Sewage Smell in the House: Causes and Fixes

Sewer gas is more than unpleasant. It is a mix of gases that can include hydrogen sulfide and methane, and both can burn. A smell that comes back means gas is finding a way in, and that path needs fixing.

Check the safety points first, then try the easy fixes in order.

Safety First

• Unsure if it's sewer or natural gas: Both can smell like rotten eggs. Utilities add a sulfur-containing chemical called mercaptan to natural gas so leaks can be smelled. If you can't tell, leave. Don't use light switches, phones or garage door openers inside. Call your gas utility or 911 from somewhere safe, and stay out until they say it's safe.
• Strong smell in a closed space: Wisconsin Department of Health Services says that if you think an enclosed space holds a high concentration of sewer gas, evacuate and call the fire department. Avoid sparks and flames there, because methane and hydrogen sulfide can ignite.
• Don't rely on your nose: Hydrogen sulfide smells like rotten eggs even at low levels. At high levels it can dull your sense of smell, so the odor stops being a reliable warning.
• Symptoms: Eye and airway irritation, dizziness, drowsiness, headache and nausea are listed health effects. Very high levels can cause unconsciousness and death. See a doctor if you have these symptoms and suspect sewer gas. Open windows and leave the room if you feel unwell. The ventilation step is our own judgment.
• Carbon monoxide and methane have no smell. Alarms are the only warning for carbon monoxide.

Try the Easy Fix First: Dry Traps

Every sink, tub, shower and floor drain has a curved pipe called a P-trap. It holds water that blocks sewer gas. If a drain goes unused, that water can evaporate, and hot or dry weather speeds this up. South Davis Sewer District lists guest bathrooms, basement and garage floor drains, laundry drains and utility sinks as common spots. Vacant homes are also prone to it.

1. Follow your nose to the likeliest drain. Basement floor drains are a frequent entry point, according to Wisconsin DHS.
2. Pour a cup or two of water into it.
3. Run water in every sink, shower and floor drain at least once a month from now on.
4. For a drain you rarely use, fill the trap with water, then slowly pour in a few ounces of mineral oil. The oil floats on top and slows evaporation. Use mineral oil, not cooking oil, which can turn rancid and smell.

Floor drains with a trap primer

Some floor drains have a trap primer, a device that tops up the trap with water. Mr. Rooter says to test it by running the fixture it serves and watching for water flowing into the trap. Signs of a failing primer include sewer odors from a rarely used drain and a trap that dries out quickly after you fill it. Replacing a primer is plumber work.

If the Smell Stays: Other Causes

Toilet seal

Service Experts says a failing wax ring can let sewer gas out, even when no water shows. Wipe the floor dry around the base. If water comes back, the seal may be failing. Loose floor bolts and a leaking supply line can also cause leaks. Replace the seal promptly, and call a plumber if you see floor or ceiling damage below a bathroom.

Roof vents

Wisconsin DHS lists leaking or blocked plumbing roof vents as a source of sewer gas, and says leaves or bird nests in a vent should be removed. Get on the roof only if it is safe to do so. Our own judgment: if you aren't comfortable on a roof, hire a plumber.

Septic tanks, sump pits and appliance drains

Wisconsin DHS names home septic tanks and cracks in the foundation as other routes for gas. We found no source for a sump pit or appliance drain test, so we only suggest checking that the washing machine and dishwasher drains still have water in their traps, which is our own judgment. If a sump or septic system smells, ask a plumber or a septic professional.

Call a Plumber Now If

• Several fixtures drain slowly or gurgle at the same time. That points to the main line, not one pipe.
• Water rises in a tub, shower or floor drain when you flush or run a washing machine.
• The smell is strong, keeps coming back after you refill the traps, or is getting worse quickly.
• You see wet floors or ceiling stains near a toilet.

While you wait, use less water and flush only when needed. Avoid chemical drain cleaners, which HomeTips says are usually ineffective on sewer clogs and can damage pipes. If sewage has backed up into the home, the EPA says to hire a professional for the cleanup, because sewage-contaminated water is a case for experts.

Frequently Asked Questions

Is sewer gas in the house dangerous?

It can be. Sewer gas can contain hydrogen sulfide, which irritates the eyes and airways and causes headaches, dizziness and nausea. Very high levels can be deadly. Methane and hydrogen sulfide can also ignite.

Why does my bathroom smell like sewage?

The most common reason is a dry P-trap in a seldom-used sink, shower or floor drain. A failed toilet seal, a blocked vent or a clogged sewer line are other causes.

How do I get rid of a sewer smell in a drain?

Pour a cup or two of water into the drain to refill the trap. For a drain you rarely use, add a few ounces of mineral oil on top of the water to slow evaporation. If the smell returns, call a plumber.

Can sewage smell mean a gas leak?

Both can smell like rotten eggs. If you can't tell which it is, leave the house and call your gas utility or 911 from outside. Stay out until you're told it's safe.

Sources

• Wisconsin Department of Health Services: Sewer Gas (sources, hydrogen sulfide smell and health effects, vents, floor drains, when to evacuate)
• South Davis Sewer District: Understanding Sewer Odors and P-Traps (dry traps, monthly water, mineral oil, when to call a plumber)
• Mr. Rooter: How to Check a Trap Primer (testing a primer, signs of failure)
• Service Experts: Signs It's Time to Replace Your Toilet Wax Ring (wax ring, sewer smell, water at the base)
• HomeTips: Sewer Line Clog Signs (multiple drains, gurgling, drain cleaners, when to call)
• Kansas Corporation Commission: Natural Gas Safety (gas smell, what to do)
• EPA: A Brief Guide to Mold, Moisture and Your Home (sewage-contaminated water calls for a professional)
• Oregon State Fire Marshal: Carbon Monoxide (carbon monoxide has no smell)

These are general guides. Local plumbing codes and your own system can differ, so a licensed plumber has the final say.