Slab stage sits at the second-highest-leverage point of a new home build, second only to waterproofing. Everything below the concrete — formwork, steel reinforcement, plumbing rough-ins, termite barrier, vapour barrier, insulation if specified — is fully visible and able to be checked the day before the pour. The day after the pour, every one of those elements is permanently encased in concrete and effectively invisible for the life of the home. Defects in formwork or reinforcement caught before the pour cost nothing to rectify. The same defects caught after the pour cost thousands in saw-cuts, epoxy injections, or — in the worst cases — slab replacement.
This is the checklist we work through on every slab inspection in Queensland — what each item looks like in practice, why it matters, and which clauses of AS 2870 and AS 3660.1 govern it.
Formwork
Formwork defines the perimeter and depth of the slab. If the formwork is wrong, the slab is wrong. We start with edge beam dimensions — depth and width — checked against the engineering drawings and verified against the minimum requirements set out in AS 2870 for the home's site classification. A shallow edge beam on a reactive H1 site is a structural compromise that the certifier should not sign off.
Next, we check square and level. Each corner is verified with the 3-4-5 method (a 3 m run on one face, 4 m on the adjacent face, and a 5 m diagonal — if the diagonal misses, the corner is out of square). The set-out is then cross-checked against the engineering drawings — not the plans-on-the-wall, the engineer's stamped drawings — to confirm the slab footprint matches the design.
Brace tightness gets a manual check at every kicker, and we sight the form joints for gaps that would let cement paste bleed during the pour. The QBCC Standards and Tolerances Guide sets the tolerances we benchmark against.
Reinforcement
Reinforcement is where most slab-stage defects sit. The steel has to be the right bar size, at the right spacing, with the right concrete cover, and supported off the ground so the cover is preserved when the pour comes. We check every element against the engineer's reinforcement drawing and AS 2870.
Bar size and spacing in the edge beam comes first — typically N12 or N16 longitudinal bars with R10 ligatures at the spacings specified by the engineer. Cover is critical: SEQ residential slabs typically run 50 mm bottom cover and 40 mm side cover to protect the steel from corrosion across the life of the home. Slab mesh laps should overlap a minimum of 300 mm (or as specified by the engineer) with the laps tied off, not just resting.
Mesh chairs are required at roughly one-metre centres so the mesh sits in the middle third of the slab depth. Mesh resting directly on the vapour barrier means zero bottom cover at that point — a structural compromise that telegraphs as cracking within the first decade.
Corner reinforcement at re-entrant angles per AS 2870 is the most-missed item — additional bars diagonally across each internal corner to control shrinkage cracking. We tick each corner against the engineer's plan.
Plumbing rough-ins
Every waste pipe and pressure line that runs through the slab is a penetration that has to be aligned, protected and sealed before the pour. The first check is alignment against the plumbing plan — drains under the kitchen island, shower wastes under the wet areas, floor wastes in laundries. A drain set out 200 mm from the planned location becomes a cabinetry problem at fixout, and the only fix is to chase through the concrete.
Depth and set-down are critical for wet areas. Waste lines need sufficient depth to maintain fall through the slab thickness, and a minimum gradient of 1:60 is typical for residential waste runs. Sleeve protection at every penetration — PVC or fitted sleeves protecting copper, PEX or other pressure lines through the slab — is required so the concrete does not bond directly to the pipe.
Termite collars at every penetration per AS 3660.1 sit on top of the slab edge or around each riser. We confirm each collar is fitted, square, sealed and continuous before the pour locks the geometry in.
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Book an InspectionTermite management system
Termite management is mandatory on every Queensland residential build under AS 3660.1. The system has to be installed before the slab pour, and once the concrete goes in, the system is either complete or it isn't — there is no retrofit that restores the 50-year warranty if the install is wrong.
For chemical barriers, the chemical has to be applied to all soil under and around the slab edge to the concentration specified by the manufacturer. The most-missed defect we see is chemical applied to the perimeter only — the underslab area left dry. Termites travel through the soil under the slab, not just around it.
For physical barriers, the system is a continuous membrane, mesh or granular layer installed without gaps under the entire slab footprint and turned up at every penetration. Common defects: barriers compromised by reinforcement chairs piercing the membrane, collars missing at internal stack penetrations, or barriers terminated short of the slab edge.
Collar termination at every riser has to be flush with the finished slab surface and sealed continuously to the barrier — no air gap, no unsealed joint. We log every collar individually against the installer's certificate.
