Quick Answer:
If you smell sewer gas after a drain clears, the water in the P‑trap may be siphoning away and losing its seal. Do a quick test: run a full basin dump and watch whether the trap refills and keeps water in the bend. If the trap refills and the odor stops, you likely have a transient draw issue. If the seal disappears repeatedly, take the following steps and consider professional help.
Why This Happens
- Siphoning occurs when a large volume of water leaves a fixture quickly and creates a vacuum that pulls water out of the trap. This can happen with tubs, sinks, or showers that drain fast.
- Blocked or partially blocked downstream pipes can amplify the effect by causing uneven flow or gurgling.
- Poor venting or a blocked vent stack can let negative pressure build in the drain line, which can let the trap drain toward the sewer. See the note about Trap not vented properly below for more detail on vents.
- A damaged or improperly installed trap can fail to hold water, while evaporative loss is only likely if the fixture is rarely used.
Step-by-Step What to Do
Step 1 — Confirm the problem
- Note when the odor occurs: only after a specific fixture drains, or all the time?
- Listen for gurgling or slurping sounds when other fixtures are used; that points to pressure issues.
Step 2 — Run a full basin dump and observe the trap
- Fill the basin, tub, or sink with water to its normal level.
- Remove the plug so the fixture drains fully (a “full basin dump”).
- Watch or feel the trap area (under the sink or accessible cleanout) to see if water returns to the bend and stays there.
- If the trap refills and the smell disappears, the seal is working for that event. If it does not refill or the water is pulled out, you have siphoning or an installation problem.
Step 3 — Check nearby fixtures and vents
- Run other fixtures (toilet, another sink) to see if the problem is isolated or system‑wide.
- Look for signs of poor venting: slow drains, regular gurgling, or multiple traps losing their seal. This is often related to a vent issue and relates to the idea of Trap holds water constantly when vents are functioning correctly.
- If you can safely access the roof, check the vent pipe for obvious blockages like leaves or debris, but do not climb if you are uncomfortable or it’s unsafe.
Step 4 — Try simple fixes
- Pour about a gallon of water into the trap to re-establish the seal after the test.
- Clear any visible debris in the drain or use a hand auger for the immediate drain—do not force chemicals into the system repeatedly.
- Install a hair screen or basket to reduce future clogs that could contribute to uneven flow.
Step 5 — Monitor and repeat the test
- Repeat the full basin dump after simple fixes. If the trap keeps refilling and the odor is gone, monitor for recurrence over several days.
- Keep notes: which fixture, what time, other fixtures in use—this helps a professional diagnose if needed.
What Not to Do
- Do not ignore a siphoning trap—if the seal disappears repeatedly, professional help may be appropriate.
- Do not pour strong drain chemicals into the trap regularly. They can damage pipes, harm seals, and mask the underlying problem.
- Do not remove roof vents, dig up piping, or attempt major vent work without proper knowledge or permits.
When to Call a Professional
- If multiple traps lose their seal or you have persistent sewer odor despite refilling traps.
- If you hear constant gurgling, slow drains throughout the house, or have sewage backflow—these can indicate a blocked main or venting problem.
- If the trap is not accessible, the piping is old or corroded, or the cause seems related to venting or main sewer line issues.
Safety Notes
- Ventilate the area if you smell sewer gas. Open windows or run an exhaust fan to reduce exposure.
- Wear gloves and eye protection when inspecting or clearing a trap. Avoid breathing directly over open drains.
- If you suspect sewer gas levels are high inside the home, evacuate and call a professional—sewer gas can contain hydrogen sulfide and methane, which are hazardous in high concentrations.
Common Homeowner Questions
- Why does the smell come back after a while? Short answer: the trap may not be holding water consistently; either it’s siphoned out or evaporated if unused.
- Can I fix venting issues myself? Short answer: Basic clearing of a roof vent if safe may help, but diagnosing venting often needs a plumber.
- Is the trap bad if it never refills? Short answer: Not always—there may be downstream blockage or vent problems; have it checked if simple refilling doesn’t hold.