In This Guide
Introduction
A higher than expected water bill. A water meter that seems to be moving faster than it should. A Thames Water letter about excessive usage. These are the signs that prompt most London homeowners to suspect a leak, but knowing how to confirm one is another matter.
The good news is that the first step costs nothing and takes 30 minutes. The water meter test is the standard method used by plumbers and water companies to confirm whether a leak is present and whether it is on the supply side or inside the property. You do not need any tools or specialist knowledge to carry it out.
This guide walks through the complete process: the meter test, the internal checks, the surface signs to look for outside, and what to do when the test confirms a leak.
What Kind of Leak Are You Looking For?
Before starting, it helps to understand that household water leaks fall into two categories, and the checks for each are different.
Leak Type | Where It Occurs | Typical Signs |
Internal leak | Inside the property, visible pipework, fittings and appliances | Dripping, damp patches, staining, visible water |
Underground leak | Supply pipe buried beneath garden, driveway or floor slab | High bill, moving meter, no visible internal sign |
The meter test tells you which category you are dealing with. Internal checks then identify the specific source if the leak is inside. If the meter confirms a supply side leak with no internal cause, the leak is underground.
Step 1: The Water Meter Test
This is always the first step. It takes 30 minutes, requires no tools, and gives you a definitive answer on whether a leak is present on the supply pipe.
How to find your meter
Your external water meter is typically located in a small rectangular chamber set into the pavement or path just outside your property boundary. The lid is usually marked “water” and can be lifted with a flat-head screwdriver or a meter key. Some properties have the meter inside the boundary near the external stop tap. If you cannot find yours, Thames Water can provide the location for your address.
How to carry out the test
- Switch off every tap, appliance and fitting that uses water. This includes the dishwasher, washing machine, garden tap, any drip irrigation, and toilet cisterns that may be mid-fill.
- Lift the meter chamber lid and read the dial. Write down every digit including any moving decimal figures, or photograph the display.
- Wait 30 minutes. Do not use any water during this period, including flushing a toilet.
- Read the meter again and compare the two figures.
What the result means
Result
| What It Indicates | Next Step |
No change | No active leak on the supply pipe | Check inside the property – see Step 2 |
Slight increase | Small or intermittent leak on the supply side | Book an investigation |
Significant increase | Active leak on the supply pipe | Book an investigation urgently |
Dial moving continuously | Ongoing leak, possibly substantial | Book an investigation urgently |
Step 2: Internal Checks
If the meter test showed no movement, or if you want to rule out internal causes before concluding the leak is underground, work through these checks in order.
Toilet cisterns
A running toilet is the single most common cause of unexplained water loss in London properties and the easiest to miss because it is often silent. Add a few drops of food colouring to the cistern and wait ten minutes without flushing. If colour appears in the bowl, the flap valve is leaking and needs replacing. This is a straightforward DIY repair or a quick job for a plumber.
Taps
Feel the spout of every tap in the property when fully closed. Even a slow drip that is not visible to the eye produces a measurable loss over a billing period. Check both hot and cold supplies at every basin, bath and sink. A worn tap washer is a simple repair.
Shower and bath fittings
Check the shower tray seal and the bath seal for gaps or deterioration. Water escaping through a failed seal goes under the tray or bath rather than into the room, and can cause significant damage before it is noticed.
Appliances
Check behind the washing machine and dishwasher for slow drips at the fill hose connections and at the drain hose. Both are common sources of slow leaks that go unnoticed until a bill arrives.
Garden tap and outdoor fittings
Turn on and off the outdoor tap and check the connection at the wall for drips. If you have a drip irrigation system, check the joins and the timer valve. These are easy to overlook and can run continuously.
Hot water cylinder and header tank
If your property has an older hot water system with a copper cylinder and a header tank in the loft, check the overflow pipe from the tank. If it is discharging water outside, the ball valve in the tank is faulty and needs replacing.
Step 3: Check Outside for Surface Signs
Even when the meter confirms a supply side leak and there are no internal causes, surface signs outside are not always obvious. But look for the following.
Surface Sign | What It May Indicate |
Patch of grass greener or growing faster than the surrounding area | Slow underground leak feeding the soil below |
Area of lawn that stays wet when everything else is dry | Water rising from a buried supply pipe |
Cracking, sinking or lifting of driveway or garden path | Soil erosion from escaping water undermining the surface |
Soft or spongy ground over the pipe run | Saturated soil from a slow ongoing leak |
Unexplained muddy area after a period of dry weather | Leak bringing water to the surface |
The absence of these signs does not rule out an underground leak. In dense clay soil or under concrete, the escaping water may never reach the surface at all.