Insulation under slab
Underslab insulation is not standard on most Queensland builds, but high-performance, passive-house or premium-spec homes increasingly include it. Where specified in the design, the insulation is typically XPS (extruded polystyrene) board laid continuously across the slab footprint and turned up at the edge beam.
We check continuity — no gaps between boards. Laps need to be tight and, where the design specifies, taped. Edge insulation runs vertically up the inside face of the edge beam to break the thermal bridge between slab and footing.
The single most common defect we see is foam compression under formwork pressure or under reinforcement chairs — the boards crush to half-thickness at point loads, eliminating the R-value at exactly the points that matter. The vapour barrier always sits on top of the insulation, never below — sealing the insulation against ground moisture would defeat the design.
Vapour barrier
The vapour barrier is the continuous plastic membrane that sits between the ground (or the underslab insulation) and the slab itself. Its job is to stop ground moisture rising into the slab and through to the finished floor — a major issue on reactive SEQ soils.
Continuity is the first check. We sight the entire footprint for tears, cuts and gaps. Reinforcement chairs commonly puncture the barrier and each puncture is a moisture entry point. Laps between sheets need a minimum 200 mm overlap and the laps should be taped continuously, not spot-taped.
Penetrations need a sealed collar — typically tape wrapped around every pipe, drain and rod that passes through the membrane. Edge wrap-up along the inside face of the form needs to reach the slab edge height so the barrier turns up to the finished slab level, not just to ground level.
Ready to book your inspection? A VG Inspect QBCC-licensed inspector attends every job.
Book an InspectionSoil engineer's report cross-check
The slab design is derived directly from the soil engineer's report. Under AS 2870, every residential slab in Queensland sits on a site classified as M (moderately reactive), H1 (highly reactive), H2 (highly reactive at depth), P (problem sites — fill, soft clay, slope) or E (extremely reactive). Each classification drives a different slab design — edge beam depth, slab thickness, reinforcement spec, joint placement.
Our cross-check is straightforward: pull the soil report from the file, confirm the site classification, then verify the slab on the ground matches the design that should follow from that classification. The defect we see most often is a design mismatch — an M-class slab detailed on the engineering drawing while the soil report on file shows H1, or vice versa. Sometimes the soil report was updated after the slab was designed and the design was never revised.
A slab designed for the wrong classification will crack, move or fail the structural warranty within the first 6 years and 6 months. Always verify site classification matches slab design before the pour.
Pre-pour readiness
Pre-pour readiness is the final sweep before we sign the inspection clear. Set-downs for wet areas need to be cast correctly — typically 20 mm below the adjacent floor level so the tiled shower or wet area finishes flush after screed and tiles. Slab steps for shower trays and recessed shower bases need to be formed in, not left to be chased out of the slab after the pour.
Sub-floor levels at wet areas need to be at the correct elevation relative to the rest of the slab so falls and drainage work as designed. Access for the concrete pump needs to be clear — no formwork or reo bent at the perimeter to allow the pump line through.
The site needs to be clean: no offcuts in the form, no plastic bags on the reo, no debris in the edge beam. Form alignment final check, final cover check on the steel, and the slab is ready to pour.
Why this matters
Slab stage is permanent. Every defect caught before the pour is a no-cost rectification — a tradie moves a sleeve, an extra chair gets slipped under the mesh, a corner bar gets added, a tape job gets redone on the vapour barrier. The pour goes ahead clean and the structure underneath the home is sound for 50 years.
Every defect caught after the pour is a structural concession at best and a saw-cut, epoxy injection or slab replacement at worst. The right slab inspection is one inspector, two hours, against the engineering drawings plus AS 2870 plus AS 3660.1 plus the QBCC Standards and Tolerances Guide. The wrong slab inspection — or no inspection at all — is a 50-year bet on a barrier you can no longer see.
Ready to book your inspection? A VG Inspect QBCC-licensed inspector attends every job.
Book an InspectionBook a slab inspection
Building in Lilywood? We inspect slabs right across the northern Brisbane growth corridor every week, and we know how local site classifications and soil reports translate into slab design corridor by corridor.
From Caboolture down through the Moreton Bay region, we cover slab-stage inspections across South East Queensland, with same-week availability timed to the day before your scheduled pour.
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A slab inspection with VG Inspect is $550 and includes a same-day digital report with AS 2870 and AS 3660.1 clause references on every item. Adam Gates personally attends every inspection (QBCC Licence 1318443), with same-week availability across South East Queensland. Rated 5.0 from 65 reviews. Call 07 3180 8041 or book a slab inspection online.