fault. The tell is a straight, uniform-width gap appearing in a regular pattern, usually with the remains of a backing rod and a perished sealant in it. The correct repair is joint preparation, correct backing rod diameter and a movement-capable sealant or a proprietary joint system. The materials make the point: cured epoxy elongation is effectively zero, while a cured acrylate gel can stretch several hundred percent. Who does it: a remedial contractor, us included — but as a sealant scope at a sealant price, not as injection. Construction joints and cold joints are a different animal entirely and are injected routinely at 200 to 400 mm packer centres.
3. Case 11 — The water would have nowhere else to goAlways ask where the water goes instead. Seal the only outlet from a saturated cavity, a planter box whose drainage outlet is already blocked, or a void beneath a slab, and you have converted a leak into hydrostatic pressure rather than removed it. That produces uplift on thin slabs, concentrates salt attack at the next weak point, or simply blows the repair back out. A planter box sealed without restoring its outlet becomes a bathtub, and a full bathtub is a load the structure was never asked to carry. Voids under slabs will also swallow effectively unlimited resin, which is why we set a hard stop of 1 to 2 litres per packer and move on rather than pumping a drum into the ground on your invoice. Who does it: restore the outlet, the agi line or the drainage path first, then seal. If nobody can answer where the water goes, that is a design problem and it needs a hydraulic or civil designer, not an injection quote.

Cases 12 and 13: nothing to grip, or nothing to be gained

1. Case 12 — The whole element is porous, with no defined pathWhere water comes through the body of a wall or slab across a broad area rather than along a line, crack injection has nothing to grip. It is common in older, poorly compacted concrete, in honeycombed zones at the base of a pour, and in walls sitting below the water table. The options are negative-side crystalline or cementitious tanking, a drained cavity system that accepts the water and channels it to a sump, curtain injection into the ground behind the wall on a 250 to 500 mm grid, or excavation and a positive-side membrane. Which one is right depends on the head of water, the finish you need and whether excavation is even possible. Who does it: still us for the tanking and the curtain work, but understand that it is a different scope with a different pricing basis — curtain injection is priced from a trial panel at a rate per point or per kilogram of gel, never as a lump sum, because you are injecting into ground you cannot see.
2. Case 13 — What is actually wanted is more strengthEpoxy injection restores monolithic action across a crack. Cured compressive strength sits in the 60 to 90 MPa range and the bond exceeds the tensile strength of the parent concrete, so a correct epoxy repair makes the concrete fail beside the old crack rather than in it. What it cannot do is add capacity. If the member is under-designed, overloaded, or has lost section to corrosion or impact, injection returns it to roughly what it originally was — and what it originally was is not enough. That needs a strengthening design: fibre-reinforced polymer wrap, bonded steel plate, or additional support. Who does it: a structural engineer designs it and a specialist remedial contractor installs it. We will happily inject as one item on that engineer's drawing. We will not inject instead of it, and any contractor offering to solve a capacity problem with a resin gun is telling you something about their understanding of the resin.

Cases 14 and 15: the two hard stops

1. Case 14 — A post-tensioned slab that has not been scannedPost-tensioned slabs contain steel tendons stressed after the concrete cured and still under enormous tension. They are common in car park decks and apartment slabs across Brisbane and the Gold Coast, and they are invisible from the surface. Drilling into one without scanning can cut a tendon, which is potentially catastrophic and is not a recoverable mistake — there is no version of this we will improvise or take somebody's word on. The sequence is: scan with ground-penetrating radar, mark the tendons, and obtain written engineer approval that nominates both where drilling is permitted and the maximum injection pressure for that element. If nobody on site can confirm whether the slab is post-tensioned, we treat it as if it is, and the scan goes in the quote as its own line. Who does it: a scanning contractor and the structural engineer, before a drill comes onto the deck.
2. Case 15 — Heritage and soft lime-mortar masonrySoft historic masonry bedded in lime mortar behaves nothing like a reinforced concrete wall. The pressures used for mechanical-packer polyurethane leak sealing run commonly from 35 to 140 bar, and that will destroy a lime bed, blow the joints and cause damage that cannot be undone. Heritage fabric usually also needs to breathe: sealing the face can drive moisture and dissolved salts back into the stone or brick and accelerate its decay rather than protect it. This is different competence, different materials, very low or no pressure, and generally heritage advice or an approval before anything is touched. Who does it: a heritage masonry specialist working with a heritage adviser or the relevant approval authority. If the building is listed, that conversation happens before the assessment, not after the damage.

All fifteen, in one table

Print this, or take it to whoever has quoted you. If the quote in your hand is for injection and your situation is in the left-hand column, ask the contractor which of these they ruled out and how.

