Why do my drains smell like sewage?
Updated August 2026
When a drain gives off a sewer odor, the most common and most harmless cause is a dried-out P-trap in a fixture nobody uses much, letting sewer gas rise straight up the empty pipe. The U-shaped trap under every drain is meant to hold a plug of water that blocks the gas, and it evaporates in a guest bath or floor drain that sits idle. Run water down it for a minute to refill the trap. If the smell persists, a failed toilet wax ring or a blocked vent is next.
Safety first: sewer gas is mostly methane and hydrogen sulfide, and while a whiff from a dry trap is harmless, a strong, persistent smell is worth taking seriously. In enough concentration hydrogen sulfide is toxic and methane is flammable, so ventilate the room, do not ignore a smell that keeps returning, and never light a flame in a space that reeks of gas. If the odor is strong, house-wide, and paired with slow drains, treat it as a possible sewer backup and get it looked at rather than living with it.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Dried-out P-trap in an idle drain | Most common | Run water for 30–60 seconds to refill the trap; add a little mineral oil in rarely used drains to slow evaporation. | Free |
| Failed toilet wax ring | Common | A leaking wax seal lets gas (and sometimes water) escape at the toilet base. Pull the toilet and set a new ring. | $5–30 part |
| Blocked or poorly vented stack | Occasional | A plugged vent pulls trap water out by siphon, releasing gas. Clear the roof vent or correct the venting. | $150–400 |
| Biofilm buildup in the drain | Occasional | Gunk coating the pipe walls and overflow rots and smells. Clean the drain and overflow with a brush and enzyme cleaner. | Free / $10–20 |
| Cracked or leaking drain line | Occasional | A cracked pipe or loose fitting in a wall or crawlspace vents gas into the house. A plumber locates and repairs it. | $150–600 |
| Developing main-line problem | Rare | A widespread sewage smell with slow drains can mean a main-line issue. A camera inspection finds it. | $250–500 inspection |
Check these first
- Find the idle drain. Sniff around guest baths, basement floor drains, laundry sinks, and unused showers, since those are where traps dry out.
- Pour a bucket of water down every suspect drain and floor drain. If the smell fades after refilling the traps, that was the cause.
- Smell around the base of each toilet. A sewer odor strongest at the floor line points at a failed wax ring, not a trap.
- Notice timing. A smell that appears on a windy day or when the furnace kicks on can mean a venting problem pulling gas indoors.
- Check for slow drains alongside the smell. Odor plus sluggish drainage across fixtures shifts suspicion toward the main line.
Prevent it next time
Most sewer smells are prevented by keeping trap water where it belongs. Run water through guest-bath and floor drains every few weeks so their traps never dry out, and pour a cup of water (or a little mineral oil to slow evaporation) into drains you rarely use. Keep drains clear of the biofilm that smells, and stay current on sewer-line cleaning and a periodic camera inspection, since a well-kept clay or cast-iron line lasts 50–60 years. The sewer-line checklist ties it together.
When to call a pro
Call a plumber when refilling the traps does not clear the smell, when a toilet base consistently reeks (a wax-ring or flange repair), or when you suspect a blocked vent or a cracked line inside a wall. Resetting a toilet with a new wax ring runs $150–350, clearing or correcting a vent $150–400, and a camera inspection to trace a hidden leak or main-line issue $250–500. A house-wide sewage smell paired with slow drains is not a wait-and-see problem; get it diagnosed before it becomes a backup.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How do I stop a floor drain from smelling like sewage?
Pour a quart or two of water down it to refill the P-trap, which is almost always the cause in a drain that sits unused. For a drain that dries out repeatedly, add a few ounces of mineral oil or cooking oil on top of the water; the oil floats and slows evaporation so the trap stays sealed for months. If the smell returns fast even with water in the trap, the problem is not evaporation but a venting issue or a cracked line, and that needs a plumber.
Is sewer gas in the house dangerous?
A faint, occasional whiff from a dry trap is harmless, but a strong, constant sewer smell should not be ignored. Sewer gas contains hydrogen sulfide, which is toxic in high concentration, and methane, which is flammable, so a persistent heavy odor in a closed space is a real concern. Ventilate the area, find and fix the source (dry trap, failed wax ring, or blocked vent), and if the smell is strong throughout the house, treat it as a possible backup and get it inspected promptly.
Why does my bathroom smell like sewage only sometimes?
Intermittent smells usually point at venting or a partial seal rather than a fully dry trap. A blocked or undersized vent lets pressure changes pull water out of the trap when other fixtures drain, so the smell comes and goes with water use. Wind and barometric shifts can push gas back down a vent on some days and not others. A wax ring starting to fail may only leak odor when the toilet is used a certain way. If refilling the trap does not end it, have the vent and wax ring checked.
Can a dry P-trap in one bathroom smell up the whole house?
It can, especially with forced-air heating or cooling moving air around. Sewer gas rising from one unused shower or floor drain gets picked up by return-air currents and spread room to room, so the source often is not where the smell is strongest. That is why the fix starts with walking the house and pouring water into every seldom-used drain and trap. Refill them all, wait a day, and the odor is usually gone without ever calling anyone.