Symptoms
- Musty or sewage smell near the floor drain
- Smell noticeable especially in warmer months
- No actual water backup or clog present
- Smell improves temporarily after pouring water down the drain
Likely causes
- CommonDried-out trap losing its water sealFloor drains are rarely used, so the trap's water can fully evaporate over weeks or months, especially in dry basements.
- PossibleBuildup of dirt and organic debris inside the drainCan add to the smell even when the trap has water in it.
- Less commonVent issue affecting the drain's ability to hold a sealLess common but possible in some plumbing layouts.
- Less commonSewer gas backflow from a more significant line issueWorth ruling out if refilling the trap doesn't help at all.
What to do
- Pour about a quart of water directly into the floor drain to refill the trap.
- Wait a day and see if the smell noticeably improves, which confirms a dry trap was the cause.
- Clean the drain grate and the visible area just below it of any dirt or debris using a brush.
- Set a recurring reminder to pour water down infrequently used floor drains every few weeks, especially in summer.
- Consider a drain trap seal product or trap primer if the drain dries out repeatedly despite regular watering.
- If the smell persists even with a full trap, treat it as a bigger vent or sewer line issue worth having a plumber check.
Tools and parts
- Bucket
- Brush
Floor drain trap sealSearch: “floor drain trap seal insert”
When to call a pro
Call a plumber if refilling the trap doesn't stop the smell, since it may point to a venting or sewer line problem.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.