Hidden water leak: find it without breaking walls

A hidden water leak gives itself away through a meter that turns while everything is off, a spreading stain, a musty smell or a rising bill. Confirm it with the meter test, then have it located by an equipped plumber: moisture meter, thermal camera, acoustic listening and tracer gas pinpoint the leak before anything is opened.

Suspect a hidden leak in Paris or the inner suburbs? Call +33 1 89 70 15 36 (01 89 70 15 36 from a French phone), 7 days a week, 24 hours a day, or use the contact page. Our leak detection and repair service locates the leak and gives you a written report.

What is a hidden water leak?

It is a leak on a pipe you cannot see: built into a wall or partition, buried in a screed or concrete slab, under a shower tray, behind a boxed-in bath or inside a service duct. It can run for weeks before leaving a mark.

Two things make it hard to find:

• the stain is almost never right above the leak. Water takes the easiest path: along a pipe, down the slope of a screed, through the thickness of a floor. A stain at the bottom of a wall can come from a fitting several metres away, or from the flat upstairs;
• nobody knows exactly where their pipes run, especially in an older flat renovated several times.

The most common causes in Paris

• a worn seal or fitting on a pipe inside a wall;
• a cracked flexible hose behind bathroom furniture or a boxed-in bath;
• a hairline crack in a copper or PEX pipe under the screed;
• a failed seal around a walk-in shower or shower tray;
• a leak in heating pipes laid in the floor;
• a shared riser pipe leaking between two flats.

In older Paris buildings, lead, copper, galvanised steel, cast iron and PEX often coexist in the same flat. Joints between different materials, and lead or galvanised steel pipes, are the weakest points.

Warning signs

• A rising water bill with no change in your habits, or an over-consumption alert from Eau de Paris, the city water utility.
• The meter turning while no tap or appliance is in use.
• Stains or rings on walls or ceilings, bubbling paint, peeling wallpaper, a wall that feels cold and damp.
• A musty smell that persists in a dry room, even after airing.
• A floor that moves: tiles lifting or sounding hollow, grout darkening or crumbling, parquet warping.
• A warm patch on the floor in one spot: a classic sign of a hot water or underfloor heating leak.
• A boiler that needs topping up every two or three days: the heating circuit is probably leaking.
• The sound of running water in a wall or under the floor when everything is off, mostly heard at night.
• A stain on the ceiling of the flat below, while nothing shows in yours.

One sign is enough to check. Several together justify a leak search without delay.

The meter test: a check you can do yourself

This test confirms that a leak exists. It does not tell you where.

1. Turn off every water outlet: taps, shower, toilet, washing machine, dishwasher, water softener.
2. Note the meter reading, including the figures after the decimal point. Also watch the small flow wheel or star.
3. Wait at least 30 minutes without using any water. For a very slow leak, run the test overnight.
4. Read the meter again. If it has moved, or the star is turning, water is escaping somewhere.

In many Paris buildings, the flat's meter is a sub-meter on the landing or in a service duct. To read the result, work out whether the leak is in your flat or further upstream, and who pays, see leak before or after the water meter.

Two simple checks before you call

• The toilet cistern: add a few drops of food colouring to the tank and wait ten minutes without flushing. If colour appears in the bowl, the mechanism leaks. It is the most common silent leak, and the easiest to fix.
• Accessible fittings: look under sinks, behind the toilet, around the water heater and its safety valve. Look for limescale, rust or mould marks.

If the meter turns and you find nothing visible, the leak is probably inside a wall or floor.

Why you should not open walls at random

Opening a wall "to have a look" causes two problems. The stain and the leak are rarely in the same place: you open up, find nothing, open somewhere else, and end up with several channels cut into the walls. And the insurer asks for a report that establishes where the leak came from: openings made without any prior investigation are harder to justify. A methodical search means opening once, in the right place.

How a plumber finds a leak without breaking anything

A technician does not start by drilling. They look, measure, then confirm. Here is what actually happens in your home.

1. Visual inspection

The technician examines the reported areas: damp wall, damaged ceiling, hollow-sounding tiles, cracked shower tray seal, limescale traces, discolouration where partitions meet. They ask questions (since when, at what time, after which use). This step steers everything else.

2. Moisture meter: measuring damp inside materials

A moisture meter (hygrometer) measures the moisture in plaster, plasterboard, wood or screed, either with pins or without contact. The technician takes a series of readings, not one: wall by the shower, adjacent partition, floor, ceiling of the flat below if possible. This map shows where moisture is highest and which way it is spreading. If the right-hand wall of a shower is saturated and the left is normal, the leak is probably on the right.

3. Thermal camera

An infrared camera shows surface temperature differences. A damp area appears colder than its surroundings; a hot water or heating leak appears warmer, often as a patch spreading beyond the line of the pipe. It works very well on underfloor heating, behind plasterboard linings and under shower trays, and nothing is opened.

4. Acoustic listening

A leak on a pressurised pipe produces a sound and vibration the ear cannot hear. Sensors amplify this signal and the technician moves them along the pipe route to the loudest point. With a correlator, two sensors compare when the sound arrives to calculate the leak position. It works on metal, PVC and PEX, including under a slab.

5. Pressure test

To check whether the leak is on the supply side, the technician isolates the system and pressurises it with a gauge. If the pressure drops, the system leaks. If it holds, the supply is sound and the cause is elsewhere: waste pipe, shower tray, shower seal. This avoids searching in the wrong place.

