Signs of house subsidence and foundation movement
The specific warning signs that a house is subsiding or its footings are moving, and what each one is telling you about the ground below.
Subsidence is the downward movement of the ground supporting a building. When one part of a footing sinks, or the slab tilts, the rigid structure above has to go somewhere, and it shows that stress as a set of recognisable symptoms. Learning to read them early is the difference between a monitored, manageable problem and an expensive surprise.
Cracks that follow the load
The most reported sign is cracking, but subsidence cracks have a character. They tend to run diagonally from the corners of window and door openings, because those are the weakest points in a wall being pulled apart. In brickwork they often step up through the mortar joints. They are frequently wider at the top than at the bottom, showing the wall is rotating as one end drops. Unlike shrinkage cracks, they usually appear on both faces of the wall and keep growing rather than settling down.
Doors, windows and floors that no longer sit true
As a footing drops, frames rack out of square. Doors and windows that used to close cleanly begin to catch, drag on the floor or leave uneven gaps down one side. Floors can develop a noticeable slope or feel bouncy where the sub-floor has moved. Skirting boards may pull away from the wall, and gaps can open where the ceiling meets the wall or where an extension joins the original house. Because extensions often sit on separate, shallower footings, the join between old and new is a classic place for movement to concentrate.
Signs outside and underneath
Outside, look for gaps opening between the house and paths, patios or a garage that were once flush, and for external brickwork cracks that line up with the ones inside. Doors and windows sticking after heavy rain or a long dry spell hint at reactive clay soil swelling and shrinking. In the sub-floor or basement, watch for leaning piers, cracked or crumbling stumps, and damp patches or eroded soil near footings that point to a leaking pipe or poor drainage washing ground away.
One symptom rarely proves subsidence on its own. It is the combination, cracks with the right direction plus jamming openings plus sloping floors, and above all steady growth over time, that separates real footing movement from the ordinary seasonal shuffle every Australian house makes. If several signs are present and worsening, book a structural engineer to diagnose the cause before commissioning any repair.
Common questions
Is subsidence the same as settlement?
Not quite. Initial settlement is the small, expected movement as a new building beds down onto its footings in its first years. Subsidence is later, ongoing downward movement caused by the ground itself changing, for example reactive clay drying out, a leaking drain washing soil away, or tree roots removing moisture. Ongoing, one-way movement is the concern.
How fast does subsidence happen?
It varies. Movement from a burst pipe or eroding fill can appear over weeks, while reactive-clay and tree-related movement usually plays out slowly over seasons and years. That is why monitoring a crack over a wet and a dry season, rather than reacting to a single reading, is so useful.
Sources
- CSIRO, Foundation Maintenance and Footing Performance: A Homeowner's Guide (BTF-18)
- NSW Fair Trading, building and renovating guidance
General information only. Confirm details for your property with a licensed structural engineer.
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