Car park slab and soffit leak sealing

What the drips, the stalactites and the rust stains are actually telling you

Nearly every car park enquiry starts with the same sentence: water is dripping onto the cars. What matters is not the drip, it is what is around it. The soffit of a suspended deck records its own history, and it is usually readable before anyone gets a scissor lift out.

Read the whole level before pricing any of it. On a deck with a separate topping slab, water enters at one point and exits metres away, which means mapping is worth more than injecting. A quote written off one visit to one wet bay is a quote for the wrong length of joint.

Before anyone drills: is this deck post-tensioned?

A post-tensioned slab contains steel tendons that were tensioned after the concrete cured and are still under enormous load. They are common in car park decks and apartment slabs built from the 1980s onward. Drilling into one without scanning can cut a tendon, and that is not a mistake anybody recovers from: it is a violent release of energy, an immediate structural issue, and an insurance event.

The controls are simple and they are not negotiable. Ground-penetrating radar over every drill position, tendons and reinforcement marked on the soffit, and written approval from a structural engineer before a bit touches concrete. The same engineer should nominate a maximum injection pressure in writing, because on a prestressed element the governing limit is the structure, not what the pump can produce.

A scan with moisture readings, crack monitoring where needed and a written diagnosis sits at $450 to $1,500 as its own line. On a post-tensioned deck it is not an optional extra you can decline to save money, and any contractor willing to skip it is telling you something about how they will do the rest of the job.

Where the structural drawings have been lost, which is common in buildings that have changed managing agents more than once, the deck is treated as post-tensioned until proven otherwise. Nobody's recollection is evidence.

Structural slab or topping? Map it before you inject

Most car park decks are not one slab. There is a structural slab that carries load, and above it a topping or screed that carries the fall to the drains, sometimes with a membrane between them. Water gets into the gap between the two, travels sideways along it until it finds a defect in the structural slab, and comes out there. That is why the wet bay and the leaking bay are frequently not the same bay.

Mapping is the difference between a programme that works and one that seals a joint and moves the leak three bays over. It costs a fraction of the injection and it is the step cheap quotes leave out, because it is the only step that produces no visible progress.

• Get the as-built structural drawings from the body corporate records and confirm slab thickness, topping thickness and the joint set-out, so packer offsets can be calculated rather than guessed.
• Correlate the drip lines on the soffit with the joint set-out above. Joints run in straight lines and so do their leaks. A drip on a curve is usually a crack, a drip on a line is usually a joint.
• Scan for thickness and reinforcement at each intended drill position, which also confirms whether you are drilling into the structural slab or only into the topping.
• Water-test or dye-test on the deck above at suspected entry points, and inspect during or immediately after rain if the season allows.
• Check the deck surface: drainage falls, ponding, failed joint sealant and cracked topping. If the water is arriving because the deck above cannot shed it, injecting the soffit is treating the symptom.

Three car park leaks we will tell you not to inject

Injection is very good at one thing: sealing a defined path through concrete. It is genuinely poor at everything else, and in a car park there are three situations where the honest answer costs us the job and saves the committee a great deal of money.

1. Chloride-induced corrosion, not a water pathCracking that follows the line of the reinforcement, with rust staining, spalling or drummy concrete, is steel expanding as it corrodes. Rust occupies more volume than the steel it came from, so it pushes the cover off. Sealing that crack traps the problem behind resin while the steel keeps expanding, and in two or three years there is more spalling and a harder repair. 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 a large deck consider cathodic protection or a migrating corrosion inhibitor. That is a different trade licence class, a different programme and a different budget, and we hand it over rather than paper over it.
2. A designed movement jointExpansion and control joints in a deck are built to move. Filling one with resin defeats its purpose and transfers the movement into the concrete beside it, which then cracks. The repair is joint preparation, correct backing rod and a movement-capable sealant, quoted per linear metre as a sealant scope. An injection contractor who cannot tell a movement joint from a cold joint on your deck should not be drilling it.
3. A failed deck membrane or a deck that cannot shed waterWhere the trafficable membrane above has debonded, blistered or reached end of life across an area, injection seals the paths water is using today and water finds the next ones. That typically buys 6 to 18 months. The correct answer is membrane removal and replacement to AS 4654.1 and AS 4654.2, with the falls corrected. Injection has a legitimate role there as staged protection for the level below while the deck programme is funded, but it should be sold as exactly that, in writing, with a stated expected life.
4. Broad-area seepage through the slab bodyIf water is coming through the concrete across an area rather than along a line, there is nothing for injection to grip. That is a coating, tanking or drainage question. Pumping resin into porous concrete produces a quote, a mess and no result.

