Basement Floor Drain Backing Up or Smelling?

Floor drains work like every other drain in the house. A trap below the opening holds a small amount of water that blocks sewer gas from coming up into the room. A Utah sewer district's explanation of the problem puts it simply: when a drain is not used often, the water in the P-trap can slowly evaporate, and once the trap dries out, sewer gases can escape into indoor spaces and cause unpleasant odors.

This is the most common explanation for a basement that smells like sewage with no standing water anywhere. It shows up most in drains that rarely see water: finished basements, guest-area floor drains, and drains near AC condensate lines that only run seasonally.

Pour a couple of cups of water down the floor drain and see if the smell is gone within a day. If it is, that is the whole fix, and running water down that drain every few weeks keeps the trap full going forward.

If the smell does not go away after refilling the trap, the problem probably is not the trap itself. That points toward a vent issue, a cracked trap, or a blockage further down the line, and it is a reasonable point to call a plumber instead of guessing further.

A floor drain that is actually filling with water, rather than just smelling, is a different problem. Basement floor drains sit at the lowest point in most homes, so when a shared line further down is blocked, water finds that opening before it finds anywhere else.

This usually shows up when you run water upstairs, flush a toilet, or run the washing machine, and water or sewage rises in the floor drain in response. That pattern means the blockage is downstream of the floor drain, not in the floor drain itself.

Treat backed-up water from a floor drain as contaminated until you know otherwise. The CDC's guidance on cleaning up safely after a flood or sewage contact recommends wearing rubber boots, goggles and gloves, and putting a waterproof bandage on any cuts before you go near it.

Keep kids and pets away from the area, stop using water elsewhere in the house so you are not adding to what is backing up, and skip chemical drain cleaners. They are not built to clear a main line blockage and will just leave caustic water sitting in a spot you may need to clean up by hand.

The most common cause is the same main line problem covered elsewhere on this site: roots, grease, or a section of pipe that has settled. Because the floor drain sits at the lowest point, it is often the first place a main line problem shows up, sometimes before a toilet even gurgles.

In Twin Cities suburbs, the property owner is generally responsible for the sewer line between the house and the street. The City of Minneapolis's own guidance states that the property owner is responsible for the sanitary service lateral connecting the property to the public sewer system, which is the general pattern most nearby cities also follow.

Run water down rarely-used floor drains every few weeks so the trap does not dry out. Keep the area around the drain and the main cleanout clear so a plumber has quick access if something does back up.

If the floor drain has backed up more than once, treat that as a pattern rather than a one-time event. Call to get connected with a local plumber who serves Eden Prairie and mention that it has happened before. That detail often changes what they check first.