A sewage smell in a house can come from something as simple as a dry drain trap or as serious as a broken drain line leaking into a crawlspace. Some causes are plumbing problems; others leave contamination that needs cleanup. This guide helps you narrow down the source.
Sewer gas versus sewage contamination
Sewer gas is the odor of gases from the sewer system entering the home through plumbing openings. It may smell like rotten eggs. There's no visible contamination.
Sewage contamination means wastewater has leaked or backed up into the building, soaking materials. It requires cleanup, not just a plumbing fix.
Common causes of sewer gas
Dry traps. Every drain has a trap that holds water to block sewer gas. Floor drains, guest bathrooms and basement sinks that aren't used can dry out. Pouring water down the drain often solves the problem.
Vent problems. Plumbing vents let gases escape through the roof. Blocked or poorly installed vents can let gases into the home or siphon water from traps.
Toilet seal failures. A failed wax ring under a toilet can let gas escape and may allow leaks.
Cracked or loose pipes. Damaged drain or vent pipes inside walls can leak gas.
Clogged drains. Buildup in drains can produce odors.
Sewer cleanouts with missing or loose caps.
Signs of sewage contamination
- Visible wastewater, staining or damp areas around toilets, drains or in basements
- Sewage backups from floor drains or toilets
- Persistent odor that doesn't go away with running water
- Damp or stained areas in crawlspaces
- Flies around drains or in crawlspaces
- Soft or damaged flooring near bathrooms
Crawlspace leaks
Broken or disconnected drain lines under homes are common in older houses, and leaks can continue for a long time unnoticed. Odor may be strongest when the HVAC system runs, especially if ductwork or equipment is in the crawlspace. A plumber should inspect, and contaminated soil, vapor barrier and insulation may need removal.
Finding the source
- Run water in every drain, including floor drains and unused fixtures, to refill traps.
- Check toilets for movement, leaks or damp flooring at the base.
- Inspect the crawlspace or basement for wet areas, staining or odor.
- Check the roof vent for obstructions, or have a plumber do it.
- Call a plumber for a smoke test or camera inspection if the source isn't obvious.
When it's a cleanup problem
If sewage has leaked or backed up, porous materials it touched, such as carpet, pad, drywall, insulation and crawlspace vapor barrier, usually need removal. Remaining surfaces must be cleaned and disinfected, and the area dried. Sewage is classified as Category 3 water under the industry's water damage standard because it can contain pathogens.
Health considerations
Sewer gas can contain hydrogen sulfide and other gases. At typical household odor levels, the main effect is unpleasantness, but high concentrations in enclosed spaces can be dangerous. If odor is strong and accompanied by symptoms such as headache or dizziness, ventilate and leave the area, and contact the utility or a plumber.
A practical example
A family notices a sewage odor that's worse when the heat runs. Refilling floor drain traps doesn't help. A plumber's camera inspection finds a separated drain joint under the house. After the plumber repairs it, a cleanup crew removes the saturated vapor barrier and insulation, treats the soil and framing, dries the crawlspace and installs a new vapor barrier. The odor is gone.
Preventing future problems
Run water in rarely used drains monthly, keep cleanout caps tight, have older drain lines camera-inspected if you notice recurring odors, and install a backwater valve where sewer backups are a risk. Ask your insurer about sewer backup coverage.
Frequently asked questions
Why does the smell come and go?
Traps dry out intermittently, wind affects vents, and HVAC operation draws air from crawlspaces.
Is a rotten egg smell always sewer gas?
It can also indicate a natural gas leak (which has an added odorant). If you suspect gas, leave and call the gas utility.
Can I fix a dry trap myself?
Yes, by running water. A small amount of mineral oil on top can slow evaporation in rarely used drains.
Does insurance cover sewage cleanup?
Sudden backups may be covered with a sewer backup endorsement; long-term leaks often aren't.
Should I use bleach in drains?
It won't fix vent or pipe problems, and shouldn't be mixed with other chemicals.
When is the smell an emergency?
When sewage is backing up into living areas or there are symptoms from gas. Stop water use and call a plumber.
Can a septic system cause sewage smells indoors?
Yes, if the tank is full, the system is failing or vents are blocked. A septic professional can inspect.
Do air fresheners help?
Only temporarily; find and fix the source.
Key takeaways
Sewer gas often comes from dry traps, vent problems, toilet seals or cracked pipes, and is fixed with plumbing repairs. Sewage contamination from backups or leaks requires removing affected materials, cleaning and disinfection. Crawlspace leaks are a common hidden source. Refill traps, inspect, and call a plumber when the source isn't obvious.
Sources
- IICRC: ANSI/IICRC S500 Standard for Professional Water Damage Restoration
- CDC: Clean Up After a Disaster
This guide is general information. It is not legal, medical, environmental or safety advice. Rules vary by state and locality. For a specific situation, consult the relevant agency or a qualified professional.