Every house founded on soil, rather than solid rock, experiences some foundation movement over its life. Footings aren't rigid anchors, they're designed to accommodate a certain amount of ground movement, and AS 2870, the Australian Standard for residential slabs and footings, classifies sites by how much movement their soil is expected to produce so footings can be designed accordingly. The question isn't really whether a foundation has moved, it's whether the movement is within what was designed for, or has exceeded it.
Signs Worth Paying Attention To
- Cracking in walls, particularly diagonal cracking running from window and door corners.
- Doors and windows that stick or no longer close squarely, a sign the frame around them has racked out of shape.
- Uneven or sloping floors, sometimes only noticeable with a level or when furniture no longer sits flat.
- Gaps opening at skirting boards, architraves, or where a floor meets a wall.
- Visible separation or movement at an extension junction, where a newer part of the house meets the original structure.
What Actually Causes It
In the Sydney basin and across much of NSW, the dominant cause is reactive clay soil movement, the shrink-swell behaviour described in more detail in our piece on how reactive clay causes house cracking. Beyond soil reactivity, foundation movement is commonly linked to tree roots drawing moisture unevenly from beneath a footing, poor site drainage concentrating water in one area, leaking plumbing or stormwater infrastructure, nearby excavation or construction, or, in older housing stock, footings that were simply under-designed for the site conditions by today's standards. The CSIRO's long-standing guide to foundation maintenance and footing performance remains one of the most widely referenced Australian resources on how homeowners can reduce the risk of movement around their own footings, particularly through consistent watering and drainage practices rather than letting conditions swing between waterlogged and bone dry.
How It's Properly Assessed
An assessment typically includes a visual inspection of the affected structure, a review of crack patterns and their distribution, and where useful, a floor level survey to measure the extent and shape of any movement across the building. That data is considered against the site's likely soil classification, any obvious triggers, and the building's age and construction, to reach an opinion on cause and, where relevant, whether the movement is ongoing or has stabilised.
When to Get It Looked At
Movement that's active, has produced cracking beyond hairline width, or is affecting the function of doors, windows, or floors is worth an independent assessment rather than assuming it will settle down on its own. It's also a standard part of a pre-purchase structural inspection, since foundation movement that looks alarming to a buyer often turns out, once properly assessed, to be old and stable rather than an active problem. See our broader guide on whether a crack is actually structural for how this fits into that wider assessment.
