AS 2870 Footing Systems for Residential Homes in Australia

What it is

AS 2870 is the Australian Standard for residential slabs and footings. It is the deemed-to-satisfy reference under the National Construction Code for Class 1 and Class 10 buildings on a foundation. The standard does two jobs: it classifies the site based on the reactivity of the soil to moisture change, then sets out how to design and detail a footing system that can handle the expected ground movement.

A site classification report and the matching footing design must sit with the building approval package. Without them the certifier or building surveyor will not pass slab inspection. The classification is normally produced by a geotechnical engineer or a registered site classifier under a boreholes, hand augers and soil identification procedure that is set out in the standard.

Site classifications

AS 2870 uses a letter system that runs from least to most reactive. The letter is paired with a characteristic ground surface movement (ys) in millimetres which feeds the footing design.

Class A

Stable, non-reactive soil. Typically rock, sand or weakly bonded materials. Characteristic ground surface movement is effectively zero. Cheapest and simplest footing options apply, including stiffened raft slab or strip footing on a sand pad.

Class S

Slightly reactive clay sites with ground surface movement up to 20 mm. Standard waffle raft or stiffened raft slabs designed to AS 2870 cover most builds without bespoke engineering.

Class M and M-D

Moderately reactive clay sites. Class M has ground surface movement between 20 mm and 40 mm. The M-D suffix is added when the depth of suction change is 3 m or greater, which is typical of dry climates such as inland Victoria, South Australia and parts of New South Wales. Deeper edge beams and additional reinforcement are required.

Class H1 and H1-D

Highly reactive clay sites with ground surface movement between 40 mm and 60 mm. Heavier slab and beam systems are required. Standard waffle pods can still be used but with deeper beams, more reinforcement and tighter detailing around set-downs and pipe penetrations.

Class H2 and H2-D

Very highly reactive clay sites with ground surface movement between 60 mm and 75 mm. Common in parts of south-east Queensland, Sydneys western basin and Adelaides reactive clay belts. Footing systems are often piered raft slabs that bear on stable strata below the active reactive zone.

Class E and E-D

Extremely reactive clay sites with ground surface movement greater than 75 mm. Site-specific engineering is mandatory. Standard waffle and stiffened raft systems are not used; designs typically use deep bored piers tied into a stiff raft or grade beams.

Class P

Problem sites that cannot be classified using the AS 2870 abnormal moisture conditions table. This includes fill of unknown depth, sites with mine subsidence risk, soft soils, landslip risk, sites within the influence of trees and sites with collapsing soils. Class P always requires site-specific engineering. The footing system is not standard.

How the classification leads to a footing

For Class A, S, M and H1 the engineer typically uses a standard footing system from AS 2870 Section 3 (deemed-to-satisfy design). For Class H2 and above, or any P site, Section 4 (engineered design) applies and a structural engineer must justify the slab and beam geometry, reinforcement, and any piers.

Common residential footing systems

• Waffle raft on void formers (pods). Cheapest where ground is flat and class is A, S, M or H1.
• Stiffened raft (conventional ground slab with edge and internal beams trenched into the natural ground). Common on H1 and H2.
• Strip footing with suspended floor (timber or concrete). Common on steeply sloped sites, Class P sites with shallow fill, and termite-prone areas requiring inspection zones.
• Bored pier and beam. Used on Class E, Class P with deep fill or trees, and reactive H2 sites with steep slopes.

Site classifier responsibilities

The classifier signs off on the AS 2870 class. The classification report must record borehole or hand-auger depths, soil descriptions, moisture conditions and the characteristic ground surface movement (ys) value used. If the classifier records the wrong class, the entire footing design may be undersized for the site. A wrong classification on a reactive clay site is one of the most common root causes of expensive structural failure in residential building.

Where footing failure shows up

The lag between footing under-design and visible failure is often years. Cracks in cornices, sticking doors, sloping floors, gaps at window heads and brick step cracks are typical first signs of differential movement. Once cracking is visible at finishing level, the slab has already moved. Remediation cost from underpinning or pier and pin systems frequently exceeds the cost of the original house frame.