Signs of a Slab Leak
The signs that point under the concrete, and the ones that only look like they do.
A water bill suddenly high, with nothing changed at home
The bill is metered, so the water genuinely went somewhere. The useful question is not whether you used it — it is whether it left the system before it reached a fixture.
Describe what the meter is doingWhat a jump on the bill actually tells you
A water bill suddenly high is a measurement, not a diagnosis. The meter counts everything that passes it, and it cannot tell the difference between a shower, an irrigation valve stuck part-open, a toilet flap that no longer seals, and a supply line weeping under the concrete. All four read as usage. That is why the bill on its own cannot point anywhere. What makes it useful is a second reading taken under controlled conditions: with every fixture off, anything the meter still registers is water leaving the system without being used. That single test splits the possibilities in half before anyone visits. The reason it matters to run that test early is that leaks do not plateau. A hidden leak large enough to move a bill is moving water continuously, and the cost of it is the smaller half of the problem — the larger half is what that water is doing to the material it is running through.
The check you can run yourself, in order
- Turn everything off — including the things that run on a timerFixtures, appliances, the ice maker, and above all the irrigation controller. Automatic irrigation is the single most common reason a meter test gives a false positive, because it can start mid-test without anyone in the house touching anything.
- Read the meter, then read it againMost meters have a small low-flow indicator — a triangle, a star, or a spinning dial — that moves with any flow at all, even flow too small to change the digits. Note the exact figures, wait fifteen to thirty minutes with the house untouched, and read again.
- Interpret what movedMovement with everything off means water is leaving the system continuously. No movement over half an hour makes a continuous leak unlikely and points instead at something intermittent: irrigation, a toilet that refills periodically, or a genuine change in use.
- Split the house from the yardIf your property has a shutoff valve where the supply enters the building, close it and repeat the reading. Movement with the house isolated puts the loss on the buried service line outside; no movement puts it inside.
- Rule out the toilets before anything elseA silently running toilet is the most common non-leak cause of a large bill. A few drops of food colouring in the tank, left twenty minutes with nobody flushing, will show up in the bowl if the flapper is passing water.
What the same bill usually means alongside other signs
- The bill jumped and the meter moves with everything offClose to conclusive for a leak somewhere on the pressurised system. The next question is which section, and that is what isolation and locating equipment answer.
- The bill jumped but the meter is still with everything offPoints at something intermittent rather than continuous. Irrigation, a refilling toilet, a seasonal change in use, or a billing period that covered more days than the last one.
- The bill jumped and one patch of yard stays soft or greenA classic pairing for a leak on the service line between the meter and the house, where nothing indoors would show at all.
- The bill jumped and one area of floor feels warm underfootA warm floor with no heating under it is a hot-water line losing water beneath the slab. Two signs agreeing is much stronger evidence than either alone.
- The bill jumped and there is a musty smell with no visible dampWater has been present long enough to affect materials out of sight. The smell marks where the moisture ended up, not where it escaped.
- The bill jumped and you can hear water running when the house is quietWater escaping under pressure through a small opening makes a continuous noise. That is also the noise acoustic locating equipment is built to follow.
Two local factors behind a bill that moves on its own
Slab-on-grade has been the standard foundation in San Diego since the tract building of the 1950s, and that changes what a jumped bill usually means. A supply line running in or under concrete has no service access — nothing to lift, no crawlspace to inspect — so it can lose water continuously with nothing visible anywhere in the house. No damp, no stain, no sound in most rooms. The bill is frequently the only evidence that exists. The water is also hard enough here that mineral scale is a normal part of how these systems age. That matters for a bill that moved on its own, because it is the usual mechanism behind a pinhole failure in copper that has carried water quietly for decades — the pipe thins from the inside rather than being damaged from outside. It is why a leak can appear on a system that has never given trouble and where nothing about the household has changed. The hardness figures and what they do to a buried line are covered on our guide to the signs of a slab leak.
What decides how expensive this gets from here
How long it runs before it is found
The water itself is the cheap part. Drying, replacing saturated material, and making good grow steadily the longer a leak is left, and they frequently end up larger than the plumbing repair.
Whether the loss is inside or outside the building
A service line in soil is reachable without touching the structure. The same volume of water inside a wall or under a floor is a building repair as well as a plumbing one.
What sits over the failure
Bare soil, a concrete slab, a finished floor and a poured driveway are four different access problems for an identical length of pipe.
Whether it is one failure or an ageing system
A single defect on sound pipe is a repair. A pipe material failing along its length by corrosion is a different conversation, and the age and material of the line is what tells them apart.
How much of the system has to be eliminated first
A loss that isolates to one zone in ten minutes is a shorter job than one that could still be on any of four systems when work starts.
Questions people ask
Could the utility simply have read the meter wrong?
It happens, and it is worth checking — but it is quick to rule out and slow to argue about. Read the meter yourself, note the figure, and compare it against the reading printed on the bill. If your own reading is climbing at a rate that matches the bill, the meter is measuring real water and the explanation is on your side of it. Estimated readings, and billing periods covering different numbers of days, are the two other things worth looking at before assuming a fault.
How large does a leak have to be to change the bill?
Smaller than most people expect, because it never stops. A drip too fine to hear runs every hour of every day, and that continuity is what accumulates rather than the flow rate. It is also why a leak that shows on a bill can still be invisible in the house — the volume is spread over weeks, not spilled in one place.
The meter moved a tiny amount. Is that normal?
No. With every fixture closed, the correct amount of movement is none. Any registered flow means water is passing the meter and not arriving anywhere useful. What varies is urgency, not whether it matters — a very small loss on an accessible line is a different priority from the same loss under a slab.
Can I just wait and see if the next bill comes back down?
You can, and it is a reasonable choice only if the meter test came back clean, because then you are waiting on something intermittent and self-correcting. If the meter moves with everything off, waiting a billing cycle means another month of water running into whatever it is running into, and that is the part that gets more expensive rather than the water.
Is paying for leak detection worth it when the repair is small?
Locating is what keeps the repair small. The alternative to finding the leak is guessing where it is and opening something to look, which produces a hole, a repair for that hole, and often an intact pipe with the original leak still running. Diagnosis is not an extra line item on top of the repair; it is the thing that decides how big the repair has to be.
Whose responsibility is the line between the street and the house?
It varies by utility, and it is worth establishing before assuming. As a general pattern the utility owns the main and the meter, and the property owner owns the service line from the meter to the building — which is precisely the stretch that a green patch in the yard usually points at. Your provider can confirm where their responsibility stops for your specific address.
Related reading
Signs of a Hidden Water Leak
The signs a leak gives off before it becomes visible, and what each one narrows down.
Water Under Flooring
Why the wet spot is almost never where the water got in, and what to do about it first.
Leak Triage Checklist
A step-by-step, no-tools-needed sequence for narrowing down a suspected leak before anyone visits.
Tell us what the meter did
What the bill did, whether the low-flow indicator moved with everything off, and whether anything else has changed — a damp patch, a warm floor, a smell. That one meter reading changes where the search starts.
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