Sewer Smell in the House: Find the Source

A sewage or rotten-egg smell usually means sewer gas is getting past a drain trap, a seal or a vent somewhere in the house. A lot of the time the fix is simple, like a basement floor drain that has dried out. Sometimes it points to a cracked pipe or a problem in the sewer line. Here's how to track it down, what's safe to check yourself and when it's time for a plumber. LocalRepairLine is a referral service, not a contractor, and this is general information, not a substitute for an in-person inspection.

Where the smell usually comes from

• A dry drain trap. Every drain has a U-shaped trap that holds water and blocks gas. In a guest bathroom, an unused shower or a basement floor drain, that water evaporates.
• A failing toilet seal. The wax ring under the toilet can let gas through. You'll often smell it right at the base.
• A blocked plumbing vent. Leaves, ice or a nest in the roof vent can push gas back into the house. Drains may gurgle too.
• The washer drain. A loose drain hose or a standpipe without a trap is a common laundry-room source.
• A cracked pipe or loose fitting. Behind a wall or in the basement, where you can't see it.
• A sewer line problem. If several drains are slow or gurgle, the issue may be past the house.
• Gunk in one sink. If it only smells at one sink, it's often buildup in the drain or the overflow hole.

Safe checks to run first

1. Find where it's strongest. Walk room by room and sniff near each drain, the toilets and the laundry area.
2. Refill the drains you rarely use. Run water for a minute in every unused sink, tub and shower, and pour a bucket into any floor drain. If the smell fades within an hour or two, a dry trap was the cause.
3. Check the toilets. Does one rock when you sit on it? Is there water or staining around the base? Either one points to the seal.
4. Clean the sink that smells. Scrub the drain opening and the overflow hole with a small brush and hot, soapy water.
5. Listen when water drains. Gurgling from a tub or sink when the toilet flushes or the washer empties suggests a vent or sewer line issue.
6. Look in the basement and yard. Damp spots, a sewage smell near the cleanout or soggy patches in the yard are worth noting.

What not to do

• Do not mix drain cleaners, or mix any drain cleaner with bleach. The fumes can be dangerous.
• Do not climb onto the roof to clear the vent in wet or icy weather.
• Do not keep running water if drains are backing up. Stop using water until it's checked.
• Do not just cover the smell with air freshener and wait. The source is usually still there.

When to call a plumber

• The smell comes back after you've refilled the traps.
• Several drains are slow or gurgling at the same time.
• Sewage backs up into a tub, shower or floor drain.
• A toilet rocks or leaks at the base. If it also keeps running, see our guide on a toilet that keeps running.
• You smell it inside a wall or ceiling, or there are wet spots in the yard.

What to tell the plumber

• Where the smell is strongest and when it started.
• Whether it's constant or shows up after showers, laundry or heavy rain.
• Any slow drains, gurgling or backups.
• What you already tried, and whether the house is on city sewer or a septic tank.

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