A dripping tap is easy to spot. A cracked pipe under your slab is not. Water from hidden leaks and poor drainage soaks into the soil under Australian homes, and that soil carries the weight of your house. When it swells, softens or washes away on one side, the footing moves and cracks follow.
This guide explains how leaks and drainage problems cause subsidence and foundation movement, how clay and sandy soils react differently, the warning signs to watch for, and what to do next.
Can a water leak cause subsidence?
Yes. A leak can cause subsidence or foundation movement by altering the moisture content of the soil beneath part of a house. On clay, the wet soil swells and lifts. On sand, water softens the soil or washes it away. Either way, the footing loses even its support, and movement and cracks appear in walls, floors, and frames.
CSIRO’s homeowner guide to foundations puts it plainly: “Even a minuscule leak can be enough to saturate a clay foundation.” A slow leak that runs for months can do more harm than a burst pipe that is fixed the same day, because nobody knows it is there.
How does leaking water move a foundation?
Your house does not sit on bare footings alone. It sits on the soil under those footings. Leaks cause trouble because they change that soil in one spot while the rest of the site stays drier. The result is uneven support, and uneven support is what cracks a building.
Clay soil: swelling and heave
Reactive clay is common across much of Australia. The QBCC notes that most Queensland homes are built on clay soils. CSIRO explains that “all clays react to the presence of water by slowly absorbing it, making the soil increase in volume.”
When a leak wets the clay under one corner, that corner swells and can push the footing up. This is called heave. The rest of the house stays put, so the walls between the two areas crack. Our guide to reactive clay and moisture explains the wet-and-dry cycle in more detail.
Sandy soil: softening and washout
Sand does not swell much, but it erodes. CSIRO notes that “all soils are prone to erosion, but sandy soil is particularly susceptible to being washed away.” It adds that even clay with a sand component of around 10% or more can erode.
A leaking pipe in sandy soil can carry fine particles away, leaving a gap under the slab or footing. The concrete then drops into the space. This is closer to a void than a swelling problem, and it often shows up as a sinking floor.
Fill and mixed soils.
Many homes sit on fill or a mix of soil types. Fill that was poorly compacted can settle further once it gets wet. Mixed soils can swell in one place and wash out in another, which is why a site assessment matters more than a guess.
Figure 1. A leak under one side of a home changes the soil there and leaves the footing with uneven support.
Which leaks cause the most trouble?
Any water that reaches the soil near your footings can cause movement. These are the common sources, including those named by CSIRO and the QBCC.
| Leak source | Where it shows up | Effect on clay soil | Effect on sandy soil | What to do |
| Water supply pipe (pressure pipe) | Higher water bill, meter moving with taps off, damp patch | Constant wetting, swelling and heaving near the leak | Softening and washout under the slab | Do a meter test, then call a licensed plumber |
| Sewer or drain pipe | Bad smells, slow drains, and soggy ground near pipe runs | Wet clay along the pipeline | Washes fine soil into the broken pipe, leaving voids | Licensed plumber, CCTV drain inspection |
| Stormwater and downpipes | Pooling after rain, downpipe discharging onto the soil | Concentrated wetting at corners | Erosion at the discharge point | Connect downpipes to stormwater, fix blocked drains |
| Hot water system or air conditioner overflow | Wet ground under the unit | Local swelling next to the wall | Local softening | Pipe overflows away from the house |
| Garden taps and irrigation | Wet garden beds against walls | Repeated wetting at one spot | Washout under edge beams | Move taps and sprinklers away from the house |
| Roof, gutters and water tanks | Overflowing gutters, leaking tank fittings | Wetting along the eaves line | Erosion beside the footings | Clear gutters, repair tank fittings |
How does poor drainage add to the problem?
Drainage problems act like a slow leak that switches on every time it rains. CSIRO warns that “downpipes not positively connected to a proper stormwater collection system will direct a concentration of water to soil.” The QBCC also advises homeowners to keep the site well-drained and to direct any water away from the house and neighbouring properties.
CSIRO recommends paving around the house that extends at least 900 mm out (more on highly reactive soil) with a fall away from the building of at least 1:60. It also suggests moving garden taps well away from the building. The QBCC adds that garden beds, edging, and gravel paths should not be set against the house, and that brick weep holes should not be set below soil level.
Figure 2. Drainage that keeps water away from footings, based on CSIRO (2024).
From leak to cracks: how the damage builds
Leak-related movement usually follows the same chain. Knowing it helps you see why fixing the leak is step one, not the whole answer.
Figure 3. How a leak turns into visible damage.
Clay takes up water slowly, so this chain can take weeks or months to show. CSIRO notes that because clay absorbs and releases water at a low rate, the change “will not usually be noticeable” at first. By the time you see cracks, the leak may have been running for a long time.
What are the warning signs of a leak under the house?
Look for two kinds of clues: signs of water and signs of movement. When both show up near the same part of the house, a leak is a strong suspect.
Signs of water
- Your water bill rises with no change in use.
- The water meter keeps moving when every tap is off.
- Damp or soggy ground, or green patches of grass, in dry weather.
- Musty smells, mould or lifting floor coverings near wet areas.
- Slow drains or sewer smells outside.
Signs of movement
- New or growing cracks in walls, especially stepped cracks in brickwork or diagonal cracks from door and window corners.
- Doors and windows that stick or no longer latch.
- Floors that slope, or tiles that crack in a line.
- Gaps between skirting boards and the floor, or between walls and ceilings.
CSIRO reproduces the damage scale from the footing standard AS 2870. Cracks under 5 mm that are easily filled, with doors and windows sticking slightly, sit in the “slight” category. Cracks of 5 to 15 mm, or several cracks of 3 mm or more in a group, are “moderate,” and pipes can fracture at that stage. That is one more reason a small leak can grow into a bigger one.
