How do I stop an overflow pipe dripping outside?

The overflow is doing its job: it is dumping water outside rather than into the house. That is the good news. The bad news is that it will keep doing so until the valve that should have shut off is dealt with.

Safety

Step by step

1. Work out which pipe it is.Trace it back. A pipe just below an upstairs window usually serves a toilet cistern. A pipe high on a gable serves a loft tank. A small copper pipe near the boiler is a pressure relief and is a different matter entirely.
2. Stop if it is a boiler or cylinder relief.That indicates an expansion vessel or pressure fault. It is not adjustable and not a DIY repair — call a heating engineer, and note that it may indicate the system is over-pressurised.
3. Open the cistern or tank and look.The water level should sit below the overflow outlet. If it is at or above it, the float valve is not closing. That is the whole diagnosis in most cases.
4. Adjust the float first.Modern valves have an adjuster screw or a clip on the float arm; older brass ones have a metal arm you bend gently downwards. Lower the level so it sits comfortably below the overflow.
5. Check the float itself has not filled.A ball float with a hole in it fills with water, sinks, and never rises enough to shut the valve. Unscrew it and shake — water inside means replace it, and they are inexpensive.
6. Replace the washer or diaphragm.If the level is right but water still trickles in, the valve is not sealing. Isolate the supply, take the valve apart, and replace the rubber washer or diaphragm. Grit on the seat is a common cause and worth cleaning off.
7. Replace the whole valve if it is old.A new float valve costs very little and takes twenty minutes. On an old brass valve with a worn seat, replacing the unit is more reliable than replacing the washer.
8. Check the siphon or flush valve too.In a toilet, a leaking flush valve lets water pass into the pan continuously, so the inlet keeps refilling. Put a little food colouring in the cistern — colour appearing in the pan without flushing confirms it.
9. Isolate before dismantling.The isolating valve on the supply pipe, or the stopcock. Then flush or drain to empty. Working on a live inlet is how people flood a bathroom.
10. Deal with it quickly in winter.A dripping overflow in freezing weather ices the wall and the path below. That is a slip hazard and can damage the render.
11. Check the overflow pipe is clear and falls correctly.It should run downhill to the outside with no dips. A blocked or wrongly-fallen overflow means water backs up inside instead, which is far worse than a drip outside.

Tips

• Turn the isolating valve slowly when restoring the supply. A sudden surge disturbs debris that then lodges on the new washer.
• Take the old valve or washer with you when buying a replacement. There are several standard types and they look similar.
• A cistern that refills by itself every few hours with no overflow is still leaking — into the pan rather than outside. The food colouring test finds it.

Common mistakes

• Bending a plastic float arm — Modern arms crack rather than bend. Use the adjuster screw or clip that is designed for it.
• Ignoring a slow drip — It wastes a substantial volume of water over months, and in winter it ices the wall and the ground below.

If it doesn't work

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