6. Tracer gas

When the leak is very small, deeply buried or in a noisy environment, the technician drains the pipe and injects a harmless nitrogen and hydrogen mix. The gas works its way through the materials and comes out at the leak, where a surface detector picks it up. It is the most precise method for a thick concrete slab.

7. Tracer dye

On a shower tray, bath or toilet, the technician applies a coloured dye to the suspect area. The dye follows the water's path and reappears where it comes out: bottom of a partition, nearby tiles, ceiling below. Some dyes glow under UV light. It gives direct visual proof, very useful in the report for the insurer.

8. Inspection camera

For waste pipes (toilet, shower, sink) and accessible cavities, a camera is fed into the pipe or duct. It shows breaks, dislodged joints and deposits, common in old Paris pipework.

When that is not enough: a targeted opening

Sometimes an opening is still needed, for example for diffuse damp behind a plasterboard lining. Thanks to the readings, the technician knows where to open: a small opening at the identified point, to confirm the source and check the condition of the materials. Photos go into the report.

Not every method is used every time: the technician picks the ones that suit your layout. For a very common Paris case, see also leaking shower tray: the invisible leak.

Special case: a leak under a floor slab

A leak under a slab is one of the trickiest: you cannot see it, it is hard to reach, and the water often ends up on the ceiling of the flat below. Typical signs: tiles warm in one spot, a dull sound under the floor, lifting tiles or parquet, a stain downstairs, a turning meter.

• Underfloor heating: the thermal camera is the first choice; it draws the pipe layout and shows the anomaly.
• Thick slab: tracer gas takes over when thermal imaging is not enough.
• Pressurised supply pipe: acoustic listening works even under a slab.

If the leak is active and significant, shut off the main water supply until the visit. The repair is then done through a small opening at the leak.

After detection: repair and report

The repair

If the leak is accessible (seal under a sink, fitting behind an access panel), it can often be repaired during the same visit. If it means opening a partition or lifting tiles, we give you a repair quote after the detection. A failed fitting is repaired locally; a pipe that is weak along its whole length may justify wider replacement, especially for lead and cast iron: see replacing lead or cast iron pipes in Paris. Making good (filling, tiling, painting) should appear clearly on the quote.

The leak detection report

At the end of the search we hand over a report: tests carried out, methods used, location of the leak, photos (including thermal and dye images), conclusions and repair recommendations. This is the document your insurer and, in a co-owned building, the syndic (building manager) will ask for. To see what a proper report contains, read what a real leak detection report looks like.

Who pays for the leak search?

Many French home insurance policies include leak search cover, with conditions and ceilings that vary. Repairing the cause is a separate question. The details are in our guide to leak detection and insurance in Paris. If you rent, report a suspected hidden leak to your landlord in writing, with photos: diagnosing and repairing a pipe built into the walls is generally the landlord's responsibility. For our prices, see our rates.

The cost of waiting

A hidden leak never fixes itself. While it runs, it:

• wastes water continuously, day and night;
• soaks plaster, wood and screed, and lifts floor and wall finishes;
• encourages mould, which harms indoor air quality;
• often reaches the flat below, turning one problem into a claim involving several households.

The earlier the search, the smaller the opening and the repair.

Preventing hidden leaks

• Read your meter once a month, at the same time of day.
• Replace ageing flexible hoses and seals.
• Have the water pressure checked: pressure that is too high wears out fittings, and a pressure reducer can be fitted.
• Maintain the water heater and its safety valve.
• Have old pipes inspected, especially before a bathroom renovation.
• A connected leak detector can alert you to an abnormal flow.

Frequently asked questions

How do I find a water leak behind a wall?

Start with the meter test to confirm the leak. Then look for signs on the wall: stain, bubbling paint, a cold patch, a musty smell. To pinpoint the exact spot without breaking anything you need a moisture meter, a thermal camera, acoustic listening or tracer gas, which is what a plumber equipped for leak detection uses.

Does repairing a pipe inside a wall always mean breaking it open?

An opening is usually still needed to reach and repair the pipe. The search beforehand keeps it to a small area in the right place, instead of several blind channels.

How long does a leak search take?

It depends on the layout. A simple search takes a few hours; a complex one using several methods can take half a day or more. The technician will tell you after the visual inspection.

Can a leak be hidden and not make the meter turn?

Yes, if it is on a waste pipe (trap, shower fitting, shower tray) or on the heating circuit: those do not pass through the water meter. The clues are then damp, smell or a boiler that keeps losing pressure.

The leak may come from a neighbour or a shared pipe. What then?

That is exactly what the search establishes. If the source is in a neighbour's flat or in the shared parts of the building, we tell you clearly and the report lets you take it up with the syndic or the insurers. See riser pipe leaks in a co-owned building.

Leak detection specialist or plumber: who should you call?

A leak detection specialist (chercheur de fuite in French) is a professional equipped to locate a leak without breaking anything: moisture meter, thermal camera, acoustic listening, tracer gas. Some only do the search and leave you to find another tradesperson for the repair. A plumber equipped for leak detection does both: finds the leak, repairs it straight away when possible, and gives you the report for your insurer. Before booking, ask which methods will be used, whether a written report is included and whether the repair is included or quoted separately.

Have your leak located

Réseau Tubulure carries out non-destructive leak searches in Paris and the inner suburbs (92, 93, 94), 7 days a week, 24 hours a day, with a detailed report after every visit. Many of our plumbers speak English.

• Phone: +33 1 89 70 15 36
• Form: contact us
• WhatsApp: from the button on the site

See our leak detection and repair service. If water is actively flowing, first read water leak emergency: who to call.