Leaking basements and below-ground walls

The kicker joint is usually the real path

The kicker joint is the horizontal construction joint at the base of a below-ground wall, where the wall was poured onto an already-set slab. Two pours placed hours or days apart never fully bond. The result is a continuous, connected water path running the entire length of every below-ground wall in the building.

It is also the path that hides. Water entering at the kicker joint tracks along it, rises into the wall through whatever imperfection it finds, and appears as a damp patch or a running crack somewhere that looks nothing like the base of a wall. Contractors who chase the visible crack seal the exit and leave the entry open, which is why so many basements are on their second or third injection repair.

Practically, this means the joint gets treated as the primary scope on most below-ground walls rather than as an extra. Drilling is set out at 45 degrees down through the wall to intersect the joint, or vertically into the slab to meet it, with packers at 200 to 400 mm centres. A crew covers roughly 8 to 15 linear metres of kicker joint per day, and that rate drops sharply with heavy flow, restricted access or a fit-out to work around.

Sequence matters. We inject a fast-gelling hydrophobic resin first to kill flow along the joint, then follow with a flexible hydrophilic or acrylate material to seal the residual weeping that a rigid foam bridges over rather than penetrates.

When it is porosity, not a crack

Some below-ground walls and slabs are not leaking through a defect. They are leaking through themselves. Water is coming through the body of the concrete across a broad area — no defined line, no single crack, no joint you can trace — because the concrete is porous and there is standing water pressure on the other side of it.

Injection has nothing to grip in that situation. There is no path to fill. Pumping resin into a wall that is generally damp achieves a set of expensive plugs surrounded by wall that is still passing water, and any contractor who quotes injection on a broad-area seepage without saying so is either not diagnosing or not telling you.

The correct answers are different scopes entirely. A negative-side crystalline or cementitious tanking system — a cement-based coating that grows crystals inside the concrete pores and blocks water within the concrete itself rather than sitting on the face as a skin. A drained cavity: a dimpled sheet fixed to the wall creating a gap behind the lining, water running down it to a channel and a sump, which accepts water rather than fighting it. Curtain injection into the soil behind the wall to build a barrier where you cannot excavate. Or excavation and an external membrane where access allows and the budget justifies it.

It is normal for a real scope to combine these. Injection kills the defined paths — the joints, the cracks, the tie holes — and a tanking system or drained cavity handles the general porosity over the top. What is not legitimate is selling injection alone against a broad-area problem and calling it waterproofing.

Voids under the slab, and the resin budget nobody publishes

Below a basement slab there is often a drainage layer, a blinding layer that has broken down, or a genuine void from washout or poor compaction. None of it is visible and all of it is connected to more of itself.

Inject into that and the resin does not build pressure and it does not seal. It simply goes. There is no natural stopping point, which is how a job that was quoted as a joint seal turns into drums of resin pumped into the ground under a building with nothing to show for it. It is one of the few ways this trade loses very large amounts of money very quickly, and the client pays for it either in the price or in the variation.

The discipline is a volume budget per packer, agreed before work starts. A hard stop of one to two litres into a single packer is the working rule: if a packer takes that much without the pressure building and without resin appearing at the neighbouring packers, you are filling a void rather than sealing a path. Stop, cap it, move on, and reassess the diagnosis rather than continuing to pump.

That rule has a commercial consequence worth putting in the scope. Where a void is suspected, the correct pricing basis is volume-based or a day rate with a provisional sum, not a per-linear-metre rate, because a per-metre rate over an unknown void is a bet rather than a price. Where the void is large, filling it with a microfine cement grout is usually the right material — cement in cement, compatible, permanent and vastly cheaper by volume than resin.

Mapping before drilling

Two things get scanned before a drill touches a below-ground structure. The first is reinforcement, using a cover meter or ground-penetrating radar, so drilling does not cut a bar. The second, and the one that is not negotiable, is post-tensioning.

A post-tensioned slab contains steel tendons that were tensioned after the concrete set and remain under enormous load. They are common in car park decks and apartment slabs across Brisbane and the Gold Coast. Cutting one is catastrophic, it is not a recoverable mistake, and no schedule pressure justifies it. On any suspected post-tensioned element we scan with GPR, mark the tendons, and obtain the engineer's written approval before drilling. If the building's records are unclear about whether a slab is post-tensioned, it is treated as though it is.

The second mapping question is structural slab versus topping. Where a suspended slab has a separate topping poured over it, water enters at one point and exits metres away between the two layers. Injecting the exit does nothing. On these, mapping the path — flush testing, dye testing where appropriate, and observation during and after rain — is genuinely more valuable than the injection that follows it.

