The fastest way to stop sewer gas is to find which drain is letting it through and seal it
Sewer gas enters your home through open or damaged drains, usually in bathrooms or kitchens. The smell comes from hydrogen sulfide and methane rising from the sewer line. You stop it by restoring the trap seal—the water barrier that sits inside every drain and blocks gas from coming back up—or by sealing drains you do not use.
Most sewer gas problems are fixable in under an hour without calling a plumber. The source is almost always a dried-out trap, a loose drain connection, or a vent pipe that is blocked or damaged. Start by identifying which room smells worst, then work through the fixes in order of simplicity.
Key Takeaways
- A dried-out trap seal is the most common cause; running water down the drain refills it and stops the smell within minutes.
- If the smell is strongest near a toilet, check the wax ring seal under the base—a failed wax ring lets gas leak into the room.
- Blocked vent pipes on your roof prevent air from flowing through the drain system, which can push gas back into the house.
- Drains you rarely use, like a basement floor drain or guest bathroom, dry out and lose their seal; pour water down them monthly to keep the trap full.
- A plumber should inspect if the smell persists after you have tried these steps, because it may signal a cracked vent pipe or sewer line damage.
Refill dried-out trap seals by running water down every drain
Every drain in your home has a P-trap or S-trap—a curved section of pipe that holds standing water. This water blocks sewer gas from rising back up. If a drain sits unused for weeks or months, the water evaporates and the seal breaks. The fix is straightforward: run water down the drain for 30 seconds to refill it.
Start with the drains you use least: the guest bathroom sink, the basement floor drain, the laundry tub, or any shower you do not use regularly. Pour a bucket of water down each one. If the smell was coming from that drain, it should fade within an hour. To prevent it from happening again, run water down rarely-used drains once a month—even just 10 seconds is enough to keep the trap full.
If you have a basement floor drain that smells, pour a gallon of water down it and then add a cup of mineral oil on top. The oil floats on the water and slows evaporation, keeping the seal intact longer between uses.
Check the wax ring seal under the toilet base
A toilet sits on a wax ring that seals the gap between the toilet base and the drain opening below. If this ring fails—usually from age, settling, or a loose bolt—sewer gas leaks into the room around the toilet. You will smell it strongest right at the base of the toilet, and you may also see water stains on the floor or ceiling below.
A failed wax ring requires removing and resetting the toilet, which is a job for a plumber unless you have done it before. The cost is usually $150 to $300. Do not ignore this problem: a failed seal also lets sewage seep into the subfloor, which causes rot and mold. If the smell is concentrated around the toilet and refilling other drains did not help, call a plumber to inspect the wax ring.
Clear blocked vent pipes on your roof
Your home has vent pipes that run from the drain system up through the roof. These pipes let air into the drains so water flows smoothly and gas flows out. If a vent pipe is blocked—by leaves, bird nests, ice, or debris—air cannot flow, and pressure builds in the drain system. This pressure pushes sewer gas back into the house through drains and fixtures.
Go to your roof and look for a 2- to 4-inch pipe sticking up. If you see leaves, nesting material, or ice around the opening, that is likely your problem. Use a plumbing snake or a stiff wire to clear the blockage, or flush it with a garden hose. If you cannot safely reach the roof, a plumber can do this in 30 to 60 minutes for $100 to $200.
After clearing the vent, the smell should stop within an hour. If it does not, the vent pipe itself may be cracked or damaged below the roof line, which requires professional inspection.
Seal unused drains with a drain cap or plug
If you have a drain you never use—an old sink in a basement, a floor drain in a rarely-entered room, or a shower in a guest space—you can seal it permanently instead of maintaining the trap. Buy a drain cap or test plug from a hardware store (they cost $5 to $15) and install it according to the package directions. Most screw in or push in from above.
Sealing a drain is permanent, so make sure you will not need it again. If you might use it someday, stick with the monthly water-pouring routine instead. A sealed drain that you later need to use can be opened by unscrewing or prying out the cap.
Inspect drain connections under sinks for leaks or gaps
Under a sink, the drain pipe connects to the wall or floor. If this connection is loose, cracked, or missing a coupling, sewer gas can escape into the cabinet. Open the cabinet under your kitchen and bathroom sinks and look for gaps, water stains, or loose fittings.
Tighten any loose connections by hand or with a wrench—turn clockwise. If you see a crack in the pipe, that section needs to be replaced; this is a plumber's job. If the connection looks fine but you smell gas from that cabinet, the trap itself may be cracked, which also requires replacement.
Check for a damaged or missing cleanout cap
Your main sewer line has access points called cleanouts—usually a 4-inch pipe with a removable cap, often in the basement or crawlspace. If the cap is loose, cracked, or missing, sewer gas leaks directly into your home. Look for the cleanout and check that the cap is tight and not damaged.
If the cap is loose, tighten it by hand or with a wrench. If it is cracked or missing, buy a replacement cap at a hardware store (they cost $10 to $30) and install it. If you cannot find the cleanout or the cap will not tighten, a plumber can locate it and seal it for $75 to $150.
When to call a plumber for sewer gas
Call a plumber if the smell persists after you have refilled trap seals, cleared vent pipes, and checked drain connections. A lingering smell may signal a cracked vent pipe inside the wall, a damaged section of the sewer line, or a failed trap that needs replacement.
Also call if you smell sewer gas throughout the house rather than in one room, or if the smell is strongest in the basement or crawlspace. These patterns suggest a problem deeper in the system that requires professional diagnosis. A plumber will use a camera to inspect the line and identify the exact source.
Frequently Asked Questions
Why does my drain smell like sewer gas even after I run water down it?
The trap may be refilling but the smell is coming from a different source—check other drains, the toilet wax ring, and vent pipes before assuming the first drain is the problem. If the smell is truly from that drain and refilling does not help, the trap itself may be cracked or the vent pipe serving that drain may be blocked.
Can I pour bleach down a drain to get rid of sewer gas?
Bleach kills some odor-causing bacteria but does not fix the underlying problem—a broken seal or blocked vent. It may mask the smell temporarily but the gas will return. Focus on refilling the trap or clearing the vent instead.
Is sewer gas dangerous to breathe?
Sewer gas is unpleasant and can cause headaches or nausea in high concentrations, but brief exposure in a home is not usually dangerous. However, it signals a problem with your drain system that should be fixed, and a failed wax ring can lead to sewage seeping into your home, which is a health hazard.
How often should I pour water down drains I do not use?
Once a month is enough to keep the trap seal intact. Pour water for 10 to 30 seconds. If you have a floor drain, pour a gallon and add mineral oil on top to slow evaporation between pours.
What does it mean if sewer gas smell comes and goes?
An on-and-off smell usually means a trap is slowly drying out, or a vent pipe is partially blocked and only backs up under certain conditions—like when multiple drains run at once or when wind direction changes. Refill the trap and clear the vent to stop the cycle.