A drain gurgles when air is being pushed or pulled through the water sitting in its trap. That happens when water can’t flow smoothly down the pipe and air has to find another way in or out. The two usual reasons are a partial clog somewhere in the drain line and a blocked or missing vent, the pipe that lets air into the drain system through your roof. If several fixtures gurgle at once, or a toilet gurgles when a sink drains, the problem is probably further down the main line.
Gurgling at the very end of a toilet flush is normal; that’s the siphon breaking. See how a toilet works.
Why drains need air
Every fixture has a trap, the U-shaped bend under a sink or built into a toilet, that holds a little water to block sewer gas. As water rushes down the drain, it needs air to follow behind it, the way a juice carton glugs if you pour it without a second hole. Plumbing systems get that air from vent pipes that run up through the roof. When the vent is blocked, or water can’t get past a clog, the moving water pulls air through the nearest trap instead, and you hear it gurgle.
The common causes
1. A partial clog in that fixture’s drain
If one sink or tub gurgles and drains slowly, the clog is probably close by: hair, soap scum or grease in the trap or the pipe just past it. Cleaning the stopper, using a drain snake, or removing and cleaning the P-trap under a sink usually fixes it.
2. A blocked vent
Leaves, birds’ nests, snow or ice can block the vent opening on the roof. Signs include gurgling in several fixtures, slow drains with no obvious clog, and sometimes a sewer smell, because air pulled through the traps can siphon them dry. Clearing a roof vent means working on the roof, so it’s usually best left to a plumber.
3. A clog in the main line
If the toilet gurgles when you run the washing machine or a sink, or several drains on the lowest floor are slow, the blockage may be in the main drain or sewer line. Tree roots, grease buildup and collapsed pipe are common culprits. A plumber can snake the line or inspect it with a camera.
4. A full septic tank or failing drainfield
In homes with a septic system, gurgling throughout the house can be an early warning. The U.S. Environmental Protection Agency notes that an overloaded drainfield can cause sewage to back up into toilets and sinks, and recommends having household tanks pumped typically every three to five years.
5. A missing or improper vent
After a remodel, a fixture sometimes ends up without proper venting. Constant gurgling at one newly added fixture may mean it needs a vent or an approved air admittance valve.
How to narrow it down
| What you notice | Most likely cause |
|---|---|
| One fixture gurgles and drains slowly | Clog in that fixture’s drain |
| Several fixtures gurgle, drains slow, no single clog | Blocked roof vent |
| Toilet gurgles when the washer or a sink drains | Clog in a shared branch or the main line |
| Lowest drains back up; gurgling everywhere | Main sewer line or septic problem |
| Sewer smell along with gurgling | Traps being siphoned dry by poor venting |
What you can do yourself vs. when to call a pro
| Safe for most homeowners | Call a plumber |
|---|---|
| Clean stoppers and sink traps | Several drains gurgling or backing up |
| Use a plunger or hand snake on one drain | Clearing roof vents |
| Run water in rarely used drains to refill traps | Main line snaking or camera inspection |
| Keep grease out of the kitchen drain | Septic pumping and inspection |
Safety first. Sewage backups carry bacteria, so wear gloves and clean affected areas thoroughly. Don’t mix drain cleaners, and don’t use them before plunging or snaking, since splashback can cause burns. Leave roof work to a professional. A strong, persistent sewer smell should be checked promptly.
Frequently asked questions
Is a gurgling drain an emergency?
Not usually, but it’s an early warning. A gurgle today can turn into a backup later, especially if several fixtures are involved.
Why does my toilet gurgle when I run the washing machine?
The washer dumps a lot of water quickly. If the shared drain or main line is partly blocked, or the vent is restricted, that surge pulls air through the toilet’s trap.
Can I fix a blocked vent from inside the house?
Generally no. Vents are cleared from the roof opening. If you’re not comfortable working safely on a roof, call a plumber.
Sources
- U.S. Environmental Protection Agency, How Your Septic System Works
- U.S. Environmental Protection Agency, How to Care for Your Septic System