Water hammer
The loud bang or shudder you hear when a tap or appliance valve closes suddenly is called water hammer. Water moving through a pipe has momentum; when a valve snaps shut, that water stops abruptly and the shock travels back along the pipe, rattling it against walls and joists.
It's most common with fast-closing valves – quarter-turn taps, washing machine and dishwasher solenoid valves – and in homes with high mains pressure. Over time, the repeated shocks can loosen joints and damage fittings, so it's worth tackling rather than living with.
Other common noises
Not every noise is water hammer.
- Knocking from the loft as a tank refills – a bouncing float valve in the cold water tank
- Rattling pipes when water runs – loose or missing pipe clips
- Creaking or ticking as the heating warms up – pipes expanding against floorboards or notches in joists
- Whistling or squealing – a partly closed valve or worn washer
- Gurgling from radiators – air in the heating system
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How plumbers fix it
The fix depends on the cause. Plumbers can add or replace pipe clips, pack pipes away from joists, fit a quieter equilibrium float valve in a loft tank, fit water hammer arrestors near appliances, or install a pressure-reducing valve if the incoming mains pressure is very high. Swapping a noisy old tap valve can also help.
When it's urgent
Most pipe noises are annoying rather than dangerous. Call a plumber promptly if the noise started suddenly and is very loud, if you notice drips or damp near pipe runs, or if the noise comes from a hot water cylinder or boiler – a boiler making kettling or banging sounds should be checked by a Gas Safe registered engineer.
