Sewer Smell in Your Basement? Dry Floor Drains, P-Traps and What Else It Could Be

You head downstairs to switch the laundry and it hits you: a rotten, sewer-ish smell that wasn’t there last week. It’s one of the most common calls we get once the weather turns, and most of the time the fix costs nothing. This is for Des Moines-area homeowners who want to find the source, fix what they safely can, and know when it’s a real sewer problem that needs a camera.

Why does my basement suddenly smell like sewer?

Every drain in your house has a trap: the U-shaped bend under a sink (a P-trap) or the trap built into the bottom of a floor drain. That trap holds a few inches of water, and that little plug of water is the only thing keeping sewer gas in the pipe and out of your basement. When it evaporates, the seal is gone and the gas comes straight up.

Basement floor drains are the usual suspect because nothing ever goes down them. In older Des Moines homes, like the Beaverdale brick houses or the four-squares around Drake and Highland Park, that floor drain might not see water for months. Fall makes it worse: once the furnace starts running, the air dries out and traps evaporate faster.

How do I fix a dried-out floor drain?

1. Lift the grate if it comes out and look down. If you see water a few inches below, the trap is full and the smell is coming from somewhere else.
2. Slowly pour in about a gallon of water.
3. Add a tablespoon or two of mineral oil (cheap cooking oil works too). It floats on top and slows evaporation, so the seal lasts months instead of weeks.
4. Give it an hour with a window cracked and see if the smell clears.

While the grate is off, look for a small threaded plug on the side of the drain bowl. Many older floor drains have a cleanout plug there. If it’s missing or cracked, sewer gas goes right around the trap no matter how much water you add. A replacement plug is a cheap fix and usually ends the smell.

Pro tip: Set a phone reminder for the first of every month from October through April to pour water into every drain you rarely use. It takes two minutes and it’s the cheapest plumbing maintenance there is.

What else in a basement can let sewer gas in?

If the floor drain trap is full and the smell is still there, work through this list. It’s roughly in order of how often we find each one.

An unused shower, laundry tub or bar sink

Same problem, different drain. The finished-basement shower nobody uses, the old laundry tub, the wet bar: each has a trap that dries out. Run water in each for 30 seconds.

A toilet with a failed wax ring

A basement toilet that rocks, or a smell strongest right at the base, points to the wax ring between the toilet and the floor flange. Once that seal breaks, gas escapes constantly. It’s a pull-and-reset job.

A loose cleanout plug on the main line

The main cleanout is a capped fitting on the main drain, usually near the front wall of the basement. If someone opened it to clear a clog and didn’t snug it back down, or the old threads have rusted, it can leak gas. You’ll smell it strongest in that corner.

Cracked cast iron

Homes built before the 1970s usually have cast iron drain and vent pipes. Cast iron rots from the inside out, and we regularly find hairline cracks along the bottom of horizontal runs or a rusted-through seam on the main stack. Look for rust streaks, damp spots or white crusty residue on the pipe.

An ejector pit or rough-in that isn’t sealed

A sewage ejector pit, the basin that pumps a below-grade basement bathroom up to the sewer, has to have a gasketed, bolted lid and its own vent. A worn gasket or a missing bolt is a common source. Newer homes around the metro often have a basement bath rough-in, with pipes stubbed out of the slab for a future bathroom. Those are supposed to be capped. If a cap has popped off, it’s an open pipe to the sewer.

A blocked plumbing vent

Drains need air from the roof vent to flow without pulling traps dry. Leaves, a bird’s nest, or frost buildup in a sub-zero cold snap can close it off. Gurgling drains plus a smell is the giveaway. That one’s a roof job, so call us rather than climbing up there in November.

Are drain flies a clue?

Yes. Drain flies are the small, fuzzy, moth-looking flies resting on the wall near a floor drain or tub. They breed in the slimy film inside drains, so a cluster usually means gunk in the drain body that smells too. Scrub the drain bowl with a stiff brush and hot water; bleach alone tends to run past the film.

If the flies keep coming back after a good cleaning and there’s a damp or smelly spot in the floor away from any drain, suspect a broken pipe under the slab. That’s worth a camera.

Could it be something other than sewer gas?

Before you chase drains, rule these out:

What you notice	Likely source	What to do
Rotten-egg smell near the furnace, water heater or gas line	Possible natural gas leak	Leave the house, then call 911 or your gas utility from outside
Sulfur smell only when the hot water runs	Water heater anode reacting with the water	An anode rod swap ($180–$300) usually fixes it
Musty, damp-basement smell	Moisture, mold, wet carpet or drywall	Find the water source; check the sump and gutters
Smell strongest at the sump pit	Stagnant water or a dry, open pit	Add water and fit a lid
Smell plus slow or gurgling drains	Partial sewer line blockage or a break	Camera inspection

Planning ranges for the Des Moines metro in 2026; your written quote depends on the equipment, access and what we find on site.

Natural gas has an odorant added that smells a lot like rotten eggs, which is close to sewer gas. If you’re near a gas appliance and can’t tell which it is, treat it as gas: don’t flip switches, don’t go looking, just get out and call from outside.

When does a sewer smell mean a sewer line problem?

The smell alone is rarely the sewer main. What we watch for is the combination: a smell plus slow drains in several fixtures, a floor drain that gurgles or backs up when the washer empties, or a smell after heavy rain. In older neighborhoods with clay tile sewer lines and big trees, roots and separated joints are common. In 1950s and 1960s homes we also find cast iron under the slab rotted through along the bottom. That’s when a camera earns its keep: we run it from the cleanout, locate the problem and show you the video.

Fix	Typical range
Kitchen or bath drain clear	$95–$200
Toilet pulled and reset on a new wax ring	$125–$275
Main sewer line clearing through a cleanout	$200–$350
Sewer camera inspection on its own	$150–$350
Hydro-jetting (camera inspection included)	$400–$800
Sewer spot repair (dig and replace a section)	$1,500–$4,500

Planning ranges for the Des Moines metro in 2026; your written quote depends on the equipment, access and what we find on site.

For more common jobs and how pricing works around town, see our Des Moines plumber cost guide.

Pro tip: Buying an older home? A sewer camera inspection before closing is some of the best money you’ll spend. A clay line full of roots won’t show up on a standard home inspection.

What can I fix myself, and when should I call?

Safe to do yourself: refilling traps, adding mineral oil, scrubbing out a floor drain, replacing a missing floor drain cleanout plug, running unused fixtures and closing up an open sump pit.

Call a plumber when the smell survives a trap refill, you see a crack or rust weeping on cast iron, a toilet rocks, an ejector pit lid needs resealing, more than one drain is slow or gurgling, or anything backs up through the floor drain. Call (515) 494-5483. We’ll trace the source, give you a written quote before any work starts, and if it’s a camera job you’ll see the footage yourself. Our drain cleaning service covers everything from one trap to the main line, and if water is actively coming up, our emergency plumbing checklist covers the first few minutes.