Why Does My Drain Gurgle When I Have the Water Running?
A gurgle is air moving through water where it should not be. Something downstream is slowing the flow, or something upstairs is stopping air getting in. Where you hear it, and what else is running at the time, tells you which.
The third cause is the vent: the pipe that lets air into the drainage system through the roof. If it is blocked by a nest or leaves, water leaving one fixture sucks air through the trap of another, which gurgles and then lets sewer smell in. That one is fixed at the roof, not at the drain.
What the Gurgle Is
Every fixture drains through a trap, the U-bend that holds a plug of water to keep sewer gas out of the house. Draining water pushes air ahead of it and pulls air behind it. When that air cannot move freely, it bubbles through the nearest trap. That is the sound.
Air Pushed Back
A partial blockage downstream narrows the pipe. Water arriving faster than it can pass compresses the air in front of it, and that air escapes back up through the trap as bubbles.
Air Pulled Through
If the drainage vent is blocked, water leaving one fixture creates suction behind it. The easiest place to pull air from is the trap of the next fixture, so that trap gurgles and can be sucked partly dry.
Why It Matters
A dry or partly dry trap is an open door for sewer gas. A gurgle that arrives with a smell is the drain telling you a trap seal has been broken.
Which Drain Gurgles Tells You Where the Problem Is
Run the fixtures one at a time and listen. The pattern is the diagnosis.
| What you hear | Where the problem is | What to do |
|---|---|---|
| One sink or shower gurgles as it drains, and nothing else is affected | That fixture's waste or trap | Clean the trap or pull the hair; plunge; hot soapy water |
| Basin and shower in the same bathroom both gurgle | The branch line they share, past the traps | Plunge one while the other is blocked off; if it returns, the line needs a machine |
| Toilet gurgles or the water level moves when the shower or washing machine runs | The main sewer line, or the vent | Stop using water and call: this is not a plunger job |
| Gurgle plus a sewer smell, especially after a bath drains | Blocked or undersized vent siphoning the traps | Check the roof vent for a nest or leaves from the ground; a plumber clears it from the roof |
| Gurgle from the floor waste or the outside gully, or water rising there | Main sewer line, usually roots or a collapsed section | Stop using water; camera inspection and jet blasting |
What You Can Do Yourself
For a single gurgling fixture, these clear most cases. Stop at step four if the gurgle is in a different fixture from the one draining.
- Shower or bath: pull the grate and hook out the hair and soap with a bent wire or a plastic hair hook. Most shower gurgles end here.
- Sink: put a bucket under the trap, unscrew it, clean it out, refit it hand-tight and run the tap to check for drips.
- Plunge with the overflow or the second bowl blocked by a wet cloth, so the pressure goes down the pipe and not out the other opening.
- Follow with a jug of hot soapy water. Skip caustic drain cleaners: they rarely reach the blockage and they burn the next person who opens the pipe.
- If the gurgle is in the toilet or another room while this fixture drains, stop here. Nothing from the supermarket reaches the sewer line, and a chemical dose makes the plumber's job dangerous.
When It Is the Vent
The vent is the pipe that pokes through the roof above the bathroom or kitchen. Its job is to let air in so water can leave without sucking the traps dry.
Signs
Gurgling in a fixture that is not draining, slow draining everywhere at once with no overflow outside, and a sewer smell that comes and goes, worst after a large volume of water leaves (a bath, the washing machine).
Causes
Birds nest in the top of the pipe, leaves and tennis balls find their way in, and a vent cowl can crush or rust closed. Older houses sometimes have vents that were never adequate for the fixtures added since.
The Fix
Clearing the vent from the roof, with a rod or a hose, and fitting a cowl that keeps birds out. This is roof work and a licensed plumber does it with fall protection. Do not go up a ladder to look.
When It Is the Sewer Line
The main line runs from the house to the street. When it is restricted, every fixture is affected, and the lowest one shows it first.
Tree Roots
The most common cause in Sydney's older suburbs. Roots enter at pipe joints, catch paper and waste, and the line narrows over months. The gurgle starts weeks before the overflow.
Broken or Bellied Pipe
A cracked, displaced or sagging section holds water and waste. Clearing it helps for a while; the camera shows whether it needs repair or relining.
How It Is Fixed
Jet blasting clears the line, then a camera shows why it blocked. If it is roots in a sound pipe, cutting them and clearing keeps it open; if the pipe is damaged, relining or a section repair is quoted as a fixed price before any work starts.
Gurgling Drain Questions, Answered
Why does my drain gurgle when water runs?
Air is moving through the water in a trap. Either a partial blockage downstream is pushing air back, or a blocked vent is pulling air through another fixture's trap. A single fixture gurgling on its own is usually its own waste or trap; a fixture gurgling while another is used points to the shared line, the vent or the main sewer.
Is a gurgling drain serious?
A single fixture gurgling is a minor blockage you can often clear. A toilet that gurgles when the shower or washing machine runs is the main sewer line restricting, which gets worse until it overflows, so it is worth acting on before that happens.
Why does the toilet gurgle when the shower runs?
The shower water is entering the main line and finding it restricted, and the air it displaces is escaping through the toilet's trap, the lowest and largest water seal in the house. It is the classic sign of a blocked sewer line or a blocked vent.
Can a blocked vent cause gurgling?
Yes. The vent lets air into the drainage system so water can leave without suction. If a nest or leaves block it, draining water sucks air through the nearest trap, which gurgles and can be partly emptied, letting sewer smell into the room.
Will drain cleaner stop the gurgling?
Rarely. Caustic cleaners do not reach a sewer line or a vent, and they make the pipe dangerous for the person who opens it next. Pulling hair from a shower waste, cleaning a sink trap or plunging works for a local blockage; anything further needs a drain machine.
How do plumbers fix a gurgling drain?
By finding which of the three causes it is: clearing the fixture's waste or branch with a machine, clearing the vent from the roof, or jet blasting the sewer line and putting a camera down it to see whether roots or a damaged section caused it. The repair is quoted as a fixed price once the cause is on screen.
Related: Blocked drains · Blocked sewers · How to unblock a drain · Smelly drains · Blocked drain cost
