Overflow pipe dripping outside
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Most likely cause & what to check
Find which cistern or tank is overflowing — the overflow pipe usually runs through an outside wall directly from the appliance. Check the toilet cistern first, then the cold water storage tank in the loft.
Remove the lid and look at the water level — it should be about 25mm below the overflow pipe outlet. If water is reaching the overflow, the float valve is letting in too much water.
For a toilet cistern: the float arm may just need bending downward slightly (on older brass valves) so the ball cuts off the water supply sooner. On modern plastic valves, adjust the float height using the adjustment screw.
If the valve keeps letting water through even when the float is up, the valve seat is worn and the whole valve needs replacing. These cost £5–10 and are easy to swap — shut off the water, disconnect the supply pipe, unscrew the lock nut, and fit the new valve.
For a loft tank: the same principle applies. Adjust or replace the ball valve inside the tank.
After adjusting, check the water level is now below the overflow and the dripping has stopped.
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Frequently asked questions
Is an overflowing overflow pipe dangerous?
Not immediately dangerous, but it wastes water continuously and can cause damp or damage to the outside wall over time. Fix it promptly.
My overflow pipe is dripping but I can't find which tank it comes from — what do I do?
Trace the pipe from outside where it exits the wall, then follow it inside. It will lead to either a toilet cistern, cold water storage tank, or hot water cylinder expansion pipe.