Enclosed below-ground spaces add a third consideration. Sumps, pits and some plant rooms meet the definition of a confined space under AS 2865, which requires an entry permit, atmospheric testing before and during entry, a standby person who does not do productive work, communications and a rescue plan. That is a legal obligation, not a best-practice option, and it adds roughly $800 to $2,000 per day to a quote. A price that does not show it either is not doing it or has not told you.

How a below-ground wall job runs

1. Diagnose and map, before quoting a metre of anythingWhere water enters versus where it appears. Whether there is a topping slab. Whether the path is a joint, a crack, a tie-hole grid or general porosity. Whether a service is leaking — a corroded sewer or a blocked stormwater line above a wall is a plumbing job and takes ten minutes to rule out.
2. Expose the substrateRender, paint, linings and drained-cavity sheeting all conceal the extent of the defect. Where a drained cavity system already exists behind a lining, injecting is often the wrong answer entirely and that has to be established before the scope is written, not after.
3. Scan, then set outCover meter or GPR on any structural element, mandatory GPR and written engineer approval on anything post-tensioned. Then 13 mm holes at 45 degrees, offset from the crack or joint line by roughly half the element thickness so the hole intersects at mid-depth, drilled to two-thirds or three-quarters of the thickness and never through the far face.
4. Flush and prove the connectionClean water through each packer proves the path is hydraulically connected from one fitting to the next, and clears the laitance, efflorescence and calcite that otherwise block resin. If water does not appear at the next packer, the set-out is wrong and gets changed before injecting.
5. Kill the worst flow firstA single gushing point is dealt with first, using a fast-gelling hydrophobic resin and, under real head, a hydraulic cement plug or oakum packing to reduce flow. Injecting the methodical sequence while one point is running washes the work away.
6. Inject in sequence, bottom to topOn a vertical crack, always from the lowest packer up, so resin displaces water out of the crack rather than trapping it. On a horizontal joint, consistently from one end to the other. Move to the next packer only when resin shows there or the packer reaches refusal.
7. Hold the volume budgetOne to two litres into a single packer without pressure building and without resin showing at its neighbours means a void, not a crack. Cap it, move on, and revisit the diagnosis rather than emptying a drum into the ground.
8. Cut back at 24 hours and make goodPackers stay in a minimum of 24 hours, longer for slow-cure products. Shafts cut 10 to 20 mm below the surface, recesses filled with a non-shrink hydraulic plug or polymer-modified mortar and finished flush. Reinstatement of render, paint or lining is a separate scope item and is quoted as one.

Getting it scoped and approved

Below-ground walls, slabs, car park decks, lift pits and planter boxes are nearly always common property, even when the water shows up inside a lot. That means the repair path runs through the committee, and the most common reason these jobs stall for a year is not cost — it is that three contractors quoted three different things and the committee had no way to compare them.

The fix is a written scope issued to every tenderer before anyone prices it. At minimum it should state the unit basis for each item, whether mechanical packers or surface ports are required, which resin is to be used and why that one suits this defect, whether making good and reinstatement are included, what verification is required at handover, and how any void or unknown quantity will be priced. Three quotes against one scope are comparable. Three quotes against three descriptions are not.

Two lines are frequently missing from the cheapest quote and present in the dearest, which makes the dearest look uncompetitive when it is actually the only complete one. The first is confined space compliance where sumps or pits are involved, at roughly $800 to $2,000 per day. The second is after-hours access in an occupied or trading building, which runs a labour penalty of roughly 30 to 100 per cent under standard Australian trade penalty structures. Both are real. Both should be shown as separate lines rather than buried.

For the committee's own record, ask for the diagnosis in writing before the repair rather than after. Where a defect might be a builder's responsibility, or where an insurer will be asked to consider gradual ingress versus sudden damage, a dated photographic record of the condition and the diagnosis is worth considerably more than a description written after the wall is dry.

What a below-ground job costs

Indicative Australian market bands, itemised the way a comparable scope should be. These are bands, not quotes, and a real price comes from the specific structure.

The single most useful thing a committee can do with these numbers is check the unit. A quote priced per crack and a quote priced per linear metre are not comparable, and normalising them is usually where the apparent price difference between contractors disappears.

How completion is proven

Verification on below-ground work is not a signature on a docket. There are four things worth requiring in the scope, and they cost the contractor time rather than money, which is why they are the first things omitted from a cheap price.

First, travel confirmed during injection: resin appearing at the next packer along is direct evidence it moved through the path rather than sitting in the hole. Second, observation for a minimum of 24 to 48 hours and, wherever the programme allows, across an actual rain event — in South East Queensland the honest test window is the November to March wet season, when the ground stays saturated for days rather than hours. Third, moisture meter readings taken at the same marked points before and after, so there is a measurement rather than an impression.

Fourth, and the one that matters in two years: a photographic record. Photographs before and after, taken from the same positions, together with a written description of what was found and what was done. If water reappears, that record shows whether it is at a path that was treated — which is a warranty matter — or at a new location, which negative-side sealing makes a genuine possibility because it redirects water rather than removing it.

