Short answer: Start with the floor drain: pour a few quarts of water into it, because a dry trap is the most common cause. Then check the cleanout cap, the laundry standpipe and any sewage ejector pit. If the smell gets worse after heavy rain or water comes up through the floor drain, the main sewer line or the city sewer may be overloaded, and you should call a plumber.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Smell near the floor drain | Dry trap | Pour water into the drain | Only if it keeps drying out |
| Smell near a capped pipe on the wall or floor | Loose or missing cleanout cap | Check the cap is tight and present | If damaged |
| Smell near a covered pit | Sewage ejector pit lid not sealed | Check the lid and its gasket | Often |
| Worse after heavy rain, water at the drain | Overloaded or blocked sewer line | Watch the floor drain during rain | Yes |
| Musty smell, not sewage | Damp basement, mold | Look for water stains and humidity | Depends |
Dry floor drain traps
Basement floor drains are often used only a few times a year, so their traps dry out. Oatey recommends running water through unused drains about once a month, and a drain seal can keep gas out between uses.
Cleanouts and ejector pits
A cleanout is a capped opening on the drain line used to clear clogs. If the cap is missing, cracked or loose, gas escapes. Homes with a basement bathroom often have a sewage ejector pit with a sealed lid; if the lid or its gasket is not tight, the basement will smell.
Smell after heavy rain
Heavy rain can overload combined or aging sewer systems and push gas, and sometimes water, back toward the house. A partially blocked main line makes this worse. If dirty water rises from the floor drain, stop using water in the house and call a plumber. Ask about a camera inspection and whether a backwater valve makes sense for your home.
When to call a professional
- Water or sewage comes up through a floor drain.
- The smell is strong or you feel unwell.
- A sewage ejector pump runs constantly or alarms.
- Several drains in the house are slow.
What it usually costs
HomeGuide (2026) lists sewer line cleaning at about $150 to $800 and sewer camera inspections at about $125 to $500. Plumbers typically charge $45 to $150 per hour plus a call-out fee.
Frequently asked questions
Why does my basement smell like sewage after it rains?
Rain can overload the sewer system and push gas back through weak seals, especially a dry floor drain or a partly blocked main line.
Is a floor drain smell dangerous?
A faint smell from a dry trap is mainly a nuisance, but strong or persistent sewer gas should be dealt with, and any sewage backup is a health hazard.
Related problems
- Sewer smell in the house
- Sewer smell in the bathroom
- Clog deep in the drain pipe
- Water stain on the ceiling
Sources
- Oatey: Sewer gas odor in your house and what to do about it
- InspectAPedia: Sewer odor source list
- HomeGuide: Plumber cost per hour (2026)
- HomeGuide: Plumbing inspection cost (2026)
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.