Smell only in basement

Basement area with odor localized around old concrete floor drain

Quick Answer:

If you smell sewer or a persistent rotten-egg odor only when you stand at the top of the basement stairs or just inside the basement, the most common causes are a dry or damaged floor-drain trap, a failed trap primer, a sump or clean-out problem, or an unfinished opening letting sewer gas in. Start by inspecting the floor drain trap, add water to restore the seal, and check the sump and clean-out before sealing anything or assuming the smell is from stored items.

Why This Happens

Basement smells are often plumbing-related because basement drains connect to the building’s lowest waste lines and traps. If a trap is dry, cracked, or missing, sewer gas can travel up through the drain. Sumps, clean-outs, and utility penetrations can also allow smells to enter the space. A malfunctioning trap primer (the device that keeps low-use traps topped with water) will let the seal evaporate.

If you want more detail on similar scenarios, see Sewer smell from basement drain for a closer look at drain-related causes and fixes.

Step-by-Step What to Do

1. Remove and inspect the floor drain grate

  • Put on gloves and remove the grate. Use a flashlight and look into the trap. Check for visible water, cracks in the drain body, or foreign objects.
  • If the trap looks dry or you can see the pipe below, it’s likely lost its water seal. If you see a large crack or broken concrete around the drain, note that for a professional repair.

2. Pour a bucket of water into the trap to restore the seal and retest

  • Slowly pour a full bucket (2–5 gallons) of water into the drain to refill the trap. Listen for gurgling that could indicate connected fixtures or vent issues.
  • Wait a few minutes and then check whether the smell has reduced. If the odor goes away, the trap seal was the likely cause. Retest over the next 24 hours—some traps lose water slowly and need a permanent fix.

3. Check the basement clean-out and sump for cross-contamination or overflow

  • Open the clean-out plug carefully. If you smell sewage or see backup, do not try to clear it with chemicals—this is a sign of a deeper problem.
  • Inspect the sump pit and pump. A flooded pit, backflow, or a direct connection between a dirty sump and a wastewater line can spread odors. If the sump has standing sewage, call a professional.

4. Look for unfinished penetrations or utility openings that could be admitting sewer gas

  • Walk the perimeter and inspect rim joists, pipe penetrations, around the furnace, and any gaps where utilities enter the basement. Unsealed gaps can let sewer gas or stale air in.
  • Note openings but don’t seal them yet—test traps and clean-outs first. Temporary covers can be used after you confirm plumbing traps are functioning.

5. Confirm any trap primer that serves basement drains is functioning

  • If a trap primer supplies water to the floor drain, run the upstream fixture or test the primer per the manufacturer’s instructions. Some primers activate when a sink or toilet is used; others are mechanical and need repair.
  • If the primer is not working, the trap will dry out between uses and allow odors back into the basement.

What Not to Do

  • Do not assume the smell is from stored items, old boxes, or furniture before checking floor drains and traps.
  • Do not cover or permanently seal openings (penetrations, vents, or the drain) until you have tested and refilled traps—sealing can hide the source and make a plumbing problem worse.
  • Do not ignore standing sewage, overflowing sumps, or structural cracks around the drain—call a professional if these are present or if adding water to traps doesn’t stop the smell.

When to Call a Professional

  • The smell persists after you’ve refilled traps and checked the sump and clean-out.
  • You find standing sewage, a clogged or overflowing sump, or evidence of sewer backup.
  • There are structural cracks around drains, a broken or corroded drain body, or a failed trap primer that you cannot safely repair.

Safety Notes

  • Wear gloves and eye protection when inspecting drains. Avoid putting your face directly into a drain opening.
  • Sewer gas can be unpleasant and, in high concentrations, hazardous. Vent the area and leave if you feel lightheaded or unwell.
  • Don’t mix or pour strong chemicals into drains to mask odors—chemicals can harm plumbing, create toxic fumes, and complicate repairs.

Common Homeowner Questions

  • Why do I only smell it when I go downstairs? Cooler basement air can trap odors near the floor and stairwell; a basement trap or opening is often the source.
  • Can I fix this myself? Yes, if the problem is a dry trap: refill it and monitor. Call a pro for backups, standing sewage, or structural issues.
  • Is sewer gas dangerous? Short exposures are usually just unpleasant; prolonged exposure can be harmful. Vent the area and get help if anyone feels ill.

More in this topic

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

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