How to Unclog a Basement Floor Drain
If sewer gas smell persists after refilling the P-trap, the trap may be cracked or the vent stack may be blocked. Call a plumber. Sewer gas contains methane and hydrogen sulfide.
What You'll Need
- 2 cups
- 2 tbsp
- 1/2 cup
- 1 cup
- 1
Step-by-Step Method
Lift or unscrew the floor drain grate. Shine a flashlight inside. Remove any visible debris (hair, dirt, sediment).
Pour 2 cups of water down the drain. If the drain smells like sewer gas, the P-trap was dry. The water refills it and blocks the gas.
Pour 2 tbsp mineral oil down the drain after the water. The oil floats on the water in the P-trap and dramatically slows evaporation.
Pour 1/2 cup baking soda followed by 1 cup vinegar. Wait 15 minutes. Flush with warm water.
- Do not ignore sewer gas smell from floor drains (indicates a dry P-trap)
- Do not seal or cover floor drains permanently (they serve as emergency overflow points)
- Quarterly maintenance: pour 2 cups of water + 2 tbsp mineral oil down every basement floor drain, even if you never notice them.
- Floor drains also serve as emergency overflow points for water heater failures and plumbing leaks. Keep them clear and accessible.
Frequently Asked Questions
Almost always a dried-out P-trap in the floor drain. Basement floor drains rarely receive water, so the P-trap water evaporates and sewer gas enters the house. Pour 2 cups of water down the drain to refill the trap. Add mineral oil to slow future evaporation.
Sources & Methodology
P-trap water seal is the standard sewer gas barrier in residential plumbing. Mineral oil evaporation barrier is a standard maintenance technique for seldom-used drains.
Last reviewed: March 20, 2026