What a trap seal is and how it fails
Under every sink is a P-trap; every tub, shower and floor drain has the same thing built in, and a toilet has one cast into the china. Each one holds roughly two to four inches of water. Sewer gas — hydrogen sulfide, methane, and whatever else is decomposing downstream — cannot cross that water, so it goes up the vent stack through the roof instead. Everything on this page is a variation on one question: where did the water go?
What does not work is anything you spray. Air freshener, bleach poured down the drain, a scented tablet — all of these mask a gas that is still entering the room. If the smell comes back within a day, you did not fix anything.
Find the fixture first
| Where you smell it | Likely fixture | First move |
|---|---|---|
| Guest bathroom, basement, an apartment after a vacation | Any drain that has not run in weeks | Run every fixture for a minute and pour a quart of water into every floor drain |
| Kitchen, worse when the disposal runs | Disposal body and splash guard | Clean under the rubber splash guard. Biofilm there smells and is not in the trap at all. |
| Bathroom sink, constant, worse when you run water | Overflow channel or the trap joints | Bottle brush through the overflow hole; check the slip joints for a slow drip |
| Around the base of a toilet, worse after a flush | Wax ring or a loose closet flange | Rock the bowl. If it moves at all, the seal is broken. |
| Laundry area | Washer standpipe with no trap, or a trap emptied by the pump's discharge | Check there is a trap on the standpipe and that the hose is not pushed too far down it |
| Basement, no fixture nearby | Floor drain, a missing cleanout cap, or an ejector pit lid | Water into the floor drain; check every cleanout has its plug in |
| Whole house, worse on windy days or before a storm | Venting | See the venting section below |
The five failure modes
| Cause | Signature | Fix | Cost |
|---|---|---|---|
| Evaporation | An unused fixture. Vacation homes, spare baths, basement floor drains, the drain under a laundry sink nobody uses. | Pour a quart of water in. For a drain that will sit for months, add a few ounces of mineral oil on top of the water — it floats and slows evaporation. Then put a monthly reminder on the calendar. | Free |
| Siphonage | Smell appears right after a large flush somewhere else — the toilet, or the washing machine dumping. The trap gurgles. | A venting problem. The draining column pulls the seal out of a nearby trap because air cannot get in behind it. Sometimes a failed air admittance valve; sometimes a partly blocked vent stack. | $15 for an AAV, several hundred for vent work |
| Leaking trap or joint | Constant low-level smell, a damp cabinet floor, white crust on a slip nut | Hand-tighten the slip nuts; if that does not do it, replace the trap. A plastic P-trap assembly is about ten dollars and goes together without solvent or tools. | $10 |
| Biofilm | Smell only when water runs, and only at one fixture. The trap is full. | The growth is in the tailpiece, the overflow channel or under the disposal splash guard, above the water. Scrub it mechanically with a bottle brush and detergent. | $5 |
| Toilet seal | Smell at floor level near a toilet, sometimes staining or a soft spot in the floor | Pull the toilet, replace the wax ring, check the closet flange is not cracked and sits at the right height, reset with new bolts. A plumber charges $150 to $300; the parts are under twenty dollars. | $20 DIY |
Venting, and why a smell moves around
Drain lines need air behind the water or they siphon. That air comes from the vent stack that runs up through the roof, and in some parts of a house from an air admittance valve — a one-way check valve, usually under a sink or in an island cabinet, that lets air in but not gas out. Both fail in ways that produce exactly the symptoms above.
A roof vent can be blocked by a bird nest, a wasp nest, leaves, or in cold climates by frost closing the opening on a very cold morning. The tell is a smell that comes and goes with the weather, or that appears in a specific room only when a large fixture drains. An AAV is a rubber diaphragm that goes stiff with age; they last years, not decades, and swapping one is a fifteen-dollar part that threads onto the pipe by hand. If you have neither symptom under control after checking traps, that is the point where a plumber earns their fee, because diagnosing a vent means going on the roof or opening walls.
Baking soda and vinegar, honestly
They fizz, which is satisfying and does very little. The two neutralize each other on contact, and what is left in the pipe is dilute sodium acetate with no meaningful cleaning power. There is a small mechanical benefit from the foam lifting loose slime, and the smell of vinegar covers the smell you were chasing for a couple of hours, which is why people believe it worked. It has no effect whatever on a dry trap, a bad wax ring, or a venting fault — which is to say, on any of the actual causes. If you want to clean a tailpiece, a bottle brush and dish detergent does the job properly.
