Short answer
A sewer smell in the house most often comes from a drain trap that has dried out, a vent problem, or a leak or blockage in the drain system. Run water in unused drains and note where the smell is strongest. A rotten-egg smell can also be natural gas. If you cannot tell, leave and call your gas utility from outside.
Smells like rotten eggs? Read this first
Utilities add an odorant to natural gas so leaks can be noticed, and Ready Illinois, the state's emergency preparedness site, says a sulfur or rotten-egg smell is a sign of a leak. If the smell is strong or you are unsure, leave with everyone, including pets, and call your gas utility from outside. Steps are in the first section below.
A sewer smell in the house is unpleasant and can also be a warning. The cause is often simple and cheap to fix, but the same smell can point to a blocked vent, a damaged pipe or a failing septic system. This guide covers common causes, a safe order of checks, what health agencies say about sewer gas, a sewage smell in the yard and when to call a plumber, septic professional or utility.
EPA says it is not possible to diagnose the exact cause of an odor remotely, so treat this as a way to narrow things down.
If it smells like rotten eggs: rule out natural gas first
Sewer gas and natural gas can both smell like rotten eggs. The federal toxic substances agency ATSDR describes hydrogen sulfide as a flammable gas that smells like rotten eggs and says it is often called sewer gas. Connecticut's utility regulator (PURA) explains that gas companies add a chemical called mercaptan to natural gas to give it that smell. Until you know which you have, treat it as a possible gas leak. These steps come from the gas safety pages of Ready Illinois and Connecticut PURA:
- Do not try to find the source of the smell.
- Avoid flames and sparks. Do not touch light switches or electrical outlets.
- Do not use phones or appliances inside, including doorbells and garage door openers.
- Open windows and doors only if you can do it quickly and easily.
- Leave quickly and take everyone, including pets.
- From a place where you no longer smell gas, call your local gas utility. Call 911 if there appears to be immediate danger.
- Stay out until the gas company says it is safe, and do not turn the gas back on yourself after a safety shut-off.
Connecticut's page allows a faint smell to be reported by a phone away from the smell, but says to leave first if it is strong or unsure. Save your gas utility's emergency number today. If the utility clears your home, or the smell is clearly coming from drains, carry on below.
Common causes of sewer smell in the house
| Cause | Typical clue | Who fixes it |
|---|---|---|
| Dry P-trap in a seldom-used drain | Smell is strongest at one drain, such as a guest bathroom, a basement floor drain or a vacant home | You, usually by adding water |
| Damaged or leaking trap or fixture connection | Smell returns soon after you add water, or moisture under the fixture | A plumber |
| Vent pipe problem (leak, crack, blockage, frost) | Smell in more than one place, gurgling, or worse at certain times | A plumber |
| Seal at the base of a toilet | Smell right at the toilet, a toilet that rocks, or moisture on the floor | A plumber |
| Slime in a slow drain | Smell from one slow drain, sometimes with small flies | You, by cleaning the drain |
| Partial blockage or damage in the main drain or sewer line | Several drains slow or gurgling, or backups | A plumber, and the utility if the public sewer is involved |
| Septic system or drainfield problem | Odor in the house or around the tank or drainfield, wet or spongy ground | A septic provider or plumber, and your health department |
The City of Phoenix Water Services Department explains that the P-shaped trap under sinks, tubs and showers holds standing water that acts as a barrier against sewer gas and pests, and that this water can evaporate if the drain is rarely used or the house is vacant. South Davis Sewer District in Utah calls a dry trap the most common cause of sewer odors and adds that hot, dry weather speeds evaporation.
Phoenix also says wastewater plumbing is vented through pipes that usually run behind the drywall and out the roof. A bad joint or cracked pipe can leak sewer gas, and a plumber can do a smoke test to find it. EPA adds that vents can clog with leaves or freeze, and wind changes can push odors indoors.
Phoenix notes that hair, dirt and grime in drains can form a slime that smells like rotten-egg sulfur, and Iowa State University Extension says drain flies breed in that slime. See drain flies and gnats in drains. If several drains smell and gurgle, read gurgling drains and toilets.
How to find where the smell comes from: a safe order of checks
None of these needs a ladder, a tool or a chemical.
- Note where and when. One room or the whole house? Phoenix says sewer odor is more prevalent at night or early in the morning, when heavy bathroom use pushes gases toward the easiest exit, such as a dry trap.
- Rule out the water. Phoenix suggests smelling a glass of cold tap water in another room. No odor in the glass means the problem is in your drains.
