Refill and test a suspect drain
This takes about five minutes and costs almost nothing.
- Find every drain.Look for the floor drain near the furnace or water heater, the laundry standpipe, any basement shower, a utility sink and the cleanout cap on the main sewer line, which is often near the front foundation wall.
- Pour slowly.Pour roughly a gallon of water into each floor drain and run water in any fixture that is rarely used. The goal is to refill the U-shaped trap that blocks gas.
- Wait a day.Give the basement a day to air out. If the smell was sewer gas from a dry trap, it should fade noticeably.
- Watch the timing.If the smell returns within a few weeks, note how long it took. Fast return points to evaporation; very fast return can mean a cracked trap or an open cleanout.
- Check the cleanout.Make sure the cleanout plug is present and tight. A missing or cracked cap is an open pipe to the sewer.
Why it happens more than people think
Every drain has a trap, a bend that holds a few inches of water as a seal. Fixtures used daily stay full. A floor drain, though, may only see water when the water heater is flushed, which in many houses means almost never. In a heated basement during a dry Colorado winter, that seal can shrink and break within weeks or a few months.
Older Lakewood homes often have a floor drain that was installed when the house was built and has been forgotten under a rug, a storage shelf or a laundry basket. Newly purchased homes are a classic case: the previous owners moved out, the house sat empty, and the new owners notice the smell in their first week.
Sewer gas is mostly an unpleasant nuisance at the low levels that come through a dry trap, but it should not be ignored. It can contain hydrogen sulfide and other gases, and it tells you that the barrier between your house and the sewer system is not working.
Ways to keep a trap from drying out
| Option | How it works | Things to know |
|---|---|---|
| Calendar reminder | Pour water in every few weeks | Free, but easy to forget during travel |
| Drop of mineral oil | A thin film on top slows evaporation | Ask a plumber whether this suits your drain; avoid cooking oils |
| Trap seal insert | A one-way flexible valve fits in the drain | Must be sized correctly and still let water through in a flood |
| Trap primer | A plumbing device adds water automatically | Installed by a plumber; ask whether a permit is needed in your city |
| Cleanout cap replacement | Seals an open pipe on the main line | Inexpensive part; confirm the correct size first |
Common questions
- Can I just tape over the floor drain?
- That blocks gas but also blocks water if a water heater or washer leaks. A proper trap seal insert is a better choice than tape.
General information for Lakewood, not a quote or professional advice. In an emergency, call 911.