Guides · Twin Cities homes
Why does my basement floor drain smell like sewer?
The short answer
Nine times out of ten, the trap under the floor drain has dried out, and sewer gas is coming straight up the pipe. Pour a bucket of water down the drain and the smell should be gone within the hour. If it returns quickly, or you also have gurgling or slow drains, the cause is a blocked vent or a partial main-line clog.
The dried-out trap
Every drain has a U-shaped trap that holds a little water. That water is the seal that keeps sewer gas on the sewer side. A floor drain in a Minnesota basement can go months without any water reaching it — the furnace and water heater don't drain there, the utility sink has its own trap — and in a dry, heated winter basement, the water in the trap simply evaporates. Once it's gone, the drain is an open pipe to the sewer, and the basement smells like it. It's the single most common "sewer smell" call we get, and the fix is a bucket of water.
To keep it from happening, pour a gallon down every floor drain once a month, or add a splash of mineral oil after the water — it floats on top and slows evaporation.
When the water doesn't fix it
- The smell comes back in a day or two. The trap may be leaking or cracked, or a trap primer (a small line that's supposed to keep it wet) has failed. Both are repairable.
- Gurgling or slow drains too. A blocked vent or a partial main-line clog is pushing gas through the seals. See our guide on toilets gurgling when the tub drains — it's the same system.
- Smell near a fixture that isn't used — a basement shower, a laundry tub, a bathroom nobody uses. Same dried-trap problem, different drain.
- Rotten eggs from the water itself, not the drain — that's a water-heater or well-water issue, and a different guide.
Why it's worth taking seriously
Sewer gas is unpleasant, and in concentration it's a health and even a fire concern. More practically, a persistent smell after the trap is refilled is the drain system telling you something is wrong upstream — and a main line that's narrowing gives this warning before it backs up. The bucket test is free and takes a minute. If it doesn't hold, don't keep masking it.
Try it yourself, or call?
Worth trying first
- Pour a gallon of water down the drain and wait an hour
- Check every rarely used drain in the basement and do the same
- Add a little mineral oil on top to slow evaporation over the winter
Time to call
- The smell returns within days after refilling the trap
- You also have gurgling, slow drains, or water at the floor drain
- The smell is strongest when the washing machine or a tub drains — that's a line or vent issue
- You can't tell where it's coming from
Common questions
Is sewer gas dangerous?
In the small amounts from a dried trap, it's mostly a nuisance. In concentration — a broken line under the slab, a disconnected vent — hydrogen sulfide and methane can cause headaches and worse. A smell that's strong, persistent, or making anyone feel unwell is a same-day call.
Why does it smell worse in winter?
Two reasons: the basement is drier, so traps evaporate faster, and the furnace runs constantly, which can pull air up through an open drain. Frost-capped vents in a cold snap also push gas out through the weakest seal in the house.
There's a small pipe running to the floor drain. What is it?
A trap primer — it drips a little water into the drain whenever a nearby fixture runs, to keep the trap wet. If yours has failed, the trap dries out and the smell starts. It's a straightforward repair.