A constantly running toilet is one of the most common household plumbing problems in the UK — and one of the most wasteful. Left unchecked, a faulty toilet can waste up to 400 litres of water every single day, adding significantly to your water bill and your environmental footprint. The good news is that most causes are straightforward to diagnose and fix, often without needing to call a plumber.
How a Toilet Cistern Works
Before diagnosing the problem, it helps to understand the basic mechanics of a toilet cistern. When you flush, a handle or button lifts a flapper valve (or flush valve) at the bottom of the cistern, releasing water into the bowl. As the water level drops, a float — attached to a float arm or fill valve — descends with it, opening the inlet valve to allow fresh water to refill the cistern.
Once the water reaches the correct level, the float rises and shuts off the inlet valve. An overflow pipe acts as a safety measure, directing excess water away if the cistern overfills. When any part of this system fails, water can run continuously — either back into the bowl or out through the overflow pipe.
Common Causes of a Running Toilet
1. Faulty Flapper Valve
The flapper valve is a rubber seal that sits at the bottom of the cistern and controls the flow of water into the bowl. Over time, rubber degrades — it can warp, crack, or accumulate limescale deposits, preventing it from forming a watertight seal. When this happens, water slowly trickles from the cistern into the bowl continuously, even when you haven’t flushed.
You can confirm a faulty flapper by adding a few drops of food colouring to the cistern water. If colour appears in the bowl without flushing, the flapper is leaking.
2. Float Arm or Fill Valve Issues
If the float arm is set too high, or the float itself has cracked and filled with water, the cistern will overfill. The inlet valve won’t shut off at the right level, causing water to run continuously into the overflow pipe. You may hear a constant trickling or hissing sound, and notice water dripping from the external overflow pipe on the outside wall of your home.
Older toilets use a ballcock float on a metal or plastic arm. Newer siphon-style cisterns use a float cup that rises and falls around a central fill valve. Both can fail in similar ways.
3. Overflow Pipe Problems
If water is dripping or flowing from the overflow pipe outside your property, this is a clear sign the cistern is overfilling. This is almost always caused by a float set too high or a worn inlet valve that won’t close fully. In hard water areas, limescale build-up inside the valve can also prevent it from seating properly. It is worth noting that under the Water Fittings Regulations 1999, a continuously running or overflowing cistern must be repaired promptly, as it constitutes a waste of water supply.
Step-by-Step Fixes
Replacing a Faulty Flapper Valve
Replacing a flapper valve is a simple DIY job that takes around 15 minutes:
- Turn off the water supply using the isolation valve on the pipe leading to the cistern, or the stopcock under the sink.
- Flush the toilet to empty the cistern, then remove the cistern lid.
- Unhook the old flapper from the overflow tube and disconnect the chain from the flush handle.
- Take the old flapper to a plumbers’ merchant or DIY store to find a matching replacement.
- Fit the new flapper, reconnect the chain (leaving a small amount of slack), and restore the water supply.
- Flush several times to confirm the cistern fills and stops correctly.
Adjusting or Replacing the Float
If the float arm is set too high, try these steps:
- On older ballcock systems, gently bend the float arm downward so the float sits lower in the water, shutting off the valve sooner.
- On newer fill valves, look for an adjustment screw or clip on the valve body and lower the float cup accordingly.
- If the float has cracked and is waterlogged, it must be replaced — a new ballcock float costs just a few pounds from any DIY store.
Descaling the Inlet Valve
In hard water areas, limescale can prevent the inlet valve from closing fully. Turn off the water supply, disassemble the valve head, and soak the components in white vinegar for a few hours to dissolve the scale. Reassemble and test. If the valve is heavily corroded or damaged, replacement is the more reliable option.
When to Replace Parts vs Call a Plumber
Most running toilet repairs are well within the capabilities of a confident DIYer. Replacing a flapper valve, adjusting a float arm, or fitting a new fill valve are all low-cost, low-risk tasks that require no specialist tools.
However, you should consider calling a qualified plumber if:
- You’ve replaced the flapper and float but the toilet is still running.
- There is visible water damage around the base of the toilet or cistern.
- The cistern is cracked or the flush mechanism is broken beyond simple adjustment.
- You are not confident working with the water supply or internal plumbing components.
A local, qualified plumber can diagnose and fix a running toilet quickly, often in a single visit. When choosing a tradesperson, look for membership of a recognised body such as the Chartered Institute of Plumbing and Heating Engineering (CIPHE) or check that they are listed on a reputable trade register. Given the water waste involved — and the obligations under the Water Fittings Regulations 1999 — it is always worth acting promptly rather than leaving the problem to worsen.
