Why does the basement smell only sometimes?

Dry traps, wind-driven vent issues, or seasonal evaporation can make odors intermittent.

How often should I refill a floor drain trap?

Monthly in dry seasons or any time you notice a musty or sewer odor returning.

Is bleach safe to pour into the floor drain?

Avoid heavy bleach in septic systems; use mild cleaners and keep the trap wet instead.

Basement floor drain odor: stop sewer gas fast

A basement floor drain relies on a water trap to block sewer gas. When that trap evaporates or biofilm builds up, odors sneak in. A few simple steps usually fix it without harsh chemicals.

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Quick fixes (under 10 minutes)

Refill the trap. Pour 2–3 cups of water into the drain, then add a tablespoon of mineral oil to slow evaporation.
Clean biofilm. Scrub the grate and upper trap with a nylon brush and mild cleaner; avoid heavy bleach in septic systems.
Check the cap. Ensure any cleanout plug in the drain body is intact and tight.

If the smell returns

Venting issues. A blocked roof vent can siphon traps dry. Clear debris or consult a plumber.
Dry seasons or infrequent use. Top up the trap monthly; keep a reminder near the utility panel.
Nearby sump pit. If a sump pit is open, add a sealed lid and vent per code to redirect odor.
High groundwater. Check for dampness around the trap that may signal seepage or a cracked drain body.

Sources

EPA guidance on indoor sewer gas prevention
Typical plumbing code notes on trap maintenance and venting

FAQ

Why does the basement smell only sometimes?: Dry traps, wind-driven vent issues, or seasonal evaporation can make odors intermittent.
How often should I refill a floor drain trap?: Monthly in dry seasons or any time you notice a musty or sewer odor returning.
Is bleach safe to pour into the floor drain?: Skip heavy bleach in septic systems; use mild cleaners and keep the trap wet instead.