If your home is in Wagga Wagga and you’ve had foundation problems, one question inevitably arises: were the original footings built to code? And if you are building new in the Riverina, what does the building code actually require for reactive clay? This guide explains the Australian Standards and NCC (National Construction Code) requirements for residential footings on reactive clay, in plain language.
The Framework: AS 2870 and the NCC
Residential foundation design in NSW is governed by two key documents:
Australian Standard AS 2870, Residential Slabs and Footings is the technical standard that specifies how to design and build residential foundations on reactive soils. It covers soil classification, slab and footing design procedures, and performance requirements.
The National Construction Code (NCC), previously the Building Code of Australia (BCA), references AS 2870 as an acceptable solution for residential foundations. When a builder or certifier approves a new home, they are confirming (among other things) that the footing design meets AS 2870 requirements.
Compliance with AS 2870 is not optional for new residential construction. It is a mandatory requirement under the NCC and enforced through the building approval process.
Step 1: Soil Classification
Before any footing depth is specified, the site soil must be classified. AS 2870 defines six reactive soil classes:
| Class | Description | Surface Movement (ys) |
|---|---|---|
| A | Non-reactive | < 10 mm |
| S | Slightly reactive | 10-20 mm |
| M | Moderately reactive | 20-40 mm |
| H1 | Highly reactive | 40-60 mm |
| H2 | Very highly reactive | 60-75 mm |
| E | Extremely reactive | > 75 mm |
| P | Problem site | Variable |
Soil classification is determined by a geotechnical investigation, soil test borings and laboratory testing of samples. The classification must be documented before an AS 2870 compliant footing design can be prepared.
In Wagga Wagga, the predominant classifications are:
- M to H1 in established suburbs (Kooringal, Lake Albert, Turvey Park)
- H1 to H2 in parts of Estella and Gobbagombalin
- Class P in floodplain areas near the Murrumbidgee (North Wagga)
Step 2: Footing Design for Each Soil Class
Slab-on-Ground Systems
AS 2870 Table 2.1 specifies minimum requirements for slab-on-ground systems by reactivity class. The key parameters are:
Edge beam depth and width: The perimeter beam (the thickened edge of a waffle-pod or raft slab) must be deep enough to provide rigidity against differential clay movement. Beam depth requirements increase with soil reactivity:
| Soil Class | Minimum Edge Beam Depth | Minimum Edge Beam Width |
|---|---|---|
| A/S | 300 mm | 300 mm |
| M | 400 mm | 300 mm |
| H1 | 500 mm | 350 mm |
| H2 | 600 mm | 400 mm |
| E | Site-specific design required | - |
| P | Site-specific design required | - |
Reinforcement: Slab reinforcement (steel bars or mesh) must be specified appropriately for the soil class. H2 class slabs require more robust reinforcement than M class slabs.
Waffle pod voids: The polystyrene pods used in waffle-pod slabs provide a “free movement zone” beneath the slab that accommodates some heave without stress. On H1 and H2 sites, appropriate void depth is specified.
Internal beam depth and spacing: Internal stiffening beams must also increase in depth and decrease in spacing as soil reactivity increases.
A properly designed H2 class slab in Wagga is substantially heavier, deeper, and more heavily reinforced than an M class slab. Attempting to save money by specifying an M class design on an H2 site is a building code violation and the primary cause of premature slab failure in newer Wagga estates.
Stump Systems
For suspended floor homes using stump supports, AS 2870 specifies:
- Minimum stump embedment depth based on soil class
- Requirements for stump pad sizes
- Requirements for connection between stump and bearer
For H1 and H2 classified sites, AS 2870 requires stumps to penetrate below the “critical depth”, the depth at which seasonal moisture variation is negligible. This critical depth (Hs) is typically 1.0-1.8 m in the Wagga region.
Stumps set only to 600 mm depth on an H1 or H2 site are not AS 2870 compliant and will move with the clay.
Strip Footings (Older Construction)
Many Wagga homes built before AS 2870 was adopted (pre-1980) used strip footings, continuous concrete strips under load-bearing walls, set at relatively shallow depths. These were designed before soil reactivity was formally classified, and many are not deep enough to meet current requirements on reactive clay.
