Why Do My Pipes Bang or Knock When I Turn Off the Water?

A loud bang when a faucet, toilet, or washing machine shuts off is usually water hammer, a pressure surge from water stopping suddenly, and it is often fixed with a water hammer arrestor or by lowering high water pressure. Softer knocking or ticking as hot water runs is usually pipes expanding against loose straps or framing, which is more annoying than harmful.

Match the noise to the cause

• One loud bang when water shuts off. That is classic water hammer. Fast-closing valves in washing machines, dishwashers, and ice makers are the usual triggers. Water moving through the pipe stops all at once and the surge slams the pipe.

• Banging at almost every fixture. Suspect high water pressure. Test it with a gauge on an outdoor faucet. High pressure makes every surge bigger.

• Rattling while water runs. A pipe is loose and moving. It may be missing a strap, or the strap may have worked loose where the pipe passes through framing.

• Ticking or creaking with hot water. Pipes expand as hot water runs and shrink as they cool. Where they rub against wood or a tight hole, they tick. It is usually harmless.

• Knocking and sputtering after plumbing work. Air got into the lines while the water was off. It usually clears after the lines are flushed.

Fixes you can do yourself

• Check your water pressure. A $10-$15 gauge on an outdoor faucet takes a minute. Roughly 40 to 80 psi is a commonly cited normal range. If you read above about 80, the fix is lowering pressure, not just quieting the pipes. See low water pressure for more on reading the gauge.

• Add arrestors at the washing machine. If you have a washing machine causing the bang, screw-on water hammer arrestors fit between the valves and the hoses. They cost about $15-$40 each and take minutes to install.

• Secure loose pipes. If you can reach the noisy pipe in a basement, crawl space, or utility room, add pipe straps and slip on foam or rubber cushions where it touches wood. Use straps that match the pipe material.

• Flush air out of the lines. Open every faucet in the house, starting with the one closest to the main and ending with the farthest, and run them until the water flows smoothly. Then close them in the reverse order.

• Recharge old air chambers (in older homes). Shut off the main, open the highest and lowest faucets to drain the system, then close them and turn the water back on. This lets air back into old air chambers. If the banging returns quickly, arrestors are the better long-term fix.

When high pressure is the real problem

If your pressure tests high, quieting the pipes with arrestors treats the symptom. The fix is a pressure reducing valve, or a new one if your home already has a valve that has failed. A plumber can install or replace one and set it to a reasonable pressure, often somewhere in the 50 to 70 psi range. If your home has a pressure reducing valve or check valve, ask about an expansion tank at the water heater too, since closed systems need somewhere for heated water to expand.