Why Are My Pipes Knocking? Water Hammer Explained
25 May 2026 · Plumbers On Duty
You shut off a tap or the washing machine stops filling, and there is a loud bang or knock in the walls. It is not ghosts, and it is not normal. It is called water hammer, and left unchecked it can damage your plumbing over time.
What is water hammer?
Water hammer is the banging or knocking noise that happens when fast-moving water is suddenly stopped, for example when a tap or a washing machine valve shuts off quickly. The moving water slams against the closed valve and the resulting shock wave travels through the pipes, making them knock against framing and fittings.
It commonly shows up when appliances like washing machines and dishwashers, which use fast-closing solenoid valves, shut off.
Why it matters
Beyond the annoying noise, repeated water hammer puts stress on pipe joints, fittings and valves. Over months and years that stress can loosen joints and contribute to leaks, turning a noise problem into a water-damage problem.
Common fixes
- Fit water-hammer arrestors. Small devices that absorb the shock wave, often installed near washing machines and dishwashers.
- Check and recharge air chambers. Many older homes have air chambers behind fixtures that can become waterlogged and stop cushioning the flow.
- Secure loose pipes. Pipes that are not properly clipped will bang against the structure. Securing them reduces noise.
- Reduce excessive water pressure. Very high mains pressure worsens water hammer. A plumber can fit or adjust a pressure-reducing valve.
Frequently asked questions
Is water hammer dangerous? It will not hurt you, but over time the repeated shock can loosen joints and fittings and contribute to leaks, so it is worth addressing.
Why do my pipes only bang when the washing machine stops? Appliances with fast-closing valves stop the water flow abruptly, which triggers the shock wave. Water-hammer arrestors are designed for exactly this.
Can I fix water hammer myself? Some fixes, like fitting arrestors at an appliance, are simple. Persistent or whole-house water hammer, especially from high pressure, is best assessed by a plumber.
Sources
Need a vetted plumber?
Every plumber on our directory is verifiable. Filter by city, service and rating, then call the one you trust. Browse plumbers near you.