- Run water in every drain you rarely use, including guest sinks and tubs, spare showers and the basement floor drain. Utilities suggest from a cup or two per drain (South Davis) to a quart (Phoenix). See whether the smell fades within a day.
- Keep the traps full. South Davis recommends running water in these drains at least monthly, and says a few ounces of mineral oil slows evaporation (it prefers it to cooking oil, which can go rancid).
- Check each toilet base for moisture or rocking, and note whether drains are slow or gurgling. Those point to a seal, vent or partial blockage rather than a dry trap.
- Smell outside. Note whether the odor is near a roof vent, in the yard or by a septic tank. Stay off the roof, since NDSU Extension warns ladders and icy roofs are treacherous.
If the odor stays away after you refill traps, set a monthly reminder. If it returns soon, South Davis says the cause could be a damaged trap, a blocked vent or a plumbing leak, and that a licensed plumber can diagnose these.
When sewer gas is a health concern
The sources we opened focus on hydrogen sulfide:
- Smell is not a reliable alarm at high levels. ATSDR's October 2024 fact sheet says people lose the ability to smell hydrogen sulfide at higher concentrations, which makes the odor an ineffective warning.
- Effects depend on dose and time. Lower, longer-term exposure can cause eye irritation, headache and fatigue. Very high levels can cause sudden unconsciousness or death.
- It is heavier than air and flammable. ATSDR says risk is greater at ground level and in enclosed spaces. Keep flames away, never climb into a septic tank, sump or pit, and take extra care in basements.
- Watch for symptoms. NDSU Extension says headache, nausea, dizziness or drowsiness can signal exposure and calls for medical care right away.
These sources do not say how much of a household whiff is harmful, and we will not guess. A strong or recurring smell, or any symptoms, is a reason to ventilate, leave the area and get advice from a plumber, your utility or your local health department. For sewage that has backed up into the house, see sewage backup: what to do.
Sewage smell in the yard and septic smell
EPA's septic pages say an odor may come from a roof vent or other vent pipe, that a drainfield that is not working properly can also cause odor, and that soft, wet or spongy soil around the drainfield without recent heavy rain is a good indication of failure. It also lists backups, slow drains, gurgling, damp spots and bright green, spongy grass over the drainfield as warning signs.
For more on these signs, read signs your septic system is failing. EPA says to contact a local septic service provider and/or a plumber about an odor. It says surfacing untreated wastewater is a direct public health hazard, so keep people and pets away. See also how often to pump a septic tank.
On a city sewer, a smell in the yard or street may involve the public system. South Davis says outdoor odors should be reported to the district, while owners are responsible for indoor plumbing including P-traps. See main sewer line location.
When to call a plumber or septic professional
- The smell returns soon after you refill traps, or more than one room smells.
- Drains are slow or gurgling, or you find moisture or a rocking toilet.
- A septic smell comes with wet ground, backups or green, spongy grass.
- The smell is strong or anyone feels unwell. Ventilate, leave if needed and get advice first.
Sewage coming up through drains or standing in the yard is urgent. Stop using water and see sewage backup: what to do.
How to find and check a company where you live
The right company and price depend on your county or state. Read the newest reviews of companies that serve your address, check the license with your state or provincial licensing board, and ask for proof of insurance and a written quote that lists the work, including what they will check and a copy of any camera video. See drain cleaning near me and septic contractors near me.
What to look up where you live
- Your gas utility's emergency number: save it now. It is the first call for a possible gas smell.
- Your city sewer utility, sewer district or public works department: how to report outdoor sewer odors and where the public pipe ends and yours begins.
- County health department or environmental health office: for sewage on the ground, septic problems and health questions.
- State or provincial plumbing or contractor licensing board: to confirm a license and check complaints.
- State environmental agency or septic regulator: see septic regulators by state or by province.
For Canadian readers. Plumbing and septic systems are regulated by each province or territory, and municipalities run sewer utilities with their own contacts. Costs are in Canadian dollars and cannot be compared directly with US figures. Check your provincial regulator and your city.
For septic, plumbing and drain company owners
Sewer smell in house is a search your customers make before they call anyone, often late in the evening. If you want to be the company they find, start with near-me searches for plumbers and septic companies and the plumber Google Business Profile guide. You can also book a call with LiveLeadsLane. LiveLeadsLane is a marketing practice for contractors, not a directory, and it does not take jobs or refer homeowners.
