Hidden water leak: 7 signs, a 1-hour test and how to find it without breaking walls

A hidden leak is water escaping where nobody looks: inside a wall, under a floor, along a buried pipe or through the float valve of a water tank. Because it happens after the water meter, every litre of it goes on your bill. To find out whether you have one, shut every point of use and watch the meter for an hour; to find the spot without breaking walls, leak detection reads sound, temperature and pressure.

In short:

• Every leak after the water meter is billed, including the one nobody can see.
• The 7 signs: a bill that rises and stays up, a meter turning with everything shut, the sound of running water, stains and mould, damp floors, weak pressure and a tank or pump behaving oddly.
• The meter test confirms the leak in one hour and shows whether it sits before or after the water tank.
• A toilet with a visible trickle loses 144 litres a day, and a dripping tap from 6 to more than 32 litres, according to the water conservation manual of ANA, FIESP and SindusCon-SP.
• To pinpoint it, leak detection combines geophone surveys, thermography and pressure tests. To catch the next one before the bill does, measure night-time consumption.

What is a hidden water leak?

It is any water loss that happens out of sight. The usual places are:

• the buried pipe between the water meter and the water tank;
• pipework inside walls and under floors, hot-water lines included;
• the tank float valve that never closes and lets water out through the overflow pipe;
• flush valves and toilet cisterns that let a constant trickle into the bowl;
• in a company, also irrigation lines, the mains feeding distant tanks and old pipework nobody has mapped.

Two losses share the same name and should not be confused. Losses in the utility’s network happen before the meter and do not reach your bill: in Brazil they came to 39.53% of treated water in 2024, according to Instituto Trata Brasil’s 2026 Water Loss Study. The hidden leak in a building happens after the meter, and you pay for it. That is the subject of this article.

How much water does a hidden leak waste?

The manual Conservação e reúso da água em edificações (water conservation and reuse in buildings), published by Brazil’s National Water Agency (ANA) with FIESP and SindusCon-SP, estimates the loss from the most common leaks inside a building:

Where it leaks	What the leak looks like	Estimated loss
Tap	Slow drip	6 to 10 litres a day
Tap	Medium drip	10 to 20 litres a day
Tap	Fast drip	20 to 32 litres a day
Tap	Very fast drip	More than 32 litres a day
Tap	2 mm stream	More than 114 litres a day
Tap	4 mm stream	More than 333 litres a day
Toilet with flush valve	Visible trickle	144 litres a day
Toilet with flush valve	Leak in the supply pipe	144 litres a day
Urinal	Visible trickle	144 litres a day

A single toilet with a trickle, at 144 litres a day, passes 4,000 litres a month. In a building with dozens of washrooms, a few of those already show on the invoice. A buried pipe has no table: it leaks 24 hours a day, at mains pressure, and the volume depends on the size of the crack. That is why the most reliable sign of a hidden leak is not your eyes. It is the meter.

What are the signs of a hidden water leak?

Seven signs appear before the water becomes visible. One is enough to justify the meter test; two or more justify a search for the spot.

1. The bill goes up and stays up

Consumption in cubic metres jumps from one month to the next and stays there, with no change in routine, occupancy or production. Always compare the volume and the daily average, not the amount, which moves with tariff increases and with the number of days between readings. The other seven causes of a high bill are covered in my water bill went up: causes and how to cut it.

2. The meter moves with everything shut

With taps, showers, toilets and machines stopped, no water should pass through the meter. If the flow indicator, the small wheel or pointer on the dial, keeps turning, water is escaping somewhere.

3. The sound of running water

A hiss in the wall, a noise under the floor or the sound of filling with everything shut. At night, with the building quiet, it is easier to hear. It is the same noise a geophone amplifies in a professional search.

4. Stains, mould and a musty smell

Blistered paint, crumbling plaster, darkened grout, mould on the skirting or on the ceiling of the floor below. Moisture travels through materials and can show up far from the leaking pipe, which is why breaking the wall where the stain is does not always find the leak.

5. Damp floors or water seeping from the ground

A floor that never dries, a puddle that keeps coming back in the car park, a patch of grass greener than the rest, water seeping onto the pavement. These are typical signs of a buried pipe.

6. Weaker pressure than usual

Taps and showers with less force than before, with no change in the neighbourhood supply, may mean that part of the water is escaping before it reaches them.

7. A water tank or pump behaving oddly

The tank takes longer to fill, the pump keeps switching on and off, or the overflow pipe drips. In a company, a tank that loses level overnight, with operations stopped, is the strongest sign of all.

How do you do the water meter test?

The test confirms the leak and tells you which section it is in. It takes an hour and needs no tools:

1. Pick a time with no use, with the water tank full. In a company, overnight or at the weekend.
2. Shut everything that uses water: taps, showers, machines, irrigation, drinking fountains and equipment that discharges water. Do not close the valve at the meter box, which sits before the meter.
3. Write down the reading and photograph the dial, with the time.
4. Wait an hour without using water and read it again. If the numbers changed or the wheel turned, there is a leak.
5. Find the section: close the valve that feeds the water tank. If the meter keeps turning, the leak is between the meter and the tank, almost always in a buried pipe. If it stopped, the leak is after the tank: float valves, toilets and the internal pipework.