Current NCC requirements for new strip footings on reactive clay are governed by AS 2870. For repairs and alterations to existing homes with strip footings, the requirements depend on the scope of work.
What “Compliant” Actually Means in Practice
AS 2870 compliance does not guarantee a crack-free foundation. The standard acknowledges that reactive clay sites will move and designs to accommodate movement within limits. A compliant design:
- Allows some differential movement but limits it to keep the structure serviceable
- Is specified for the particular soil class of the site
- Assumes the moisture regime around the foundation approximates the conditions assumed in the design
What invalidates compliance (even of an originally compliant design):
- Trees planted near the foundation altering the moisture regime
- Leaking pipes introducing concentrated moisture
- Removal of large trees causing unexpected clay heave
- Fill added or removed near the foundation changing drainage patterns
- Extensions built without appropriate tie-in to the original foundation design
Checking Whether Your Wagga Home’s Footings Were Built to Code
For existing homes, the original building approval documents (DA and CC) should include a footing design or specification. These are held by Wagga Wagga City Council for the life of the building and can be requested under government information access provisions.
If the original documents cannot be located, a structural engineer can assess the existing footing using visual inspection, physical probing, and sometimes limited excavation adjacent to the footing to determine depth and configuration.
If the footing is found to be non-compliant with AS 2870 for the actual soil class, this does not automatically mean the footing must be replaced, the assessment is whether the existing footing is performing adequately and whether repair or supplementation is required.
Implications for Repairs and Extensions
When carrying out significant repairs or extensions on a Wagga home:
Like-for-like repairs (replacing stumps with equivalent stumps): Generally do not trigger full AS 2870 reassessment, but should meet the minimum requirements of the original design.
Foundation upgrades (underpinning, changing footing type): Must comply with current AS 2870 requirements for the soil class. This is typically administered through a building approval.
New extensions: The new footing must be designed for the actual soil class and must be tied in appropriately to the existing footing. See our guide to extension foundation tie-in in Wagga.
Significant alterations: If an alteration increases loads on the existing footing (adding a second storey, for example), the existing footing must be checked against the new loads.
Frequently Asked Questions
Q: My builder says the soil was tested and classified as M, but the house is cracking like it’s on H2, what happened? A: Several possibilities: the soil test was taken from a less reactive part of the site, there is a localized zone of higher reactivity under the house, or post-construction changes have altered the moisture regime. Get an independent geotechnical reassessment.
Q: I want to add a second storey to my 1970s Wagga home, does the existing foundation need upgrading? A: Almost certainly yes for a full second storey. The existing foundation was designed for a single-storey load. Adding a second storey significantly increases column loads and wall loads. A structural engineer must assess whether the existing footing can carry the additional load or whether upgrades are required before building approval will be granted.
Q: The concrete in my new home’s slab edge beam is only 350 mm deep but we’re on H1 clay, is that compliant? A: Minimum edge beam depth for H1 class under AS 2870 is 500 mm. A 350 mm beam on an H1 site is potentially non-compliant. Review your building approval documentation, the design may have been done to a specific engineered design that achieves equivalent performance differently, or there may have been an error. Raise with your builder and certifier.
Q: Is there a minimum footing depth for all buildings in NSW or just residential? A: AS 2870 covers Class 1 and Class 10 buildings (houses and associated structures). Commercial buildings and other classes are designed under separate standards (AS 3600 for concrete, AS 4100 for steel), with site-specific geotechnical investigation and structural engineering typically required.
Q: If my footings don’t meet current code, can I still sell the house? A: Yes. Existing buildings are not required to be upgraded to comply with current code unless significant alterations are being made. However, if the footings are performing poorly (causing visible cracking and movement), this is a disclosure obligation in NSW and may affect value. See our pre-purchase inspection guide.
Need to understand whether your Wagga home’s foundation meets current requirements? Contact Wagga Foundation Repair for an assessment, we work with registered engineers to assess existing footings against AS 2870 standards.