Why a Dried-Out Floor Drain Lets Sewer Gas Into Your Home

TL;DR: A floor drain relies on a small amount of standing water in its P-trap to seal off sewer gas from the rest of your house. That water evaporates in 2 to 4 weeks with no use, especially in a dry basement or a rarely-used utility room, and once it's gone, methane and hydrogen sulfide gas can rise straight through the pipe into your living space. The fix costs nothing: pour a quart of water down every unused floor drain once a month, or use a trap seal device if the drain sits empty for longer stretches.

_Last reviewed: July 2026 · 6 min read_

That sour, rotten-egg smell coming from the basement floor drain isn't your imagination, and it's not "just an old house smell." It usually means the water seal in that drain has evaporated, and your home's plumbing vent system is now venting sewer gas directly into your basement air.

Okoniq Property Hub helps you log which drains in a property need a seasonal water top-off, so a dry trap doesn't turn into a recurring odor complaint or a missed inspection item.

What is a floor drain's P-trap and why does it need water in it?

Every drain in your home, floor drains included, has a curved section of pipe called a P-trap that holds a small pool of water at all times. That water acts as a physical seal, blocking sewer gas from traveling back up through the pipe and into the room.

Floor drains almost always sit in basements, garages, laundry rooms, and mechanical closets, spaces that don't get regular water flow the way a kitchen sink or shower does. A sink gets used daily, which constantly refreshes its trap. A floor drain might go months without a drop of water running through it unless the washing machine backs up or the sump pump discharges nearby. Without that refresh, the trap's water supply has nowhere to come from, and it slowly disappears through evaporation.

How does a floor drain trap actually dry out?

Evaporation, not a leak, is the usual cause, and it happens faster than most homeowners expect. In a heated basement with low humidity, the water in a floor drain trap can fully evaporate in 2 to 4 weeks. Run a furnace or dehumidifier nearby and that timeline shortens, since both pull moisture out of the air and speed up evaporation from any open water surface, including a trap.

Seasonal homes and vacation properties are especially prone to this. If you're closing up a seasonal home for months away, every floor drain in it will dry out completely before you return, guaranteed. The same applies to a rental unit sitting vacant between tenants, or a basement utility room in a house where the washer drains into a standpipe instead of the floor drain itself.

What does sewer gas smell like and why is it dangerous?

Sewer gas smells like rotten eggs or a sulfur odor, and that smell comes from hydrogen sulfide gas, a byproduct of decomposing waste in the sewer line. At low concentrations it's mostly a nuisance and an unpleasant one. At higher concentrations in a poorly ventilated space, hydrogen sulfide can cause headaches, nausea, and eye irritation, and methane, which is also present in sewer gas, is flammable.

The real risk isn't usually acute poisoning from a single dried trap in a house, but the fact that persistent sewer gas exposure signals a plumbing system that isn't sealed the way it should be. If you smell it and can't trace it to an obvious dry floor drain, it's worth ruling out a cracked vent stack or a failing seal elsewhere in the system, sometimes tied to aging cast iron drain pipe that's corroded enough to let gas escape from a joint rather than the trap.

| Cause | Smell pattern | Fix | |---|---|---| | Dried-out floor drain trap | Constant, strongest near the drain | Pour water down it | | Cracked vent pipe or joint | Comes and goes, hard to pinpoint | Needs a plumber | | Dry trap under a rarely-used sink | Localized to that fixture | Run the tap |

How do you fix a dried-out floor drain trap for good?

Pour about a quart of water directly into the drain, and the smell should disappear within minutes once the trap reseals. If the odor returns quickly, check that the drain grate isn't cracked and that there's no debris holding the trap open, since a clogged trap can behave the same way a dry one does. For a stubborn clog rather than a dry trap, our guide on when to snake a drain yourself versus calling a plumber covers how to tell the difference.

For long-term prevention, set a reminder to pour water into every floor drain once a month, especially in basements, garages, and any room with a basement dehumidifier running, since that unit will dry out nearby traps faster than normal. If a drain sits unused for a season or longer, a trap seal device, a rubber or mechanical one-way valve that fits over the drain opening, can hold the seal without needing manual refills.

Which drains in your home are most likely to dry out?

Floor drains aren't the only culprits. Any fixture that isn't used regularly, a guest bathroom sink, a laundry room standpipe, a basement wet bar, or a drain behind a water heater, can dry out the same way. Landlords managing a vacant unit should walk every drain in the property before showings, since a sewer gas smell during a walkthrough is one of the fastest ways to lose a prospective tenant's confidence in the property.

FAQ

How often should I pour water into a floor drain?

Once a month is enough in most homes, and more often, roughly every 2 weeks, in a heated basement running a dehumidifier or in a dry climate.

Can a dried-out floor drain actually make you sick?

At typical household concentrations, sewer gas is more of a nuisance and an early warning sign than an acute health hazard, but prolonged exposure can cause headaches and nausea, and methane buildup in an enclosed space is a fire risk.

Why does my basement smell like sewer gas even though the floor drain has water in it?

A cracked trap, a loose drain grate, or a failing seal on a nearby cast iron pipe joint can all let gas escape even with water present, so if refilling the drain doesn't stop the smell within a day, the problem is likely elsewhere in the system.

Do all floor drains have a P-trap?

Nearly all do, since it's required by plumbing code, but very old homes or drains added without a permit sometimes lack one, which means the smell will never fully go away no matter how much water you add.

Will a trap seal device work instead of pouring water regularly?

Yes, a trap seal device sits in the drain and creates a mechanical or rubber-flap seal that doesn't rely on standing water, making it a good option for drains in vacant units or seasonal properties.

This is educational information, not plumbing or safety advice. Call a licensed plumber if a sewer gas smell persists after refilling the trap, or if you suspect a cracked vent or drain pipe.