Why Do My Pipes Bang When I Turn Off the Water? Water Hammer Explained

The fix depends on which of three noises you actually have, so start there.

Graphic: summary of this guide, made by our team from the cited sources.

Which noise is it?

What you hear	When	Likely cause	First fix
One loud bang or thud	The instant a valve closes (faucet, washer, dishwasher, toilet fill)	Water hammer	Check pressure, then add an arrestor at that valve
Rattling or knocking that continues	While the water is running	Loose or unsupported pipe	Add straps and cushion where the pipe touches framing
Ticking or creaking	As hot water starts or stops flowing	Hot pipe expanding and rubbing on supports	Add sleeves or cushioned clamps where pipes touch wood
Groaning or chattering	While water runs through one valve	Partly closed or worn valve	Open the valve fully, or replace the washer or cartridge

Oatey's guide to banging pipes lists the same family of causes: water hammer, loose pipes, high water pressure, sediment in a water heater, and partly closed valves. If the knocking comes from the water heater itself and not the walls, that's sediment, which is a different job.

Test 1: is your water pressure too high?

High pressure makes every hammer worse because the water is moving faster when the valve shuts. Oatey says pressure above 80 psi can make pipes vibrate and bang. The model residential code sets the same ceiling. IRC section P2903.3.1 says static pressure shall not be greater than 80 psi, and it requires a pressure-reducing valve where the main pressure is higher.

Checking takes five minutes. Screw a hose-thread pressure gauge onto an outdoor hose bib or the washer's cold valve, make sure nothing else is running, and open the valve. Take a second reading at a different time of day, because supply pressure is not constant.

Graphic: made by our team from IRC P2903.3.1 and Oatey's pressure guidance (linked above).

If you read over 80 psi, a pressure-reducing valve (PRV) is the real fix. If you already have one and the pressure is high anyway, it may be failing. It is normally installed on the main line near where water enters the house. High pressure can also make a water heater's relief valve weep, which our guide to a dripping water heater relief valve covers.

Test 2: find the valve that slams

Water hammer is loudest at quick-closing valves. These are valves that snap shut instead of turning slowly: the solenoid valves in washing machines, dishwashers and refrigerator ice makers, single-handle faucets slapped closed, and toilet fill valves. Run each one alone and listen.

• Bangs only when the washer or dishwasher fills or stops: a point-of-use arrestor at that appliance is the standard answer.
• Bangs when you close a single-handle faucet quickly: try closing it slowly. If that silences it, the faucet is fine and the line needs an arrestor or better support.
• Bangs after a toilet finishes filling: watch the fill valve. A worn valve that shuts abruptly, or one that sings while filling, is covered in our guide to a toilet fill valve that whistles.

The fix code calls for: a water hammer arrestor

The model codes require an arrestor wherever quick-closing valves are used. IRC section P2903.5 says a water-hammer arrestor shall be installed where quick-closing valves are utilized, installed per the manufacturer's instructions, and conforming to ASSE 1010. ASSE 1010 is the product standard, so look for that mark on the package.

Inside, an engineered arrestor has a sealed cushion of air or gas kept separate from the water by a piston, a bellows or a diaphragm. When the valve closes, Sioux Chief explains, the moving water pushes up into the arrestor and compresses that charge until the momentum dies away, so the pressure spike never builds. Sioux Chief calls the piston style the most widely used. For homes, its guideline is any ASSE 1010 AA size arrestor on both the hot and cold lines at ice makers, dishwashers, washing machines and tub or shower valves.

At a washing machine, this is a DIY job. Oatey's steps: turn off the supply valves and disconnect the hoses, check the hoses and fittings for damage, thread the arrestor onto the shutoff valve by hand and snug it with a wrench without overtightening, then reconnect the hose and turn the water back on. Do both hot and cold. In-wall arrestors on soldered copper or PEX branch lines are a plumber's job.

What about the old air chambers?

Many older houses have short capped pipes standing up behind the fixtures, meant to trap a pocket of air as a cushion. Oatey notes that, unlike modern piston arrestors, these traditional air chambers can lose their air and fill with water. Once they do, the banging returns. Sioux Chief's white paper quotes the National Plumbing Code of Canada, which says air chambers made from a piece of vertical pipe don't provide acceptable protection. Because an open chamber loses its air, a sealed ASSE 1010 arrestor at the slamming valve is the lasting fix.

Loose pipes: when the bang keeps rattling

If the noise goes on while water flows, or a single hammer turns into a series of knocks, the pipe is moving. Oatey says pipes should be supported at least every 16 inches and at every change in direction. Push gently on exposed runs in the basement or crawl space. Anything that moves easily needs a strap, a J-hook or a cushioned clamp. Where a pipe passes through a joist or stud, a foam or plastic sleeve stops it from knocking against the wood. Fastening loose runs won't stop the pressure spike, but it often makes the sound much quieter.

Why it is worth fixing, not just ignoring

Sioux Chief lists the parts that wear out early from uncontrolled water hammer: water heaters, temperature and pressure relief valves, pressure reducing valves, backflow preventers, faucets, solenoid valves, fittings and pipe hangers. In other words, the noise is the cheap symptom. The expensive part is the washer valve or water heater that fails a few years early. If the pipes also lose pressure at one tap, see our guide to low water pressure at one faucet.

When to call a plumber

• Your gauge reads over 80 psi and you have no PRV, or your existing PRV no longer holds pressure down.
• The bang comes from inside a finished wall with no exposed valve or pipe to work on.
• You added arrestors at the appliances and the hammer continues, which points to a branch line that needs a properly sized arrestor.
• You see a weeping joint or stain near where the pipe bangs.