Pipes bang or shudder when you turn off a tap or the washing machine fills? That is water hammer. What causes it, what you can check and how a plumber fixes it.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
A bang or shudder in the pipes when a tap shuts off, or when a washing machine or toilet stops filling, is called water hammer. Fast-flowing water stops suddenly and the shock travels along the pipe. It can loosen joints over time, so it is worth fixing. The usual fixes are securing loose pipes, replacing a worn valve, or fitting a shock arrestor.
Water near electrics? Do not touch wet switches, sockets or light fittings. Switch off at the consumer unit only if you can reach it without standing in water. Water pouring in? Turn off the stopcock: find your stopcock →
What it could be and what to do
Water hammer Engineer needed
Likely cause: Aspect explains that closing a tap or valve quickly causes a sudden pressure surge that makes the pipes vibrate. The faster the flow, the stronger the shock.
What to do: Note which tap or appliance causes it. Closing taps more slowly is a short-term fix. A plumber can fit a shock arrestor or slow-closing valves.
Worn tap or valve You may be able to fix this
Likely cause: Worn valve seating lets pressure rise quickly. Aspect notes a faulty tap downstairs can cause noise upstairs.
What to do: If one tap always causes it, that tap or its valve probably needs a new washer or cartridge.
Loose pipework Engineer needed
Likely cause: Pipes that are not clipped properly move and knock. Aspect warns that vibration stresses joints and fixings, and noise that gets louder means it is getting worse.
What to do: Look where you can see pipes, under sinks and in the airing cupboard, for loose clips. Pipes under floors need a plumber.
Banging only when the heating is on You may be able to fix this
Likely cause: Knocking that only happens with the heating on is usually the heating system, not the water supply.
What to do: See our radiator and boiler noise pages.
Safe checks you can do first
These are the checks we suggest before a plumber arrives. Stop if anything feels unsafe.
Stop the water: use the small isolation valve on the pipe to the fitting if there is one, or the main stopcock, usually under the kitchen sink. Stopcock finder →
Electrics: keep away from anything wet and electrical. Get an electrician to check before switching back on.
Is it just you? Ask neighbours. No water or brown water in the whole street is usually Scottish Water: 0800 0778 778, free, 24 hours.
Contain it: buckets, towels and moving belongings limit the damage. In a flat, warn the neighbour below.
Photos: take photos and video before you clear up, for your insurer or landlord.
Need a plumber now? Call us any time.
Our callout is from £75 in standard hours (out of hours £100–£180) for attendance and diagnosis. Any repair and parts are agreed with you before work starts.
Our emergency line is open 24 hours a day, 7 days a week.
Every property is different. This page is general guidance, not a diagnosis.
YOUR QUESTIONS, ANSWERED
Helpful answers before you call.
Why do my pipes bang when I turn off the tap?
That is water hammer. Fast-moving water stops suddenly and the shock makes the pipes vibrate. A plumber can fix it by securing loose pipes, replacing worn valves or fitting a shock arrestor.
Is water hammer dangerous?
Not immediately, but repeated shocks loosen joints and fixings and can lead to leaks. If the banging is getting louder, get it checked.
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