Guide
What polyurethane injection is, in plain English
Updated 17 August 2026
If water is coming through a concrete wall at your place, somebody has probably already told you the answer is polyurethane injection. That phrase explains nothing to anyone outside the trade, and most of what is written about it online is either a product brochure or three paragraphs of marketing. This page assumes you know nothing about it and does not skip the parts that are awkward.
Here is the whole thing in one sentence. We drill small holes at an angle so they cross the crack inside the wall, screw fittings into those holes, and pump in a liquid resin that reacts with the water already in the crack, foams up, and sets into a solid plug running through the full thickness of the concrete. No excavation, no digging up the garden, no removing the driveway.
Everything below is the detail behind that sentence: what happens on the day, the actual measurements and pressures involved, what the resin can and cannot do, and a translation of every term you are likely to see on a quote. Where injection is the wrong answer for a situation, we say so on this page rather than in a disclaimer at the bottom.
The bit that matters: it goes through the wall, not on it
The difference between a repair that lasts and one that fails in a season is almost entirely about where the resin ends up. A proper injection puts resin inside the crack, through the full thickness of the concrete, so that the seal sits in the middle of the wall where the water is trying to get through. A cheap repair puts resin on the face of the wall, over the crack, where the water pressure behind it simply pushes it off.
That is why the holes are drilled at an angle instead of straight at the crack. A hole drilled at 45 degrees, offset from the crack line by roughly half the thickness of the wall, intersects the crack plane at about mid-depth. On a 250 mm wall that means the hole starts about 125 mm to one side of the crack. Alternate the holes left and right of the crack in a stitch pattern and you have a set of entry points that reach the crack from both sides at its middle.
The second thing that matters is that concrete walls leak from the point of least resistance, not from the most obvious crack. Water tracks sideways between a membrane and the concrete, or between a structural slab and its topping, and comes out wherever the wall is weakest. That is why the first job on any leak is working out where the water gets in, not where it comes out.
Why a wet crack is the right condition, not the wrong one
Polyurethane injection resin is a single-component MDI-based prepolymer, which is a technical way of saying it is a liquid that stays liquid in the drum and reacts the moment it meets water. The water is a reactant, not an ingredient: it triggers the reaction and the finished plastic contains none of it. The reaction gives off carbon dioxide, which is what blows the liquid into a closed-cell foam and drives it along the crack.
So a wall with water running down it is exactly the condition the material is designed for. If a crack is bone dry we flush water into it first to give the resin something to react with. This is the opposite of epoxy, which is a structural glue that needs a dry, still crack and which running water will emulsify into a soft, useless plug.
How fast it sets is set on site, not at the factory. An accelerator is dosed into the resin at roughly 1 to 10 per cent by volume, which gives anything from about 20 seconds to several minutes before it gels. A gusher gets a fast dose so the resin does not simply wash away down the crack. A fine, damp hairline gets a slow dose so the resin has time to travel before it stiffens.
One piece of honesty that manufacturers are quiet about: polyurethane does not bond well to wet concrete. The film of water on the crack face acts as a bond breaker. The seal you end up with is a mechanical plug keyed into the irregular shape of the crack, not something glued to it. That is a perfectly durable way to stop water, and it is also why the crack has to be filled properly rather than just skinned over.
What actually happens on the day
Eight steps, and the first one involves no equipment at all. That ordering is deliberate: diagnosis is the highest-value step in the whole job and the one most often skipped.
- Work out where the water is getting inBefore any drill comes out. That means checking the things that are not the concrete: blocked subsoil drains and weep holes, a driveway falling toward the building, a downpipe discharging at the footing, a failed sealant joint, a leaking pipe. A very large share of enquiries that start as "we need injection" end as a drainage job. If nobody can explain how water gets in, nobody can promise injection will stop it.
- Expose the crackRender, paint and linings hide the true extent of a crack, and you cannot inject what you cannot see. Stripping back is usually quoted as its own item, because guessing at it is how contractors lose money and how clients get surprised by a variation.
- Measure the crack and decide whether it is still movingWidth, length, orientation, and whether it passes through the full element. If it is unclear whether the crack is active, crack monitors get fitted and read over 4 to 12 weeks across a temperature and moisture cycle. That feels slow. Injecting a moving crack is money spent twice.
