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Slab on ground construction: residential build sequence

Step-by-step slab on ground construction for Australian residential builders: site classification, AS 2870, vapour barriers, hold points, tolerances, NCC 2022.

By Dan Waring. Last updated 7 May 2026. Verified 7 May 2026.

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TL;DR

Slab on ground is the dominant residential foundation system in Australia: a reinforced concrete slab cast directly on prepared ground, designed under AS 2870:2011 and compliant with NCC 2022 Volume Two. The soil report (geotech) classifies your site from Class A through to P, and that class drives every number: slab thickness (85 to 110 mm panel), edge beam depth (300 to 600 mm), reinforcement mesh grade, and concrete strength (N20 minimum, N32 on reactive soils). The single biggest cost driver is site classification: a straightforward Class M stiffened raft on metro clay runs around $100 to $150 per m2 ex-GST (2026); step up to H2 and expect $180 to $250+ with full engineering. The hold point that determines whether the slab pours is the pre-pour inspection: missed reinforcement cover or an untreated termite barrier means demolition and re-pour once concrete is in. No engineer’s sign-off on the inspection, no pour.

When you do this

Slab on ground runs between site cut and fill (earthworks) and frame. It is a hold-point-dense stage: inspections happen at subgrade, pre-pour, and sometimes at 28-day break testing. The slab is on the critical path; a missed concrete truck booking or an RFI on the engineering drawings can cost you one to two weeks.

Who’s involved

RoleResponsibility
Structural / geotech engineerSoil report, site classification, slab design, drawings, concrete specification
ConcretorFormwork, reo placement, concrete placement, finishing, curing
Certifier (private or council)Pre-pour inspection and sign-off (mandatory hold point)
Pest managerPre-pour termite treatment and certification (mandatory hold point)
Sparky, plumberServices rough-in through slab before pour (conduits, pipes, waste penetrations)
SurveyorSlab set-out and RL (reduced level) checks

Steps

1. Soil report and site classification

Engage a geotechnical engineer before quoting the slab. The report classifies the site per AS 2870:2011 into one of seven classes based on expected ground movement from moisture changes:

ClassTypical movement (Ys)Soil type
Aup to 10 mmNon-reactive sand or rock
Sup to 20 mmSlightly reactive clay
Mup to 40 mmModerately reactive clay (most eastern-states metro sites)
H1up to 60 mmHighly reactive clay
H2up to 75 mmHighly reactive clay, severe
Emore than 75 mmExtremely reactive (engineer design required)
PvariableProblematic: fill, soft soils, abnormal conditions (engineer design required)

Source: AS 2870:2011 Residential Slabs and Footings (verified 2026-05-07).

For Class E and P sites, the NCC Housing Provisions prescriptive tables do not apply: full engineering design to AS 2870 is mandatory (NCC 2022, ABCB Housing Provisions Part 4.1).

2. Slab design and engineering drawings

The structural engineer produces slab drawings specifying:

  • Slab type (stiffened raft, waffle pod, or strip footing system)
  • Panel thickness (typically 85 to 100 mm for Class A/S/M; 100 to 110 mm for H1/H2)
  • Edge beam depth and width (300 mm minimum on Class A, up to 600 mm on H2 sites)
  • Internal beam layout, spacing, and depth
  • Reinforcement: mesh grade and bar sizes (SL72 through RL818 depending on class)
  • Concrete strength: minimum N20 (20 MPa at 28 days per NCC 2022 Housing Provisions 4.2); N25 or N32 typically specified on reactive sites
  • Hold points and inspection schedule

The two dominant slab types in Australian residential work:

Stiffened raft slab: flat panel with deepened edge and internal beams, cast monolithically. Suits Class A through H2 with standard AS 2870 tables for A to M; H1/H2 usually needs engineering calcs. Most common system on metro sites.

Waffle pod slab: polystyrene void formers create a rib grid, elevating the slab slightly off ground. Preferred where reactive soil movement must occur beneath without loading the slab directly. Standard on highly reactive (H1/H2) sites and increasingly common in VIC and QLD.

