Melbourne homes move because the ground beneath them changes with the seasons, and reactive clay is the main reason. Reactive clay swells when it takes on water and shrinks as it dries out, so a house built on it sits on ground that lifts slightly in winter and drops slightly in summer. Over years, that movement is rarely even: one corner of the house typically moves more than another, and the walls crack where the difference is greatest. This single mechanism, repeated season after season, underpins most of the work we do across Melbourne.
What makes this hard to notice day-to-day is the timescale. The movement in any single season is often only a few millimetres, well within what a home can absorb without visible damage. It is the accumulation over many wet and dry cycles, combined with uneven drying across a block (shaded side versus sun-exposed side, garden bed versus paved area), that eventually produces the differential movement that shows up as cracking, sticking doors or an out-of-level floor. This is also why symptoms often become noticeable after a run of unusually dry or unusually wet weather, rather than gradually and evenly over time.
Soil movement potential is measured using the AS 2870 site classification system, which rates a block from stable to extremely reactive. Class A is sand or rock with negligible seasonal movement. Class S is slightly reactive clay, up to about 20mm of surface movement. Class M is moderately reactive, about 20 to 40mm, and common across much of Melbourne. Class H1 is highly reactive clay at about 40 to 60mm, enough to crack masonry, and H2 is very highly reactive at about 60 to 75mm. Class E is extremely reactive, more than about 75mm, and Class P covers problem sites such as fill, soft soil, landslip or mine subsidence. A “-D” suffix indicates deep-seated moisture change, more common in drier inland climates than metropolitan Melbourne. (Source: AS 2870, Residential Slabs and Footings.)
Three other causes account for most of the rest. Thirsty tree roots draw moisture out of the clay on one side of the house, which is often why the worst cracking shows up on the side nearest a large tree. Drainage failure, such as leaking stormwater, a blocked downpipe or a broken sewer line, saturates one corner of the site. And poorly compacted fill and ageing timber stumps are more common in older Melbourne housing stock.
As a general tendency rather than a claim about any specific address, Melbourne’s western and northern suburbs commonly sit on basalt-derived clay, which tends to be heavier and more reactive, while bayside and south-eastern suburbs often have sandier profiles that behave differently through the seasons. Your own site classification report, or a free assessment, is the only reliable way to know what applies to your property. If you want to understand more about the symptom side before choosing a repair, our Melbourne house subsidence page covers how to assess what you are seeing.
