I did bathroom reno and replaced bathtub and shower faucets locations and extended copper pipes a little. That's not the first time I weld copper pipes and I didn't have any problems in past. What I noticed Is that when I turn off the water (cold or hot) I hear knocking like pipes knock on something. I examined all the places where I changed the plumbing and everything looks secure and doesn't touch anything. But I feel that knocking inside the pipes. It's like a pressure issue inside.

Probably it was before my work. I recently bought the house but it annoys me. Should I be concerned about it and can that knocking damage my pluming?

  • It's called "water hammer" and you can buy arrestors that can be installed to dampen it. Commented Sep 13, 2021 at 20:51
  • 2
    So the old faucets required several turns to close and the new ones are 1/2 or 1/4 turn. Water hammer is due to the inertia forces generated by the moving fluid being forced to stop quickly - so just turn the faucets slower… cheaper that buying and fitting arrestors.
    – Solar Mike
    Commented Sep 13, 2021 at 21:00
  • 2
    Sounds like the walls are still open, so arrestors should be a good option in this case.
    – JPhi1618
    Commented Sep 14, 2021 at 15:08

1 Answer 1


That's called "water hammer" and comes about when a valve closes quickly and the water running through the pipe kind of "bounces" back in the opposite direction. That creates a pressure wave that vibrates or bounces the pipes and you get an audible sound when the "dancing pipes" bang against stuff.

It usually doesn't do harm to the plumbing unless it's really strong or especially frequent. In which cases it can cause leaks and so forth in the pipes.

It's worth fixing because it's "fingernails on a chalkboard" annoying.

Plumbers install air chambers above where water lines for sinks, bathtubs, etc. are roughed in. This serves to provide a shock absorber for the "bouncing water".

If the walls are still open you could add this fairly easily.

Installation of air chamber in water line - credit: Gene Hayes

The easiest way to "fix" it if the walls are closed up is to install water hammer arrestors at the point where your faucets attach to the supply lines or elsewhere in the supply lines. They consist of a cylinder with a piston kind of thing that absorbs the pressure wave when the water is redirected back on itself.

Cutaway drawing of water hammer arrestor

Illustration of water hammer arrestor (From Family Handyman - via Pinterest)

  • Potentially dumb question... If the water supply is pressurized (and it is or it wouldn't flow up hill), how does it not fill that 18" air chamber? Won't the pressure simply push the water any and every where it possibly can until it hits a closed valve? I understand the hammer arrestor because there's air pressure pushing back against the piston, but not just an extra piece of pipe. Oh, there's air trapped in there when it's built, so it acts like the arrestor, just without the piston, right?
    – FreeMan
    Commented Sep 14, 2021 at 18:03
  • Yeah, I think that's the gist of it. Not sure if the piston in the arrestor allows the air tube to be shorter...or what.
    – gnicko
    Commented Sep 14, 2021 at 20:09
  • FreeMan: When the 18" is installed, all the air that's in it is trapped, being lighter (ie. less dense) than water. You'd have to ask the chemistry or physics forum if the air will dissolve through the water over time. IDK.
    – Doug Null
    Commented Oct 22, 2022 at 21:49

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