Basement Floor Drain Smells Like Sewer? How to Fix It

Pour a gallon or two slowly into the drain now. If the smell is gone within an hour or so, that was it.

Graphic: summary of this guide, made by our team from the cited sources.

How to refill the trap the right way

1. Find every drain in the basement. Floor drains, laundry or utility sinks, a shower or toilet in a basement bathroom, and any garage drain. Minneapolis says plumbing fixtures need regular use so water stays in their traps (City of Minneapolis).
2. Pour slowly. A gallon of water is enough to fill a trap, according to North Royalton's wastewater department (North Royalton). Minneapolis suggests a bucket (City of Minneapolis). Pouring slowly lets the water settle into the trap instead of rushing past.
3. Watch for backup. If water rises in the drain instead of going down, the drain line is clogged. Stop and see the backup section below.
4. Set a reminder. Refill rarely used drains on a schedule. If the smell comes back sooner each time, something is pulling water out of the trap or bypassing it.

The water seal in a trap is not deep. Residential plumbing code requires a seal of at least 2 inches and no more than 4 inches (IRC P3201.2 via UpCodes). A few weeks of evaporation in a warm, dry basement can lower it enough to let gas through.

If the smell comes back

Clue	Likely cause	Fix
Smell returns after weeks	Trap evaporates between uses	Refill on a schedule, or add a trap seal device
Smell returns in a day or two	Trap leaking, or seal being pulled out	Plumber to inspect the trap and vent
Missing or loose cap inside the drain	Open cleanout plug may let gas bypass the trap	Replace the cleanout plug
Gurgling in the drain when upstairs fixtures run	Blocked plumbing vent	Check the roof vent; call a plumber
Water backing up out of the drain	Clog in the drain line or sewer lateral	Call a plumber or your city sewer department

Sources: dry traps and vents from City of Minneapolis and Wisconsin DHS; trap seal protection from IRC P3201.2 via UpCodes.

Look for a cleanout plug

Some basement floor drains have a small cleanout opening inside the drain bowl, closed with a plug or cap. Shine a flashlight under the grate and check whether there is one. If a plug is missing, cracked or loose, sewer gas may be able to bypass the water in the trap. Bring the old plug or the measurement of the opening when you shop for a replacement, and ask a plumber to look if you are not sure what you are seeing.

Stop the trap from drying out

Building codes treat this as a known problem. The residential code requires floor drain traps to have a trap primer or a deep-seal design, says trap seal primer valves connect to the trap above the level of the seal, and recognizes barrier-type devices that protect a floor drain trap seal from evaporation (IRC P3201.2). For a homeowner, that means:

1. Refill on a schedule. North Royalton says a gallon of water is enough to fill a trap and recommends running water through rarely used floor drains, garage drains and utility sinks (North Royalton).
2. Add a barrier-type seal. These insert into the drain and help keep the trap seal from evaporating. Ask your plumber or local building department which devices are approved where you live.
3. Ask a plumber about a trap primer if the drain is in a dry, warm area and dries out quickly.

What the code says about floor drains

The residential code treats floor drains as a known trouble spot. It says traps for floor drains must have a trap primer or be a deep-seal design (IRC P3201.2 via UpCodes). Many older homes have neither. If your drain dries out quickly, a plumber can tell you whether a primer, a deep-seal trap or a barrier insert fits your drain.

Is it sewer gas, or a different basement smell?

Not every basement odor comes from the drain. Use the smell itself as a clue:

Smell	Likely source	What to do
Rotten eggs, strongest at the drain	Sewer gas through a dry trap or open cleanout	Refill the trap, check the plug
Rotten eggs through the house, not just at the drain	Possible gas through a blocked vent or foundation crack	Check the roof vent; call a plumber
Musty or earthy	Mold or dampness	Find the moisture source and dry it
Sewage smell plus water on the floor	Drain backup	Stop using water; call a plumber or the city

Sources: sewer gas entry points from Wisconsin DHS; mold and moisture from EPA.

For musty smells, the EPA says to dry wet materials within 24 to 48 hours after a leak, and to keep indoor humidity below 60 percent, ideally between 30 and 50 percent (EPA). A humidity meter costs about $10 to $50 at hardware stores, according to the EPA (EPA). A dehumidifier helps in most basements.

If the drain backs up

A backup is a different problem from a smell. Minneapolis says most sewer backups happen when roots grow into a cracked or broken private sewer lateral. Pipes in poor condition, ground movement, heavy rain and misuse of the sewer can also cause them (City of Minneapolis).

What to do:

• Stop running water, washing laundry and flushing until the drain is clear.
• Call your city's sewer department. In Minneapolis, a crew checks whether the blockage is in the city sewer or your private line at no charge (City of Minneapolis). Many cities offer something similar.
• If the clog is in your private line, call a drain cleaning service.
• After a sewage backup, Minneapolis suggests a professional cleaning company to make sure no health hazards remain (City of Minneapolis).

Check the vent

The Wisconsin Department of Health Services says sewer gas can also enter through a leaking or blocked roof plumbing vent or through cracks in the foundation, and suggests occasionally checking the vent for debris like leaves or bird nests (Wisconsin DHS). Do that from the ground with binoculars, or have a plumber or roofer check it; don't climb onto the roof yourself.

If other drains smell too, the same causes apply; see why a bathtub drain smells for the water-heater and stopper causes.

When it's a safety issue

The Wisconsin Department of Health Services says sewer gas is a mix that can include hydrogen sulfide, ammonia and methane, and that hydrogen sulfide smells like rotten eggs. Headaches, dizziness, drowsiness and nausea can be signs of exposure, and you should see a doctor if you have them and suspect a leak. If you believe sewer gas levels are high in an enclosed space, leave the building, call the fire department, and avoid sparks or flames (Wisconsin DHS).