If your basement smells like sewer gas, rotten eggs, or sulfur, the answer is almost always one of these eight things: a dried-out floor drain, a bad toilet wax ring, a cracked or loose pipe, a clogged plumbing vent, a failed sewer ejector pump, a clogged or damaged main sewer line, a malfunctioning sump pump, or an open cleanout. Nine times out of ten, it is the floor drain. That one you can usually fix in two minutes with a glass of water.
The hard part is figuring out which of the eight you have. The rest of this guide walks through each cause, how to identify it, how to fix it, and what it typically costs if you need a plumber. There is also a section on when this smell becomes an actual health emergency, because sewer gas can be dangerous at high concentrations and some homeowners wait too long to act.
Read the next section first if you want to skip diagnosis and just figure out whether the smell is dangerous right now.
Is Sewer Gas in My Basement Dangerous?
Mostly no. Sometimes yes. Depends what’s in the air and how much.
Sewer gas is a cocktail. Hydrogen sulfide gives you the rotten egg smell. Methane comes along too, which becomes a real problem if it builds up enough to ignite (anywhere from 5 to 15 percent of the air volume in the room). Carbon dioxide rides along. So does ammonia. Plus various other compounds your sewer line picks up.
At the levels you can smell in a normal basement, none of these are about to kill you if you’re a healthy adult. That’s the honest answer. But “not about to kill you” isn’t the same as “harmless.” Headaches show up first. Then nausea. Dizziness. Burning eyes. Sometimes a weird metallic taste sits on your tongue for hours after.
There’s a strange thing about hydrogen sulfide you should know. At really high concentrations, it actually shuts down your sense of smell. So the smell vanishes right when the gas is most concentrated. If you’re standing in your basement and the smell suddenly disappears but your spouse still gripes about it when they come down, that’s not good news. That’s worse news.
Leave the basement and call someone if any of these are true:
- You’re feeling dizzy, queasy, or your head’s starting to pound
- The smell is bad enough you don’t want to be in the room
- Anything is hissing
- It hit you suddenly and it’s intense
- Sewage is coming up anywhere in the house
- Kids, older folks, or anyone with breathing issues is in the house and showing symptoms
- You smell anything that could also be natural gas (more on this in a second)
If none of that’s happening and the smell is just there in the background, fine. You’ve got time. Keep reading.
How to Tell Sewer Gas From Other Bad Smells
Worth confirming what you’re actually smelling before you start tearing apart your basement. A bunch of other smells get mistaken for sewer gas, and the fix for each one is completely different.
Sewer gas. Rotten eggs. Sulfur. Sometimes plain old “sewage.” Worse near drains, toilets, sump pits, floor drains.
Natural gas. Also smells like rotten eggs, which is the whole problem. Gas companies add a chemical called mercaptan on purpose, so a gas leak smells like sewer gas. Real difference: natural gas is dangerous in ways sewer gas isn’t. Flammable. Explosive. If the smell is anywhere near your furnace, water heater, dryer, gas range, or you can’t pin it on a plumbing source, get out of the house. Don’t flip switches. Don’t open the garage door. Call your gas utility from outside.
Carbon monoxide. No smell. None. If you’ve got headaches, dizziness, or confusion with no smell to explain it, suspect CO. Get working detectors on every floor if you don’t already have them.
Radon. Also no smell. Radon’s a separate test and a separate problem.
Dead animal. Sweet. Thick. Rotting-meat smell, not sulfur. Builds for a couple of weeks then fades. Sewer gas doesn’t have that arc. It just sits there until you fix it.
Mold or mildew. Musty. Damp. Earthy. Sometimes called “old basement smell.” That’s not sewer gas. That’s moisture.
Sweet or chemical smell. Could be refrigerant from your AC or furnace. Could be off-gassing from something you’ve got stored down there.
Ruled out everything except sewer gas? Read on.
The 8 Causes of Sewer Gas Smell in a Basement
In rough order of how often we actually see them.
