Short answer: Most sewer smells come from sewer gas getting past a plumbing seal. The usual suspects, in order: a dry P-trap in a drain you rarely use (basement floor drain, guest shower, laundry standpipe), a toilet with a failed wax ring, a blocked or damaged roof vent, and a cracked or backed-up sewer line. If the smell is strong, comes with gurgling drains or backups, or makes you feel sick, ventilate the house and call a plumber.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Smell near a floor drain or unused shower | Dry P-trap (the water seal evaporated) | Pour a few quarts of water into the drain | Only if refilling does not help |
| Smell around the base of one toilet, maybe a wobble | Failed wax ring or loose toilet | Rock the toilet gently; look for water at the base | Yes, if you are not comfortable resetting a toilet |
| Smell when it rains or when fixtures drain, gurgling sinks | Blocked or damaged plumbing vent | Listen for gurgling when the tub or washer drains | Yes (roof work) |
| Smell plus slow drains in several fixtures | Partial blockage in the main sewer line | Check whether more than one drain is slow | Yes |
| Rotten-egg smell only from hot water taps | Water heater reaction, not sewage | Compare hot and cold water at the same tap | See our water heater guide |
| Smell under a sink, in a cabinet or wall | Loose slip joint, cracked pipe or open pipe | Look and sniff under sinks with a flashlight | If the leak is in a wall |
1. A dry P-trap (the most common cause)
Every drain has a curved pipe, the P-trap, that holds a little water. That water is the seal that keeps sewer gas out of the room. In a drain that is rarely used, such as a basement floor drain, a guest bathroom shower or a laundry standpipe, the water slowly evaporates and the gas comes straight in.
Oatey, a plumbing products manufacturer, lists dry traps first among the causes of sewer gas odor and recommends running water through unused drains regularly, about a gallon a month.
- Pour two to three quarts of water into every drain you do not use often, including floor drains.
- Wait an hour. If the smell fades, the trap was dry.
- For drains that dry out again quickly, a little mineral oil on top of the water slows evaporation, or a one-way drain seal sized to the pipe can be installed.
2. A toilet that is not sealed
Toilets have no visible trap under the floor. The seal is a wax ring between the toilet and the floor flange. If the toilet rocks, was recently reset, or the flange sits too low, sewer gas can leak around the base.
Signs: the smell is strongest near one toilet, the toilet moves when you sit, or you see staining or water at the base. Replacing the wax ring means removing the toilet; many homeowners do it, but if the flange is broken or the floor is soft, it is a job for a plumber.
3. A blocked or damaged plumbing vent
Vent pipes run from the drain lines up through the roof. They let air into the system so that water flowing down a drain does not suck the water out of nearby traps. Leaves, nests, snow or a disconnected pipe in the attic can block or break a vent.
Typical clues: sinks or tubs gurgle when another fixture drains, the smell shows up when the washing machine empties or during windy or rainy weather, and traps keep losing their water even though the drains are used. Clearing a roof vent means working on the roof, so most people call a plumber or roofer.
4. A problem in the main sewer line
If several drains are slow, a floor drain or tub backs up when you flush or run the washer, or the smell is worst near the basement cleanout, the main line may be partly blocked by roots, grease or a broken pipe. This is the cause that gets worse and more expensive if it waits.
A plumber can run a camera through the line to see what is inside. Sewage backups are a health hazard: keep people and pets away from the water and do not try to clean up a backup without protection.
5. Other places the smell can come from
- Sink overflow holes: soap and gunk in the overflow channel of a bathroom sink can smell like sewage. Clean it with a bottle brush and hot water.
- Loose or open pipes: a slip nut under a sink that worked loose, or an old drain pipe that was left open or stuffed with a rag during a remodel.
- Hot water only: a rotten-egg smell that only comes from hot taps is usually the water heater, not the sewer. See hot water smells like rotten eggs.
- Not plumbing at all: natural gas has a rotten-egg odor added on purpose. If you think it could be gas, leave the house and call your gas utility from outside.
Smell only when it rains, after a shower or when doing laundry
When it rains: pressure changes and heavy flow in the sewer can push gas past weak seals, and a blocked vent makes this worse. Check vents and floor drains first.
After a shower or doing laundry: a large volume of water draining at once can siphon nearby traps when venting is poor. Gurgling at the same moment is the giveaway.
Comes and goes: traps that slowly dry out between uses, or a vent issue that depends on wind and weather.
When to call a professional
- The smell is strong, makes anyone dizzy or sick, or you cannot find the source.
- Sewage or dirty water comes up in a tub, shower or floor drain.
- More than one drain is slow or gurgles.
- You suspect a roof vent problem or a cracked pipe inside a wall or under a slab.
- You smell something like gas: leave first, then call the gas utility.
What it usually costs
Refilling a dry trap costs nothing. HomeGuide (2026) lists most plumbers at $45 to $150 per hour, with call-out fees usually $50 to $200. Clearing a clogged drain runs about $100 to $275, sewer line cleaning about $150 to $800, and a sewer camera inspection about $125 to $500. Prices vary a lot by region.
Frequently asked questions
Is sewer gas dangerous?
In the small amounts that usually cause a household smell, the main issue is the odor and what it says about your plumbing. Higher concentrations can cause headaches, nausea and dizziness, and sewer gas can contain methane. Ventilate, find the source and get it fixed.
How do I know if it is a dry trap?
Pour water into the drain you suspect. If the smell fades within an hour or so, the trap was dry. Rarely used floor drains and guest showers are the usual culprits.
Why does my house smell like sewage but there is no leak?
Gas can escape through a dry trap, a loose toilet seal or a blocked vent without any water leaking. The smell itself is the sign of a broken seal somewhere in the system.
Related problems
- Sewer smell in the bathroom
- Sewer smell in the basement
- Clog deep in the drain pipe
- Hot water smells like rotten eggs
Sources
- Oatey: Sewer gas odor in your house and what to do about it
- InspectAPedia: Sewer odor source list
- InspectAPedia: Drain piping odor FAQs
- HomeGuide: Plumber cost per hour (2026)
- HomeGuide: Plumbing inspection cost (2026)
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.