That last point should be said plainly at scoping stage rather than discovered afterwards. A warranty on injection can honestly cover the crack or joint that was sealed. It cannot cover a new path opening three metres away, and no contractor who understands the physics will pretend otherwise.

What it costs

Indicative only, and excluding GST. Build a line-by-line estimate for your own job with the price book, or read the working behind the rates in the 2026 cost guide.

Questions we actually get asked

Our basement was injected two years ago and it is leaking again about four metres away. Was the first repair a failure?

Possibly not. Sealing from the dry side stops water passing at that point; it does not remove the water from the ground. Blocking one path can genuinely cause the next weakest point in the same wall to start passing water, and that is physics rather than an excuse. What tells you which situation you are in is the record of the original works: if the new leak is on a path that was treated, that is a defect in the repair; if it is at a location that was never treated, it is a new path and the diagnosis needs revisiting. Before-and-after photographs of the original repair, taken from the same positions, together with a written description of what was found and what was done, are usually enough to establish which.

The leak only appears two or three days after heavy rain. Why the delay?

Because you are seeing groundwater, not rainfall. Surface water arriving during a storm reaches you within minutes and usually comes over a threshold or down a wall face. Water that arrives days later has had to soak into the ground, raise the water table around the structure, and build enough pressure to push through a joint. That lag is actually a useful diagnostic: it tells you the path is below ground and under head, which is the pattern injection is designed for, and it rules out most surface water and plumbing explanations.

The quote says resin is charged 'as required'. Is that reasonable?

It is reasonable only where the scope also states how the quantity is controlled. Voids beneath a slab or behind a wall can absorb effectively unlimited resin, so an open-ended resin line with no cap is a blank cheque. What a properly written scope contains is a volume budget per packer with a hard stop — one to two litres is the working figure — a statement of what happens when that stop is reached, and either a provisional sum or a rate per litre with a not-to-exceed figure. Ask for those three things in writing rather than asking for a lower rate.

Can the work be done without closing the car park or emptying the plant room?

Usually yes, in stages, and the constraints are what drive the cost rather than the injection itself. Staging means only part of a level is out of use at a time. Vehicles are protected before anything is pumped, because cured polyurethane on a bonnet costs more to resolve than the repair being carried out. Where daytime access is impossible, night or weekend work carries a labour penalty of roughly 30 to 100 per cent. We itemise those lines rather than absorbing them into a rate, so the committee can decide whether the disruption saving is worth the premium.

Is a sump and pump a legitimate answer, or is that giving up?

It is a legitimate engineering answer and on some structures it is the correct one. A drained cavity system fixes a dimpled sheet to the wall, creating a gap behind the lining so water runs down into a channel at floor level and away to a sump. You are managing water rather than fighting it. That approach makes sense where the concrete is generally porous rather than defective, where the water table sits above the floor level for parts of the year, and where sealing would simply move the problem. It has an ongoing maintenance obligation — pumps fail, and a pump that fails unnoticed is worse than no pump — so it belongs in the building's maintenance schedule, not just in the repair.

Do we need an engineer involved, and at what point?

For straightforward leak sealing in a below-ground wall or a lift pit, no. You do need one before any drilling into a post-tensioned slab, where the approval must be written rather than verbal. You need one to nominate a maximum injection pressure on thin or slender elements. And you need one where the cracking might be structural rather than a durability problem — diagonal cracking, cracks widening toward the top, cracking in beams, columns or other primary members, or any lean, bulge or change in floor level. A structural assessment typically runs $1,200 to $3,500, and where it is needed it should be commissioned before quotes are sought, not after one arrives.

There is rust staining and flaking concrete along the crack. Does that change the scope?

Completely. Rust staining, spalling and hollow-sounding areas along a crack line mean reinforcing steel is corroding, and corroding steel expands and pushes the concrete apart. Injecting that crack seals over an expanding problem rather than fixing it. The correct scope is concrete repair: break out to sound concrete behind the bar, clean or replace the steel, apply a passivating primer and reinstate with a compatible repair mortar, with a corrosion inhibitor or cathodic protection considered on larger structures. That is a different trade licence class and a different budget, and a contractor who quotes injection over visible corrosion is either not looking or not saying.

What documentation should end up in the body corporate's records?

The pre-works diagnosis with dated photographs, the written scope that was issued to tenderers, photographs before and after taken from the same positions with a written description of what was found and what was done, before-and-after moisture readings at marked points, the safety data sheets for every product used, the contractor's QBCC licence number and currency of insurance, and the warranty document stating what it covers and what it excludes. That set answers almost every question that comes up later, including the ones asked by an insurer, an incoming committee, or a lot owner who wants to know why a levy was raised.