What causes cold water pipes to knock?

Updated April 17, 2017

If you’ve ever been using cold or hot water and turned off the faucet suddenly, you may have heard a “knocking” sound somewhere in your pipes. This is not an uncommon problem and can be remedied with some simple do-it-yourself fixes.

What Happens when You Turn on the Faucet?

Water mains that deliver water to your home are carrying a lot of water that is under pressure. When you turn on the faucet in your home, the pressure of the water behind it is released, letting the water flow freely.

What Happens when You Turn off the Faucet?

As the water flows freely through your faucet, the pressure holding it is released. When you turn off that faucet, all that water flowing under pressure is stopped (imagine trying to stop the flow of a river). It has nowhere to go, except back into the water pipes.

Where Does the Sound come From?

When all that water under pressure flows back into the pipes, it can cause vibration, banging or knocking sounds in the pipes. The sound will be amplified in pipes that are not secured properly.

How to Fix It

The way to prevent your pipes from knocking when you turn off the faucet is to ensure they’re secure. You can install a water hammer arrester (follow directions on package or hire a plumber) that dissipates the noise caused by the returning water pressure. It’s also wise to install bracing and brackets to support and stabilise your pipes. These two tactics will remove the knocking sound from your pipes.

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About the Author

Andre Zollars started writing in 1999, when she worked in the editorial department at "The Missoulian." She has been published in "Endovascular Today," "High Country Angler," "Outside Bozeman" and "Western Ag Reporter." She also has written for online magazines New West, Hunting and Fishing USA. Zollars holds a Bachelor of Arts in international studies from the University of Washington.