1. A Dried-Out Floor Drain (The Most Common Cause)
This is the answer about half the time. Maybe more. Look at your basement floor. See a small grated drain in the concrete? Underneath that grate, the pipe makes a U-turn. That U-turn is called a P-trap. It holds water. Always. That water is the seal that keeps sewer gas down in the pipes where it belongs.
When the water evaporates out of the trap (and it does, eventually) the seal is gone. Gas comes straight up through the open drain like nothing’s in the way.
Where this happens:
- Basements nobody uses
- Floor drains in storage rooms, mechanical rooms, laundry
- After the family’s been away a couple weeks
- Dry climates, or homes with the humidity cranked low
- Drains near the furnace, where the heat speeds up evaporation
Fix: Pour half a gallon of water down every floor drain in the basement. Wait an hour. Smell gone? You found it. Want it to stay gone? Float a tablespoon of mineral oil on top of the water after refilling. Floats on the water, slows evaporation way down. Costs three dollars. Lasts months. There’s also a thing called a trap primer you can pick up at any hardware store that adds water automatically, but for most homes the mineral oil trick works fine.
Cost: Free. If you’ve got a drain that won’t stay wet no matter what (it happens), a plumber will install an automatic trap primer for $150 to $300.
2. A Failed Toilet Wax Ring
Every toilet sits on a wax donut. Squishy beige ring. Costs about ten bucks at any hardware store. Its job is sealing the connection between the toilet base and the drainpipe in the floor. When that ring fails (usually because the toilet’s been there fifteen years and nobody’s touched it, or somebody moved the toilet for tile work and didn’t replace the ring), sewer gas leaks up around the base.
How to tell:
- Smell is strongest right at the toilet, not the rest of the basement
- Floor around the toilet is stained, warped, or feels soft
- Toilet rocks when you sit on it
- Ceiling below has water marks (if there’s a finished ceiling under that bathroom)
- Toilet’s been there a decade or more without anyone messing with it
Fix: New wax ring is ten dollars at any hardware store. Job is: shut off water, drain the tank, disconnect the supply line, unbolt the toilet, lift it off the floor (heavy, get help), scrape the old wax off, set the new ring, put the toilet back. If you’ve ever set a toilet before, you can do this. If you haven’t, it’s a doable project but get a friend who has. Plumbers charge $150 to $400 depending on whether the flange under the toilet is also rotted out, which adds time and parts.
3. A Cracked, Loose, or Damaged Pipe
A pipe somewhere has a crack, a loose joint, or a section that’s corroded through. Sewer gas leaks out wherever the break is. Most common in old cast iron drain pipes that have been corroding from the inside for fifty years. Also shows up in homes where something settled, something shifted, or someone did amateur repair work that didn’t hold.
What you’ll notice:
- Smell stays in one specific spot, often near a wall or behind something
- Comes and goes, sometimes worse when water’s running upstairs
- Moisture, staining, or rust streaks somewhere on the pipe
- Older house (40+ years) with original cast iron
Fix: Not really a DIY job unless you’ve done plumbing work before. Cast iron repair means cutting the bad section out and connecting PVC to what’s left using rubber couplings called Fernco connectors. $300 to $1,500 for a spot repair. Full drain stack replacement if it’s all corroded: $2,000 to $8,000.
4. A Clogged or Broken Plumbing Vent Stack
Your house has pipes that go UP through the roof. Not just down to the sewer. The up pipes are vents. They do two things: let sewer gas escape harmlessly above the roofline, and let air into the drain system so water flows properly when you flush a toilet or empty a sink.
When something blocks the vent (leaves, ice, a tennis ball your kid threw on the roof, a bird that decided to nest there), the gas has nowhere to escape. It backs up into the lowest point in the system, which is usually the basement.
