AS 2870-2011: residential slabs and footings

TL;DR

AS 2870-2011 is the Australian Standard for residential slab and footing design, used on every Class 1 and Class 10a build. It is the direct route to satisfying NCC 2022 H1P1 for footings and slabs, alongside the ABCB Housing Provisions 2022 Part 4.2. The standard defines seven site classes (A, S, M, H1, H2, E, P) based on reactive soil movement: Class A is stable sand or rock, Class P is a problem site requiring custom engineering. Get the soil report before quoting a slab. A Class M raft slab is meaningfully cheaper than an H2 waffle raft, and the difference is real money. H, E, and P sites fall outside the Housing Provisions deemed-to-satisfy tables: they require a structural engineer working directly to AS 2870.

In plain English

AS 2870:2011 is the national standard that governs how residential slabs and footings are designed and built in Australia. It covers Class 1 (single dwellings, townhouses) and Class 10a (garages, carports, sheds) buildings that sit on the ground, not vertically above or below another dwelling (verified 2026-05-08, Standards Australia AS 2870:2011).

The standard does two main things:

Site classification: defines a methodology for classifying the soil under a site as one of seven classes (A through P), based on how much the ground is likely to move due to moisture changes in reactive clay soils.
Footing and slab design: provides prescriptive (deemed-to-comply) designs for stiffened raft slabs, waffle pod slabs, strip footings, pad footings, and piled footings matched to each site class.

The NCC 2022 Volume Two picks up AS 2870 under H1D4. Performance Requirement H1P1 requires that a building’s structure resists the loads and actions to which it is reasonably subjected. For footings and slabs, H1D4 is satisfied by complying with AS 2870, AS 3600, or Part 4.2 of the ABCB Housing Provisions (verified 2026-05-08, NCC 2022 H1D4).

The ABCB Housing Provisions 2022 Part 4.2 is a free alternative path and provides DTS prescriptive solutions for Class A, S, and M sites. For H, E, and P sites, Part 4.2 explicitly defers back to AS 2870 for design and construction (verified 2026-05-08, ABCB Housing Provisions Part 4.2).

Site classification table

The site class determines which footing design applies. Classification is performed by a geotechnical engineer under AS 2870 Section 2 (verified 2026-05-08, ABCB Housing Provisions Part 4.2).

Class	Soil type and behaviour	Characteristic surface movement (ys)	DTS path available
A	Most sand and rock; little or no ground movement from moisture changes	Negligible	Yes (Housing Provisions 4.2)
S	Slightly reactive clay; only slight ground movement	ys up to ~20 mm	Yes (Housing Provisions 4.2)
M	Moderately reactive clay or silt; moderate ground movement	ys 20 to ~40 mm	Yes (Housing Provisions 4.2)
H1	Highly reactive clay; high ground movement	ys ~40 to 60 mm	Engineer to AS 2870
H2	Highly reactive clay; very high ground movement	ys ~60 to 75 mm	Engineer to AS 2870
E	Extremely reactive clay; extreme ground movement	ys greater than ~75 mm	Engineer to AS 2870
P	Problem sites: soft or loose soils, fill, landslip, mine subsidence, abnormal moisture conditions, or sites that cannot otherwise be classified	Variable or unquantifiable	Engineer to AS 2870

A “-D” suffix (M-D, H1-D, H2-D) indicates deep-seated moisture changes characteristic of dry climates with design suction change depths of 3 m or more. These also require engineering under AS 2870, not the Housing Provisions (verified 2026-05-08, NCC 2022 H1D4).

What it requires

Site classification report first

Before any footing or slab can be designed, the site must be classified. A soil report prepared by a geotechnical engineer covers bore or test pit logs, soil descriptions, the site class recommendation, and footing guidance. Most certifiers require the report before issuing a Construction Certificate. The report is also a prerequisite for the structural engineer to complete their footing design.

Typical cost for a residential site classification report: $500 to $1,200 ex-GST, depending on the number of test holes, site access, and location (verified 2026-05-08, Smolders Geotechnical).

