Subsidence and cracked walls: when water is moving the house

Not every cracked wall is a water damage problem. Some are, and those are worth catching early. On reactive clay, the ground under a house swells when it gets wet and shrinks when it dries. Footings are designed for the seasons; they are not designed for a supply pipe that has been leaking under one corner for months, or a downpipe that empties against the slab. When the water is the cause, fixing the crack without fixing the water just schedules the next one.

Reactive clay: how much the ground is expected to move

The QBCC’s homeowner guide says the majority of Queensland homes sit on reactive clay soils, and that footings designed to AS 2870 are intended to accommodate the movement that seasonal wetting and drying causes, under normal conditions. Its table of site classes shows how much that is:

The site class comes from the site investigation for the footing design and may be in the building approval documents; ask the builder or the certifier if you cannot find it. The higher the class, the more a change in moisture under one part of the slab matters.

Where water comes into it

The QBCC lists the abnormal conditions, other than seasonal change, that may lead to movement and damage ranging from minor to extreme: the effect of trees, poor surface drainage and/or leaking plumbing (QBCC, A guide to preventing structural damage). Two of the three are water. In practice that means:

• A leaking supply pipe under or beside the house, wetting one area of clay continuously.
• Leaking stormwater or sewer drains, including a downpipe connection that has broken below ground.
• Surface water ponding against the footings, from ground that falls toward the house, garden beds and edging that trap water, or sprinklers next to the walls.

The guide also describes a loop worth knowing about: movement in paths can stress pipes and inspection openings and break them, and the resulting leaks can make the clay under the house swell, which causes more movement (QBCC, A guide to preventing structural damage). It tells owners of houses under warranty to check and maintain plumbing, drainage and stormwater regularly, carrying out immediately carrying out repairs to leakages or breakages when observed — leaks it says usually show as seepage or greener lawns — or when cracking exceeding 3 to 5 mm appears in walls or ceilings.

So the first checks are cheap. Do a meter test with everything turned off; South East Water says to wait for 20 minutes (South East Water, Leaks). Walk the outside of the house after rain and look for ponding, downpipes discharging at the wall, and soil built up over the weep holes, which NSW Health and the QBCC both say should be kept uncovered (NSW Health, Mould factsheet). Look for one patch of lawn that is greener than the rest.

How serious is the crack?

Crack width is the main measure. Western Australia’s Guide to Standards and Tolerances reproduces the AS 2870 classification of damage to walls:

For new building work, WA’s guide says category 3 or greater damage to walls is defective and needs investigation, stabilisation, monitoring and rectification, and that Category 2 cracks are to be monitored for 12 months (Building and Energy WA, Guide to Standards and Tolerances 2019, 3.02 and Table 3.02). For an owner, the useful part is Category 3’s description: service pipes can fracture. Cracking that wide is both a possible result of a leak and a possible cause of one. Measure a crack with a ruler or crack gauge, mark its ends in pencil with the date, and photograph it; whether it grows over weeks is what an engineer will ask.

Insurance: subsidence, the leak, and wear and tear

Subsidence is one of the events listed for standard home building cover, in the same subparagraph as storm, tempest, flood (within the meaning given by section 34), the action of the sea, high water, tsunami, erosion or land slide or subsidence (Insurance Contracts Regulations 2017 (Cth) reg 19(1)(a)(xiv)). How a particular policy treats it is in its wording and its Key Facts Sheet, which the insurer must keep on its website (Insurance Contracts Regulations 2017 (Cth) reg 13(4)). Check the definition of subsidence and any condition about what caused it before assuming either way.

Where a leak caused the movement, two more provisions come in. The escape of water from a fixed pipe is a listed event (Insurance Contracts Regulations 2017 (Cth) reg 19(1)(a)(vi)); wear and tear, rust or corrosion is taken back out of standard cover (Insurance Contracts Regulations 2017 (Cth) reg 19(2)(b)). A slow leak that ran for months is where an insurer will look at the second. The Australian Financial Complaints Authority’s approach is that the policyholder has to show an insured event caused the damage, and the insurer has to prove an exclusion it relies on (AFCA, The AFCA Approach to general insurance claims handling (July 2026), s 2.4). That makes the plumber’s written cause, the dated crack photographs and an engineer’s report the evidence that decides it.

For a newer house, defective footings or drainage are a building-work question first: the builder, and the state’s statutory home building insurance or warranty where the builder cannot be pursued. Water damage repair cost covers walls and floors once the cause is fixed, and water leak in a wall covers finding a pipe leak. No published Australian price for underpinning or footing repair could be sourced for this page.

Common questions