How we seal a soffit joint on a deck that is still being used

1. Mark the run before the drill comes outChalk the full joint run, not just the wet section, and mark every packer position. Scan each marked position. On a deck where the drawings are missing, this is where the programme either gets safe or gets dangerous.
2. Drill at 45 degrees, into the joint, never through it13 mm holes at 45 degrees to the soffit, alternating either side of the joint line, offset from it by roughly half the element thickness so the hole meets the joint at mid-depth. On a 250 mm slab that is an offset of about 125 mm. Drill to two-thirds to three-quarters of the slab thickness and never break through the far face. On a soffit, breaking through means resin into the bay above.
3. Mechanical packers at 250 to 400 mm centresJoints are wider and better connected than cracks, so they take more resin and closer packers. As a working rule, spacing should not exceed the thickness of the element. Mechanical packers seal inside the slab and take working pressure; surface ports glued to a wet soffit will not adhere and will blow off.
4. Water-flush every packer before injectingFlushing proves the joint is hydraulically connected from one packer to the next, and it clears the laitance and calcite that would otherwise stop resin dead. If water pumped into one packer does not appear at the next, the set-out is wrong or the joint is not continuous, and that is far better to learn with water than with resin.
5. Kill any single gusher first, then work the joint in sequenceA running point is dosed with a fast-gel hydrophobic polyurethane, accelerated at roughly 1 to 10 per cent by volume to give anything from about 20 seconds to several minutes of gel time. Once the flow is down, the joint is injected consistently from one end to the other rather than jumping to wherever the drip is loudest.
6. Start low on pressure and watch the structure35 to 140 bar is the normal working band for mechanical-packer polyurethane, but the limit is the slab, not the pump, and on a prestressed deck it is whatever the engineer nominated. Over-pressure announces itself: the joint visibly opening, resin appearing somewhere unexpected, spalling around a packer, or a sudden pressure drop with no resin return.
7. 24 hours, then cut back and make goodPackers stay in place a minimum of 24 hours. They are then cut 10 to 20 mm below the surface, the recess filled with a non-shrink hydraulic plug and ground flush. Any resin bleed is ground off the soffit, because nothing will bond to cured polyurethane afterwards. On a deck programme this is nearly always a second visit, and it should appear in the programme rather than being discovered at the end.

What 15 to 30 linear metres a day means for your deck

A crew in accessible conditions seals 15 to 30 linear metres of joint per day. Off a scissor lift, working overhead, with traffic management running and bays being cleared and released around you, the real rate lands at the bottom of that band or below it. That single number lets a committee sanity-check any deck quote before it goes to a vote.

Take a mapped programme of 180 linear metres of soffit joint. At 15 to 30 metres a day that is 6 to 12 crew days of injection, plus mobilisation, plus a return visit for cut-back and make good, plus the days lost to staging if only two bays can be closed at a time. If a quote in front of you implies 60 metres a day, either it is not using mechanical packers, or the metres are wrong.

Access equipment runs $450 to $1,600 per day delivered, with a scissor lift at the cheap end and a knuckle boom at the dear end. Headroom in a car park frequently rules out the machine you would prefer, which is worth confirming on site before it is priced.

Staging is a genuine trade-off and it belongs to the committee, not to us. Closing more of the deck at once shortens the programme and reduces the number of traffic management days. Closing less keeps residents parked but stretches the job, and the daily costs run for longer. We price both when a committee asks, because the cheaper answer is not always the one that inconveniences fewer people.

The lines other quotes bury

Traffic management is frequently the largest single line on a car park deck quote, and a lump-sum quote lets a contractor hide it, absorb it, or produce it later as a variation. Every one of these appears separately on our quotes so a committee can see what it is buying and can compare three tenders against each other line by line.

These are indicative South East Queensland rates, published so you can hold any quote against them. They are not a quote, and long repetitive runs on a single deck genuinely price below the bands shown because the crew stops setting up and starts producing.

One drip of resin on a bonnet costs more than the job

Cured polyurethane does not come off duco. It has to be ground or cut away, which on a car means a panel and a respray, and the conversation that follows it is not one any committee wants in the minutes. Everything about how we set up a deck is built around that fact.

Bays under and adjacent to a work zone are cleared and signed, not just coned. Residents get notice with dates and bay numbers, and a bay that cannot be cleared is a bay we do not work over that day. Drip containment goes under every packer position, surfaces are masked before injection rather than cleaned after it, and the crew carries the means to stop work in under a minute if a vehicle appears where it should not be.