Footing systems

AS 2870 covers six footing types:

Footing type	Common use case	Notes
Stiffened raft slab	Class A, S, M sites; most common residential system	Flat panel with edge and internal beams; DTS designs in Housing Provisions 4.2 tables for A/S/M
Waffle pod slab	Class H1, H2 sites; reactive clay; VIC and QLD common	Polystyrene void formers create rib grid; voids allow soil movement without lifting slab
Strip footings	Class A, S, M sites; masonry or brick veneer	Continuous concrete beam below loadbearing walls
Pad footings	Isolated column loads; deck posts; Class A/S/M	Individual concrete pads under point loads
Piled footings	Class E, P sites; severely reactive or problem ground	Steel or concrete piles bypassing poor surface soils
Slab on ground (unstiffened)	Class A only	Rarely specified; limited to very stable sites

DTS designs for Class A, S, and M

For these three classes the ABCB Housing Provisions Part 4.2 provides prescriptive tables covering beam depths, reinforcement mesh types and sizing, concrete grade (N20 minimum per Housing Provisions 4.2), slab thickness, and concrete cover. Using these tables satisfies H1P1 without a full engineering design.

Example depth ranges for stiffened rafts from Part 4.2:

Site class	Edge beam depth (approx.)
Class A	300 mm
Class S	400 to 450 mm
Class M	450 to 600 mm

These are indicative. The actual depth depends on wall construction type (brick veneer, full masonry, or timber/steel frame) and whether the slab is external. Use the Housing Provisions tables directly.

Engineer designs for Class H, E, and P

Classes H1, H2, E, and P are outside the scope of the Housing Provisions prescriptive tables. A structural engineer must design the footing system directly under AS 2870. AS 2870 provides two design methods for stiffened and waffle rafts on reactive sites:

Deemed-to-comply method: uses AS 2870 tables (larger than the Housing Provisions tables) for H1, H1-D, H2, and H2-D sites.
Maximum differential deflection method: full engineering analysis for sites outside the tables or where non-standard dimensions are required.

On Class H2 sites, stiffened raft edge beam depths commonly exceed 600 mm. Waffle pod slabs are often preferred over stiffened rafts at H1 and H2 because the void beneath the slab panel accommodates reactive soil heave more effectively.

Vapour barrier and damp-proofing

Under the Housing Provisions and AS 2870, a vapour barrier must be installed under slab-on-ground construction. The standard barrier is 0.2 mm polyethylene sheet (branded “AS 2870 Concrete underlay, 0.2 mm High impact resistance”), with 200 mm lap joints and sealed around all penetrations. Sheets extend beneath edge beams to ground level (verified 2026-05-08, ABCB Housing Provisions Part 4.2).

Failure to correctly lap and seal the vapour barrier is a common defect that contributes to moisture ingress under slabs and is identified in post-construction inspections.

What it doesn’t cover

AS 2870 applies to residential Class 1 and Class 10a buildings. Exclusions:

Class 2 buildings (multi-storey apartments): AS 3600 and structural engineering under NCC Volume One apply
Class 3-9 buildings: commercial/industrial; separate standards and NCC Volume One
Retaining walls and basement construction: separate geotechnical and structural engineering scope
Piling on Class P sites: AS 2870 provides guidance but deep piling usually requires site-specific geotechnical design beyond the standard

Practical implications

When to get a soil report

Order the soil report at pre-DA stage if the site is in a known reactive soil area (inner Melbourne, Brisbane clay, Adelaide hills, parts of Sydney). For greenfield estates in stable sandy areas (coastal NSW, Perth sands), Class A is common but should still be confirmed. A soil report is cheap insurance relative to the cost of underspecifying a slab.

Quoting without a soil report

Quoting a concrete slab without knowing the site class is a known loss. A Class M raft slab involves deeper edge beams and more reinforcement than a Class S slab. An H2 waffle raft adds formwork cost, additional engineering fees, and commonly 15 to 30 percent more concrete volume. If you quote Class S and the site classifies H1, you wear the difference unless you included an explicit allowance in the contract.

