Where the smell is coming from and how to stop it
Sewer gas smell usually means one of five things: a dry P-trap under a sink or floor drain, a cracked vent pipe on your roof, a broken sewer line, a clogged drain, or a loose toilet wax ring. The fastest way to find out which is to locate where the smell is strongest—a single bathroom points to that toilet or sink, while smell throughout the house usually means the vent stack or main line. Once you know the location, most fixes are straightforward enough for a homeowner, though a cracked sewer line or vent pipe will need a plumber.
The smell itself is hydrogen sulfide gas, which your sewer system produces naturally. It normally vents safely up through the roof vent stack. When that system fails—when a trap dries out, a vent gets blocked, or a seal breaks—the gas backs up into your home instead.
Key Takeaways
- A dry P-trap under a sink or floor drain is the most common cause and the easiest fix: run water down the drain for 30 seconds to refill it.
- Check your roof vent stack for blockages (leaves, nests, ice) and make sure it extends at least 12 inches above the roof line.
- A loose or worn toilet wax ring can be tightened by a plumber in under an hour, or replaced if it has failed.
- If the smell is strong and comes from the ground around your home or from multiple drains at once, the sewer line may be cracked and needs a plumber to inspect.
Check and refill dry P-traps under sinks and drains
Every sink, shower, and floor drain has a P-shaped bend in the pipe below it called a P-trap. This trap holds standing water that blocks sewer gas from rising back up. If a drain sits unused for weeks or months, the water evaporates and the trap dries out. Gas then flows straight into your home.
To fix this, run water down the drain for 30 seconds. Hot water works better than cold because it flows faster and fills the trap more completely. If you have a floor drain in a basement or crawlspace that you rarely use, pour a bucket of water down it once a month to keep the trap full. If the smell returns within a few days of running water, the trap itself may be cracked or the drain may be clogged further down—both require a plumber.
Look for blockages in your roof vent stack
Your home's plumbing vents through a pipe that runs up through the roof. This vent stack lets sewer gas escape safely into the air and also lets air into the pipes so water drains properly. If the vent gets blocked, gas backs up into the house and drains slow down or gurgle.
Go to your roof (safely, on a clear day) and look at the vent pipe. It should extend at least 12 inches above the roof line. Check for leaves, bird nests, ice buildup, or debris blocking the opening. Clear away anything you can reach by hand. If the vent is clogged further down the pipe, or if you are not comfortable on a roof, call a plumber—they can snake the vent or use a camera to see what is blocking it.
In winter, ice can form around the vent opening and trap gas. If you live in a cold climate and the smell appears only in winter, this is likely the cause. A plumber can install a vent cap designed to shed ice and snow.
Inspect the wax ring under your toilet
The wax ring is a rubber-and-wax seal that sits between the toilet base and the flange (the fitting that connects to the sewer line). Over time, this ring hardens and shrinks, or the toilet rocks slightly and breaks the seal. When the seal fails, sewer gas leaks out around the base of the toilet.
If the smell is strongest right around the toilet, and especially if you notice water stains on the floor or ceiling below, the wax ring has likely failed. This is not a repair you should attempt yourself—the toilet has to be removed and reset, which requires turning off the water, draining the bowl, unbolting the toilet, and installing a new wax ring. A plumber can do this in under an hour. The cost is usually $150 to $300 depending on your area.
Check for cracks in your sewer line
If the smell is strong and comes from multiple drains in your home, or if you notice wet patches in your yard or smell sewer gas near the ground outside, your main sewer line may be cracked. Tree roots often grow into sewer lines, or the line can crack from age or ground settling. When this happens, gas leaks out into the soil and into your home.
A cracked sewer line is not something you can fix yourself. Call a plumber and ask them to run a camera inspection (called a sewer scope). This lets them see exactly where the crack is and how bad it is. Repair costs vary widely depending on the location and severity—anywhere from $3,000 to $25,000 or more if the line has to be dug up and replaced. Some cracks can be sealed from inside the pipe without digging, which costs less.
If you suspect a sewer line problem but the smell is mild and localized, start with the simpler fixes first (dry traps, vent blockages, wax ring). A plumber can help you narrow down the cause before you commit to an expensive inspection.
Seal gaps around pipes and clean your drains
Sewer gas can also enter through small gaps where pipes pass through walls or floors. Look under sinks and in basements for holes or cracks around pipes. Seal these with caulk or expanding foam. This is a quick fix that costs almost nothing and can eliminate smell that leaks in from outside the main plumbing system.
A slow or clogged drain can also trap water and create conditions where gas builds up. If you notice drains running slowly or smell coming from a specific drain, try pouring boiling water down it, or use a plunger to clear a blockage. If the drain stays slow, a plumber can snake it to remove buildup.
When to call a plumber
Call a plumber if the smell persists after you have refilled dry traps and cleared the vent stack. Also call if you see water pooling around the toilet base, if multiple drains smell at once, if the smell comes from outside your home, or if you hear gurgling sounds from drains. These are signs of a problem that requires professional tools or informed—a camera inspection, a vent snake, a wax ring replacement, or a sewer line evaluation.
Most plumbers can diagnose the source of sewer gas smell in one visit. They will ask where the smell is strongest, check the obvious spots (traps, vent, toilet), and run a camera if needed. This diagnostic visit usually costs $100 to $200 and will tell you exactly what needs to be fixed.
Frequently Asked Questions
Can sewer gas make me sick?
Sewer gas in small amounts is unpleasant but not dangerous. Hydrogen sulfide has a rotten-egg smell and can irritate your eyes and throat at high concentrations, but you would need to be exposed to very high levels for it to cause serious harm. If the smell is mild, it is a nuisance and a sign something needs fixing, but not an emergency. If you feel dizzy, nauseous, or have trouble breathing and the smell is very strong, leave the house and call a plumber or your local health department.
Why does the smell come and go?
Smell that appears and disappears often points to a dry trap. If a drain is rarely used, the water evaporates over a few weeks. Running water down the drain refills it and the smell stops—until the water evaporates again. Weather can also affect it: wind can push gas back into the house, or temperature changes can affect how gas moves through pipes. If the smell is constant, the cause is usually a vent blockage, cracked line, or failed wax ring.
Is it safe to use drain cleaner to fix sewer gas smell?
Drain cleaner will not fix sewer gas smell unless the smell is caused by a clogged drain. If the problem is a dry trap, vent blockage, or wax ring, drain cleaner will not help and can damage pipes or create toxic fumes. Run water down the drain first. If the smell goes away, the trap was dry. If it stays, the problem is elsewhere and you should call a plumber rather than use chemicals.
How much does it cost to fix sewer gas smell?
Refilling a dry trap costs nothing. Clearing a vent blockage costs $150 to $300 if a plumber has to go on the roof. Replacing a wax ring costs $150 to $300. A sewer scope camera inspection costs $300 to $500. Repairing a cracked sewer line costs much more and depends on how deep the line is and whether it can be sealed from inside or has to be dug up.