How to check for a supply pipe leak
Both tests below use equipment you already have. Between them they answer the only question that matters at this stage, which is whether the water is going somewhere inside the house or under the ground outside it.
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Test one: the meter test
Use this whenever the property has a working internal stop tap. It is the test an engineer runs first on arrival, and running it yourself beforehand saves time on the visit.
What you need
- Your water meter, usually in a chamber at the boundary under a small lid marked WATER or W.
- Your internal stop tap, usually under the kitchen sink.
- Half an hour when nobody is using water.
Shut everything down
Taps off, washing machine and dishwasher off, garden irrigation off, ice maker off. Wait until every toilet cistern has finished refilling, because a cistern topping itself up will move the meter and it is not the leak you are looking for.
Read the meter, including the small dial
Write down the whole reading. On a slow leak the large digits will not change at all in half an hour, and the evidence sits entirely in the last figures or the small sweep hand. This is the step people skip and then have to repeat.
Close the internal stop tap
Turn it clockwise until it stops, gently. An old tap that has not moved in years can seize or start weeping when forced, so if it will not shut easily, leave it and use the second test instead.
Wait thirty minutes and read it again
Longer is better on a very slow loss. Leave it an hour if you can, and check that nobody has opened a tap in the meantime.
Read the result
- The meter moved with the house isolated. Water is escaping from the buried supply pipe outside the house. That is the pipe you own.
- The meter did not move. The supply pipe is holding. Any loss is on the plumbing inside, and the search moves indoors.
- The stop tap would not shut properly. The test is inconclusive, because water is still reaching the house. Worth mentioning when you call, because it changes how the visit is planned.
Test two: the cup test
For a property with no internal stop tap, or one that no longer shuts. It works the other way round, by shutting the water off at the boundary and watching whether the pipe pulls water back out of a glass.
What you need
- Two people, one at the tap and one at the boundary.
- A pint glass.
- A stop tap key, sometimes called a toby key, to reach the valve in the boundary chamber.
Fill the glass at a cold tap and keep the spout under the water
The tap stays running and the end of it stays submerged. That is what allows the pipe to draw water back.
Shut the supply off at the boundary
The second person closes the valve in the meter chamber with the key.
Watch the water level for five minutes
If the level in the glass drops, the pipe is siphoning water back out of it, which means there is an opening in the pipe below. A steady level means the supply is holding.
Two cautions
Do not force a valve that will not turn. Boundary stop taps are often decades old and a snapped one becomes the water company's emergency rather than your test.
Shutting the boundary valve takes the water off the whole property, and on a shared supply it may take it off next door as well. Tell them first.
What to write down before you call
Five minutes with a pen turns a vague description into something an engineer can plan a visit around.
- Both meter readings and the gap between them.
- Whether the internal stop tap shut properly.
- What is on top of the pipe run: lawn, block paving, tarmac, a concrete slab, a shared drive.
- Roughly when the bill or the pressure changed.
- Anything your water company has sent you about it.
- The age of the property, which is the best available clue to what the pipe is made of.
Read out what you wrote down
That list is enough for us to say whether it sounds like the supply pipe, what the visit involves and what it costs, before anyone travels.
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