Why Does My Basement Smell Like Sewage?

A sewer smell in the basement usually means sewer gas is getting past a seal that should stop it, most often a floor drain trap that has dried out, a loose cleanout plug, a damaged vent, or an ejector pit lid that no longer seals. Oatey, a plumbing products manufacturer, also lists failed toilet wax rings, broken sewer lines, and sulfur bacteria in water heaters. Less often, the smell is real sewage backing up after heavy rain, which Pittsburgh Water says can happen when combined sewers are overwhelmed.

Your nose is a very sensitive detector for this gas, but not a reliable one for long. OSHA puts the odor threshold for hydrogen sulfide, the rotten egg part of sewer gas, at 0.01 to 1.5 parts per million, and the CDC's Agency for Toxic Substances and Disease Registry (ATSDR) says people usually smell it between 0.0005 and 0.3 ppm. Both agencies also say that at about 100 ppm, people lose the ability to smell it after 2 to 15 minutes. Below, we cover the common sources in order of how easy they are to check, the difference between a sump pit and an ejector pit, what plumbing codes require, and who to call for each problem.

What is sewer gas, and why does it end up in a basement?

Sewer gas is the mix of gases that forms in drain lines and sewers, which Oatey describes as containing hydrogen sulfide, methane, ammonia, and other compounds.

Every drain in your home is sealed off from that gas by a trap, the curved section of pipe below a drain that holds a plug of water. The American Society of Plumbing Engineers (ASPE) says traps protect occupants "from sewer gases, odors, and potential health hazards." Basements hold the drains people use least: floor drains, laundry tubs, a guest shower, and cleanout plugs nobody has checked in years.

Is a dry floor drain trap causing the sewer smell in your basement?

Start here, because it costs nothing to check. A floor drain that rarely sees water loses its seal to evaporation, leaving an open path from the sewer to your basement air. Oatey says P-traps typically dry out completely in 2 to 4 weeks, depending on climate and ventilation, and recommends pouring about a gallon of water down infrequently used drains each month. For longer absences, Oatey suggests a small amount of mineral oil on top of the water to slow evaporation.

If the smell fades within a day of refilling the drain, you found it. If it comes back in a few weeks, the drain needs a more permanent fix.

What the plumbing code says about floor drain traps

The model codes treat this as a known problem. The International Residential Code (Section P3201.2.1 in the 2018 edition published by the International Code Council) says trap seals of emergency floor drain traps and traps subject to evaporation "shall be protected by one of the methods" the code lists. ASPE's overview of the International Plumbing Code lists the approved methods, including trap seal primer valves (ASSE 1018), wastewater-supplied trap primer devices (ASSE 1044), and barrier-type trap seal protection devices (ASSE 1072). ASPE also notes each fixture trap must hold a water seal of at least 2 inches and no more than 4 inches.

Pennsylvania adopted the 2021 International Codes effective January 1, 2026, according to the law firm Cohen Seglias, and Allegheny County enforces its own plumbing code, Article XV. A licensed plumber can tell you which device fits your drain and what your municipality requires.

Could the sump pump pit be the source of the smell?

Yes, although a sump pit is not a sewage pit. A sump pit collects groundwater from drains around or under the foundation, and it is part of the storm drainage system, not the sewer. The International Residential Code's storm drainage chapter (Chapter 33) says a subsoil sump "shall not be required to have either a gas-tight cover or a vent."

That open design is why a sump pump smell can still show up. Water that sits in a pit for weeks can go stale, and an uncovered pit lets that air drift into the room. A stale sump smell tends to be earthy or swampy rather than a sharp rotten egg odor, and it overlaps with other moisture problems covered in what to do when your basement smells earthy. For the basics of how these pits work, see whether your basement needs a sump pit.

Sump pit vs. ejector pit: how to tell them apart

A sewage ejector pit collects wastewater from basement toilets, showers, or laundry that sits too low to drain to the sewer by gravity, and pumps it up to the main line. Because it holds sewage, the code treats it very differently from a sump.

• A sewage ejector pit must be fitted with a gas-tight removable cover, according to the International Residential Code's sanitary drainage chapter (Chapter 30).

• A groundwater sump is not required to have a gas-tight cover or a vent, per the same code's storm drainage chapter.

Look at your pit. A bolted, sealed lid with pipes passing through it, sitting near a basement bathroom or laundry, usually means an ejector. A loose lid, or no lid, over clear water usually means a sump. If an ejector lid has a cracked gasket, a missing bolt, or a loose seal where a pipe or cord passes through it, that gap alone can fill a basement with sewer odor.