How to spot a vent problem:
- Drains gurgle all over the house, not just the basement
- Toilets bubble or the water level drops when you run a faucet upstairs
- Smell gets worse after somebody uses a lot of water (long shower, full bathtub, dishwasher cycle)
- Smell shows up worse in winter, especially after a cold snap (ice plug in the vent)
Fix: Sometimes you can run a drain snake through from the lowest cleanout and clear the vent that way. Usually it’s a roof job. Plumber charges $150 to $400 to climb up and clear it. If the vent itself has corroded apart inside a wall, that’s a bigger job. $400 to $1,500.
5. A Failed Sewer Ejector Pump (Basement-Specific)
Basement bathroom? Basement laundry? Anything down there that drains? Then you’ve got a sewer ejector pump in a sealed pit somewhere in the basement. The pump’s job is moving wastewater UP to the main sewer line, since the main line is above the basement floor. The pit is sealed with a gasketed lid. It has its own vent.
Pit gasket dries out. Pump fails. Vent clogs. Any of those, and sewer gas escapes out of the pit straight into your basement air.
What it looks like:
- Smell is worst right next to the pit (it’s a round lid with bolts around the edge, sitting in the floor)
- Lid looks warped, cracked, or like it’s not sitting flat
- Bolts on the lid are loose, missing, or rusted to nothing
- Pit makes weird noises, or the pump cycles constantly, or never seems to run
Fix: Replacing the gasket on the lid is doable as a DIY if you can pry the lid off without breaking it. Word of caution: the smell when you open that pit is going to be much worse than what you’re dealing with now. Ventilate hard. Replacing the pump itself is more involved. Costs: $30 to $80 for a new gasket if you’re doing it yourself. Plumber gasket replacement and reseal runs $200 to $400. Full pump replacement: $700 to $2,500 depending on the pump size and how much rebuilding the pit needs.
6. A Clogged or Damaged Main Sewer Line
Your main sewer line is the big pipe that carries everything from your house out to the municipal sewer (or septic, if you’re rural). When it clogs or breaks, waste backs up. Gas escapes from every plumbing opening in the basement at once because nothing is flowing correctly.
Signs:
- Drains slow all over the house. Not one drain. Multiple.
- Sewage actually backing up somewhere visible (toilet, floor drain, shower, washing machine)
- Smell is everywhere in the basement, not just one spot
- Wet patches in the yard where the sewer line runs
- Smell gets noticeably worse after heavy rain (groundwater is getting into the cracked pipe and pushing gas out)
Fix: Not DIY. Plumber runs a camera down the line, figures out what’s wrong, then either clears the clog with hydro jetting (high-pressure water) or repairs the pipe. Hydro jetting runs $400 to $900 for a typical clog. Sewer line repair starts around $1,500 for a small spot fix and climbs hard from there if the whole line needs replacement. Trenchless pipe lining is $4,000 to $15,000 depending on how long the run is. Full excavation replacement can hit $25,000 in bad cases.
7. A Sump Pump Pit With Stagnant Water
A sump pit is supposed to collect groundwater. Not sewage. Different thing entirely. But sump pits can still smell. Water sits too long. Bacteria grow. Leaves and dirt get in there. Mineral deposits build up on the walls.
What it looks like:
- Smell is coming straight from the pit
- Water in the pit is dark, oily-looking, or has stuff floating in it
- Smells more like rotting than classic sulfur
- Pump hasn’t run in a while because there hasn’t been much rain
Fix: Empty the pit. Scrub the walls with bleach and water mixed about 1 to 10. Let it dry. Make sure the pump still works. Costs you $5 to $20 in supplies. Plumber service of a sump pit: $150 to $300.
8. An Open or Damaged Cleanout
Every plumbing system has cleanouts. These are capped access points that plumbers use to inspect and clear pipes. Cap goes missing, cracks, or gets installed wrong, and sewer gas vents right out of it. Usually there’s one in the basement somewhere, and one outside near the foundation.
Signs:
- Smell is concentrated in one specific spot, usually next to a wall or in a corner
- You can see a visible cap, plastic or metal, often with a square nut on top
- Cap looks broken, missing, or has crumbling Teflon tape on the threads
- Smell started after somebody did plumbing work in the house
Fix: Replace the cap. Caps cost $5 to $15. Wrap fresh Teflon tape on the threads before tightening. If the cleanout body itself is broken (not just the cap), plumber repair runs $150 to $400.
