Underpinning advice
AS 2870 soil classes, decoded
Published
If you’ve ever had a soil report, a geotechnical assessment, or an engineer’s letter about your house, you’ve probably seen a single letter or code buried in it — A, S, M, H1, H2, P or E — and moved on without really knowing what it meant. That code is the site’s classification under AS 2870, the Australian Standard for residential slab and footing design, and it is the single number that determines how a house is footed, how deep piers need to go if underpinning is required, and how much movement is considered “normal” for that block.
It’s referenced constantly on our underpinning service page and in soil reports across Melbourne’s west, and almost never explained. Here’s what each class actually means.
What the classification measures
AS 2870 classifies a site by how much its soil is expected to move — shrink and swell — as moisture content changes through the seasons. That movement is what drives most residential footing and slab design in Australia, and it’s the reason two houses built to the same plan, one in Hoppers Crossing and one in Melton, can need genuinely different footing systems.
The classification isn’t about soil “quality” in any general sense — a Class E site isn’t worse dirt, it’s more reactive dirt, and reactive clay is exactly what much of Melbourne’s western growth corridor sits on. See reactive clay in Melbourne’s west, explained for the geology behind why.
The classes, from least to most reactive
Class A — Most stable. Sand and rock sites with little to no ground movement expected. Footing design is comparatively simple and cheap. Genuinely rare across the suburbs this site covers.
Class S — Slightly reactive clay. Some shrink-swell movement, but limited — up to about 20mm of expected surface movement. Standard slab and footing designs handle it without special measures.
Class M — Moderately reactive clay or silt. More movement than Class S — roughly 20–40mm expected. This is a common classification across parts of Melbourne’s middle-ring suburbs and requires a stiffened slab or deeper footings to manage.
Class H1 — Highly reactive clay. Expected movement in the 40–60mm range. This and H2 are the classes that show up constantly in soil reports across Caroline Springs, Point Cook and Werribee — reactive basalt-derived clay with real shrink-swell behaviour through a typical Melbourne wet-to-dry cycle.
Class H2 — Highly reactive clay, more severe. Expected movement of 60–75mm. Sites here need engineered stiffened slabs or deep footing systems designed specifically for the movement expected, and this classification is common through parts of Melton and Tarneit where the clay is at its most reactive.
Class P — Problem sites. Not a soil reactivity rating as such, but a catch-all for sites with additional complications — uncontrolled fill, mine subsidence risk, landslip, soft or collapsing soils, or abnormal moisture conditions (a leaking pipe, a nearby tree, poor drainage). A site can be Class P on top of any of the reactivity classes above it. Much of the engineered fill through the deeper-fill estates of Cairnlea and Wyndham Vale gets flagged this way pending confirmation of proper compaction.
Class E — Extremely reactive clay. The most severe standard classification — expected movement over 75mm. Sites this reactive need the most robust footing or piering systems available, and if a Class E site develops cracking, the movement involved is rarely something a homeowner should attempt to manage with anything less than a proper engineered fix.
What your classification actually means for you
If your house is newer (built after roughly 1990), the classification was almost certainly already used to design its slab and footings — a stiffened slab on H1 or H2 ground, for instance, is standard practice, not an admission of a problem. Cracking on a correctly designed slab that matches its actual site class usually points to something else: a moisture source disturbing the assumptions the design was based on, most often a downpipe discharging against the slab edge, a leaking under-slab pipe, or a tree root system pulling moisture from one side of the footprint unevenly. See why new homes on clay still crack for more on that specific pattern.
If your house is older, or you’ve never seen a soil report for it, the classification is worth getting — it’s often the missing piece that tells an engineer whether the movement you’re seeing is within the range the site was ever going to produce, or well beyond it. A geotechnical report referencing your specific block, not a suburb-wide estimate, is what an underpinning design gets based on.
If you’re buying, checking the site classification (or getting one done) before you commit is one of the more overlooked steps in a due diligence process focused mostly on the building inspection — a Class H2 or E site with an undersized or aging footing system is a materially different risk than the same house on Class M ground.
Where this fits into an actual repair decision
Classification alone doesn’t tell you whether you need resin injection or traditional underpinning — that comes down to the extent and pattern of movement your specific house has developed, assessed on site. But it is the number an engineer designs against once piers are warranted, and it’s worth asking for by name if a contractor hasn’t mentioned it.
Don’t have a soil report, or not sure what yours means for your house? Book a free assessment — we’ll talk you through what your site classification means in practice, not just the letter on the page.
Common questions
How do I find out my site's AS 2870 classification?
Three places, in order of ease. The soil report or site classification prepared before your house was built is the definitive source — the builder, the original owner, or sometimes the council building file will hold a copy. Failing that, a geotechnical engineer can classify the site from boreholes for roughly $800 to $2,500. Some councils and land developers also publish indicative classifications for an estate. Bear in mind the classification describes the site as it was assessed, and things like a mature tree or an altered drainage regime can change how that site actually behaves years later.
Is a Class H2 or E site a bad place to buy a house?
No — it is a site that demands a properly engineered slab and disciplined moisture management, which is a different thing. Large parts of Melbourne's western and south-eastern growth corridors are H1, H2 or E, and the great majority of homes on them perform perfectly well. What a high classification tells you is that the margin for error is smaller: a downpipe discharging at the slab edge, a garden bed watered daily against a wall, or a large tree planted too close will cause problems on an E site that the same mistake would not cause on a Class S site.
Can a site's classification change over time?
The geology does not change, but the way the site behaves certainly can. Removing a large tree lets the clay it had been drying out rehydrate and swell. Adding paving or an extension changes how much of the ground is covered and therefore how it dries. Altering drainage, installing irrigation, or a leaking pipe all change the moisture regime the classification assumed. This is why a house can sit quietly for twenty years and then start moving after a landscaping job — the soil class was never the whole story, the moisture conditions were.
Think this is happening at your place?
We assess before we quote — floor levels measured, cracks mapped, and a straight answer on whether you need work at all. Read more aboutunderpinning & foundation repair, or get in touch.