The same discipline applies to the crew. A basement is an enclosed space where nothing disperses. Drilling concrete releases respirable crystalline silica, with an Australian workplace exposure standard of 0.05 mg per cubic metre over an eight-hour period, so drilling runs with on-tool extraction or wet, not with a broom afterwards. Uncured polyurethane contains isocyanates, which are respiratory sensitisers with an exposure standard of 0.02 mg per cubic metre as an eight-hour average, and sensitisation is permanent. That means fit-tested respiratory protection and forced ventilation in an enclosed basement, and safety data sheets available on site for every product. A building manager is entitled to ask for those sheets, and should.

What the committee gets at the end

Injection is invisible once the packers are cut off, so the record is the deliverable. Ours goes to the body corporate, not just to the building manager's inbox, because the committee that funded it is the party that will need it in three years.

The handover pack contains photographs before and after, taken from the same positions, a written description of what we found and what we did, moisture meter readings at marked points before and after, and the observation period result. We observe for a minimum of 24 to 48 hours and through a rain event wherever the season allows, because a dry deck in October proves nothing.

There is an honest limit to what any of that warrants, and it should be stated plainly before the work rather than argued afterwards. Sealing from the dry side redirects water, it does not remove it. A warranty on injection covers the joint or crack we sealed. It cannot cover a new path opening somewhere else, because the water is still there and still under head. The value of the before and after photographs and the written description is that when something reappears, it takes minutes rather than a dispute to establish whether it is our repair or a new source, and if it is our repair we come back.

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

How do we find out whether our car park deck is post-tensioned?

Start with the structural drawings, which the body corporate should hold in its records and the managing agent can retrieve. On the building itself, post-tensioned slabs usually carry stamped anchor plates or patched anchor pockets along the slab edge, often in a regular line. If the drawings cannot be found, which is common in buildings that have changed agents, the answer is a ground-penetrating radar scan over the intended drill zone and the assumption that the deck is post-tensioned until the scan proves otherwise. A maintenance contractor's recollection is not evidence and no one should drill on it.

Can the car park stay open while you work?

Usually yes, in stages. A typical programme closes two to four bays at a time behind certified traffic management, works that zone, then releases it and moves on. The trade-off is real: staging keeps residents parked but stretches the programme, and traffic management is charged per day at $900 to $2,500, so a longer programme costs more in daily rates. We price both a staged and a full-level closure when a committee asks, so the disruption decision is made with the numbers in front of it rather than after.

There are stalactites hanging off our ceiling. How long has this been leaking?

Longer than the drip suggests. Calcite forms when water carrying dissolved lime from inside the concrete emerges and evaporates, and building a visible stalactite takes months to years of steady flow. Two things follow. The joint has probably been leaking through more than one wet season, so the run to be sealed is usually longer than the wet section anybody points at. And the joint itself will be partly blocked with the same deposits, which is why every packer gets water-flushed before resin goes anywhere near it.

Our last contractor sealed the soffit and it came back two bays away. Was that a failed repair?

It may not have been. Sealing from the dry side redirects water rather than removing it, so a genuinely successful repair on one joint can push water along to the next weakest point. The way to tell is the record. Ask the previous contractor for their photographs and a written description of which run they treated. If the new leak is on the run they treated, that is a repair issue and it is theirs. If it is on a joint they never touched, it is a new path and a new scope. If they cannot tell you what they treated at all, you have learned something else worth knowing before you engage them again.

Why is night work 30 to 100 per cent dearer, and is it ever worth it?

The loading follows standard Australian trade penalty structures: roughly time-and-a-half for evenings and Saturdays, double time for Sundays and nights, plus supervision, security escort and induction time that daytime work does not need. It is worth it more often than people expect. Night work on a commercial car park can remove several days of traffic management at $900 to $2,500 each, avoid clearing bays that tenants are actively using, and shorten the total programme. We quote it both ways on any deck where daytime access is genuinely constrained, because occasionally the after-hours version is the cheaper of the two.

Our quote just says crack injection, car park, and a number. Is that enough to put to an AGM?

No, and it is not enough to compare against another quote either. A deck scope should state the mapped length of joint and crack in linear metres, the packer type and spacing, whether the deck is post-tensioned and who is paying for the scan and the engineer's approval, whether traffic management and after-hours loadings are inside or outside the number, what make good covers, and the observation and documentation you receive at the end. Three quotes written that way are comparable. Three lump sums are not, and the cheapest lump sum is almost always the one with the fewest of those items inside it.