What the honest answer costs you, in dollars

The reason contractors avoid this conversation is that diagnosis is unbillable time on a job you might not win. The counter-argument is arithmetic: a wrong repair is paid for twice, and the second payment usually includes making good the first.

These are indicative South East Queensland bands, not quotes, and they move with access, height, occupancy and distance from base.

How to use this list on the quote already in your hand

You do not need us to apply this. Take whichever quote you are holding and ask the six questions below. The answers separate a contractor who diagnosed the problem from one who priced the symptom, and they work equally well against our own quotes — which is the point.

If you are a body corporate committee or a strata manager putting this out to three contractors, the same questions belong in the scope document so that every quote you receive is for the same work. Quotes that are not comparable are not really quotes.

• Which of these fifteen did you rule out on site, and what did you look at to rule them out?
• Did you check the drainage — ag line discharge, weep holes, downpipe discharge, falls — before you looked at the crack?
• Are you drilling and installing mechanical packers that seal inside the wall, or gluing surface ports to the face? Surface ports will not adhere to an actively leaking wall.
• Which resin are you using, and why that one for this crack rather than the alternatives?
• Is making good included, and is the price per linear metre, per crack, or per job?
• What does your warranty exclude, and specifically does it cover a new leak that opens somewhere else after this one is sealed?

Questions we actually get asked

You have just told me not to inject. Will you still come and look at it?

Yes, and it is most of what an assessment is for. A quote-only visit is free for most straightforward enquiries. If the situation needs instruments — moisture readings at marked points, crack monitors installed and read over 4 to 12 weeks, water testing, scanning — that is a paid investigation in the $450 to $1,500 range and you get a written finding you can take to whoever ends up doing the work, including a competitor.

Another contractor has quoted injection for something on this list. Who is right?

Possibly them. This list is about what the evidence on site shows, not about a rule that applies from a photograph. Ask them the specific question rather than relaying our opinion: for a suspected membrane failure, ask what area of membrane is debonded and how they established it; for a suspected drainage problem, ask where the ag line discharges and whether they watched it run. A contractor who has done the diagnosis will answer immediately. A contractor who priced the symptom will change the subject to warranty.

How long does injection actually buy me if the membrane really has failed?

Broadly 6 to 18 months, and less than that where root pressure, blocked outlets or fertiliser salts are working against the seal, which is the normal condition inside a planter box. That can still be the right commercial decision — deferring a one to three week rebuild past a settlement date, a wedding, or the end of a financial year is a legitimate reason to buy time. What is not legitimate is selling it as a repair. If we quote an interim measure we write down that it is one, and roughly how long we expect it to hold.

Can you fix the drainage and inject at the same visit?

Sometimes, and often the drainage alone changes the answer. Clearing a silted ag line or reopening rendered-over weep holes can drop the water head against a wall enough that the seepage stops without any injection at all, which is why we look at drainage first and price the injection second. Where both are genuinely needed, the drainage goes first: sealing the face of a retaining wall before restoring its drainage raises the retained head against a wall that may not have been designed for saturated backfill.

My crack is about 0.3 mm, it is dry, and it has not changed. Do I need to do anything?

Probably not. AS 3600, the Australian concrete structures standard, allows for crack widths broadly in the 0.1 to 0.4 mm range at serviceability depending on the element and its exposure classification — fine cracking as concrete shrinks and cures is expected rather than defective. The question is not whether a crack exists but whether water passes through it and whether it is getting wider. A dormant hairline in a dry internal wall genuinely needs nothing. The same crack passing water into a basement wants sealing, because water sitting against reinforcement becomes a corrosion problem that costs several times more later.

Does publishing this mean you turn down work?

It means we turn down the wrong work. Fifteen disqualifications still leaves the large majority of below-ground leak enquiries in South East Queensland squarely inside what injection does well — cracks, cold and construction joints, kicker joints, tie-bolt holes, lift pits and car park soffits. Our commercial argument is not that injection is always the answer. It is that we are the people who will tell you when it is not, and that is a more durable business than one repair sold badly.

I am on a body corporate committee. How do I stop three contractors quoting three different things?

Write the scope before you invite the quotes, and put the diagnosis in it rather than the remedy. State the symptom, the location, what has been ruled out and by whom, and require every quote to state packer type, packer spacing, the make-good extent and the pricing unit. Three quotes written against one scope are comparable. Three quotes written against a photograph and a phone call are not, and the cheapest of them is almost always the one that priced the least work.

Where to next

• concrete cancer repair
• rising damp treatment
• balcony membrane replacement
• scope it properly before you tender
• resin injection versus slab re-levelling
• run the leak diagnostic
• how concrete crack injection works