Troubleshooting
Why do my drains smell?
Updated · Editorial team; not yet technically reviewed
Short answer
Most drain smells come from a trap that has lost its water seal, usually a floor drain, guest shower or basement sink that isn't used, so sewer gas rises into the room. Other causes are buildup in the drain itself, a blocked vent that siphons traps dry, a failed toilet wax ring, or a cracked drain pipe. Pouring water into unused drains fixes the most common one.
How the trap keeps gas out
Every fixture has a U-shaped trap that holds a few inches of water. That water plug blocks sewer gas. Evaporation, a missing vent or a crack can remove it.
Find the source
| Where it smells | Likely cause | Try |
|---|---|---|
| Basement floor drain or unused shower | Dry trap | Pour a bucket of water in; add a spoon of mineral oil to slow evaporation |
| Kitchen sink, musty or rotten | Food and grease buildup | Clean the trap and disposal |
| Around a toilet base | Failed wax ring | Reset the toilet with a new ring |
| Whole house, worse in wind | Vent problem | Have vents checked |
| Strong, persistent, with damp spots | Cracked drain pipe | Smoke or camera test |
When it isn't sewer gas
A rotten-egg smell near a gas appliance or stove could be natural gas. Leave the house and call the gas utility from outside. For hot water that smells like sulfur, see rotten egg smell in hot water.
What not to do about a drain smell
Masking a smell with bleach or air freshener hides the clue without fixing the cause. Bleach also doesn't clear the biofilm that builds up on drain walls; a stiff drain brush and hot water do more. Never mix drain cleaners with each other or with bleach, which can release toxic gases. If the smell is like rotten eggs and you have gas appliances, leave the house and call the gas utility; the smell may not be sewer gas.
Sewer gas is mostly methane and hydrogen sulfide. At household levels it usually causes headaches and nausea rather than serious harm, but a persistent smell means an open path into the house that should be closed. Common culprits a homeowner might miss include a dry floor drain, a loose toilet wax ring, a washing machine standpipe without a trap, and a vent pipe cracked in the attic. A smoke test finds the leak quickly when nothing visible explains it.
Common questions
Why does my basement floor drain smell?
Usually its trap has dried out; pour water in to restore the seal.
How do I stop a trap from drying out?
Run water in unused drains every few weeks, or add a little mineral oil on top of the water to slow evaporation.
Can a blocked vent cause sewer smells?
Yes; without air from the vent, draining water can siphon traps dry.
Why does my bathroom smell like sewage around the toilet?
A failed wax ring under the toilet can leak gas and water at the base.
Does mineral oil really keep a floor drain trap from drying?
A thin layer on the water slows evaporation in drains that are rarely used.
How do plumbers find a cracked drain causing smells?
With a smoke test or a camera inspection of the drain lines.
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