Banging pipes come in two distinct flavours, and telling them apart takes about ten seconds once you know what to listen for.

A single sharp bang when something shuts off is water hammer. Ticking, creaking or knocking that builds as the heating warms up is expansion. They have completely different causes and completely different fixes.

What a plumber actually checks, and in what order

1. When exactly does it happen?

If it bangs the instant a tap, washing machine or dishwasher shuts off, that is water hammer. Moving water has been stopped abruptly and the shock has nowhere to go.

If it ticks, creaks or knocks gradually as the radiators heat up and again as they cool, that is pipework expanding and contracting against something solid. It is not water hammer and no amount of arrestors will fix it.

2. Which appliance triggers it?

Washing machines and dishwashers close their solenoid valves almost instantly, which is why they are the usual trigger. Quarter-turn lever taps do the same thing.

Run each appliance in turn and listen. Pinning it to one outlet narrows the job enormously.

3. Is the pipework properly clipped?

This is the single most common cause of both noises. Pipe that is unsupported in a floor void, or clipped too far apart, has room to move and slap against a joist when the pressure changes.

Where the noise is under a floor, lifting a board near the sound usually finds a length of unclipped pipe or a pipe passing through a tight notch in a joist with no room to expand.

4. Is there an arrestor fitted, and does it still work?

A shock arrestor absorbs the pressure spike. Older systems often never had one. Where one is fitted, it can waterlog over time and stop working, at which point the banging returns after years of quiet.

5. Is the mains pressure too high?

High incoming pressure makes hammer far worse. If the pressure is well above what the house needs, fitting or adjusting a pressure reducing valve can solve it outright and is kinder to every fitting and appliance in the house.

6. Is a float valve chattering?

If the noise comes from a toilet cistern or a loft tank while it is filling, a worn float valve vibrating as it closes will set the whole run humming or knocking. Replacing the valve is a small job.

What it usually turns out to be

Water hammer is most often unclipped pipework plus a fast-closing appliance valve, and the fix is usually mechanical rather than clever: clip the pipe properly and, if needed, fit an arrestor near the offending outlet. Heating knock is nearly always a pipe binding where it passes through a joist or against a bracket, and easing that clearance fixes it permanently.

When to stop and call someone

Frequently asked questions

Is water hammer dangerous?

Not immediately, but it stresses joints and fittings and over years it causes leaks. It is worth fixing.

Why did it only start recently?

Either something changed on the system, a new appliance with a fast valve, higher mains pressure, or an arrestor that has waterlogged and stopped absorbing the shock.

Can I fix banging pipes myself?

If you can safely reach the pipework and it is obviously unclipped, adding a clip may sort it. Under floors and in walls it is a job for someone with the tools to get at it without wrecking the floor.

Is knocking when the heating comes on the same thing?

No. That is expansion, and it needs clearance rather than an arrestor.

Need a plumber in Herts, Beds or Cambs?

Vantage Plumbing and Heating covers Hertfordshire, Bedfordshire and Cambridgeshire. The line is answered by a person at any hour of any day, and you get a price before anyone starts. Call 01462 506374 or request a callback.

Related: general plumbing repairs · boiler making a banging noise · central heating repairs.