Source: NCC 2022 ABCB Housing Provisions Part 4.2 (verified 2026-05-07); AS 2870:2011 (verified 2026-05-07).

3. Site preparation: cut, fill, and compaction

Excavate to design subgrade level. Any fill placed under the slab must be compacted in controlled layers per NCC Housing Provisions Part 3.2:

  • Sand fill: compacted in layers no more than 300 mm deep using a vibrating plate or roller
  • Clay fill: compacted in layers no more than 150 mm deep using a mechanical roller
  • Sand blow count test (AS 1289.6.3.3): 7 or more per 300 mm to confirm compaction

Remove all vegetation, topsoil, and organic matter from the footprint. Pegs or profiles set by the surveyor establish the slab boundary and floor RL (reduced level).

Source: NCC 2022 ABCB Housing Provisions Part 3 Site Preparation (verified 2026-05-07).

4. Services rough-in through slab

All pipes, conduits, drains, and ducting that penetrate or pass under the slab go in now, before formwork is set up. Coordinate plumber, sparky, and any HVAC subcontractor so penetrations are in the right position and at the right invert. Pressurise water supply lines before the pour to catch leaks. Mark all penetrations against the hydraulic and electrical drawings.

5. Termite management: pre-slab treatment

For Class 1 buildings, a termite management system complying with AS 3660.1:2014 is mandatory under NCC 2022. The system must have a design life of at least 50 years for a non-temporary Class 1 building. Options include:

  • Chemical soil treatment (applied by a licensed pest manager to the subgrade and perimeter, pre-pour)
  • Physical barriers: stainless steel mesh, termite-resistant polymer sheeting, or graded stone
  • Combination systems

After pour, the slab edge must remain exposed a minimum of 75 mm above finished ground level as a visual inspection zone, per AS 3660.1:2014. This 75 mm zone is a NCC hold point: soil or garden beds banked against the slab edge void the inspection zone and breach the system.

Source: NCC 2022 Housing Provisions Part 3.4 Termite risk management (verified 2026-05-07); AS 3660.1:2014 (verified 2026-05-07).

The pest manager provides a certificate of installation (certificate of treatment). Do not pour before this is in hand.

6. Formwork and reinforcement

Set up formwork to the engineer’s dimensional requirements. Lay reo (reinforcement mesh and bars) per the drawings, with chairs (bar chairs, plastic stools) maintaining the specified concrete cover:

  • Minimum 40 mm cover where concrete is in contact with the ground directly
  • Minimum 30 mm cover where a 0.2 mm polyethylene vapour barrier is installed per Housing Provisions 4.2.8

Check reinforcement against the engineering drawings: mesh grade, bar spacing, lap lengths, and chair heights. Reinforcement defects (wrong cover, sagged mesh) are the most common pre-pour finding by private certifiers. Correct before calling for inspection.

7. Vapour barrier (damp-proofing membrane)

A continuous vapour barrier is mandatory under all slab-on-ground construction for Class 1 buildings (NCC 2022 Housing Provisions 4.2.8). Specification:

  • 0.2 mm nominal thickness polyethylene film, marked “AS 2870 Concrete underlay, 0.2 mm High impact resistance”
  • Minimum 200 mm laps at all joints
  • Taped at all laps and sealed around all service penetrations
  • Must extend under edge beams to finish at ground level

Place the vapour barrier after compacted fill and services are in, before reo placement. Any tears must be patched before pour.

Source: NCC 2022 ABCB Housing Provisions 4.2.8 (verified 2026-05-07).

8. Pre-pour inspection (mandatory hold point)

Do not pour concrete until the certifier has attended and signed off the pre-pour inspection. This is a mandatory hold point on most Construction Certificates. The certifier checks:

  • Subgrade compaction and condition
  • Formwork dimensions, beam widths, beam depths, and levels against the engineering drawings
  • Reinforcement mesh and bar: grade, spacing, lap lengths, concrete cover (chair heights)
  • Vapour barrier: installed, lapped 200 mm, sealed at penetrations
  • Termite treatment: pest manager’s certificate on site
  • Services: plumbing, conduits, and penetrations in correct position
  • Concrete mix design specified on the delivery docket matches the engineer’s spec

After the certifier signs off, the concretor books the concrete truck. Any change to the pour date that follows a significant weather event or a delay may need re-inspection.

