Where the smell is coming from and what to check first
Sewer smell outside usually comes from one of four places: a dry or damaged P-trap under a floor drain or outdoor fixture, a cracked sewer line, a backed-up vent stack, or a loose or missing cleanout cap. The smell is hydrogen sulfide gas, which rises from the sewer system when air can flow the wrong way or when water that normally seals the trap has evaporated.
Start by walking around the outside of your house and sniffing near the foundation, around any floor drains in a garage or basement, near the sump pump discharge, and around the main sewer cleanout (usually a 4-inch plastic or metal cap in the yard). If the smell is strongest in one spot, that is your leak point. If it is everywhere, the vent stack on your roof may be blocked or damaged.
Check whether you have used the fixture recently. A floor drain that has not been used in weeks will lose its water seal, and the smell will come straight up. A toilet that is not flushed regularly, a shower drain in a guest bathroom, or an outdoor sink can all do this.
Key Takeaways
- A dry P-trap under a floor drain or unused fixture is the most common cause and the easiest to fix — pour water down the drain to refill the seal.
- If the smell comes from one spot in the yard, the sewer line may be cracked or the cleanout cap may be loose or missing, and you will need to call a plumber.
- A blocked vent stack on your roof prevents sewer gas from venting properly and causes smell throughout the yard; a plumber can clear it or a roofer can inspect it.
- Hydrogen sulfide gas is harmless in the small amounts that escape outdoors, but it signals a problem that will get worse if you do not address it.
Refilling a dry P-trap to seal the smell
If you have a floor drain in a basement, garage, or crawlspace that you rarely use, pour a gallon of water down it slowly. The water fills the U-shaped bend (the P-trap) and creates a water seal that blocks gas from rising. Do the same for any outdoor sink, shower, or drain you have not used in more than a month.
If the smell stops, you have solved it. To keep it from happening again, pour a quart of water down the drain once a month, or run water through the fixture weekly if you use it. If you have a floor drain that will never be used, you can cap it permanently with a rubber drain plug (available at any hardware store for under $10) or have a plumber install a one-way valve that lets water out but blocks gas from coming back up.
If pouring water does not stop the smell, the trap itself may be cracked or the drain line may be damaged further down. Move to the next step.
Checking the sewer cleanout and vent stack
The main sewer cleanout is usually a 4-inch cap in the yard, often near the foundation or at the property line. Walk around and look for it — it may be plastic (white, black, or green), metal, or concrete. If the cap is loose, cracked, or missing, sewer gas will escape right there. Tighten it by hand or with a wrench, or call a plumber to replace it if it is damaged.
Next, look at your roof. The vent stack is a 2- to 4-inch pipe that sticks up through the roof, usually near the center or back of the house. If it is blocked by leaves, bird nests, ice, or debris, sewer gas cannot escape upward and backs up into the yard. Climb a ladder and look down the pipe (do not stick your head directly over it). If you see blockage, you can try to clear it with a plumbing snake or call a plumber. If the pipe is cracked or rusted, it needs to be replaced.
A blocked vent stack often causes smell throughout the yard rather than in one spot, and the smell may be worse during heavy rain or after the toilet flushes.
Repairing or replacing a cracked sewer line
If the smell is strongest in one area of the yard and does not come from a dry trap or loose cleanout cap, the sewer line itself may be cracked. Roots, ground settling, or age can split the pipe, and gas escapes into the soil. You will need a plumber to diagnose this — they use a camera on a flexible rod (called a sewer scope) to look inside the line and confirm the crack.
Repair costs vary widely depending on where the crack is and how deep the line runs. A small crack near the surface may cost $3,000 to $5,000 to dig up and repair. A deep crack under the driveway or far from the house can cost much more. Some plumbers offer trenchless repair, where they insert a new pipe inside the old one without digging, which is faster but also more expensive upfront.
If you suspect a cracked line, call a plumber for a sewer scope before you spend money on other fixes. The camera will show exactly what is wrong and what your options are.
Sealing gaps and cracks around the foundation
Even if the sewer line is intact, small cracks in the foundation or gaps where pipes enter the house can let sewer gas seep out. Walk around the outside of your foundation and look for visible cracks or gaps. Seal them with concrete caulk (for cracks) or expanding foam (for gaps around pipes), both available at hardware stores for under $15.
Pay special attention to areas where the sewer line enters the house or where it exits toward the street. These are the most common places for small leaks. If you see standing water or soft soil near the foundation, that is a sign of a leak that needs plumber attention.
When to call a plumber
Call a plumber if pouring water into the drain does not stop the smell, if the smell comes from the cleanout or a specific spot in the yard, if you see cracks in the foundation or sewer line, or if the smell is everywhere and you suspect a blocked vent stack. A plumber can run a sewer scope camera for $300 to $500, which will show exactly where the problem is and what it will cost to fix.
Do not ignore the smell. A small leak or blockage will get worse over time, and a cracked sewer line can contaminate groundwater or create a health hazard. Fixing it now is cheaper than waiting for the line to fail completely.
Preventing sewer smell in the future
Use all your drains regularly. A toilet flushed weekly, a shower used monthly, and a floor drain watered monthly will all keep their seals intact. If you have a fixture you never use, cap it or install a one-way valve.
Keep your vent stack clear. In fall, trim tree branches away from the roof and check for nests. In winter, watch for ice buildup. After heavy rain, listen for gurgling sounds in the drains — that is a sign the vent is blocked.
Have your sewer line inspected every 5 to 10 years if your house is more than 30 years old, or sooner if you have had problems. Roots and age are slow processes, and catching a crack early is much cheaper than replacing the whole line.
Frequently Asked Questions
Is sewer smell outside dangerous?
Hydrogen sulfide gas in small outdoor amounts is not a health hazard — you smell it long before it reaches dangerous levels. However, the smell means gas is escaping, which signals a problem that will get worse. If you smell it indoors, that is more serious and needs when ready attention.
Can I fix a cracked sewer line myself?
No. Sewer lines are buried 3 to 5 feet deep and often run under driveways or landscaping. Digging and repairing them requires heavy equipment and a permit. A plumber will handle the permit, the work, and any damage to your yard.
How much does it cost to fix sewer smell?
Refilling a dry trap costs nothing. Replacing a cleanout cap costs $50 to $200. Clearing a blocked vent stack costs $200 to $400. A sewer scope inspection costs $300 to $500. Repairing a cracked line costs $3,000 to $25,000 depending on depth and length. Get a quote from a plumber before committing to any repair.
What if the smell comes back after I pour water down the drain?
The trap may be cracked, the drain line may be blocked further down, or there may be a second problem you have not found yet. Call a plumber to scope the drain and see what is wrong. Do not assume it is the same issue.
Can I use bleach or baking soda to get rid of sewer smell?
Bleach and baking soda can mask the smell temporarily, but they do not fix the underlying problem. The smell will return. Find and repair the source — a dry trap, loose cap, or blocked vent — and the smell will stop for good.