The banging sound in your walls when you close a tap quickly is called water hammer. It is not just annoying - over time it damages fittings and joints.
What causes water hammer
Water has mass and momentum. When a tap is closed quickly, that momentum has to go somewhere - it transfers into the pipe as a pressure wave that travels back through the system. This wave is what causes the banging sound. Fast-acting solenoid valves in washing machines and dishwashers are particularly aggressive causes because they close almost instantly.
Why it matters beyond the noise
Repeated pressure waves stress joints, fittings and the pipe itself. Over time, hammer can loosen compression fittings, crack older soldered joints and eventually cause small leaks. In older galvanised systems, it accelerates corrosion at joints. It can also damage the solenoid valves that cause it.
How to fix it
A plumber installs a water hammer arrestor - a device with a sealed air chamber that absorbs the pressure wave. These are installed close to the offending valve, typically at the washing machine or dishwasher connection. In some cases, reducing the supply pressure at the main reduces the severity. Loose pipes rattling against the wall structure also need to be re-secured with proper pipe clips.
If it started suddenly
If water hammer develops suddenly in a house that didn't have it before, a check valve or air chamber may have waterlogged and stopped working. Water hammer arrestors have a finite service life and may need replacement. Sudden hammer can also indicate a pressure surge from the municipal supply.
Frequently Asked Questions
Is water hammer dangerous?
Occasional light hammer is unlikely to cause immediate damage. Severe, frequent hammer damages fittings over time and should be fixed.
Can I fix water hammer myself?
Tightening loose pipe clips is something you can do. Installing a hammer arrestor or adjusting supply pressure needs a plumber with the right fittings and pressure gauge.