Sewer smell from basement drain

Basement floor drain with metal grate and concrete floor

Quick Answer:

If you smell a sewer odor while walking through the basement, start by keeping the area ventilated and then check the floor drain’s water seal. Most often a dry or contaminated P‑trap is the cause; pour a gallon of water into the floor drain, remove the grate and inspect the trap and nearby clean-out, and do a simple smell-test to localize the source before calling a plumber.

Why This Happens

Sewer gas normally stays out of living space because of water seals (P‑traps) and intact drain bodies. If the trap dries out, is clogged with debris, or the drain body is cracked, odors rise up through the floor. Other reasons include a failed trap primer line that doesn’t refill the trap, a blocked or leaking main clean-out, or slow flow in the line that leaves sewage in a low spot. Seasonal effects and house pressure can make smells more noticeable in certain conditions — for example, if the issue is confined to the basement you’ll want to read the guide for Smell only in basement, and if it fluctuates with cold weather it can resemble the patterns in Smell worse in winter.

Step-by-Step What to Do

Step 1 — Refill the P‑trap

  • Pour about a gallon of clean water slowly into the basement floor drain to refill the trap. This restores the water seal that blocks sewer gas.
  • Wait a few minutes and recheck the odor; if the smell disappears, a dry trap was the likely cause.

Step 2 — Remove the grate and inspect the trap

  • Use a screwdriver or pry tool to lift off the grate. Wear gloves and eye protection.
  • Look down with a flashlight: is the trap visibly dry, full of debris, or coated with a slime or scum? Remove loose debris with gloved hands or a small scoop.
  • If the trap is dry despite recent refilling, look for a leaking trap primer line or evaporation from infrequent use.

Step 3 — Check the basement clean-out

  • Locate the clean-out plug on the basement floor or wall near the drain. Carefully loosen the cap—use a rag and be prepared for some odor or liquid.
  • Check for standing sewage at the clean-out or slow drainage when you open it. Standing sewage or very slow flow indicates a partial or full blockage further down.

Step 4 — Do a smell-test to localize the source

  • Move your nose close to the trap area, then to the clean-out, and finally around nearby joints and the drain body. Note where the odor is strongest.
  • If the smell is strongest at the clean-out, the problem is likely downstream (blockage or sewage back-up). If it’s strongest at the trap, the trap or its seal is suspect.

Step 5 — Inspect the drain body and trap primer line if water seals are fine

  • If the trap stays full of water but odor remains, look for hairline cracks in the drain body or around the drain flange that allow gas to escape.
  • Check for a trap primer (a small water feed line to keep the trap full). If present, ensure it has water and isn’t pinched or broken.
  • If you find a crack or a failed primer line, seal or replacement will usually require a plumber.

What Not to Do

  • Avoid blindly pouring bleach or strong drain cleaners into the floor drain—these can damage rubber seals, harm drain finishes, and hide diagnostic cues by masking odors without fixing the cause.
  • Do not ignore signs of sewage backup, persistent smell after refilling traps, or gurgling at other fixtures—these are reasons to stop and call a professional.

When to Call a Professional

  • Call a plumber if the smell persists after you refill traps and inspect the drain, if you see standing sewage or slow drainage at the clean-out, or if other fixtures in the house gurgle or back up.
  • If you find cracks in the drain body, a leaking trap primer line, or you’re uncomfortable removing the clean-out cap, schedule a professional inspection and possible camera scan of the sewer line.
  • Also call immediately if you detect widespread sewer gas inside the house or any health symptoms from the odor—those can indicate a more serious sewer leak or venting problem.

Safety Notes

  • Sewer gas can contain harmful compounds—avoid prolonged inhalation. Ventilate the area by opening windows and running a fan if possible.
  • Wear gloves, eye protection, and a mask when inspecting drains or removing the grate. Treat any liquid as contaminated—clean and disinfect hands and tools afterward.
  • Do not use an open flame for testing or lighting near suspected sewer gas. If you smell gas strongly, leave the area and call a professional.

Common Homeowner Questions

  • Can a dry floor drain make the whole basement smell?
    Yes. A dry P‑trap allows sewer gas to come up through the drain and can make the basement smell strongly.
  • Will pouring water fix it permanently?
    Sometimes—if the trap is simply dry. If the trap keeps losing water or the odor returns, there’s a leak, failed primer, or downstream problem that needs fixing.
  • Is sewer gas dangerous?
    Short exposures are usually unpleasant; prolonged exposure can cause health issues. Treat it seriously and call a pro for persistent or strong odors.

More in this topic

For more related fixes and similar symptoms, see Sewer Gas Smell from Floor Drains.

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