Get the footings wrong for the soil and the house cracks — which is why AS 2870 makes you classify the site before you design the slab. Here is the site-class system (A through P), the footing systems that match each, and who is responsible for getting it right.
AS 2870 and the site classes
AS 2870-2011 "Residential slabs and footings" is the primary standard for residential footings, and the NCC calls it up as the DtS pathway. It matches the slab and footing system to the expected ground movement from soil moisture changes (the shrink-swell of reactive clays) plus bearing capacity. The site classes, from stable to problem:
| Class | Reactivity |
|---|---|
| A | Non-reactive (sand, rock — little movement) |
| S | Slightly reactive |
| M | Moderately reactive |
| M-D | Moderate + deep/abnormal moisture changes (trees, deep fill) |
| H1 | Highly reactive |
| H2 | Very highly reactive |
| E | Extremely reactive |
| P | "Problem" site (uncontrolled fill, soft clays, landslip) — always specific engineering |
The "-D" suffix flags abnormal moisture conditions needing extra design.
How a site gets classified
Classification comes from a geotechnical investigation early in the project: boreholes or hand augers log the soil profile, groundwater, fill and slope; lab tests (Atterberg limits, moisture, shrink-swell) plus field observations let the geotechnical engineer calculate the characteristic surface movement and check minimum bearing. The output is a site classification / geotech report — the basis for the footing design. Declining a soil test does not remove the risk; the engineer just uses conservative (more expensive) assumptions, and the owner carries the risk of the unknown.
Footing systems by class
AS 2870 gives standard (DtS) footing systems per class, and it is "footing-agnostic" as long as the system meets the performance for the class:
- A and S — a wide range works (conventional strip/pad, stiffened rafts, waffle rafts) because movements are small.
- M and M-D — stiffened or waffle rafts detailed for the M range (minimum slab thickness, beam depths, articulation); conventional footings need deeper beams and heavier reinforcement than S.
- H1 and H2 — DtS rafts still apply but with significantly increased beam depths, slab thickness, reinforcement and edge-beam stiffness, plus articulation and moisture control.
- E — often pushed to specific engineering: stiffened rafts with deep beams, articulated masonry, strict drainage, sometimes piles.
- P — always specific engineering from first principles (deepened footings, ground improvement, bored or screw piles).
Who is responsible — and what the surveyor needs
- The geotechnical engineer does the investigation, testing and AS 2870 classification.
- The structural engineer relies on that report to design and certify the footings (a regulated design role).
- The builder procures and coordinates the investigations and design but does not usually classify the site.
Before approving the slab, the building surveyor needs: the geotech site classification report (the class plus any "-D"), the structural footing/slab design certified by an engineer referencing the AS 2870 class, and footing-stage inspection evidence. Without these, they refuse the permit or issue a stop-work.
What goes wrong when the footing does not match
- Technically: excessive differential movement → slab and masonry cracking, doors and windows jamming, and in severe cases structural instability — exactly what AS 2870 exists to prevent.
- Legally: AS 2870 and the NCC are part of the regulatory framework, so a Class S footing on an actual H2 site is a structural-failure and litigation risk. The common failures — no soil test, footing depths not suited to the class, generic designs with no site-specific engineering — leave the builder and designer exposed to statutory warranty claims, insurance disputes and professional-negligence findings. Building without (or contrary to) the classification undermines your defence if cracking appears.
Common mistakes
- Skipping the geotech report to save cost (the risk does not go away).
- Reusing a generic footing detail across sites with different classes.
- Building on a P or E site without specific engineering.
- No footing-stage inspection before the pour.
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