What It Is
A floor drain is a drain fixture installed flush with the floor surface to collect water that accumulates on the floor and direct it into the drainage system. They are common in basements, laundry rooms, utility areas, and garages. A floor drain serves as both a safety overflow point for water heater leaks, washing machine overflow, and flooding - and, because it is the lowest drain in the house, the first place sewage appears when the main sewer line backs up.
What It Does
The floor drain's primary job is to route any water on the floor into the drain system quickly. In a basement, this means water from a leaking water heater, a sump pump that overflows, a washing machine discharge, or groundwater that seeps in during heavy rain can all exit through the floor drain rather than pooling on the floor.
Because floor drains connect directly to the sewer line, they also have a trap - a curved section of pipe that holds water and prevents sewer gas from entering the room. This trap must stay wet to function. In rooms where the floor drain rarely receives water, the trap can evaporate and allow sewer odors into the space.
Where It Is Located
Floor drains are typically installed at the low point of the floor so water flows toward them by gravity. In a basement, there is often one central floor drain, sometimes near the water heater or washing machine. Older homes may have multiple floor drains throughout the basement. The drain cover is a flat grate or strainer set flush with the concrete floor.
How It Works
Water flows across the floor, into the drain opening, through the trap below the floor, and into the drain-waste system. The trap holds a small reservoir of water that seals the drain against sewer gas entry - the same principle as the p-trap under a sink. Some floor drains have a primer valve that automatically adds water to the trap periodically to prevent evaporation. Others simply rely on water from normal use to keep the trap wet.
Common Problems or Signs of Failure
- Sewer odor coming from the floor drain - usually indicates a dry trap
- Water or sewage coming up through the floor drain - indicates a main sewer line backup
- Slow-draining floor drain that pools water before it drains
- Floor drain that is completely sealed with concrete or covered with stored items (a code issue and a flood risk)
Can a Homeowner Check It?
- If the floor drain smells, pour a gallon of water into it - this refills the trap and should eliminate the odor within a day. If the smell returns quickly, the trap may not be holding water properly.
- Check that the drain cover is not permanently sealed or obstructed. A floor drain covered by stored items can't serve its safety function.
- If water has appeared around the floor drain without a clear source, that's a sewer backup indicator - stop using water in the house and call a plumber.
When to Call a Plumber
Call if sewage or water is coming up through the floor drain - that is a main sewer blockage. Also call if pouring water in the drain doesn't resolve odors, which may indicate a failed or missing trap. Some older basement floor drains have degraded traps that no longer hold water and need to be replaced.
Related Plumbing Problems
- Why Is Water Coming Up Through My Basement Floor?
- Why Does My Drain Smell?
- Why Is My Basement Flooding?
Related Plumbing Terms
Related Gray's Services
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