The most common cause is a dry floor-drain trap. Every drain has a trap that holds a water seal, usually 2 to 4 inches deep, to block sewer gas, and a drain that is never used lets that water evaporate. Pouring a gallon of water into the floor drain fixes that in seconds. If the smell comes back with full traps, the usual suspects are a loose cleanout cap, a failed toilet wax ring, a vent problem, or a cracked drain line, and a camera inspection finds the last one.
What causes a sewer smell in a basement?
| Cause | Clue | Fix |
|---|---|---|
| Dry floor-drain trap | Smell near a drain nobody uses | Pour water in; repeat monthly or add a trap primer |
| Loose or missing cleanout cap | Smell strongest near the cleanout pipe | Tighten or replace the cap |
| Failed toilet wax ring | Smell around a basement toilet base | Reset the toilet with a new seal |
| Blocked or missing vent | Gurgling drains, traps that seem to empty | Clear or correct the vent |
| Cracked or broken drain line | Smell persists with full traps; damp spots | Camera locate, then line or repair |
| Partial sewer blockage | Slow drains, odor after heavy water use | Hydro jet and camera the line |
How do you check a dry trap?
Find every drain in the basement, including the floor drain, a utility sink, an unused shower or a laundry standpipe. Pour about a gallon of water into each one. If the smell fades within an hour, a dry trap was the cause. Floor drains in finished basements are the usual culprit because they only see water when something leaks. On the Front Range the dry air speeds up evaporation, so a drain that holds its seal for months somewhere humid can dry out here in a few weeks, especially with the heat running in winter.
What if the smell comes back with water in the traps?
Then gas is getting past the system somewhere other than the traps. Check these in order:
- The cleanout. The capped pipe where the main line can be accessed should be sealed tight. A cracked or loose cap vents sewer gas straight into the room.
- Basement toilets. A rocking toilet or staining at the base points to a failed wax ring.
- Gurgling. Drains that gurgle or traps that seem to lose water when other fixtures run point to a venting problem or a partial blockage pulling the seal out.
- The line itself. A crack or separated joint under the slab lets gas and moisture out. You cannot see it, but a sewer camera inspection can, and it shows exactly where.
Is sewer gas dangerous?
At the low levels that cause a household odor, it is mainly a sign that something needs fixing, but do not ignore it. Sewer gas contains hydrogen sulfide, the rotten-egg smell, and OSHA notes that at higher concentrations it can deaden your sense of smell, so a smell that fades without a fix is not proof the problem is gone. Ventilate the space, and if anyone feels dizzy or sick, get out and call for help.
When does a smell mean a bigger repair?
When it comes with slow drains, recurring backups at the floor drain, or damp spots on a slab, it usually points to the line, not the fixtures. Roots, offsets and cracks in older pipe all leak gas before they fail outright. Our camera finds the spot, and many cracked lines can be sealed with trenchless epoxy lining through the existing cleanout, with no slab cut. If the pipe has broken apart, we will say so and quote the repair instead.
A lot of the smell calls we get end with a bucket of water, and we will tell you that over the phone before we ever roll a truck. When the smell keeps coming back, we run the camera instead of guessing, because the fix for a cracked line under the slab is very different from the fix for a dry trap, and you should see which one you have before you pay for anything.
What should you do next?
Refill the traps first. If the smell returns, call (303) 476-3160 or get your free quote and we will find the source on camera. Related reading: 7 signs your sewer line is failing and why drains back up in winter.
Frequently asked questions
Why does my basement smell like sewer only sometimes?
An intermittent smell usually means a trap seal that is marginal: it dries out between uses, or it gets pulled down when other drains run because of a venting problem or partial blockage. Refill the traps and note when the smell returns; that timing points to the cause.
How do I stop a floor drain from drying out?
Pour water into it every few weeks, more often in winter when the heat is on and Colorado air is driest. For a permanent fix, a plumber can install a trap primer, which refills the trap automatically, or a trap seal insert that lets water down but keeps gas from coming back up.
Can a sewer smell come from a cracked sewer line?
Yes. A crack or separated joint under the slab lets sewer gas and moisture escape into the soil and up into the basement. If the smell persists with full traps and a sealed cleanout, a camera inspection is the way to confirm a crack and find its exact location.
Is it safe to stay in a house with a sewer smell?
A mild household odor is usually a nuisance rather than an emergency, but treat it as a problem to fix. Ventilate the area, refill traps, and if the smell is strong, getting worse, or anyone feels dizzy or ill, leave the house and call for help.