Influence zone: trees and drainage

AS 2870 includes provisions for the influence of trees on reactive sites. A tree’s root zone can extend to 1 to 1.5 times its mature height from the trunk, and moisture removal at depth by tree roots increases the potential for differential movement. The soil report should identify any trees within the influence zone and flag implications for the site class.

Poor site drainage (overland flows, poor falls away from the slab, leaking services) also drives moisture change in reactive soils. AS 2870 footing designs assume the drainage conditions stated in the soil report are constructed and maintained.

State variations

AS 2870 is a national standard called up consistently across all states via the NCC. However, soil reactivity patterns vary:

Victoria (VIC) and South Australia (SA): widespread Class M, H1, and H2 sites, especially on expansive Tertiary clay formations. Waffle pod slabs are the default in many Melbourne suburbs.
Queensland (QLD): coastal sandy soils are commonly Class S to M, but black soil plains in the Darling Downs and inland QLD are H1 to H2.
New South Wales (NSW): variable; Sydney basin has significant Class M areas; inner-west and outer-west clay soils run H1.
Western Australia (WA): Perth sandy soils are predominantly Class S or A; not a reactive clay environment. Coastal sites may have specific dune sand considerations.
Northern Territory (NT): reactive cracking clays common around Darwin. Class H1/H2 sites frequent.
Tasmania (TAS): generally stable soils in coastal and highland areas; reactive clay less prevalent, but site-specific reports still required.
Australian Capital Territory (ACT): granitic and clay soils; reactive conditions occur in lower-lying areas; Class M to H1 sites are found in established suburbs.

No state variation to AS 2870 itself. The standard applies uniformly; the soil is what changes.

Common defects and site holds

Defect or hold	Cause	Consequence
Wrong site class specified	Soil report not ordered or ignored; engineer uses lower class to reduce beam depth	Structural cracking as soil moves beyond footing capacity
Vapour barrier not lapped or sealed	Speed or oversight; penetrations not collared	Moisture under slab, timber flooring movement, concrete degradation
Edge beams poured without inspection	Footing inspection hold point not cleared by certifier	Defect not identified before concrete locked in place
Tree removal after classification	Trees removed after soil report; report assumed their influence on moisture	Moisture rebound under slab as tree root zone dries out then rewets; possible heave
Drainage falling toward slab	Site levels not checked; finished paving draining toward building	Concentrated moisture change adjacent to edge beams; increased reactive movement
Incorrect mesh or bar size	Cost reduction; wrong reo substituted	Non-compliant slab; potential insurance consequences

Source link

Standards Australia: AS 2870:2011 product page (verified 2026-05-08). The standard is paywalled. The ABCB Housing Provisions 2022 Part 4.2, which covers the DTS provisions for Class A, S, and M sites, is free at abcb.gov.au.

References

Standards Australia, AS 2870:2011 Residential slabs and footings (product page). https://store.standards.org.au/product/as-2870-2011 (verified 2026-05-08).
Australian Building Codes Board, NCC 2022 Volume Two, Part H1 Structure. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h1-structure (verified 2026-05-08).
Australian Building Codes Board, ABCB Housing Provisions 2022, Part 4.2 Footings, slabs and associated elements. https://www.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions/4-footings-and-slabs/part-42-footings-slabs-and-associated-elements (verified 2026-05-08).
Australian Building Codes Board, Part 4.1 Scope and application of Section 4, Housing Provisions 2022. https://www.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions/4-footings-and-slabs/part-41-scope-and-application-section-4 (verified 2026-05-08).
Smolders Geotechnical, Soil testing cost. https://smoldersgeotechnical.com.au/soil-testing-cost/ (verified 2026-05-08).

NCC 2022 Volume Two, the residential building code that calls up AS 2870 under H1D4
Slab on ground construction, the on-site procedure for forming, pouring, and inspecting a residential slab
Soil report (geotech), what a soil classification report covers and how to read the site class output
Stiffened raft slab, the most common residential slab type in Australia, designed under AS 2870
Waffle pod slab, the reactive-site alternative using polystyrene void formers on H1/H2 sites
Footing, plain-English definition of residential footings