Likely sources, from simplest
| Source | Clue | Fix |
|---|---|---|
| Dry floor drain trap | Smell strongest at an unused floor drain | Pour a few quarts of water in; repeat every few weeks |
| Unused basement shower or sink | Odor near a fixture nobody uses | Run water to refill the trap |
| Loose or missing cleanout cap | Smell near the cleanout | Replace or tighten the cap |
| Failed wax ring under a toilet | Odor at the base of one toilet, sometimes staining | Plumber resets the toilet with a new seal |
| Blocked or disconnected vent | Gurgling drains plus odor; traps siphon dry | Plumber checks roof vent and connections |
| Cracked drain line | Persistent odor, damp spots, no obvious fixture source | Smoke test or camera inspection |
| Leftover contamination | Odor after a past backup, worse in humid weather | Check for missed drywall, pad or insulation |
A simple search
- Refill every trap.Run water briefly in every sink, tub, shower and floor drain, including ones nobody uses. Wait a day.
- Check caps and seals.Look at the cleanout cap, around toilet bases, and at any open pipe left from past remodeling.
- Note the pattern.Does the smell come with wind, with rain, with the furnace running, or after showers? Write it down for a plumber.
- Call a plumber for a smoke test.A smoke test pushes harmless visible smoke into the drain system to show leaks at joints, vents or cracked pipe.
When the smell started after a backup
A sewer odor that appeared or never went away after an earlier backup is a different problem. Odor can linger when contaminated materials were left in place, such as carpet tack strip, the bottom of a wall cavity, the underside of a vinyl floor, or a piece of furniture that was cleaned instead of discarded. It also lingers when concrete was not thoroughly cleaned and dried. Humid days and a running dehumidifier can make the smell come and go.
If you suspect leftover contamination, ask whoever did the earlier cleanup what was removed and what was left. Ask for any moisture readings and photos they took. A second assessment that opens one or two inspection holes at the base of walls can confirm whether the cavity is clean and dry.
Terms you may hear
- Trap
- The U-shaped bend under a drain that holds water to block sewer gas.
- Vent stack
- Pipe through the roof that lets air into the drain system so traps do not get siphoned dry.
- Smoke test
- Diagnostic that fills drain pipes with visible smoke to find leaks.
- Hydrogen sulfide
- A gas produced by decaying sewage with a rotten-egg smell; strong concentrations are hazardous.
General information for Thornton, not a quote or professional advice. In an emergency, call 911.