Dry Floor Drain Traps and Basement Sewer-Gas Smells

Words you will hear

Trap: A curved section of pipe that holds water as a barrier against sewer gas.
Trap primer: A device that adds a little water to a floor drain automatically.
Cleanout: A capped access point on a drain used for inspection or clearing.
Backflow: Water or waste moving the wrong way in a drain line.
Vent: A pipe that lets air into the drain system so water flows and traps hold.

Why basements are prone to this

A floor drain sees water rarely, since most basements are dry. In Denver's dry climate the water in a trap can evaporate over a few weeks, especially near a furnace. Once the seal is gone, air from the drain line rises into the room, and the result is a smell that people often describe as damp, sour or sewer-like.

The fix is to refill the trap and check whether the smell stops. If it does and stays gone for weeks, the trap was the cause. If the smell returns quickly, the trap may be leaking, the drain may be venting poorly or the drain line itself may be damaged. A camera inspection is the next step for the last case.

Some drains are fitted with a trap primer, but those need periodic checking. Others have a cap or plug, which should seat tightly.

Add the drain to a monthly routine. Setting a reminder to pour a cup of water takes seconds and prevents a recurring smell.

Testing and keeping a drain sealed

Smell at the drain with the cover off, then at floor level a meter away.
Pour in a quart of water and wait a few minutes to see if the smell fades.
Look for a grate or cover that is loose, rusted or missing.
Check for floor moisture near the drain that suggests a leak.
Set a reminder to refill the trap after long dry stretches.
Mention any gurgling or backups to a plumber.

Reading the result

IfThe smell stops after refilling and stays goneThenA dry trap was the cause, so add a routine.
IfThe smell returns in daysThenSuspect a leak, venting problem or damaged line.
IfThe drain backs up or gurglesThenCall a plumber promptly and avoid heavy water use.

Common questions

How often should I refill it?

Check monthly at first. If it dries out quickly, the schedule should be tighter.

General information for Hilltop, not a quote or professional advice. In an emergency, call 911.