- Scan, then drillGround-penetrating radar or a cover meter on any structural element, and mandatory scanning plus written engineer approval before drilling any post-tensioned slab. Then 13 mm holes at 45 degrees, alternating sides of the crack, drilled to two-thirds or three-quarters of the wall thickness and never through the far face. Drilling right through loses all the injection pressure into the ground or the room beyond.
- Fit the packersA packer is the fitting that goes in the hole so a hose can be connected. Mechanical packers have a rubber sleeve that is squashed against the sides of the hole by tightening a nut, so they seal inside the wall body and take high pressure. Spacing runs 150 to 400 mm apart, tighter for fine cracks, wider for open cracks and construction joints.
- Flush the crack with waterClean water is pumped through each packer before any resin. This does three jobs at once: it proves the crack is actually connected from one packer to the next, it washes out the dust, laitance and salt deposits that would otherwise block the resin, and it wets a dry crack so the resin has water to react with. If water does not appear at the next packer, the set-out is wrong and it gets corrected before anything expensive goes in.
- Inject from the bottom upOn a vertical crack, always start at the lowest packer and work upward. Resin pushes the water up and out of the crack rather than trapping it, the crack fills from a sealed base, and resin appearing at the next packer up is direct proof it has travelled. On a horizontal crack or joint, work consistently from one end to the other. Move on when resin shows at the next packer or when the crack refuses to take any more.
- Leave it 24 hours, then cut back and make goodPackers stay in the wall for a minimum of 24 hours. Removing them the same day to finish the job in one visit lets water back in before the seal has developed. Then the packer shafts are snapped or cut off 10 to 20 mm below the surface, the recess filled with a non-shrink hydraulic plug or polymer-modified mortar, and ground flush. Render and paint are a separate scope, quoted rather than assumed.
The numbers behind each of those steps
None of these figures are arbitrary. Each one exists because of what happens when you get it wrong. Where a figure varies by product or supplier, it is given as a band rather than a false precision.
| What | Number | Why it is that number |
|---|---|---|
| Drill hole diameter | 13 mm | Matches the standard 13 mm mechanical packer, which is the workhorse size. Bit-to-packer pairing varies slightly by brand, so it is confirmed against the supplier's data rather than assumed. |
| Drill angle | 45 degrees | So the hole crosses the crack plane inside the wall rather than meeting it at the surface. |
| Offset from the crack line | About half the element thickness | Roughly 125 mm on a 250 mm wall. That offset is what puts the 45-degree hole through the crack at mid-depth. |
| Drill depth | Two-thirds to three-quarters of the thickness | Deep enough to reach the crack at mid-depth, never through the far face. Breaking through loses all pressure. |
| Packer spacing | 150 to 400 mm centres | Tighter at 150 to 200 mm for hairline cracks, wider at 300 to 500 mm for open cracks and cold joints. A common rule is that spacing should not exceed the thickness of the element. |
| Gel time | About 20 seconds to several minutes | Set on site by dosing accelerator at roughly 1 to 10 per cent by volume. Fast for a gusher, slow for a fine crack. |
| Injection pressure, PU through mechanical packers | Commonly 35 to 140 bar | Single-component pumps are typically rated to around 200 bar. Always start low and escalate only if the crack needs it. The limit is the structure, not the pump. |
| Injection pressure, epoxy through surface ports | Roughly 0.7 to 3.5 bar | Surface ports are a low-pressure method. That is their whole limitation. |
| Injection pressure, acrylate gel and curtain work | Roughly 2 to 15 bar | Higher pressure fractures and displaces the ground instead of permeating it. |
| Packers left in place | At least 24 hours | Longer in cold weather or with a slow-cure product. |
| Real expansion inside a crack | Roughly 3 to 8 times | Manufacturer free-rise figures of 8 to 30 times are measured in an open bucket with nothing pushing back. Estimating how much resin a job needs from a free-rise figure gives you a wrong number. |
Every word in the quote, translated
This is the vocabulary you will hear on site and read on a quote. None of it is complicated once somebody tells you what it means, which nobody usually does.
