Symptoms
- Loud bang or thud right when the tank finishes filling
- Pipes vibrate or rattle briefly after the sound
- Happens consistently with every flush
- May also happen with other fixtures shutting off quickly
Likely causes
- CommonFill valve closing too abruptlyModern fill valves can shut off very quickly, sending a pressure shock through the pipe.
- CommonMissing or failed water hammer arrestorAir chambers or arrestors that absorb this shock can be missing, waterlogged, or worn out.
- PossibleHigh water pressureExcess pressure makes any sudden valve closure more forceful and noisy.
- PossibleLoose pipes not properly strappedUnsecured pipes can bang against framing when a shock wave passes through.
What to do
- Confirm the timing of the bang lines up with the fill valve shutting off, not the initial flush.
- Check household water pressure with a hose-end gauge; above 80 psi is a likely contributor.
- Install a water hammer arrestor on the toilet's supply line, which is a small device that screws in between the shutoff valve and the supply line.
- If pressure is high house-wide, consider a pressure-reducing valve at the main line as a broader fix.
- Check for loose pipes in accessible areas like a basement and secure them with pipe straps if they're rattling.
Tools and parts
- Adjustable wrench
- Pressure gauge
Water hammer arrestorSearch: “toilet supply line water hammer arrestor”
When to call a pro
Call a plumber if a house-wide pressure-reducing valve is needed, or if banging continues after installing an arrestor.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.