Does movement stop once the leak is fixed?
Not always straight away. Fixing the leak removes the cause, and CSIRO notes that cracking from erosion “will usually continue to develop until the original cause has been remedied.” But wet clay releases water slowly, so the soil can take time to settle back. Soil that washed away does not return, so any void stays until it is filled.
That is why we suggest a brief monitoring period after the plumbing repair. Mark the ends of cracks with a pencil and the date, and take photos. If cracks keep growing or doors keep jamming, the ground or the pipe needs attention.
Slab homes and strip footings: how the damage differs
On a concrete slab, a leak beneath it can lift or lower part of the floor. You may see cracked tiles, a sloping floor or a gap under the skirting. On strip footings with a timber floor, the brick walls usually show it first, with stepped cracks and sticking doors. Either way, the cause is the same: one area of soil has changed more than the rest.
How to reduce the risk: a prevention checklist
- Do a water meter test once or twice a year, and any time your bill jumps.
- Keep every downpipe connected to the stormwater system. Fix any that discharge onto the soil.
- Keep gutters clear, so they do not overflow beside the walls.
- Check that paving next to the house falls away from it, not towards it.
- Pipe hot water system and air conditioner overflows away from the footings.
- Move garden taps and sprinklers away from the walls. Keep garden beds off the house line.
- Keep weep holes clear and above soil level.
- Watch the trees near the house. CSIRO notes that roots near footings “will absorb much of the moisture in the foundation soil, causing shrinkage or subsidence.” Get advice before removing a large, established tree, too.
- Have a plumber inspect older drains with a camera if you notice slow drains, smells or soggy ground.
What to do if you suspect a leak is moving your house
- Do a meter test. Sydney Water suggests turning off all taps and appliances that use water, noting the meter reading, and not using any water for at least an hour. If the numbers have changed, you leak.
- Call a licensed plumber. Sydney Water says a leak inside your home or on your property will probably need a licensed plumber. Pipe leak detection and repair is plumbing work, so start there.
- Record what you see. Photograph cracks, mark their ends with the date, and keep plumbing invoices and reports.
- Contact your insurer early if you plan to make a claim, and ask what they need from you.
- Get the ground assessed. Once the leak is fixed, have the movement checked by a foundation specialist or a structural engineer. You can check an engineer on the National Engineering Register.
Where water is entering the building itself, such as through slab joints, basement walls, or retaining walls, our water-stopping and leak management service seals those entry points. It is not a plumbing service, and a licensed plumber should still repair any broken pipes.
How is leak-related subsidence repaired?
We find what is moving the house, fix the cause, and then the damage. The right repair depends on the soil, the footing type and how far the house has moved.
- [Resin injection](https://raiseandrelevel.com/resin-injection/): polyurethane resin injected into the ground to strengthen weak soil and lift the slab or footing back towards level.
- [Void filling](https://raiseandrelevel.com/void-filling-and-sinkholes/): filling gaps left by washed-out soil so the slab is supported again.
- [Slab jacking](https://raiseandrelevel.com/slab-jacking/): lifting sunken concrete such as paths, driveways and garage slabs with resin.
- [Underpinning](https://raiseandrelevel.com/underpinning/): extending footings down to firmer ground where the soil near the surface cannot be relied on.
Cracks are repaired once the ground is stable. Patching them first, while the soil is still moving, usually means they come back. If the movement is minor and has stopped, you may not need any structural work at all, and we will let you know.
Who pays: insurance and builder’s responsibility
Does home insurance cover subsidence from a leaking pipe? Sometimes. Insurance may respond to damage caused by a sudden insured event, while damage that develops gradually is treated differently by most policies. Your product disclosure statement decides it. Moneysmart explains what to check when you compare home insurance.
If the home is fairly new, the builder may be responsible. In Queensland, the QBCC says builders are responsible for fixing issues caused by their building work for 6 years and 6 months from the start of the work. Other states have their own rules, so check with your state building regulator.
Frequently asked questions
Can a small leak cause subsidence?
Yes. CSIRO says even a minuscule leak can be enough to saturate a clay foundation. Small leaks are often worse than big ones because they run unnoticed for months. The constant wetting swells the clay or slowly washes away sand under one part of the house, and that uneven support causes cracking.
How long after a leak does subsidence show?
There is no fixed timeframe. Clay absorbs water slowly, so movement can take weeks or months to show as cracks or sticking doors. A large leak in sandy soil can wash out support faster. If you see new cracks near a wet area, check for leaks straight away rather than waiting.
Will movement stop once the leak is fixed?
Fixing the leak removes the cause, but movement may not stop on the same day. Wet clay dries back slowly, and soil that washed away leaves a void that stays until it is filled. Monitor cracks for a few months after the repair. If they keep growing, get the ground assessed.
Who pays for subsidence caused by a leak: the plumber, the insurer, or the builder?
It depends on the cause and your policy. Insurance may respond to sudden insured events, while gradual damage is treated differently by most policies, so read your product disclosure statement. A builder may be liable for defects in newer homes. A plumber may be liable if their faulty work caused the leak.
Can I fix subsidence myself after fixing the leak?
You can do the drainage and maintenance jobs yourself, such as clearing gutters, moving garden taps, and checking for cracks. Re-levelling a house or strengthening the ground is specialist work that requires the right equipment, licences, and a soil assessment. Patching cracks in a moving house will not hold.
Final thoughts
Water is a common reason Australian homes move, and a leak is one of the easier causes to fix early. Check your meter, keep downpipes connected, keep water away from the walls, and act on new cracks before they grow.
If you have found a leak and the cracks remain, talk to our team about a subsidence assessment. We will tell you what is moving, whether it needs repair, and what the options are. Call [1300 053 835] or book online.
