How do I stop my water pipes banging?

Find the tap or appliance that triggers it, then fix the cause: clip any pipes that move, fit a shock arrestor near a washing machine or dishwasher, replace a bouncing float valve, and check the incoming water pressure is not too high. Most cases need only one or two of these.

Can water hammer damage pipes?

Over time, yes. Each shock stresses the same joints, and repeated hammer loosens fittings and can crack joints, leading to slow leaks under floors or behind walls. It also wears tap cartridges and appliance valves faster.

Why do my pipes bang when the washing machine fills?

The machine uses a solenoid valve that snaps shut almost instantly when it has taken enough water. That sudden stop creates the pressure shock. A shock arrestor fitted on the supply close to the machine usually cures it.

Why do my pipes knock when the toilet refills?

That rapid knocking is usually the float valve in the cistern, or in the loft tank, bouncing on and off its seat as the water rises. Replacing the valve with a quieter type, or its washer and float arm, normally stops it.

Is it water hammer or air in the pipes?

Water hammer is a bang at the moment water stops flowing. Air gives spitting, spluttering taps and gurgling while water is flowing, often after the supply has been off. Our guide to an airlock in hot water pipes covers the second.

Why did my pipes start banging after a new boiler or cylinder?

A combi or unvented cylinder puts the hot pipes at mains pressure, where before they were fed at low pressure from a loft tank. Pipework that was never clipped for that pressure starts to move. Clipping, an arrestor or a check of the pressure reducing valve usually sorts it.

Water Hammer: Why Your Pipes Bang and How to Stop It

Water hammer is the bang or thud you hear in the pipes when a tap, washing machine, dishwasher or toilet cistern shuts off quickly. Moving water has weight, and when a valve stops it dead the energy turns into a pressure shock that shakes the pipe against whatever it touches. The usual causes are fast closing valves, pipes that are not clipped properly, and water pressure that is higher than the house needs. It is rarely dangerous on the day, but it loosens joints over time, so it is worth fixing. Most cases are cured with pipe clips, a small shock arrestor near the appliance, or a pressure reducing valve that has been checked and set correctly.

What water hammer actually is

Water in a pipe is a column with real mass moving at speed. Open a tap and the whole column starts to move. Close the tap slowly and it slows down gently. Close it in a fraction of a second, which is what a solenoid valve in a washing machine does, and the column slams to a stop against the closed valve. That sudden stop becomes a pressure spike that travels back along the pipe, reflects, and travels again until it dies away.

You do not hear the water. You hear the pipe moving as that shock passes, knocking against a joist, a floorboard, a wall or a loose clip. That is why the bang often seems to come from somewhere nowhere near the tap you just used. The pipe is only noisy where it is free to move.

Match the noise to the cause

When the bang happens and how it sounds narrows it down quickly. Listen for it a few times before anyone takes up a floorboard.

One sharp bang when a washing machine or dishwasher finishes filling: the appliance solenoid valve, which closes almost instantly.
A bang when you snap a lever or quarter turn tap shut: the tap is closing faster than the old screw down tap it replaced.
A rapid knocking or juddering while a toilet cistern or loft tank is refilling: a float valve bouncing on its seat, not true hammer.
A rattle along a whole run of pipe rather than one bang: missing or loose pipe clips.
Banging that started after a new appliance, tap or boiler went in: the new valve closes faster, or the pipework was disturbed and not reclipped.
Banging plus a cold tap that blasts out harder than it used to: the incoming water pressure is higher than it should be.

Banging pipes when the heating comes on is usually something else

Ticking, creaking and the odd crack as the heating warms up and cools down is thermal expansion. Copper grows as it heats and slides against notches in joists. It is annoying but harmless, and the fix is a bit of slack or a pipe sleeve where it rubs, not a shock arrestor.

A loud bang or rumble from the boiler or the hot water cylinder while it heats is a different problem again. On a cylinder that is often scale on the element or coil, called kettling, and our guide to a hot water cylinder making noise explains how to tell the sounds apart and which ones mean switch off now.

Checks you can make yourself

These are all safe and cost nothing, and between them they usually point at the cause.

Find where the bang is loudest. Put a hand on accessible pipes under sinks, in the airing cupboard and behind the washing machine while someone runs the offending tap or appliance.
Look for pipes that move freely. Any run you can push by hand more than a few millimetres needs a clip.
Close the tap more slowly. If the bang disappears when you ease a lever tap shut, the tap closing speed is the trigger.
Turn the stopcock down a little, not off. If the banging is noticeably softer with the stopcock partly closed, the pressure coming in is part of the problem. Our guide on where is my stopcock helps if you are not sure where yours is.
Watch the loft tank or toilet cistern while it refills. A float arm bouncing up and down as the water rises is the knocking you are hearing.