| Term | What it actually means |
|---|---|
| Injection resin | The liquid pumped into the crack. It starts liquid so it can travel, then sets hard or rubbery inside the crack and blocks the water path. |
| Packer (also called a port or an injection nipple) | The small metal or plastic fitting installed in a drilled hole so a hose can be connected. It has a one-way valve, so resin goes in and nothing comes back out. |
| Mechanical packer | A packer drilled into the concrete and locked in place by tightening a nut, which squashes a rubber sleeve against the sides of the hole. It seals inside the wall and takes high pressure. This is the correct type for nearly all leak sealing. |
| Surface port | A packer glued to the face of the wall over the crack, with a bead of filler capping the crack between ports. Cheaper and quicker, low pressure only, and it will not stick to a wall that is actively leaking. Reasonable for some dry structural epoxy work, a warning sign on a wet basement wall. |
| Gel time (also rise time or cream time) | How long the resin stays liquid after it starts reacting, before it sets. Adjusted on site with accelerator: seconds for a gushing leak so it does not wash away, minutes for a fine crack so it has time to travel. |
| Free-rise expansion | How much a foam resin expands in an open container with nothing holding it back. This is the big number on the data sheet, often 8 to 30 times. Inside a crack, squeezed on all sides, real expansion is closer to 3 to 8 times. Never estimate resin volume from a free-rise figure. |
| Refusal | The point during injection when the crack will not take any more resin: pressure climbs, holds, and nothing goes in. It means that section is full, and it is the signal to move to the next packer. |
| Flushing | Pumping clean water through the packers before injecting. It clears dust, laitance and salt out of the crack, proves the crack connects from one packer to the next, and wets a dry crack so the resin has something to react with. |
| Bottom-to-top sequence | Injecting a vertical crack starting from the lowest packer and working upward, so resin pushes water up and out of the crack instead of trapping it. |
| Cold joint (also construction joint) | The line where one pour of concrete met the next. It is a joint, not a crack, and because the two pours never fully bonded it is the most common leak path in a basement or car park. |
| Kicker joint | The joint at the bottom of a wall where the wall meets the floor slab. In a leaking basement or undercroft this is usually the real entry point, even when a crack higher up the wall is the thing you can see. |
| Negative side and positive side | Positive side is the wet side, the outside face of a below-ground wall where the water is. Negative side is the dry side, the face you can stand on. Negative-side waterproofing means sealing from the inside, which is what injection makes possible and why nothing has to be excavated. |
| Head (hydrostatic pressure) | The push of standing water. The higher the water sits above the leaking point, the harder it pushes water through any gap. It is why a basement leak gets dramatically worse after several days of rain rather than after a shower. |
| Hydrophobic | Water-fearing. A resin that uses water to set but does not absorb it, so the cured foam does not shrink or swell as the wall wets and dries. Best for fast, heavy leaks and for anything that dries out seasonally. |
| Hydrophilic | Water-loving. A resin that seeks water out, chases the leak into fine cracks, and bonds well to damp concrete. It holds water inside itself, so it can shrink if the crack dries out completely for a long period. |
| Acrylate gel (acrylic gel) | A watery liquid that turns into a flexible rubbery jelly. Because it is about as thin as water it reaches places thicker resins cannot, including soil and hairline cracks. It has no structural strength at all; it is purely a water barrier. |
| Epoxy | A two-part structural glue. It sets rock hard with no expansion and effectively welds the two faces of a crack back into one piece. It needs a dry, still crack and it has no give in it whatsoever. |
| Active crack and dormant crack | An active crack is still opening and closing with temperature, moisture or building movement. A dormant crack has stopped moving. A rigid repair in an active crack simply cracks again beside itself. |
| Tell-tale (crack monitor) | A simple gauge fixed across a crack with a scale on it, read over weeks or months, showing whether the crack is actually moving and by how much. Cheap, and it prevents an expensive mistake. |
| Tie-bolt hole (form-tie hole) | A hole running right through a concrete wall, left by the steel rods that held the formwork together during the pour. It is a straight pipe through the wall and therefore a very direct leak path. They appear in a regular grid. |
| Efflorescence | The white powdery or crusty deposit on concrete and brickwork. It is salt left behind when water carrying dissolved minerals evaporates. Not damage in itself, but proof that water has been moving through. |
| Making good (reinstatement) | The tidy-up: cutting the packers off below the surface, filling the holes, grinding flush and repairing render or paint. Worth confirming in writing whether it is included, because it frequently is not. |
| GPR scan | Ground-penetrating radar, run over the concrete to map where reinforcement and any post-tensioning cables sit before anybody drills. |
| Post-tensioned slab | A slab containing steel cables that were tensioned after the concrete set and are still under enormous tension. Drilling one without scanning first can cut a cable. Common in car park decks and apartment slabs. |
| Ag line (subsoil drain) | The perforated pipe in gravel behind a retaining wall or beside a footing that collects groundwater and takes it away. When it is missing, crushed or silted up, water builds up against the wall, and that is very often the real cause of a leak. |
| Weep hole | A small deliberate gap left in a retaining wall or brick wall to let trapped water escape. Rendered over or buried by a garden bed, it stops working. Clearing them occasionally fixes the problem on its own. |
Six things injection cannot do
Injection seals a defined path through concrete. It is very good at that and it is useless at everything else. These are the six situations where somebody selling you injection is selling you the wrong thing.