How to Diagnose Your Specific Cause
Pattern matching. When the smell shows up, where it shows up, and what else is happening tells you which of the eight you’re looking at.
Mild smell that comes and goes: Floor drain or trap dried out. Water down the drain first.
Strongest near a specific toilet: Wax ring.
Strongest near the sump or ejector pit: Lid seal or pump issue.
Multiple drains gurgling or running slow: Vent stack or main sewer line.
Worse after rain: Cracked sewer line letting groundwater in.
Worse in winter: Frozen vent stack on the roof. Common in cold weather, especially after a few days below freezing.
Worse after the family was gone for a weekend or longer: Dried-out drains. Almost guaranteed.
Worse after somebody runs a lot of water upstairs: Failing wax ring, loose connection, or vent problem.
You can actually see sewage: Emergency. Stop using water. Call a plumber today, not tomorrow.
Started right after recent renovation: Loose connection, missing cap, wax ring not seated right when the toilet went back in.
Brand new home (under 2 years old): Often a vent that was done wrong, an AAV installed badly, or a trap that hasn’t been filled enough yet from regular use. Builder’s warranty probably covers it. Call the builder before you pay a plumber.
What About AAVs (Air Admittance Valves)?
Newer homes (and remodels in the last 30 years) sometimes use air admittance valves instead of running a full vent pipe up through the roof. AAV is a one-way valve under a sink or in a wall. Lets air into the drain. Doesn’t let air out.
They fail. When they fail open, sewer gas pours straight into the room.
Smell coming from under a sink, behind a kitchen wall, or near a basement bathroom that doesn’t have an obvious source? Have a plumber check the AAV. Part costs $30. Job takes ten minutes.
Less Common Causes Worth Knowing About
A handful of things that show up rarely but are worth checking if you’ve eliminated everything above.
Dishwasher and washing machine standpipes. Both have their own traps. Either can dry out. Either can be installed without a proper trap if the original plumber cut corners. If the smell is near these appliances, check whether there’s water in the drain line.
Basement HVAC return picking up the smell. Furnace or air handler in the basement, with a sewer leak somewhere nearby? The return air system can pull that smell in and distribute it through the whole house. If the smell hits the upstairs every time the furnace runs, that’s the clue.
Slab leaks. Drainpipe cracked under the concrete floor. You won’t see anything inside. Smell rises through any tiny crack in the slab. Diagnosis takes specialized leak detection equipment. Repair often means breaking the concrete. Not fun, not cheap.
Failed backflow preventer. Homes in flood-prone areas often have a backflow preventer on the main sewer line. When it fails, sewer gas can move the wrong direction. Plumber job, $300 to $800.
Roof flashing failure around the vent stack. Rubber boot around the roof vent cracked. Rain gets in. Carries sewer gas down through the attic to lower floors. You won’t think to look here. Most people don’t.
DIY vs. When to Call a Plumber
Honest line in the sand.
Fix it yourself if: smell is mild, you can pin down what’s causing it, and the fix is pouring water down a drain, swapping a cap, tightening a sump lid, or installing a basic wax ring. Weekend stuff.
Call a plumber if: smell is sticking around after DIY attempts, you see any sewage backing up, multiple drains are slow, you suspect the main sewer line, the source is inside a wall, or your ejector pit needs attention. Not DIY territory unless you’ve done it before.
Call right now if: anyone in the house is feeling sick from the smell, the smell is overwhelming, you hear hissing, or you think you might be smelling natural gas instead.
Frequently Asked Question
Why does my basement smell like rotten eggs?
Rotten egg smell is hydrogen sulfide. It’s the main gas in sewer gas. Almost always means sewer gas is escaping somewhere in your basement plumbing. Most common cause: floor drain that dried out. Pour water down it. That’s usually the fix.
Is sewer gas in the basement dangerous?