Partly closing the stopcock is a test, not a fix. Leaving it half closed starves combi boilers, unvented cylinders and showers of the flow they need, and some will stop working properly.

The fixes that actually stop it

Almost every case of water hammer is solved by one or a combination of the following. They are ordinary plumbing jobs rather than major works.

Clip the pipework. Adding clips at the spacing the pipe needs, and packing loose runs where they cross joists, stops the pipe moving so the shock has nothing to bang against. This alone cures a lot of houses.
Fit a shock arrestor. A small sealed arrestor fitted on the supply close to a washing machine or dishwasher absorbs the spike at the source. It is the standard fix for appliance hammer. If you are wondering whether that appliance should be on the hot or cold supply at all, see does a washing machine need hot water.
Change the float valve. A bouncing ball valve in a loft tank or cistern is replaced with a quieter equilibrium type, or has its float arm and washer sorted out.
Check the incoming pressure. Where the mains is high, a pressure reducing valve on the incoming supply brings it down to a level the house is happy with.
Replace the tap cartridge or tap. Some lever cartridges shut harder than others, and a softer closing replacement can end the problem on a single outlet.

Why unvented and mains pressure systems bang more

In an older house where the hot water comes from a tank in the loft, the hot side runs at very low pressure, so there is little energy in the pipe to cause a shock. When that system is converted to an unvented cylinder or a combi, the hot pipes are suddenly at mains pressure as well as the cold. The same washing machine and the same taps that were quiet for years can start banging the week the new system goes in.

On an unvented cylinder, the inlet group has its own pressure reducing valve that holds the supply at the level the cylinder is designed for. If that valve fails, pressure in the whole hot system can creep up, and banging taps are one of the signs, alongside a tundish that drips after the house has been quiet. Our guide to the pressure reducing valve lists the other signs that it has failed. A dripping tundish is a safety valve opening, so treat that combination as a job for an engineer with the G3 unvented qualification rather than something to live with.

Is water hammer dangerous?

A bang now and then will not burst a sound pipe. The damage is cumulative. Every shock flexes the same joints, and over months compression fittings loosen, push fit joints creep and soldered joints can crack. Leaks that eventually show up as a stain on a ceiling have often been hammered for a long time first. Appliance valves and tap cartridges also wear faster when they are fighting a pressure spike every time they close.

So it is not an emergency, but it is not something to ignore either. If you notice damp, a drip under a floor or a stain on the ceiling below where the banging happens, turn the water off at the stopcock and get the leak found. Our leak detection and repair service covers leaks you cannot see.

When to call us

Call us on 020 4652 5111 or send an enquiry if the banging is getting worse, if it started after a new cylinder, boiler or appliance went in, if you have an unvented cylinder and the tundish drips as well, or if you cannot find where the noise comes from. Tell us which tap or appliance sets it off and whether it is one bang or a rapid knock, because that usually tells us what to bring. We have 20 engineers across London, and our rate is on the pricing page, agreed before anyone attends.

Questions we get asked

How do I stop my water pipes banging?: Find the tap or appliance that triggers it, then fix the cause: clip any pipes that move, fit a shock arrestor near a washing machine or dishwasher, replace a bouncing float valve, and check the incoming water pressure is not too high. Most cases need only one or two of these.
Can water hammer damage pipes?: Over time, yes. Each shock stresses the same joints, and repeated hammer loosens fittings and can crack joints, leading to slow leaks under floors or behind walls. It also wears tap cartridges and appliance valves faster.
Why do my pipes bang when the washing machine fills?: The machine uses a solenoid valve that snaps shut almost instantly when it has taken enough water. That sudden stop creates the pressure shock. A shock arrestor fitted on the supply close to the machine usually cures it.
Why do my pipes knock when the toilet refills?: That rapid knocking is usually the float valve in the cistern, or in the loft tank, bouncing on and off its seat as the water rises. Replacing the valve with a quieter type, or its washer and float arm, normally stops it.
Is it water hammer or air in the pipes?: Water hammer is a bang at the moment water stops flowing. Air gives spitting, spluttering taps and gurgling while water is flowing, often after the supply has been off. Our guide to an airlock in hot water pipes covers the second.
Why did my pipes start banging after a new boiler or cylinder?: A combi or unvented cylinder puts the hot pipes at mains pressure, where before they were fed at low pressure from a loft tank. Pipework that was never clipped for that pressure starts to move. Clipping, an arrestor or a check of the pressure reducing valve usually sorts it.