Two quick checks complete the meter test:

• Toilets. With a close-coupled cistern, put a few drops of dye inside the cistern, not the bowl, and wait 15 minutes without flushing: if the colour shows in the bowl, the seal is letting water through. With a flush valve, drop a little coffee grounds into the water in the bowl: normally they settle; if they move, a trickle is passing.
• Water tank. With the tank full and nothing in use, check whether water keeps coming in or leaves through the overflow. A float valve that does not close is a leak that goes straight down the drain.

In a company, the manual test has a limit: with several buildings, night shifts and buried mains, consumption never reaches zero, and the leak hides inside normal use. The professional version of the test is to follow night-time consumption every day. With operations stopped, it should fall close to zero; when it does not, there is a loss.

How does leak detection find the spot without breaking walls?

Once the leak is confirmed, the spot is still missing. Breaking walls to look for it is slow and sometimes useless, because the damp shows up far from the pipe. So the search is done from the surface, with equipment that reads sound, temperature and pressure. Listening sticks, geophones and leak noise correlators are among the most widely used tools in the world for locating leaks in buried pipes, according to research at Unesp funded by FAPESP.

Method	What it reads	Where it works best
Geophone survey (geophone and listening stick)	The noise of water escaping under pressure, amplified through the floor, the wall or the ground	Pressurised pipework inside walls, under floors and underground
Leak noise correlator	The time difference with which the noise reaches two sensors, one on each side of the section	Long runs of buried pipe
Thermography	The temperature difference that moisture causes on the surface	Walls, floors and slabs, especially with hot water
Hydro-pneumatic testing	The pressure drop in an isolated section pressurised with water or air	Confirming which section leaks before opening anything
Tracer gas	Gas injected into the pipe, which escapes through the crack and is detected at the surface	Small leaks and places where noise does not help

In practice the methods are combined: one confirms that the section leaks, another points to the place. T&D uses three of them in its Electronic leak detection: geophone surveys, thermography and hydro-pneumatic testing. With the spot located, the repair opens only the section needed, instead of a whole wall or floor, and operations suffer less from the works.

Who pays for a hidden leak?

The general rule: the network and the service pipe up to the meter belong to the water utility, and everything after it belongs to the property. Water that passes through the meter is billed, whether it was used or lost. Two situations change the picture:

• A leak before the meter, on the pavement or at the meter box: tell the water utility. The repair is theirs, and that water does not pass through your meter.
• A hidden leak already repaired: some utilities review the bill for the leak period, under their own rules and limits. Ask whether yours does, keep the repair invoice and photos, and request the review with a protocol number.

If the bill went up for another reason, such as an estimated reading or a wrong registration, the path is different, and it starts with reading the water bill line by line.

How do you stop the leak from coming back?

A repaired leak comes back when the cause stays. Four habits reduce the chance and, above all, the time until it is found:

1. Maintenance of the parts that leak most. Flush valves, float valves, flexible hoses, stop valves and tap washers, checked on a schedule and not only when they fail.
2. Pressure under control. High pressure strains joints and speeds up leaks. Where the mains arrive strong, a pressure-reducing valve protects the installation.
3. Frequent meter readings. Every week, on the same day and time, in a spreadsheet. The jump shows up weeks before the invoice.
4. Continuous monitoring. Telemetry reads consumption hour by hour and warns when it leaves the usual pattern. T&D’s TDRR sends alerts by app, SMS, email and WhatsApp, with analysts watching 24 hours a day, and delivers an average 10% cut in consumption from monitoring alone, mostly from finding leaks and faults early.

Monitoring tells you there is a leak; leak detection tells you where it is. How remote metering works is explained in remote water consumption monitoring. For the full programme, from baseline to target, see the guide to water savings for businesses, and for the other sources of loss, the four invisible causes of water waste.

Frequently asked questions

Does a hidden leak increase the water bill?

Yes, because all water that passes through the meter is billed. A toilet with a visible trickle loses 144 litres a day, according to the manual of ANA, FIESP and SindusCon-SP, and a buried pipe leaks all day long, at mains pressure.

How do I know whether the leak is before or after the meter?

Close the valve at the meter box, the one before the meter, and watch the wet spot. If water keeps appearing with that valve closed, the leak is before the meter, in the utility’s network or service pipe. If it stops, the leak is on the property. The meter answers too: it only turns for a leak that sits after it.

Does leak detection mean breaking walls?

Not to search. The location is found from the surface, with a geophone, a thermal camera and pressure tests. Once the spot is found, the repair opens only the section needed.

Does a toilet leak show on the meter?

It does. The trickle from a flush valve or cistern passes through the meter like any other consumption, which is why the meter test catches leaks nobody sees. To confirm it comes from the toilet, do the dye test in the cistern or the coffee grounds test in the bowl.

Will the water utility give a discount for a hidden leak?

It depends on each utility’s rules. Some review the bill for the leak period after the repair, with their own rules and limits. Ask through the official channel, keep the repair invoice and photos, and file the request with a protocol number.

Sources

• Conservação e reúso da água em edificações (ANA, FIESP and SindusCon-SP, 2005), leak loss table on p. 34, reproduced in Souza et al., 21st Brazilian Water Resources Symposium (ABRHidro, 2015), in Portuguese
• Water Loss Study 2026 (SINISA, 2024), Instituto Trata Brasil and GO Associados, in Portuguese
• Research on ground vibration measurements for leak detection in buried water pipes using cameras, Unesp Bauru, funded by FAPESP, in Portuguese
• T&D Electronic leak detection: geophone surveys, thermography and hydro-pneumatic testing
• TDRR, T&D remote monitoring: an average 10% cut in consumption from monitoring alone