Toilet Leaking or Constantly Running

Worn cistern flap valve

The rubber flap at the base of the cistern wears and allows water to trickle into the pan continuously. You'll hear a constant gentle hiss or running sound. Very common in toilets over 5 years old.

Failed fill valve (ball valve)

The fill valve controls water entry to the cistern. When it fails, water doesn't shut off properly or the float sits too high, causing water to run down the overflow.

Failed pan-to-soil-pipe seal

The rubber seal between the toilet pan and the soil pipe fails over time, allowing water to seep around the base of the toilet when flushing.

Cracked cistern or pan

Hairline cracks in the ceramic, usually caused by impact or frost, allow slow water seepage. Look for a hairline crack on the outside of the cistern or pan.

Practical checks and next steps

Replacing a cistern flap valve is within most DIY abilities — it involves turning off the water, flushing to empty the cistern, and fitting a replacement flap. A leaking pan seal or cracked pan needs a plumber.

• Check if water is running into the pan (listen for a hiss) or overflowing from the cistern outlet.
• Check for water around the base of the toilet when flushing (pan seal).
• Turn off the isolation valve on the supply pipe to the cistern (slotted screw on the pipe).
• Flush to empty the cistern, then inspect the flap valve — if perished, replace it.
• If water is coming from the base of the pan, call a plumber.