In an older house on piers, a leaking water line under the floor drips into a subfloor. Somebody eventually goes under there, or smells it, or notices the ground is wet. It is not pleasant but it is discoverable, and there is a space where the evidence can accumulate harmlessly for a while.
The estates on the old farmland out through Leppington, Austral, Edmondson Park, Middleton Grange, West Hoxton and Denham Court are slab-on-ground. There is no subfloor. A water line under a slab has nowhere at all to show itself, so the first evidence most owners get is the flooring, and by then the water has been going somewhere for a while.
A slab leak has no route to become visible, so you are watching for indirect signs instead: a warm patch on the floor, the sound of running water with everything off, a water bill that has drifted up, and a meter that moves when nothing is being used. The meter test is the definitive one and it takes ten minutes.
- Do the meter test: everything off, read, wait an hour, read again
- Listen with the house quiet, particularly at night
- Feel the floor for a warm patch, which points at the hot line
- Watch the bill rather than only the floor
- Get it located properly before anybody cuts anything
On this page
1. Why there is nothing to see
A slab-on-ground house sits directly on a concrete raft, and the water lines serving the house run through or under it. There is no accessible space between the pipe and the floor you walk on.
So when a line under there develops a leak, the water has three options: into the ground beneath the slab, along the slab and out at an edge, or up through the slab itself. Only the third produces the thing everybody is waiting for, and it is the slowest of the three.
Which means the leak that eventually shows up as lifting flooring or a damp patch on a wall has usually been running for a long time before that. It is not that it started recently. It is that it only just found a route to be seen.
People reasonably conclude that no visible water means no leak. In slab construction that inference simply does not hold.
2. The signs, before the flooring tells you
- A warm patch on the floor. A hot water line leaking under a slab warms the concrete above it. It is the most specific sign there is and people find it with bare feet.
- Running water you can hear with everything off. A quiet house at night is a surprisingly good instrument. A faint continuous hiss or trickle is worth taking seriously.
- A bill that has crept up. With no change in the household, that is water going somewhere.
- Pressure that has dropped without explanation.
- A damp patch at the edge of the slab, or on a wall low down, or on an external path beside the house.
- Flooring that lifts, cups or discolours in one area. The classic sign, and the late one.
The first two are the ones worth actively checking, because they are free and they arrive early.
3. The test that settles it
- Turn off every tap in the house, and make sure nothing is refilling: no cistern running, no ice maker, no irrigation timer about to start.
- Go to the meter and read it, including the small dial that measures fine usage.
- Do not use any water for an hour.
- Read it again.
- Any movement means water is leaving the system somewhere.
That is the whole test and it is definitive. It does not tell you where, and it does not tell you whether it is under the slab, but it converts a suspicion into a fact, which is the thing a slab leak otherwise denies you.
If you want to narrow it further before ringing anyone: close the isolation valve to the hot water system and repeat. If the meter stops moving, the loss is on the hot side, which is genuinely useful to know.
4. Locating it, before anybody cuts anything
This is where the money is, and where the difference between plumbers shows up clearly.
A leak under a slab is repaired by getting to it, and getting to it means opening the slab or working around it. Both of those are expensive enough that being approximately right is not good enough. Cutting the wrong two square metres of a living room floor is not a slightly worse outcome, it is a second job.
So the sequence is: confirm there is a loss with the meter, isolate which side it is on, then locate it precisely with acoustic and thermal equipment before any concrete is touched. Anybody proposing to open a floor without having done that is proposing an experiment.
There are also repairs that avoid opening the slab at all, by rerouting a line rather than repairing it in place. Whether that suits depends on the property, and it is worth asking about rather than assuming cutting is the only option.
Meter moving with everything off?
We will locate it properly before anybody suggests cutting a floor.
5. The other thing that turns up in these estates
While we are on newer construction, the drainage in these houses fails differently too, and it is worth knowing because it saves an unnecessary worry.
The drainage is solvent-welded PVC. Roots cannot get into it the way they get into the jointed earthenware in the older estates, so the root story that dominates drainage advice around here simply does not apply to a newer house.
What does block these lines is usually what went down them during construction: sand, render, mortar and general debris washed in before anybody lived there. It tends to present early in the life of the house rather than late, and it is a clearing job rather than a structural one.
The other consideration on these blocks is space. The house covers most of the lot, so a sewer run frequently sits under a driveway or a paved courtyard because there is nowhere else for it to go. That is exactly why relining so often wins the comparison here: excavation is a demolition and reinstatement job before it is a plumbing one.
6. Worth doing this month
- Do the meter test once, even with no symptoms. It gives you a baseline and takes ten minutes.
- Find your main isolation valve and check it actually turns.
- Walk the house barefoot occasionally. It sounds odd and it is how most warm patches get found.
- Keep an eye on the bill rather than only on the floor.
- Replace ageing flexible hoses under sinks and basins, because in a slab house a hose failure has nowhere to drain either.
- If you have never had the drainage filmed, one baseline inspection is worth having, particularly if the house is still young enough for construction debris to be the likely cause.
- Slab-on-ground construction has no subfloor, so a leak has no route to become visible.
- By the time flooring lifts, the leak has usually been running for a long time.
- A warm patch on the floor points at the hot line and is the most specific early sign.
- The meter test is definitive: everything off, read, wait an hour, read again.
- Isolating the hot water system and repeating the test halves the search for free.
- Locate precisely before cutting. Being approximately right is a second job, not a near miss.
Slab leaks FAQs
How do I know if I have a slab leak?
You confirm there is a leak with the meter test, then narrow it. A warm patch on the floor, the sound of running water in a quiet house, or a bill that has drifted up are the early signs. Lifting flooring is a late one.
Does the slab have to be cut?
Not always. Rerouting a line rather than repairing it in place avoids opening the floor and is sometimes the better option. Which suits depends on the property, and it is worth asking rather than assuming cutting is the only answer.
How accurate is leak location?
Good enough to be worth the cost, which is the relevant test. Acoustic and thermal equipment narrows it to a small area rather than a room, and on a floor you would otherwise be opening on a guess that difference is the whole job.
Could it be the drainage rather than the water line?
It can, and they behave differently. A water line under pressure loses continuously, so the meter moves with everything off. A drainage problem only produces water when something is used. That distinction is worth making early.
My newer house has a blocked drain. Is it roots?
Unlikely. Solvent-welded PVC does not give roots a joint to enter through. In a newer house the usual cause is construction debris washed in before anybody moved in, which is a clearing job.
Why does relining come up so often on these blocks?
Because the house covers most of the lot, so the sewer frequently runs under a driveway or a paved courtyard. Excavating means demolition and reinstatement before any plumbing happens, and that is what makes the comparison so one-sided here.