Short definition
A floor drain is a trapped, grated drain set into a basement, laundry-room, or garage floor. Its job is to catch incidental water — washer overflow, water-heater leaks, melting snow off boots — and route it to the building drain. Like every drain, it has a trap; the trap needs water in it to keep sewer gas out, which means an unused floor drain will eventually start to smell.
What it is
A floor drain is a 2-inch-or-larger drain with a grate at floor level and a trap directly underneath. The trap may be integral to the body or a separate P-trap below the floor. By UPC 1002, the minimum trap size for a floor drain is 2 inches, and the drainage fixture unit (DFU) value is 2 — small enough that a single floor drain rarely changes the sizing of a branch.
In WA basements and laundry rooms, the floor drain often shares its trap with the laundry standpipe (the upright pipe that catches the washer’s drain hose). That sharing is allowed by code as long as the standpipe is properly trapped and vented.
Why it matters to a homeowner
The floor drain is your basement’s safety net. When a water heater fails, a washer hose splits, or a hose bib drips off a boot, the floor drain is what keeps a small problem from becoming a big one. But the trap that protects you from sewer gas needs water in it, and an unused basement floor drain will dry out — usually around late summer, when humidity drops and any residual water evaporates. The classic symptom is a sewer smell that “comes and goes” in the basement, especially in summer.
The fix is the simplest possible plumbing maintenance: pour a quart of water into the floor drain once a month. In a vacation home or rental that sits empty for stretches, ask whoever checks on the place to do this every visit.
Common failure modes
- Trap dries from disuse — sewer gas enters through the dry seal. Fix: pour a quart of water in.
- Sediment-clogged grate — backs up during washer drain or heavy rain. Fix: lift the grate, vacuum out the debris.
- Backflow during heavy rain — in homes connected to a combined sewer, water can come up through a basement floor drain when the city main surcharges. Fix: a backwater valve on the building drain.
- Wrong cover — a non-matching grate restricts flow.
Washington note
In Puget Sound homes connected to combined-sewer mains (the older Seattle neighborhoods, downtown Tacoma), the floor drain is the most likely point where sewage will surface during a heavy-rain main backup. If you’re in one of those neighborhoods and you have a basement floor drain or a basement bathroom, a backwater valve is the standard protection. Confirm the connection type with your utility before assuming you don’t need one.