- Fix a drainage problem. A blocked or missing subsoil drain, rendered-over weep holes, a driveway falling toward the building or a downpipe discharging at the footing. Sealing the wall in that situation raises the water head against it and moves the leak. Clear the drainage first.
- Replace a failed membrane. Once a membrane on a deck, balcony or planter box has debonded, blistered, split or aged out across the whole area, injection can seal a path but cannot restore a sheet. The correct answer is removal and replacement to AS 4654.1 and AS 4654.2 externally, or AS 3740:2021 for internal wet areas. Injection there buys somewhere in the order of six to eighteen months.
- Stop rising damp. Rising damp is groundwater climbing through porous masonry by capillary action, showing as a tide mark up to about a metre with salt bloom and perished plaster. It is treated with a chemical damp-proof course, a silane or siloxane injected into the mortar joints low down. Different product, different job, same confusing word.
- Fix concrete cancer. Cracking that follows the line of the reinforcement, with rust staining, spalling or drummy patches, is caused by steel expanding as it corrodes. Sealing the crack hides the problem while the steel keeps expanding. That needs a concrete repair scope: break out, treat the steel, reinstate.
- Cure condensation. Damp appearing on cold surfaces in still, humid, poorly ventilated spaces with no correlation to rain is very often condensation, not ingress. The fix is ventilation, dehumidification and insulation. Injection achieves nothing and you are worse off by the cost of it.
- Hold a crack that is still moving. If the element is still moving, foam gets torn open and epoxy holds while the concrete cracks again a few millimetres alongside. Fit crack monitors, read them over 4 to 12 weeks, find out why it is moving, then seal.
Is it safe to have this done in an occupied building?
Yes, with the qualification that the risk sits almost entirely with the people handling the uncured resin, not with the people in the building.
Uncured polyurethane resin contains isocyanates. They are respiratory sensitisers, which in plain terms means repeated exposure can trigger a permanent asthma-like condition. Once the resin has reacted and set, which takes minutes, it is an inert plastic and it is not hazardous. In practice that means technicians in fitted respirators, gloves and eye protection, ventilation in enclosed spaces, and people, pets and food kept out of the immediate work area during injection and until the space has been aired. There is a chemical smell during the work that clears with ventilation.
Drilling concrete releases respirable crystalline silica, which is a permanent lung disease risk and is worse in an enclosed basement or lift pit where the dust has nowhere to go. Proper work uses on-tool dust extraction or wet drilling.
You are entitled to ask for the safety data sheet for every product being used on your property, and building managers should ask as a matter of course.
What the wall looks like when we leave
A line of small patched holes running at an angle either side of the crack, spaced somewhere between 150 and 400 mm apart, plus the crack itself, filled. The packer shafts are cut 10 to 20 mm below the surface and the recesses filled with a non-shrink hydraulic plug or a polymer-modified repair mortar, then ground or trowelled flush.
On bare concrete in a car park, plant room or undercroft, that is the finish. On a wall you look at every day it will need re-rendering and painting, and that should appear on the quote as its own line rather than being quietly assumed to be included.
One practical point about finishes: cured polyurethane is very difficult to remove and neither paint nor render will bond to it. If any resin has bled out onto the face it has to be ground off before anything is coated over it. Masking and protecting surfaces beforehand is far easier than cleaning afterwards, so tell the contractor up front what the final finish is going to be.