Step 4: The Internal Isolation Test
If the external meter test showed movement and you want to confirm whether the leak is on the supply pipe or inside the property, use your internal stop tap to isolate the internal pipework.
Locate your internal stop tap, typically under the kitchen sink or where the supply pipe enters the property. Close it fully. Go back to the external meter and observe the dial. If the meter stops moving with the internal stop tap closed, the leak is inside the property. If the meter continues to move, the leak is on the supply pipe between the meter and the internal stop tap.
What to Do When the Test Confirms a Leak
If the leak is internal
Work through the internal checks in Step 2 to identify the source. Running toilets, dripping taps and leaking appliances are all straightforward to diagnose and repair. For leaks behind walls or under floors, an acoustic or moisture meter investigation by a qualified plumber will locate the failure point without speculative opening of the structure.
If the leak is on the supply pipe
The leak is underground. The next step is locating the exact failure point before any excavation begins. At 0800 Homefix we use acoustic listening equipment and tracer gas detection to confirm the failure location to within centimetres before recommending any access or excavation.
A Note on Responsibility
The supply pipe from the external stop tap to your internal stop tap is the homeowner’s responsibility to maintain and repair. Thames Water is responsible for the main in the street and for the pipework up to and including the external stop tap.
If the leak is confirmed to be on the supply pipe, check your buildings insurance policy for trace and access cover before assuming you face the full cost. Most UK buildings insurance policies include this cover, which pays for locating and accessing a hidden or underground leak and reinstating the surface after the repair.
Frequently Asked Questions
How do I know if my water meter is working correctly?
If you suspect your meter is faulty rather than there being a genuine leak, contact Thames Water and request a meter accuracy test. Thames Water will test the meter and replace it if it is found to be running fast. However, a meter running fast is significantly less common than a genuine leak. Carry out the 30-minute test first. If the meter moves consistently across multiple tests at different times of day with all water use stopped, a leak is the more likely explanation than a meter fault.
What if my meter is moving but I cannot find any leak inside?
If the meter moves with all water use stopped and you cannot find any internal cause, the leak is almost certainly on the supply pipe between the meter and the internal stop tap. This section of pipe is buried underground. The water escapes into the surrounding soil rather than onto a visible surface, which is why there are often no internal signs. Professional underground leak detection using acoustic and tracer gas methods is needed to locate the failure point.
How much water does a running toilet waste?
A running toilet cistern with a faulty flap valve can waste between 200 and 400 litres per day depending on the severity of the leak. At Thames Water’s current metered rate that is approximately £100 to £200 per month added to the bill. It is the most common internal cause of a high water bill and the easiest to test for using the food colouring method.
Can I carry out the meter test at any time of day?
Yes, but early morning or late evening when household water use is naturally stopped is the easiest time to ensure no water is being used during the test. Avoid carrying out the test when appliances with automatic fill cycles, such as a dishwasher on a timer, are likely to run.
What is a stop tap and where do I find it?
A stop tap is a valve that controls the flow of water into the property. The external stop tap is located at or near the property boundary, usually in a small chamber in the pavement. The internal stop tap is inside the property, typically under the kitchen sink or where the supply pipe enters through the floor or wall. Turning the internal stop tap clockwise closes it and stops water flowing into the internal pipework.
My meter is in the pavement and I cannot open the lid. What should I do?
Meter chamber lids can sometimes be stiff or corroded. A flat-head screwdriver inserted into the slot on the lid and turned should release it. If the lid is damaged or the chamber is flooded, do not force it. Contact Thames Water who are responsible for maintaining meter chambers. Alternatively, if you have access to a meter key, which is a T-shaped tool available from most plumbers merchants, this makes lifting the lid significantly easier.
How do I check for a leak if my property has no external meter?
Some older London properties are on unmetered supplies and are charged a flat rate rather than by consumption. Without a meter it is not possible to carry out the isolation test. If you suspect a leak, the visible internal checks in Step 2 are the starting point. For suspected underground leaks on an unmetered supply, professional acoustic and tracer gas detection is required to confirm and locate the failure.
When to Call 0800 Homefix
If your meter test has confirmed a leak on the supply pipe and you cannot identify a cause inside the property, call 0800 Homefix before arranging any excavation. We locate underground water leaks using acoustic and tracer gas methods, confirm the failure point before recommending any access, and carry out the repair in the same visit in most cases.
No call-out fee. Fixed quote before we start. Written report within 48 hours.
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Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