Does the smell get worse after rain?

A sewage smell in the house after rain points to the sewer itself rather than a dry trap. Pittsburgh Water (PWSA) says about 75 percent of Pittsburgh is served by combined sewers that carry both sewage and stormwater, and that when heavy rain overwhelms those pipes, "combined sewage and stormwater can also back up into basements." ALCOSAN, which treats wastewater for 83 municipalities in Allegheny County, says older communities have combined systems while newer areas built since the 1940s have separate sanitary and storm lines.

Your own sewer lateral matters too. 3 Rivers Wet Weather says house laterals can deteriorate with age or settling, or clog with tree roots, which can let groundwater in or cause "a back-up of sewage into your basement." 3RWW adds that homeowners maintain their lateral, though how far that runs depends on your community: to the property line in some places, all the way to the main sewer in others.

If water or sewage comes up through a basement drain during storms, read why water backs up through basement drains, and call a licensed plumber to inspect the line.

Is sewer gas in the basement dangerous?

It depends on how strong it is and how long you are exposed. The OSHA table on hydrogen sulfide lists these effects by concentration:

• 2 to 5 ppm: prolonged exposure may cause nausea, tearing eyes, headaches, or loss of sleep

• 20 ppm: possible fatigue, headache, irritability, poor memory, and dizziness

• 100 ppm: coughing, eye irritation, and loss of smell after 2 to 15 minutes

ATSDR adds that long-term exposure at 5 to 50 ppm can irritate the eyes, nose, and throat. Treat a basement odor as a sign to find the source. If the smell is strong, or anyone feels sick, get fresh air and have it checked promptly.

Who fixes which problem: plumber or waterproofer?

Most sewer smells are plumbing problems. Call a licensed plumber for:

• Trap primers or barrier devices on floor drains

• Cracked vents, loose or missing cleanout plugs, and failed toilet wax rings

• Ejector pit lids, gaskets, vents, and pumps

• Sewer lateral inspection, root clogs, and backups

A basement waterproofing company is the right call when the odor is tied to water coming into the basement: a sump pit that fills with stale water, damp walls, or a smell that shows up after storms without any sewage backup. Those odors are often musty rather than sewer-like; see why a house smells musty after a storm. For ongoing dampness, basement waterproofing addresses the water, and home ventilation systems such as EZ Breathe move stale basement air out once the source is fixed.

FAQ

How do I get rid of sewer smell in my basement?

Pour water into every floor drain and rarely used fixture first, since a dry trap is the easiest cause to fix. If the odor returns, check cleanout plugs and the ejector pit lid, then call a licensed plumber to look for vent, trap, or sewer line problems.

Why does my basement smell like sewage but there are no leaks?

Sewer gas can escape without any visible water. A dry trap, a loose cleanout plug, a damaged vent, or an ejector lid that no longer seals can all release odor while the floor stays dry.

Why does a sewer smell come and go?

A trap that dries out slowly can cause a smell that returns every few weeks and vanishes after someone runs water. Odors that appear during storms can point to an overloaded sewer or a damaged lateral.

Is a sump pump smell the same as sewer gas?

Usually not. A sump pit holds groundwater, not sewage. Stale water can smell swampy or earthy, while a sharp rotten egg odor points toward a drain, vent, or ejector pit.

Should a sump pit have a sealed lid?

The International Residential Code does not require a gas-tight cover on a groundwater sump, but it does require one on a sewage ejector pit. A cover on a sump pit is still an option to discuss with a waterproofing professional if the pit smells stale or adds damp air to the basement.

Who is responsible for a sewer backup into my basement?

Homeowners maintain their own house lateral, according to 3 Rivers Wet Weather, but the part you are responsible for varies by community. Check with your municipality and your homeowners insurance provider about your specific situation.

A Better Choice, Inc. can help with basement odors tied to water

Since 1996, A Better Choice, Inc. has served over 26,000 customers in the Greater Pittsburgh area, including homes across Westmoreland, Allegheny, and Butler counties. We do not do plumbing work, so a trap, vent, or sewer line problem belongs with a licensed plumber. If the odor seems tied to damp walls, a stale sump pit, or water entering your basement, schedule a free inspection, and we will explain your options.

Visit A Better Choice, Inc. or call 1-800-343-2357 today. Contact us for more information.