Short answer
Why do pipes bang when a tap shuts? The water is moving fast, and it is stopped or slowed suddenly. This causes a pressure shock that sounds like a hammer blow. The handbook SP 35 names quick-closing and self-closing taps and valves as common causes. A plumber can check the taps, secure the pipes and look at the air chamber or pressure reducing valve.
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A pipe that bangs when a tap shuts is one of the most common complaints in a house. The bang is called water hammer. In practice, it is usually not a fault in the pipe itself. It is a pressure shock made when moving water is stopped or slowed suddenly. This page explains what causes it, what the Indian handbook SP 35 and the water supply code IS 2065 say about it, and what a plumber can check. It gives no pressure figures, no pipe speeds, no sizes and no brands.
Why does a pipe bang?
SP 35 says the pressure rise is “manifest as a series of shocks sounding like hammer blows” (clause 4.9.1). The shock does not need the water to stop completely. Any sudden change in its speed can create it, to a greater or lesser degree.
The handbook names the usual causes in clause 4.9.2: “the sudden closing of valves or taps particularly of the automatic self-closing type and the quick-closing types”, and “the sudden stoppage of a pump”. So the tap that closes fast is a common culprit. A pump that stops suddenly can also cause the shock.
Clause 4.9.1 says the strength of the shock varies directly with the speed of the water in the pipe. Water that moves faster gives a harder bang.
What do the standards say about fixes?
SP 35 clause 4.9.2 gives the other methods of avoiding water hammer:
- Slow closing valves and taps. The handbook gives screw-down types as the example.
- A pressure reducing valve on the supply line to the source of the hammer. The handbook says this reduces the speed of the water in the pipe.
- An air chamber. The handbook says an air chamber may control water hammer. It also says that provision should be made for renewing the air in the chamber.
| Cause | What the standard names | Who should act |
|---|---|---|
| Quick-closing or self-closing tap or valve | Slow-closing valves and taps, such as screw-down types (SP 35 cl 4.9.2) | A plumber |
| Sudden stop of a pump | An air chamber or pressure reducing valve, listed as general methods in SP 35 cl 4.9.2 (not specific to pumps) | A plumber |
| Pipe loose on the wall (practice) | Not named in the standards read; a clamp or cushioned support is practice | A plumber or mason |
| Shock from the street main | An air chamber on the service pipe as it enters the building (SP 35 cl 4.9.2) | A plumber, with the water supplier for the main |
Clause 4.9.2 also gives a point about the street main. If the hammer starts in the water main in the street, the house plumbing may be protected by an air chamber on the service pipe as it enters the building. This is a building-owner question for a plumber or engineer. It is not a job on the street main, which belongs to the water supplier.
For new work, IS 2065 clause 14.3 says: “When the service line is complete, it shall be slowly and carefully charged with water, allowing all air to escape and avoiding all shock or water hammer.” That is a testing and filling rule for the plumber. Ask that it is followed when a new pipe is filled.
What can I check at home?
These steps are practice, not standard clauses. Do them with care.
- Note which tap or valve bangs. Run it and shut it several times. If the bang comes every time one tap shuts, that tap is the first suspect.
- Look at the pipe. A loose pipe that is not clipped to the wall can bang against the wall when the shock passes. Secure it with a clamp or a cushioned support.
- Check the tap. A worn tap can shut very abruptly. A plumber can fit a slow-closing tap in its place.
- Ask about the air chamber. An open air chamber can fill with water and stop working. Ask the plumber whether the house has one, and how it is renewed.
- Ask about a pressure reducing valve. If the supply pressure is high, a plumber can check whether a reducing valve is needed on the supply line.
- Consider a sealed arrestor. Sealed arrestors are sold for this purpose. The standards read here do not name or size them, so ask the plumber which one suits the pipe.
What mistakes should I avoid?
These are practice points, not standard clauses.
- Ignoring a bang that grows louder or comes every time. Ask a plumber to look at it, especially if a fitting feels loose.
- Working on the street main. SP 35 treats the main as the source of the hammer, and the air chamber point is for the building side only.
- Fixing a pipe by pulling it out of its clips. A pipe pulled out of its clips can leak at the joint.
- Relying on draining the house to recharge the air chamber. That is a homeowner’s report, not a standard fix.
Frequently asked questions
Is water hammer dangerous?
SP 35 clause 4.9.1 says the shock “may have sufficient magnitude to rupture the pipe or damage connected equipment”. The standards read give no numeric damage limit. In practice, a quiet bang from one quick tap is often a nuisance, while repeated strong banging should be checked by a plumber.
Do I need an arrestor?
SP 35 names air chambers and pressure reducing valves. Sealed arrestors are a practice choice; ask the plumber which one suits the pipe.
What pressure should my house pipe have?
This page gives no pressure figure. Ask the plumber to measure the supply pressure and say whether it is high.
Related guides
- Taps and valves in house plumbing: bib tap, pillar tap, angle valve, stop cock and gate valve
- Low water pressure on the upper floor or in the shower: causes and fixes
- Dripping tap: why taps leak, washer, seat, gland and what fixes each
This guide is general information. Read the disclaimer. A licensed plumber decides the fix for your house.
Sources
General information only. It is not a structural design or professional advice. Ask a licensed structural engineer or architect to check your plans and site.