How drains are supposed to work
Every fixture has a trap, the U-shaped bend that holds a plug of water to keep sewer gas out of the house. Drain pipes also connect to vent pipes that run up through the roof and let air into the system, so water can flow without sucking the traps dry.
When a drain gurgles, air is being pulled through a trap because the water cannot move freely or the vent is not supplying air. When a room smells like sewer gas, something is letting that gas past a trap or out of a pipe.
Symptom and likely cause
| What you notice | Likely cause | Next step |
|---|---|---|
| Smell from a guest bath or unused floor drain | Dry trap | Run water for a minute to refill it |
| Toilet gurgles when the washer or tub drains | Partial main line or branch clog, or venting problem | Have the drain checked before it fully backs up |
| Sink glugs and drains slowly | Clog in its trap or branch, or a blocked vent | Clean the trap, then call if no change |
| Smell near the outdoor cleanout | Loose or missing cap, or a backup in the line | Check that the cap is in place; call if the line is full |
| Smell around the base of a toilet | Failed wax ring or seal | The toilet may need to be reset |
| Persistent indoor smell with no obvious source | Cracked drain or vent pipe, or a dry trap you have not found | A plumber may use a smoke test or camera |
Things to check yourself
When to call
- IfThe smell stops after refilling a trapThenNo further action needed. Run that fixture occasionally.
- IfGurgling comes with slow drains throughout the houseThenCall soon. It is likely a developing main line blockage.
- IfGurgling happens but drains are fastThenA vent may be partly blocked. A plumber can check the vent from the roof or with a camera.
- IfSmell persists and you cannot find a sourceThenAsk about a smoke test, which shows where gas escapes from the drain-waste-vent system.
Terms
- P-trap
- The curved pipe under a fixture that holds water to block sewer gas.
- Vent stack
- A pipe that runs through the roof and lets air into the drain system.
- Smoke test
- Harmless smoke blown into the drains to reveal where gas is escaping.
- Wax ring
- The seal between the toilet and the floor drain flange.
Common questions
Is sewer gas dangerous?
At typical household levels it is mainly unpleasant, but a strong or persistent smell should be found and fixed. If you feel unwell or are unsure whether it is natural gas, leave and call for help from outside.
Can a roof vent really get blocked?
Yes. Leaves, debris and occasionally nests can restrict a vent opening, which causes slow drains and gurgling even when the drain line is clear.
Why do I only smell sewer gas when it is windy?
Wind across the roof vent can pull or push air in the system and siphon a weak trap. A trap that is low on water or a venting issue may be involved.
General information for Anaheim, not a quote or professional advice. In an emergency, call 911.