Underpinning advice

Can a leaking pipe make a house sink?

Published 4 September 2026

Yes. A leaking pipe is one of the most common causes of foundation movement in Melbourne homes, and it is one of the very few causes that can be fixed rather than merely engineered around.

It also produces the most frustrating outcome in this industry: a homeowner spends $30,000 underpinning a house, nobody finds the cracked sewer line under the slab, and three years later the cracks come back.

Two opposite failures, both caused by water

The direction a house moves depends on what is under it, which is why the same leak produces different symptoms in Footscray and in Tarneit.

Washout — the house drops. Where the soil is sandy, silty or poorly compacted fill, escaping water carries the fine particles away with it. The soil loses volume, a void opens under the footing or slab, and the structure settles into it. This is the classic “sinking house,” and it is what most people picture.

Heave — the house lifts. Where the soil is reactive clay — the western basalt plain, the south-eastern corridor, much of the middle east — water does the opposite. Clay absorbs it and swells. A leak keeps one patch of soil permanently wetter than the surrounding ground, that patch expands, and the slab above it is pushed up while the rest of the house stays put. The differential is what cracks the building. This mechanism is set out in more detail in what is slab heave.

Both look like “the house is cracking.” They need opposite repairs. Getting the direction wrong is the most expensive diagnostic error available.

Which pipes are the usual offenders

Sewer lines are the most damaging and the hardest to spot. They run under or immediately beside the slab, they carry water continuously, and a cracked earthenware joint releases it directly into the soil under the house with nothing visible at the surface. Older suburbs with original vitrified clay sewer runs — much of Melbourne’s inner west, inner north and inner east — are where we find these most often. Tree roots find the joints, open them, and the leak follows.

Stormwater is the most common cause overall, simply because there is so much of it. A downpipe that discharges onto the ground beside the slab instead of into a stormwater connection can put thousands of litres into one square metre of soil during a single storm. A cracked or disconnected underground stormwater line does the same thing invisibly. This is the cause we find more than any other, and it is often a few hundred dollars to fix.

Water supply lines leak under pressure, so they do damage faster, but they also announce themselves through the water bill and are usually found sooner.

Irrigation counts. Drip lines running along a garden bed against the house, on a timer, every day through summer, are functionally a slow leak — and a legal one nobody thinks to check.

The signs that point at a leak rather than general ground movement

Ordinary seasonal clay movement affects a whole side of a house fairly evenly and reverses with the seasons. A leak is localised and persistent, so it looks different:

• Movement concentrated in one area rather than distributed along a wall — cracking that radiates from a single point near a wet area, a bathroom, a laundry or a downpipe.
• It does not reverse. Seasonal cracks open in summer and close in winter. Leak-driven movement keeps going in the same direction regardless of season, because the moisture source never turns off.
• A patch of ground that is always damp, or greener and more vigorous grass over one line in the lawn during a dry spell.
• A water bill that has crept up without a change in household usage.
• Damp at the base of the wall, salt staining or rising-damp symptoms appearing on the inside of one wall only.
• A sewer smell in the yard or near a floor waste, intermittently.
• Sunken paving or a driveway dipping in one spot near the house — see slab lifting.

None of these are conclusive on their own. The combination of localised, non-seasonal, persistent movement near a wet area is what makes a leak the leading suspect.

How a leak is actually found

In order of what gets used, and roughly in order of cost:

1. The visual and drainage inspection. Downpipes traced to their outlets, stormwater pits checked, the ground around the house walked after rain. A surprising proportion of cases are solved here, at no cost, because a downpipe is simply discharging onto soil.
2. The water meter test. Turn off every tap and appliance, note the meter, come back in an hour. Movement in the dial with nothing running means a supply-side leak. This one you can do yourself this afternoon.
3. CCTV drain camera. A camera fed through the sewer and stormwater lines. This is what finds cracked joints, root intrusion and collapsed sections under a slab, and it is the single most valuable diagnostic on a house that is cracking near a wet area. Typically $300 to $700.
4. Acoustic leak detection and tracer gas. For pressurised lines where the location is not obvious. $400 to $1,200.
5. Dye or smoke testing. Confirms whether a suspected line is the one actually discharging.

Do the plumbing before the underpinning

This is the practical point of the whole article.

If a leak is driving the movement, the correct sequence is: find the leak, fix the leak, then wait and monitor before deciding on structural work. Soil moisture takes months to re-equilibrate. Once the water source is removed, some houses stabilise on their own, some partially recover, and some have moved permanently and still need underpinning — but you cannot know which until the water stops.

Underpinning first means:

• you may be paying for structural work that was never needed
• the piers are being designed against soil conditions that are about to change
• if the leak stays, the movement continues and the underpinning gets blamed

A contractor who insists on underpinning without anyone having looked at the drainage is either not curious or not honest, and neither is what you want on a $30,000 job.

Who pays

Worth knowing before you start:

• Home insurance generally excludes gradual foundation movement, but many policies do cover damage caused by a sudden escape of water from a pipe. The distinction between “sudden” and “gradual” is where these claims are won and lost, so document the date you noticed it and keep the plumber’s report.
• The pipe itself is usually the homeowner’s responsibility on your side of the property boundary. Beyond it, the water authority’s.
• Tree root damage to a sewer is typically the owner’s problem even when the tree belongs to the council, which surprises most people.

Photograph everything, keep the CCTV footage, and get the plumber’s report in writing. It is the evidence for any claim and for any argument about who caused what.

The short version

A leaking pipe can absolutely make a house move — down where the soil washes out, up where the clay swells. It is common, it is localised, it does not reverse with the seasons, and it is cheap to investigate relative to the cost of getting it wrong.

If your house is cracking and you have not had the drains camera’d, that is the next thing to do — before you get underpinning quotes, not after. Our free assessment includes the drainage check, and the cost of underpinning explains what you are avoiding if it turns out to be a plumbing job.

Common questions

How much does it cost to find a leak under a slab?

Less than most people expect, and far less than the structural work it may make unnecessary. A CCTV drain camera through the sewer and stormwater lines typically runs $300 to $700 and is the single most valuable diagnostic on a house cracking near a wet area. Acoustic leak detection or tracer gas for pressurised lines runs $400 to $1,200. The water meter test — turn everything off, read the meter, read it again an hour later — costs nothing and you can do it this afternoon.

Will fixing the leak repair the damage already done?

It stops the cause; it does not reverse the effect on its own. Where the mechanism was heave, removing the moisture source lets the clay give up water over months and the lifted section can settle back a meaningful amount. Where the mechanism was washout, the soil that was carried away is gone and the settlement is permanent unless the void is filled or the load is transferred to competent ground. Either way, the correct sequence is the same: fix the water, monitor across a season, then decide what structural work is genuinely still needed.

Who pays if a council tree's roots cracked my sewer?

Typically you do, which surprises most people. The section of sewer on your side of the property boundary is generally the owner's responsibility to maintain and repair, regardless of whose tree sent roots into it. Beyond the boundary it becomes the water authority's. Some councils will consider a claim where a street tree is clearly implicated, but it is not a given and the process is slow. Keep the CCTV footage and the plumber's report either way — it is the evidence for any claim and for any insurance question about the resulting damage.

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