Water Hammer When Closing a Tap
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →If water hammer is accompanied by visible pipe movement or you can see a pipe has worked loose from a clip, address the loose pipe immediately. Repeated hammering can cause joint failures and leaks behind walls.
Water hammer — that sudden bang or series of thuds when you turn off a tap — is one of the most common plumbing complaints in UK homes, and it's far more than just an annoying noise. When water flowing through pipes is stopped abruptly, its momentum has to go somewhere, creating a pressure shock wave that travels back through your pipework. Left unchecked, this repeated hammering can gradually loosen joints, damage clips, and eventually cause leaks behind walls or under floors — sometimes in places you won't notice until serious damage has already occurred. The good news is that water hammer is usually straightforward to diagnose and fix, whether the cause is high mains pressure, unsupported pipework, or a missing air cushion in the system. This guide walks through the three main culprits and shows you how to tackle each one. If you've checked everything and the problem persists, or if you spot pipes that have visibly moved or worked loose, it's time to call a qualified plumber to rule out more serious structural issues.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
Water hammer occurs when the flow of water through a pipe is suddenly stopped or reversed — most commonly when a ceramic disc tap or solenoid valve (such as a washing machine valve or dishwasher) closes rapidly. The kinetic energy of the moving water has nowhere to go and creates a pressure spike that travels back through the pipework as a shock wave. You hear this as a thud, bang, or series of knocks. The three main causes are: high mains pressure, loose or unsupported pipework, and the absence of an air cushion in the system.
Check the mains water pressure at a stopcock. In the UK, mains pressure is typically between 1 and 5 bar, but it can be higher in some areas. The Water Industry Act requires suppliers to maintain a minimum pressure of 1 bar (10 metres head) at the boundary, but there is no upper legal limit. Pressure above 3 bar significantly increases the risk of water hammer. A pressure gauge screwed into a tap fitting (available from Screwfix for around £12) will give a reading — take readings at different times of day as pressure varies. If pressure consistently exceeds 3 bar, fit a pressure reducing valve (PRV) on the incoming mains immediately after the stopcock. A PRV suitable for domestic use costs £25–60 at Screwfix or Plumb Center and should be set to 2.5–3 bar.
Check all visible pipework for loose clips. Pipework should be clipped at regular intervals — for 15mm copper pipe, every 1.2m horizontally and 1.8m vertically; for plastic push-fit pipe (such as Hep2O or JG Speedfit), every 500mm horizontally. Any section of pipe that can move when you push it by hand should have an additional clip fitted. Pay particular attention to pipes behind kitchen plinths and under baths — these are frequently under-clipped. Plastic pipe clips and fixing screws cost pennies at any DIY retailer.
For persistent water hammer not resolved by the above, fit a water hammer arrestor. This is a small device containing a spring-loaded piston that absorbs the pressure spike. Arrestors are fitted on the supply pipe close to the problem tap or appliance — typically within 600mm of the tap body or appliance valve. They are available in 15mm and 22mm sizes with push-fit or compression ends. Screwfix, Toolstation, and Plumb Center all stock them for £15–30 each. Arrestors are particularly effective for appliance supply valves (washing machines, dishwashers) where the solenoid closes very rapidly.
On older properties (pre-1970), the system may originally have relied on air chambers — sealed lengths of vertical pipe adjacent to tap positions that provided an air cushion. These air chambers gradually waterlog over time as the air dissolves into the water. If your property has capped vertical pipe stubs near taps or cisterns, these may be old air chambers. To recharge them: turn off the water, open all taps to drain the pipes, then close all taps and restore the supply. Air will be trapped in the chambers again. This is a temporary fix as the chambers will waterlog again within months — fitting a proper water hammer arrestor is the permanent solution.
If water hammer only occurs when the central heating boiler fires up, the cause is a different mechanism — check valve chatter or thermal expansion in the heating circuit — rather than a tap issue. Similarly, if the knocking occurs continuously while a tap is open (rather than at the moment of closing), this suggests a different fault: loose internal washers, worn cartridge ceramics vibrating in the flow, or a phenomenon called "singing" pipes caused by excessive velocity in undersized pipework. Replacing the cartridge in the offending tap usually resolves the latter.
Not confident doing this yourself?
Post the job and we'll match you with vetted local engineers. Free, no obligation.
🛠 Tools & materials you may need
🔗 Some links above are affiliate links — if you buy through them we earn a small commission at no extra cost to you. This keeps the site free.
Was this guide helpful?
Frequently asked questions
Is water hammer dangerous, or is it just annoying?
It's genuinely worth taking seriously. Whilst the noise itself is harmless, the pressure spikes that cause it can fatigue pipe joints, crack solder seals, and work clips loose over months or years. The real danger is a slow leak developing inside a wall or under your kitchen units where you won't see it until it causes significant damage.
Why does my dishwasher or washing machine cause more water hammer than my taps?
Washing machines and dishwashers have solenoid inlet valves that close extremely rapidly — far faster than you can close a manual tap by hand. This sudden stop creates a much sharper pressure spike, which is why appliances are often the main culprits and why a water hammer arrestor fitted near the appliance valve is so effective.
Can water hammer fix itself, or does it always need treatment?
It won't fix itself — in fact, it typically gets worse as pipe clips work loose and joints gradually fatigue. Even if the noise seems to come and go, the underlying damage is still happening, so it's best to address the cause rather than ignore it.
My house is old and has capped pipes near the taps — what are those?
Those are almost certainly old air chambers from when the house was built, designed to absorb pressure shocks naturally. Over time they fill with water and stop working, which is why older properties often develop water hammer. You can try recharging them by draining the system, but fitting a modern water hammer arrestor is the reliable permanent fix.
How much does it cost to fix water hammer?
If it's high mains pressure, a pressure reducing valve costs £25–60 fitted yourself, or £100–150 for a plumber to install. Additional pipe clips are pennies each. A water hammer arrestor costs £15–30 to buy. If you need a plumber to diagnose and fix the issue, expect a call-out fee of £80–150 plus parts, depending on your area and what's needed.