A Leak Under Concrete: How It Shows Up and What It Costs to Reach

A leak under a concrete slab, path or driveway is the one that goes unnoticed longest. There is no puddle to find, no drip to hear, and nothing visible until either the water finds an edge or the bill arrives.

It is also the one where the repair is the easy part. Reaching it is what costs money.

How would I even know?

Five tells, roughly in the order people notice them.

The water bill. Usually the first sign, because the leak runs continuously and nothing else changes. Working out whether a high bill is a leak starts at the meter.
A patch of concrete that stays damp when everything around it has dried, or a dark seam along a joint days after rain.
The sound of running water with everything off, often faint and hard to place.
Water appearing at the edge of a path or driveway, or coming up through a crack.

Confirm it at the meter first

Everything else follows from this. Turn off every tap and appliance, then watch the meter.

We had a job in Sunnynook where we had gone out for a toilet repair. On completion we checked the meter, as we do at the end of every job, and found it running at high speed with nothing in the house being used. That was a leak outside, under the concrete, that nobody had known about. The toilet was the reason we were there. The meter is what found the real problem.

If the meter is moving with the house shut off, you have a leak. The one minute check is how to do it properly.

Why can you not just find it?

Because finding a leak is largely about listening to it, and concrete and soil both work against you.

Acoustic detection picks up the hiss of water escaping under pressure. That works well on an exposed pipe and much less well through a slab, where the sound is damped and spread out. Depth makes it harder again.

Sometimes it works first go. At a house in Henderson the owner had a high bill and a suspected leak. Our initial inspection could not confirm the exact point, so the client arranged leak detection, and that pinned it: the pipe had burst under the concrete near the outdoor hose tap. From there it was straightforward. We cut and broke out the concrete, replaced the burst section, tested it, and came back to reinstate the area and finish the concrete.

That job is the good version. Two things made it work: the leak was fairly shallow, and there was a sensible starting hypothesis in the outdoor tap line.

What does the work actually involve?

Four stages, and it is worth understanding that they are separate.

Confirm there is a leak, at the meter. Free.
Locate it. Sometimes us, sometimes a specialist detection company. This is charged and it does not fix anything yet.
Access and repair. Cut the concrete, expose the pipe, repair or replace the section, pressure test.
Reinstate. Backfill, re-concrete, finish. On the Henderson job that was a separate return visit.

We quote the stages as they become knowable rather than pretending to know the total on day one. Anyone giving you a fixed price to fix a leak under concrete before it has been located is guessing.

Detection marks sprayed on concrete near an outdoor tap before breaking out

Repair it, or run a new line around?

This is the decision that saves the most money, and it turns almost entirely on one question: do we know exactly where the leak is?

If the leak is pinpointed, repair it. That was the Henderson job. Detection put a mark on the concrete, we cut one hole, replaced the burst section and reinstated it. One cut, one repair, done.

If it cannot be pinpointed, and the pipe runs a long way under concrete, re-routing usually wins. This is the case people find hardest to accept, and it is worth spelling out. Say the line runs twenty metres under a concrete driveway and the meter proves water is escaping, but nobody can say where. The alternatives are:

Keep paying for detection attempts and exploratory cuts, with no guarantee any of them lands on it, and a driveway with several patches in it at the end.
Abandon the buried line and run a new one around the concrete, along a fence or through a garden, where nothing has to be broken out at all.

The second is frequently cheaper, and it is certainly more predictable. The new pipe does not have to follow the old path, and the dead section simply stays in the ground.

Depth and age push the same way, because both make detection harder and both mean the old pipe is likely to fail again somewhere else. But the trigger is the not knowing. If we can find it, we fix it. If we cannot find it and reaching it means breaking up a driveway on a guess, we would rather run you a new line.

If your concrete stays wet or your bill has jumped with nothing to show for it, send us a photo and tell us what the meter is doing. That is enough to start.

Common questions

Will my insurance cover breaking up the concrete?

Sometimes, and it is worth asking specifically about the access cost rather than just the repair. Many policies cover damage caused by the escaping water and some contribute to the cost of locating and gaining access to the pipe, while the pipe repair itself usually sits with you. Because these leaks are slow and concealed by nature, expect the gradual damage question to come up, which is why the plumber’s report and the dates matter.

Can I claim the wasted water back?

Very likely worth applying. An under-concrete leak is exactly the hidden, unnoticed kind that Watercare’s leak allowance exists for, and on one of these the water charge can be larger than the repair. Fix it first, then apply with the evidence of the repair.

How long has it probably been leaking?

Longer than you think. Because nothing surfaces, these run for months and occasionally years, quietly climbing on each bill. That is not a failure of attention on your part; there is genuinely nothing to see. It is the argument for doing the meter check once a season rather than waiting for a symptom.

Does the concrete get damaged by the water itself?

The water washes out the material under the slab rather than attacking the concrete, and over a long period that can leave voids beneath it. Most of the time the practical effect is minor, but a leak that has been running for years under a driveway can lead to settlement and cracking, which is another reason not to sit on it once it is found.

Who does the concrete finishing afterwards?

We reinstate the area and complete the concrete work on straightforward jobs, as we did in Henderson. Where the surface is decorative, exposed aggregate, tiled or part of a larger slab, matching it is specialist work and we will tell you up front that the finish needs a concreter rather than a plumber. That way the cost sits in the quote instead of arriving afterwards.