Why Does Your Basement Smell Like Sewer?
One of the most common reasons a basement smells like sewer is a dried-out drain trap, usually on a floor drain, laundry sink, or basement shower that rarely gets used. Oatey, a plumbing products manufacturer, explains that when water in a trap evaporates, the gas barrier disappears and sewer gas can rise into the room. Oatey's list of other causes includes blocked sewer lines, improperly installed toilets, damaged plumbing vents, and sulfur bacteria in water heaters.
The plumbing code shows why floor drains are the usual suspect. The International Residential Code (Section P3201.2) requires every fixture trap to hold a water seal of at least 2 inches and no more than 4 inches, and Section P3201.2.1 requires traps subject to evaporation, like floor drains, to be protected by a trap primer or a barrier-type seal device. A floor drain without either one depends entirely on whatever water happens to be sitting in the trap. This post covers how to track down the source, the quick fixes you can try, and when a sewer smell points to a bigger problem.
What Is Sewer Gas?
Sewer gas is the mix of gases that builds up in drain and sewer lines. Oatey says it contains hydrogen sulfide, methane, ammonia, and other compounds, and that it can be dangerous to breathe in high concentrations, with effects that can include headaches, nausea, dizziness, and respiratory irritation.
The rotten egg part of the smell comes from hydrogen sulfide. If the smell is strong, you feel unwell, or you are not sure whether you are smelling sewer gas or natural gas, get fresh air first and sort it out from outside.
How Does a Drain Trap Stop Sewer Gas?
A drain trap is the curved section of pipe below a drain that holds water. That standing water blocks gas from the sewer side from coming up through the drain.
The code sets the depth of that water seal at 2 to 4 inches for every fixture trap. When the water evaporates, the seal is gone. Basements are prone to this because floor drains, utility sinks, and basement bathrooms often go weeks or months without use.
How Do You Find the Source of a Basement Sewer Smell?
Start with the drains that get used least, then work toward the stronger possibilities:
Floor drains: Check whether you can see water in the trap. If it looks dry, that is the likely culprit.
Unused fixtures: Check laundry tubs, basement showers, and bar sinks.
Toilets: A smell around the base of a toilet can point to a failed seal. Oatey lists improperly installed toilets as a common cause.
Visible pipes: Look for cracks, wet spots, or corrosion on drain lines and cleanouts.
If the smell comes with slow drains throughout the house, gurgling, or backups, the problem is more likely in the main sewer line. Our guides on what causes slow drains and surprising causes of sewer backups cover those warning signs.
How Do You Fix a Dry Floor Drain Trap?
Refilling the trap is the first fix and often the only one needed. Oatey recommends:
Pouring about a gallon of water down the drain monthly to reprime the trap
Adding 1 to 2 tablespoons of mineral oil to the trap before a long absence, since it floats on the water and slows evaporation
Using drain seals made for infrequently used drains
For a longer-term fix, the code lists approved ways to protect traps that dry out: a potable water trap seal primer valve, a wastewater-supplied trap primer device, or a barrier-type trap seal protection device. A plumber can tell you which one fits your drain.
Could the Smell Come From the Sewer Line or Vents?
Yes. Oatey lists blocked sewer lines and damaged plumbing vents among the main causes of sewer gas odor. A smell that shows up in more than one room, or keeps coming back after the traps are full, points past a single drain.
Some homes also have a fresh air vent on the house drain. In Bethel Park, the municipality's dye test and camera lateral inspection, required before a home is sold, includes locating the fresh air vent to make sure it is not taking in groundwater, along with checking where downspouts, driveway drains, and sump pumps discharge. If a smell persists after the traps are filled, a camera inspection of the lateral can show cracks, root intrusion, or other damage. Our post on whether tree roots can crack your sewer line explains one common cause. Keep in mind who owns that pipe: 3 Rivers Wet Weather, the regional group working on sewer overflows in Allegheny County, says homeowners are responsible for maintaining and repairing the house lateral that connects the home to the public sewer.
What If the Smell Is Only in Your Hot Water?
A rotten egg smell that shows up only when you run hot water is usually not a drain problem. Penn State Extension explains that the magnesium anode rod in a water heater can react with sulfates in the water to form hydrogen sulfide. That is a water heater fix, often an anode rod swap, rather than a drain or sewer repair.
When Should You Call a Plumber About a Sewer Smell?
Oatey recommends professional help when you cannot find the source, when the problem continues after repairs, or when you suspect hidden leaks inside walls or under floors.
Call right away if the smell comes with any of these:
Sewage backing up into a floor drain, tub, or toilet
Multiple slow or gurgling drains at once
Wet spots, stains, or damp soil near the sewer line path
Headaches, nausea, or dizziness, which Oatey lists among the effects of sewer gas exposure
If you are buying a home, a camera inspection before closing can reveal sewer line trouble early. Our guide on whether to get a sewer camera inspection before buying covers what it shows.
FAQ
Why does my basement smell like sewer only sometimes?
An intermittent smell often points to a trap that dries out between uses, such as a floor drain that only gets water occasionally. Refill it and see if the smell stops.
How do I stop a floor drain from smelling?
Pour water into it to refill the trap. Oatey suggests about a gallon monthly and a little mineral oil before long absences. A trap primer or barrier device is the permanent fix.
Is sewer gas dangerous?
Oatey warns that sewer gas can be dangerous to breathe in high concentrations and can cause headaches, nausea, dizziness, and respiratory irritation.
How much water should a drain trap hold?
The International Residential Code requires a trap seal of at least 2 inches and no more than 4 inches.
Can a toilet cause a sewer smell in the basement?
Yes. Oatey lists improperly installed toilets among the main causes of sewer gas odor.
Does Bethel Park inspect sewer connections?
Bethel Park requires a dye test and camera lateral inspection before a property is sold or deeded, unless one was completed and certified within the last two years.
Drain and Sewer Line Help in Bethel Park
All Pro Plumbing Heating & Cooling diagnoses drain and sewer problems across Bethel Park and the South Hills using video camera inspection, clears blockages with professional equipment including hydro-jetting, and offers both traditional excavation and trenchless sewer line repair. Founded in 2006 by Registered Master Plumber John Oakes, the company now works out of its own office on Library Road with 10 local employees, and it provides upfront pricing for every repair option. See drain cleaning and sewer line repair.
Visit All Pro Plumbing or call 412-595-8204 today. Contact us for more information.
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