Sink Drain Odor
Updated February 20, 2026
Foul smell from the drain? Bacterial biofilm in the pipe, dried-out P-trap letting sewer gas through, or food debris in the disposal or overflow. Most drain odors fixed in under 30 minutes with household items.
Overview
Unpleasant but rarely a plumbing emergency. Three common causes: biofilm coating the drain pipe (musty or rotten smell), dried P-trap letting sewer gas in (sulfur or rotten eggs), or decomposing food in the disposal or overflow (localized to kitchen). Each has a different fix, all simple and cheap. Under 30 minutes.
Symptoms
- Rotten or sulfur smell from the drain, especially noticeable leaning over the sink
- Strongest in the morning or after hours of no use
- Rarely used bathroom or guest sink -- classic dry P-trap
- Kitchen sink worse after the disposal runs -- trapped food waste
- Odor present even with the drain flowing freely -- not a clog
Common Causes
- Biofilm -- slimy layer of bacteria, soap, hair, organic matter coats the drain walls. Musty or rotten smell. Most common in bathroom sinks.
- Dry P-trap -- curved pipe holds water that blocks sewer gas. Unused for weeks, water evaporates, seal breaks. Guest baths and basement sinks most common.
- Food in the disposal -- particles under the splash guard, on impeller blades, in the grind chamber. Starchy foods, meat, dairy that were not flushed through.
- Overflow channel biofilm -- small overflow hole on bathroom sinks connects to the drain. Collects mildew, rarely cleaned. Persistent musty smell.
- Blocked vent -- bird nest, debris, or ice in the roof vent. Negative pressure siphons water from the P-trap. Less common but check if other fixes do not work.
What You'll Need
How to Fix It
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Check and Refill the P-Trap
Sink not used in a while? Run the faucet 30 seconds. Refills the P-trap, water seal blocks sewer gas immediately. Wait a few minutes. Odor gone? That was the whole problem.
Tip: Infrequently used sink? Tablespoon of mineral oil after running water. Floats on the P-trap water, slows evaporation dramatically. Months instead of weeks. -
Clean the Drain with Baking Soda and Vinegar
Half cup baking soda, half cup vinegar. Fizzing breaks up biofilm. Sit 15-30 minutes, flush with a kettle of very hot water (boiling if metal pipes). Effective for light to moderate biofilm.
Tip: Stubborn biofilm? Let baking soda sit dry for an hour before adding vinegar. Longer contact absorbs odors and loosens the film. -
Scrub the Drain Opening and Stopper
Remove the stopper or strainer. Biofilm accumulates on the stopper rod and pivot ball. Toothbrush inside the drain opening as far as you can reach. Kitchen: lift the disposal splash guard, scrub the underside. Surprising amount of decomposing food there.
Warning: Disposal off and unplugged before cleaning around the splash guard. Do not put your hand inside the chamber. -
Clean the Overflow Channel (Bathroom Sinks)
Overflow hole connects to a narrow channel -- hidden source of biofilm and mildew. Bottle brush or pipe cleaner in the hole, scrub back and forth. Baking soda and vinegar into the overflow, fizz 15 minutes. Hot water flush. Smell improvement may surprise you.
Tip: Funnel or turkey baster to pour directly into the small overflow opening without spilling. -
Clean the Garbage Disposal (Kitchen Sinks)
Run the disposal with cold water 15 seconds. Handful of ice cubes and half cup rock salt, run 30 seconds -- scours the chamber and blades. Half a lemon or lime with cold water to deodorize. Persistent? Fill basin with warm water and dish soap, pull stopper, run disposal to flush the whole line.
Tip: No bleach in the disposal. Damages splash guard and seals over time. Ice, salt, and citrus is just as effective and safe. -
Address Persistent Odor (Vent Check)
Everything cleaned and smell persists? Vent may be blocked. Signs: gurgling when drains empty, slow draining at multiple fixtures, sewer smell that comes and goes. Roof: check vent pipe for debris, bird nests, ice. Garden hose flush from the roof. Not comfortable up there? Plumber.
Warning: Roof work carries fall risk. Sturdy ladder, someone spotting you, non-slip shoes. Never on a wet or icy roof.
When to Call a Pro
Call a plumber if the smell persists after all cleaning steps, if multiple drains have the same odor (main vent or sewer line), if drains gurgle when other fixtures run (vent blockage), or if you are not comfortable on the roof. Smoke test or camera inspection pinpoints where sewer gas is entering.
Prevention Tips
- Run seldom-used sinks weekly. Keeps the P-trap full. Set a reminder.
- Monthly baking soda and hot water flush. Prevents biofilm buildup.
- Overflow hole every few months with a bottle brush. Most overlooked odor source.
- Cold water 15 seconds after the disposal stops. Flushes all particles through.
- Never leave food in the disposal overnight. Run it and flush before bed.
- Used less than monthly? Mineral oil in the drain after running water. Slows P-trap evaporation.