What water hammer means
Water hammer is the bang in the pipes when a valve shuts quickly and the moving water stops all at once, sending a pressure wave down the line.
Where it matters on a plumbing job
Quick closing valves on washers, dishwashers and ice makers are the usual cause. An arrestor near the valve fixes it, and high house pressure makes it worse, so check pressure too.
What the model code says
- A water-hammer arrestor is required where quick-closing valves are used, and arrestors conform to ASSE 1010. (IRC 2021, Chapter 29)
Check which code edition your building department enforces for water hammer before you quote.
Work it out
- Licensing: Plumbing license rules by state
- Tools, templates and guides: Free plumbing resources
Sources: 2021 International Residential Code, Chapter 29, Water Supply and Distribution (ICC). Checked October 9, 2026.