At the levels that produce a smell, usually not dangerous for healthy adults. At higher concentrations, yes. Symptoms are headache, nausea, dizziness, eye irritation. Methane in sewer gas can be explosive if it builds up enough in a closed space. Anyone with breathing issues, kids, and older folks should clear out of the basement until the smell is resolved.
How do I get rid of sewer gas smell in my basement fast?
Pour half a gallon of water down every floor drain. Open the windows. Check around every toilet for stains or movement. If the smell isn’t gone in an hour, you’re looking at something beyond a dried trap and you should call a plumber.
Why does my basement smell like sewer only sometimes?
Usually one of three things. Drain trap that dries out then gets refilled (most common). Sewer line that smells worse when water’s running upstairs. Vent stack that’s partially blocked but not fully. The pattern of when it shows up tells you which.
Why does my basement smell like sewage after it rains?
That’s a cracked main sewer line. Rain gets in through the crack, raises the water level inside the pipe, pushes sewer gas through any weak point in the system. Not a DIY fix. Get a sewer camera inspection.
Can I pour bleach down a basement drain to get rid of sewer smell?
Bleach helps with mild bacterial smells in a sump pit. Doesn’t fix anything else. Dried P-trap? Just water. Bad wax ring or cracked pipe? Bleach does nothing useful and will damage rubber gaskets over time.
How much does it cost to fix a sewer gas smell in a basement?
Depends entirely on the cause. Dried drain: free. Wax ring with a plumber: $150 to $400. Cracked sewer line: $1,500 to $25,000+. Most basement sewer smells get fixed for under $500 because the most common causes are cheap fixes.
Can sewer gas make you sick?
Yes. Headaches, nausea, dizziness, eye and throat irritation, fatigue. Higher concentrations or longer exposure makes it worse. People with asthma or COPD react sooner. Kids and pets too, since they spend more time closer to the floor where the gas concentrates.
Why does my new construction basement smell like sewer?
New homes have a few specific issues. Vents installed wrong. AAV that wasn’t seated correctly. Trap that hasn’t been used enough to stay full of water. Builder warranty covers this stuff. Call the builder first, not a plumber.
Will a sewer smell in the basement go away on its own?
Almost never. Only case where it might is a dried trap getting refilled by accident when somebody runs water through the right fixture. Everything else needs intervention. Been smelling it more than a few days? It’s not going anywhere without you doing something.
Should I leave my house if my basement smells like sewer gas?
Usually no. At smellable levels it’s unpleasant, not dangerous. Get out and call for help if you’ve got symptoms (headache, nausea, dizziness), the smell is overwhelming, anything is hissing, you think it might be natural gas, or if anyone in the house has respiratory issues and is reacting to it.
Can a plumbing inspection find the source of a sewer gas smell?
Yes. Smoke testing, sewer cameras, leak detection equipment. A licensed plumber can usually track down even a hidden leak. Diagnostic visit runs $100 to $300. Most cases get pinpointed within an hour.
Final Thought From a Plumber
50 years doing this in homes of every age and condition, my honest advice: don’t panic, but don’t sit on it either. Sewer gas in a basement is rarely a true emergency. But it almost always means something in your plumbing isn’t working right.
Start with the easy stuff. Water in the drains. Check the toilets. Look at the sump pit. Sometimes the fix is five dollars and ten minutes. If that doesn’t get it, bring in someone before the small problem turns into a five-thousand-dollar one.
The smell is your house trying to tell you something. Don’t tune it out.
Need a Plumber in Manitoba?
If you’re in Winnipeg, Steinbach, Morris, or surrounding areas, Brown’s Plumbing & Heating has been handling exactly this kind of issue for 50 years. Sewer line repair, drain cleaning, ejector pump service, full plumbing diagnostics.
Give us a call:
- Steinbach: 204-818-8330
- Winnipeg: 204-813-1133
- Morris: 204-500-0820
Schedule a service or request an instant estimate online.
Brown’s Plumbing & Heating. Family-owned since 1974. BBB A+ rated.