9. Concrete placement, compaction, and finishing

Concrete is placed in one continuous pour where practical to avoid cold joints. Key requirements per NCC 2022 Housing Provisions 4.2:

  • Concrete strength: minimum N20 (20 MPa at 28 days); confirm the docket matches the specification before accepting the truck
  • Maximum aggregate size: 20 mm
  • Concrete vibrated and compacted immediately after placement
  • Surface finished to the specified level per the engineer’s drawings

Residential slabs are typically screeded and power-trowelled to a smooth or broom finish depending on the floor covering to follow. If polished or finished concrete is specified, tolerances are tighter: discuss with the concretor before pour.

10. Curing

Moist cure for a minimum of 7 days after placement per NCC 2022 Housing Provisions 4.2. Methods include:

  • Wet burlap or hessian kept damp, covered with polyethylene sheeting
  • Proprietary curing compounds sprayed or rolled on immediately after finishing
  • Curing blankets

Do not load the slab (foot traffic, construction loads) for a minimum of 7 days. The slab reaches its specified 28-day design strength after the full cure period.

Tolerances and acceptance

Slab tolerances for residential construction are split between the NCC and the HIA Guide to Materials and Workmanship.

ElementStandardRequirement
Slab level (floor flatness)HIA GuidePer current HIA Guide to Materials and Workmanship. Verified numerical value pending HIA member access. [HIA-032]
Edge beam depthAS 2870:2011Must match engineer’s drawings; no under-dig tolerance in the standard; any deviation requires engineer sign-off
Concrete cover to reoNCC 2022 HP 4.230 mm minimum with vapour barrier; 40 mm minimum without
Slab edge exposure (termite zone)AS 3660.1:201475 mm minimum exposed slab edge above finished ground level
Concrete strengthNCC 2022 HP 4.2N20 minimum at 28 days; confirmed by delivery docket; cylinder break tests on request

For floor flatness under finished flooring, tile or stone work, tolerances are often tighter than the standard residential bar: confirm with the tiler or flooring contractor before pour and screed.

Documents needed

  • Soil report (geotech) with site classification
  • Structural engineering drawings (slab, edge beams, reo schedule)
  • Concrete specification from the engineer (strength grade, admixtures, slump)
  • Construction Certificate or building permit with inspection schedule and hold points
  • Pest manager’s pre-pour treatment certificate
  • Hydraulic and electrical drawings (penetrations through slab)
  • Surveyor’s set-out and RL marks

Common holds

  • No geotech before the quote. Slab cost quoted without a soil report is guesswork. A Class H2 site can cost 50% more than a Class M. Subcontract the risk, not the unknowns.
  • Pre-pour hold point missed. Concrete truck arrives before the certifier signs off. Either turn the truck around (concrete readymix has a limited workable window, typically 90 minutes from batching) or pour and face a direction to break up.
  • Reo placement issues. Wrong mesh grade, sagged mesh from workers walking on it without chairs, insufficient lap length. Standard finding at pre-pour; fast to fix before concrete goes in; impossible to fix after.
  • Vapour barrier tears. Reo installation traffic tears the membrane. Any hole paths moisture under the slab. Tape and patch before pour; the certifier looks for this.
  • Services not tested before pour. A pressurised plumbing failure discovered after pour requires core-drilling and invasive repair. Test all services before the certifier attends.
  • Termite barrier bridged after construction. Soil or mulch banked against the slab edge covers the 75 mm inspection zone. A building defect by the owner that can void the termite system warranty.
  • Cold joint. A concrete pour stopped and restarted creates a cold joint, a structural discontinuity. On residential slabs, aim to complete in one continuous pour. If a break is unavoidable, use a purpose-formed construction joint per the engineer’s detail.

References

See also

Last updated: 2026-05-07. Verified: 2026-05-07. Quarterly review for currency.

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