Tracing a leak without opening up the whole wall

Close every tap, switch off the dishwasher and the washing machine, and note the meter reading to the last decimal. Come back two hours later without using water in between. If the reading changed, you are losing water from a pipe under pressure.

If the meter stands still and the damp patch still grows, it is not a water pipe. Then you look elsewhere: a drain that only leaks while water runs through it, rainwater entering along a window frame or a roof penetration, or condensation against a cold wall.

This distinction is half the work and it costs you two hours and nothing else. A leak detection company starts with exactly the same question, so if you already have the answer you are cheaper off.

Leak detection costs between 150 and 500 euro, and almost all of that difference is the method needed. A leak that a thermal camera shows in ten minutes is a different bill from a pipe that first has to be emptied and filled with tracer gas.

Note what is NOT included: this is the tracing only. Opening up, repairing and closing again are separate, and under a tiled floor that is quickly the largest part of the bill.

• Simple visible leak, no equipment: 50 to 75 euro
• Thermal imaging or acoustic, the most common: 150 to 400 euro
• Tracing a roof leak: 75 to 150 euro, on a complex roof 300 to 400
• Tracer gas, when nothing else works: 400 to 500 euro
• Repair afterwards, plumber's rate: 50 to 90 euro per hour plus materials

This test costs you two hours and nothing else, and it decides whether you need a leak detection company or a roofer. A leak detection company starts with exactly the same question, so if you already have the answer you are cheaper off.

1. Close every tap and switch off the dishwasher and the washing machine.
2. Note the water meter reading to the last decimal. Photograph it, then you cannot copy it down wrong.
3. Use no water for two hours. Do not flush the toilet either, because the cistern refills afterwards.
4. Read the meter again and compare.
5. If the reading changed, a pressurised pipe is leaking. Call a leak detection company.
6. If the meter stands still while the patch stays wet, it is not a water pipe. Then look in this order: a drain that only leaks during use, rainwater from outside, and condensation.
7. If in doubt, close the main tap and repeat. If the meter still runs, the leak is before the meter and it is the water company's problem.

Thermal imaging is the most common method. Warm water in a pipe shows against the colder floor or wall, even through tiles. It works well with underfloor heating and hot water pipes, and less well with cold water in a well-insulated floor.

Acoustic detection listens for the sound water makes as it sprays from a small hole under pressure. With a sensitive microphone that is audible through concrete. It only works on pipes under pressure, so not on a drain.

Tracer gas is the heaviest method: the pipe is emptied and filled with a harmless gas mixture that rises through the floor at the leak. That is what remains when thermal imaging and acoustics find nothing, and it is also the most expensive.

Ask in advance what happens if they do not find the leak. Some companies then charge only the call-out, others the full rate. That difference is hundreds of euro.

Leak detection itself usually costs 200 to 450 euro, depending on the method and how long it takes. That is only the tracing; opening up, repairing and closing again are separate.

For the repair itself, count on a plumber's rate of 50 to 90 euro per hour plus materials, and on restoration of tiles or plaster if anything had to be opened.

In an emergency where the water cannot be stopped, the same evening and weekend surcharges apply as with other plumbing emergencies. If you can close the main tap and do without water for a while, waiting until morning saves a lot.

The main rule with most Dutch buildings and contents policies: the DAMAGE caused by the water is covered, REPAIRING the pipe is not. A soaked floor is reimbursed, the new coupling in the pipe is not.

The cost of tracing is often included, precisely because you cannot limit the damage without it. That differs per policy and it is usually stated literally under tracing costs. Call your insurer before you give the order, not after.

Slow leaks you have let run for months are regularly rejected because you should have limited your damage. A damp patch that keeps growing is not something to watch for another season.

With a damp patch on a ceiling below a bathroom, the sealant around the shower tray or the drain penetration is the first suspect, not the pipe. That is good news: sealing is an hour's work.

With a wall that is only wet after rain, look outside: a blocked gutter, a crack in the pointing, a broken roof tile or loose lead flashing. A leak detection company for water pipes is then the wrong company.

Patches that appear in winter and disappear in summer are often condensation rather than a leak. The answer is ventilation and insulation, not demolition.