The smells that are not a drain
Two of them mean you stop reading and act.
A rotten-egg or sulfur smell is also what natural gas and propane are odorized with, deliberately, because the fuel itself has no smell. If it is sharp, if it is strongest near an appliance, a meter or a gas line rather than a drain, or if you have any doubt at all, leave the building without touching a light switch or a garage door opener, and call the gas utility from outside. They come out for this at no charge and would far rather it be nothing.
Sewer gas itself is not only unpleasant. Hydrogen sulfide is toxic at high concentration, and it deadens your sense of smell as the concentration rises, so a strong smell that seems to fade is not reassuring. Methane in a confined space is flammable. A basement full of sewer gas from a broken line is a ventilate-and-leave situation, not something to work in.
Before you call
| Check | What it rules out |
|---|---|
| Ran every fixture and poured water into every floor drain | Evaporation, the single most common cause. If the smell goes, you are done. |
| Looked in the cabinet under each sink with a flashlight | Leaking traps and loose slip joints |
| Tried to rock every toilet | Wax rings and loose flanges |
| Cleaned the disposal splash guard and each sink overflow | Biofilm |
| Noted when it happens — after a flush, on windy days, at night | Points at venting rather than a trap, which changes who you call and what you ask for |
| Checked every cleanout has its plug and the ejector pit lid is sealed | An open pathway that no trap can protect against |
If you live in an apartment or a condo, one more question is worth asking before you spend anything: does the unit next door or above have the same smell? A shared stack problem is the building's to fix, and a plumber working inside your unit alone will not find it.
Start with the water. It is free, it takes four minutes, and it solves more drain smells than every other item on this page put together. Related reading: clearing a clogged drain, since a partly blocked line and a smelly one often turn out to be the same job, and drain slope if a line has never drained cleanly since it was installed.
Questions people ask
Why does my guest bathroom smell like sewer?
The trap dried out. A P-trap holds a couple of inches of water and that water evaporates over weeks in a fixture nobody uses, opening a direct path from the drain line into the room. Run the sink and shower for a minute each, flush the toilet, and pour a quart of water into any floor drain. The smell usually goes within the hour. For a bathroom that stays unused, put it on a monthly reminder, or float a few ounces of mineral oil on the trap water to slow evaporation.
The smell only appears when someone flushes upstairs. Why?
That is siphonage, and it points at venting. A large volume of water dropping through a drain line pulls air behind it. If the vent cannot supply that air fast enough, it takes it from the nearest trap instead, and that trap loses its seal. The gurgle you hear is the sound of it happening. Causes include a blocked roof vent — nests, leaves, frost — or a failed air admittance valve under a sink. An AAV is a fifteen-dollar part you can thread on by hand; a blocked stack is a plumber going on the roof.
Does baking soda and vinegar fix a drain smell?
No. The two neutralize on contact and leave a dilute salt solution with essentially no cleaning power. The foam lifts a little loose slime and the vinegar smell masks the sewer smell for a few hours, which is why it feels effective. It does nothing at all about a dry trap, a leaking joint, a failed wax ring or a venting fault, which between them account for nearly every drain smell. Diagnose the cause first; if you do want to clean a tailpiece, use a bottle brush and dish detergent.
I smell sewer gas near the toilet, not the drain. What is it?
Almost certainly the wax ring between the toilet and the closet flange in the floor. Try to rock the bowl gently — any movement at all means the seal is compromised and gas is coming up around the base. The repair is pulling the toilet, scraping the old wax, checking the flange is intact and at the right height relative to the finished floor, and setting it on a new ring with new bolts. Parts are under twenty dollars; a plumber runs $150 to $300. If the floor around the base feels soft, stop and get someone to look at the subfloor, because it has been leaking for a while.
Is sewer gas in the house dangerous?
At the concentrations most household trap failures produce it is mainly unpleasant, but it is not harmless. Hydrogen sulfide is toxic at higher concentrations and it fatigues your sense of smell as it gets stronger, so a smell that seems to fade is not evidence that it is clearing. Methane is flammable in a confined space. Ventilate, find the cause, and treat a strong smell in a basement or crawl space as a reason to open up and leave rather than to work in it. And be sure it is sewer gas: natural gas and propane are odorized with a sulfur compound that smells similar. If there is any doubt, leave without touching a switch and call the gas utility from outside.