Pipes knocking when water is turned on is almost always water hammer: a pressure shock that hits the pipework when fast-flowing water is stopped suddenly by a tap or valve. The usual culprits are high mains pressure, fast-closing appliance valves and loosely fixed pipes. Most cases are fixed by securing pipes, fitting a pressure-reducing valve, or adding a water hammer arrestor.
That bang or shudder when you turn on a tap, or when the washing machine fills, is one of the most common plumbing noises in UK homes. It is usually harmless to start with, but left alone it can loosen joints and stress fittings over time. The cause is nearly always one of a short list, and most fixes are straightforward. Here is what is actually happening and how to stop it.
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Water hammer: the usual cause
The technical name for knocking pipes is water hammer. Water moving through a pipe has momentum. When a tap or valve shuts quickly, that fast-moving water is forced to stop almost instantly, and the energy has to go somewhere. It rebounds as a pressure shock wave that travels back along the pipe, making it knock, bang or shudder against its fixings. Plumbing guides describe it the same way: a sudden stop in flow creating a pressure wave through the pipework (PlumbingAdvice; serviceteam).
It is most noticeable on the cold mains supply, because that is where pressure is highest. The faster the water and the harder the valve slams shut, the louder the bang.
The real reasons your pipes knock
Knocking when you turn the water on usually comes down to a handful of causes, and often more than one is involved.
| Cause | Why it makes pipes knock |
|---|---|
| High water pressure | The higher the mains pressure, the bigger the shock wave when flow stops. UK homes typically run around 2 to 3 bar, but incoming mains can be higher. |
| Fast-closing valves | Solenoid valves in washing machines and dishwashers shut in a fraction of a second, giving the water no time to slow down. |
| Loose or unsupported pipes | Pipes that aren't clipped tightly are free to vibrate and bang against joists, walls and brackets. |
| Partly closed valves | A stop tap or isolation valve that isn't fully open creates turbulence and pressure changes that show up as noise. |
Pressure is worth a closer look, because it sits behind so many cases. Your water company must, by law, supply a minimum of 7 metres static head (about 0.7 bar), and most aim for at least 1 bar, per the Ofwat Guaranteed Standards Scheme and WaterSafe. The recommended working pressure inside the home is roughly 2 to 3 bar. If yours runs well above that, water hammer becomes much more likely.
Why it happens when an appliance fills
One of the most common complaints is pipes banging the moment a washing machine or dishwasher starts to fill. These appliances use solenoid valves that snap fully shut in milliseconds. On a high-pressure supply, that instant stop is a textbook trigger for water hammer. Older appliances often closed their valves more slowly, which is why a new machine can suddenly bring on the noise. Fitting a small water hammer arrestor at the appliance's supply valve usually quietens it for good.
DIY fixes you can try first
Before calling anyone, there are a few sensible checks that fix a lot of cases.
- Secure any loose pipes. Where you can see exposed pipework (under a sink, in an airing cupboard, along a loft run), add or tighten pipe clips so the pipe can't vibrate against timber or masonry. This is the most common easy fix.
- Open valves fully. Check that your main stop tap and any isolation valves are fully open, not part-closed. A throttled valve is a frequent, easily missed cause.
- Note when it happens. If it only knocks with the washing machine or dishwasher, the fix points to that appliance. If it knocks on every cold tap, pressure is the likely driver.
These steps are low-risk and need no special tools. If the noise survives them, it is usually a pressure or arrestor job.
Permanent fixes for stubborn knocking
When knocking persists across the house or keeps coming back, two devices do the heavy lifting. Both are best fitted by a qualified plumber.
Pressure-reducing valve (PRV)
If high incoming pressure is the root cause, a pressure-reducing valve fitted on the mains brings the whole system down to a steadier working level. That smaller pressure means a smaller shock wave every time a valve closes, so it tackles the problem at source rather than treating the symptom.
Water hammer arrestor
A water hammer arrestor is a small sealed device, fitted close to the worst-affected tap or appliance, that absorbs the pressure wave before it can rattle the pipes. Modern arrestors use a sealed air cushion or piston and are pre-charged through a Schrader valve like the one on a car tyre (Connexion Developments). They are reliable, compact and the go-to fix for appliance-related water hammer.
When knocking points to a hidden leak
Knocking on its own is a pressure-and-fixings problem, not usually a leak. But it is worth a second look when it comes alongside other symptoms. If your boiler keeps losing pressure, your water bill has risen with no visible leak, or you've spotted damp patches you can't explain, there may be a hidden leak in the system as well as the noise.
It is also easy to mistake one problem for another. A damp patch from condensation, for example, isn't the same as an escape of water, as our guide on condensation versus hidden leaks explains. If anything points to water going somewhere it shouldn't, professional water leak detection pinpoints the source without tearing the house apart, and the same applies to your heating circuit through our central heating leak detection.
Frequently asked questions
What is water hammer?
Water hammer is the banging or knocking you hear when fast-moving water is stopped suddenly, usually by a tap or valve closing quickly. The flow has nowhere to go, so it sends a pressure shock wave back through the pipes, making them shudder and bang against their fixings.
Why do my pipes knock when water is turned on?
The most common reasons are high mains water pressure, fast-closing valves on modern appliances, and loosely fixed pipes that are free to rattle. Sometimes a partly closed stop tap or isolation valve is to blame. Often more than one of these is happening at once.
Can I fix knocking pipes myself?
Often, yes. Securing loose pipes with extra clips and making sure your stop tap and isolation valves are fully open are simple jobs. Fitting a pressure-reducing valve or a water hammer arrestor is more involved and is usually best left to a qualified plumber.
Why do my pipes bang when the washing machine fills?
Washing machines and dishwashers use solenoid valves that snap shut almost instantly. On a high-pressure supply that sudden stop creates water hammer. Fitting a small water hammer arrestor at the appliance's supply valve usually solves it.
What water pressure should my home have?
UK domestic systems typically run at around 2 to 3 bar, while incoming mains can be higher. Your supplier must provide a minimum of 7 metres static head (about 0.7 bar) and most aim for at least 1 bar. Pressure much above the normal working range makes water hammer more likely.
Can knocking pipes be a sign of a leak?
Knocking is usually pressure or fixings rather than a leak. But if it comes with falling boiler pressure, damp patches or a rising water bill, there may be a hidden leak as well. If anything points that way, it is worth having the system checked properly.
Knocking pipes, and worried there's a leak behind it?
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