What water hammer is
When a fast-closing valve stops water suddenly, the moving water slams against it and sends a shock wave through the pipes. You hear it as a bang or knock, often behind walls or under the floor.
Common causes
In most homes, one or more of these is to blame:
- Water pressure above about 80 psi
- Quick-closing valves in washing machines, dishwashers and ice makers
- Pipes that aren't secured and move when water stops
- Old air chambers that have filled with water
Quick checks
Test pressure with a hose-bib gauge. Note which fixture causes the bang; if it's the washer, an arrestor on those connections usually fixes it.
How plumbers fix it
Fixes include water hammer arrestors at the offending fixture, a pressure-reducing valve at the main, and re-strapping loose pipes in crawl spaces or basements.
Why it's worth fixing
Repeated shocks stress joints, fittings and appliance valves, which can lead to leaks over time.
Why Do My Pipes Bang When Water Shuts Off: when to call a pro
- Weak shower or faucet flow
- Pressure drops when another fixture runs
- Pressure dropped suddenly
- Banging pipes (pressure too high is also a problem)
Why Do My Pipes Bang When Water Shuts Off: common questions
Is water hammer dangerous?
Not immediately, but repeated shocks can loosen joints and damage appliance valves.
Why did the banging start suddenly?
A failing pressure-reducing valve or a new appliance with fast-closing valves are common causes.
Every home differs; editorial standards.