A sharp bang or thud right when a tap, washing machine valve, or toilet fill valve shuts off is water hammer — a pressure shockwave caused by fast-moving water being stopped abruptly. It has a specific mechanical cause and a specific fix, and it is worth addressing because repeated hammering stresses pipe joints and fittings over time.
Why it happens
Water moving through a pipe has momentum. When a valve — a tap, a washing machine solenoid valve, a toilet fill valve — snaps shut quickly, that moving column of water has nowhere to go and slams to a stop almost instantly. The shockwave from that sudden stop travels back through the pipework and shows up as a bang, sometimes felt as a shudder in the walls.
Common causes
| Cause | How likely | What to look for |
|---|---|---|
| Missing or waterlogged air chamber / arrestor | Very common | Older homes without modern hammer arrestors fitted |
| Fast-closing appliance valves (washing machines, dishwashers) | Very common | Bang timed exactly with the appliance’s fill cycle ending |
| Loose pipe straps allowing pipes to shift | Common | Banging paired with a knocking or rattling sound, not just a single thud |
| High water pressure | Common | Hammer happens throughout the house, at multiple fixtures |
| Long, unsupported pipe runs | Occasional | Older or larger homes with long straight pipe sections |
How to fix it
- Check your water pressure
If you have a pressure gauge or can borrow one, check it against a tap outside. Household pressure above about 80 psi makes hammer more likely and is worth having a plumber address regardless.
- Locate air chambers
Older homes often have short vertical sections of capped pipe near fixtures, designed to cushion pressure shocks with a pocket of air. Over time these can fill completely with water and stop working.
- Drain the system to recharge air chambers
Turn off the main water supply, then open the highest and lowest taps in the house to drain the system fully. This lets air chambers refill with air as the water drains out. Close the taps and turn the supply back on.
- Check and tighten loose pipe straps
Where pipes are accessible — in a basement or crawl space — look for straps that have come loose, allowing the pipe to move slightly when hammer occurs.
- Consider a mechanical hammer arrestor
These are inexpensive devices that install at problem fixtures (very commonly at washing machine valves) and absorb the shock with a sealed air cushion or spring mechanism — no draining required, unlike a traditional air chamber.
When to call a plumber
If draining the system does not resolve it, if pressure is confirmed high and needs a pressure-reducing valve fitted, or if the hammer is severe enough that you are concerned about strain on fittings and joints, have a plumber assess and fit proper hammer arrestors at the source.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