Roughly what it costs, and why the range is wide
These are indicative South East Queensland market bands, not a quote. The range is wide because the cost drivers genuinely swing the number: wall thickness, how fast the water is running, how wide the crack is, whether render has to come off and go back on, and whether the work is at ground level or off a scissor lift in an operating car park.
| Item | Indicative band (AUD) | Notes |
|---|---|---|
| Site attendance minimum, business hours, metro | $450 to $900 + GST | Covers travel, set-up, plant and a minimum block of labour. Very little legitimate injection work happens below this. |
| Polyurethane crack injection, per linear metre | $150 to $450 per lm | Accessible, straightforward work. Long repetitive runs on one site drop toward $80 to $200 per lm. |
| The same work with difficult access | $450 to $900 per lm | Height, confined space, occupied premises, protection and reinstatement. |
| Per packer installed and injected | $35 to $95 each | Useful for pricing discrete defects and variations rather than a run of crack. |
| Single basement or undercroft wall crack, complete | $900 to $2,500 | The most common residential job. Rises sharply if render or tiling has to come off and go back. |
| Construction or cold joint, per linear metre | $200 to $500 per lm | Higher than a crack because joints are wider, better connected, take more resin and need closer packers. |
| Diagnostic investigation with a written report | $450 to $1,500 | Moisture readings, crack monitors, water testing, scanning. A quote-only visit is usually free; real diagnosis is not. |
Three questions that separate a real quote from a cheap one
If you take nothing else from this page, take these. They are answerable in a sentence each, and the answers tell you more about what you are buying than any amount of brochure copy.
- Are you drilling and installing mechanical packers, or gluing ports to the surface? On an actively leaking wall, surface ports rely on a surface seal that will not adhere to wet concrete.
- Which resin are you using, and why that one for this crack? A hydrophobic foam, a hydrophilic foam and an acrylate gel behave completely differently, and an answer that names the product and the reason tells you the crack was actually assessed.
- Is making good included, and is the price per linear metre or per crack? A price per crack sounds cheaper right up until the crack turns out to be four metres long.
Questions we actually get asked
- Is this the same resin injection used to lift a sunken driveway or slab?
- No, and the shared name causes real confusion. Slab re-levelling injects a high-density expanding polyurethane or geopolymer into the subgrade underneath the concrete, so the expansion pressure compacts the soil and lifts the structure. It goes into the ground, not into the concrete, and it will not stop a single drop of water or seal a crack. Leak-sealing injection goes into the concrete itself and does nothing about settlement. They are two different trades that happen to share a word.
- Why drill at an angle instead of straight into the crack?
- Because a hole drilled straight at a crack only reaches it at the surface, which is exactly where you do not need the resin. Drilling at 45 degrees, offset from the crack line by roughly half the wall thickness, makes the hole intersect the crack at about mid-depth. On a 250 mm wall that is an offset of around 125 mm. Alternating the holes left and right of the crack means the crack is reached from both sides through its middle, which is what gives you a seal through the full thickness rather than a skin on the face.
- How much resin will my crack actually take?
- Less than the data sheet suggests, and nobody can tell you exactly until they start. The expansion figures printed on a technical data sheet, commonly 8 to 30 times, are free-rise numbers measured in an open container. Confined inside a crack under back-pressure, real expansion is closer to 3 to 8 times. The exception that changes the arithmetic completely is a void behind the element, an unfilled block core or a drainage layer, which will swallow effectively unlimited resin without ever sealing the face. That is why a volume budget per packer, with a hard stop of around 1 to 2 litres, matters more than any calculation.
- Does the render or paint have to come off first?
- Usually yes, at least along the crack line and a margin either side. Render, paint and linings hide how far a crack actually runs and whether there is a second one beside it, and you cannot inject what you cannot see. Expect exposure and reinstatement to be quoted as separate items. If a quote covers a rendered wall without mentioning either, ask what happens when the crack turns out to be twice as long as the visible part.
- Does the weather or the time of year change anything?
- For polyurethane, not much. Gel time is temperature-sensitive and gets adjusted on the day, which is routine. Acrylate gel is far more temperature-sensitive and a mix that behaves at 8am can go off in the hose by 2pm on a hot Brisbane afternoon, so gel times get re-dosed through the day. Epoxy is the fussy one: it needs 24 to 72 hours to cure and slows down badly below about 10 degrees. Rain is actually useful, because watching a repair through a decent rain event is one of the better ways to prove it worked.
- Can we stay in the house, or keep the car park open?
- Generally yes. For residential work it means keeping people, pets and food out of the immediate work area during injection and until the space has been ventilated, which is usually the same day. For an operating car park it means staged works so only part of the deck closes at once, certified traffic management, and protecting parked vehicles, because a single drip of resin on a bonnet costs more than the injection did. Those things are real costs and should appear as their own lines on the quote rather than being buried in a rate.