Short answer: In a bathroom, check four things in this order: the shower or tub drain (dry or dirty trap), the sink overflow hole, the toilet seal at the floor, and the vent. If the smell comes and goes with flushing or with other fixtures draining, a venting problem is likely and a plumber should look at it.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Smell from the shower drain, guest bath | Dry trap or hair and soap buildup | Run water for a minute; clean the drain | Rarely |
| Smell from the sink | Dirty overflow channel or P-trap | Clean the overflow hole with a brush | Rarely |
| Smell at the base of the toilet | Failed wax ring or loose bolts | Gently rock the toilet; check for water at the base | Often |
| Smell after flushing or when the tub drains, gurgling | Vent problem siphoning traps | Listen for gurgling in other fixtures | Yes |
| Smell comes and goes | Trap slowly drying, or weather-related venting | Note when it happens | If it continues |
Step-by-step: find the source
- Run water in the shower, tub and sink for a minute each to refill the traps.
- Clean the sink overflow hole with a bottle brush and hot water; gunk in the overflow often smells like sewage.
- Sniff around the toilet base. If it rocks or the caulk is stained, the wax ring is a prime suspect.
- Flush the toilet and drain the tub while listening to the sink. Gurgling means air is being pulled through the trap, which points to a vent problem.
- Check the exhaust fan works; a bathroom with no ventilation holds odors longer.
The toilet wax ring
The wax ring seals the toilet to the drain flange. It can fail after the toilet is moved, if the bolts loosen, or when the flange is set too low, according to Oatey. Replacing it means shutting off the water, draining the toilet and lifting it. If the flange is cracked or the subfloor is soft, call a plumber.
Venting problems
A blocked or undersized vent lets a draining fixture pull the water out of other traps in the bathroom. The tell-tale signs are gurgling and a smell right after a flush or after the tub drains. Fixing it can involve the roof vent or adding an air admittance valve, which a plumber should handle.
When to call a professional
- The toilet rocks, the floor around it is soft, or water appears at the base.
- Fixtures gurgle when others drain.
- The smell continues after refilling traps and cleaning drains.
- Sewage backs up into the tub or shower.
What it usually costs
Cleaning drains and refilling traps is free. A plumber typically charges $45 to $150 per hour plus a call-out fee, and toilet repairs commonly run $100 to $300, according to HomeGuide (2026).
Frequently asked questions
Why does my bathroom smell like sewer only sometimes?
The smell that comes and goes is usually a trap that slowly dries out or a vent problem that depends on how much water is draining or on the weather.
Can a bathroom sink overflow smell like sewage?
Yes. Soap scum, hair and bacteria collect in the overflow channel and can smell like sewer gas. Cleaning it often solves the problem.
Related problems
Sources
- Oatey: Sewer gas odor in your house and what to do about it
- InspectAPedia: Drain piping odor FAQs
- HomeGuide: Plumber cost per hour (2026)
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.