Sewer Smell in the House: Where It Comes From

A sewer smell in the house most often comes from a drain trap that has dried out, letting gas from the sewer line drift up through the drain. Other common causes include a worn toilet seal, a clogged drain, a blocked plumbing vent and, less often, a clog or crack in the main sewer line.

Most of these are easy to narrow down by working through the house one drain at a time. This guide covers where the smell usually comes from, what you can safely check, when to bring in a plumber and what the fixes typically cost as of 2026.

Is it sewer gas or a gas leak?

Start here, because the two can smell alike. Natural gas has no smell of its own, so utilities add a sulfur odor to it so leaks get noticed (This Old House). Sewer gas contains hydrogen sulfide, which also smells like rotten eggs (Wisconsin Department of Health Services).

If the smell is near a gas stove, furnace, water heater, dryer or gas line, or you can't tell where it's coming from, treat it as a gas leak:

• Leave the house now and take everyone with you.
• Don't flip light switches, use appliances, light a match or use a phone inside.
• From outside, call 911 or your gas utility's emergency line.
• Don't go back in until they say it's safe.

That's the advice from the American Gas Association. It costs you nothing to be wrong, and the utility would rather check a false alarm than miss a leak.

If the smell is very strong, or anyone feels dizzy or sick, leave the building as well. Wisconsin's health department advises getting out, calling the fire department and avoiding anything that could make a spark when sewer gas may be at high levels in an enclosed space (Wisconsin DHS).

Is sewer gas harmful?

At the low levels most homes see, sewer gas is mostly an irritant, but it's still worth fixing quickly. The Wisconsin Department of Health Services describes it as a mix of gases from decaying waste, with hydrogen sulfide and ammonia as the most harmful parts (Wisconsin DHS).

According to the same page:

• Hydrogen sulfide can irritate the eyes and airways and cause headaches, dizziness, nausea or drowsiness.
• At high levels it can dull your sense of smell, so a smell that seems to "fade" isn't always good news.
• Methane and hydrogen sulfide are both flammable.
• Levels tend to be highest in basements.

If people in the house have headaches, dizziness or nausea and you suspect sewer gas, the department suggests seeing a doctor. This Old House lists the same kinds of symptoms and recommends getting major problems repaired promptly (This Old House).

What are the most common causes of a sewer smell?

Here are the usual suspects, roughly from most to least common. A plumber will confirm which one applies; this list is guidance to help you narrow it down.

1. A dried-out drain trap

Every drain has a curved section of pipe, called a trap, that holds a little water. That water forms a seal that keeps sewer gas from coming up the drain. When a drain isn't used for a while, the water evaporates and the seal disappears.

This Old House notes that floor drains are especially likely to dry out, because unlike sink traps, they rarely get water flowing into them (This Old House). Likely spots:

• A basement floor drain
• A guest bathroom shower or tub nobody uses
• A laundry room or utility sink
• Any drain in a home that sat empty for a few weeks

The fix is often just running water. Pour water into the drain or run the tap for a minute to refill the trap. Wisconsin's health department gives the same advice for floor and sink drains (Wisconsin DHS). If the smell comes back within days, the trap may be losing water another way, which is worth a plumber's look.

2. A worn toilet seal

A toilet sits on a wax ring (or a foam seal) that seals it to the drain pipe in the floor. If that seal cracks, shifts or was never seated right, sewer gas can leak out around the base. This Old House lists a damaged toilet seal among the causes of sewer smells and says it's a job for a plumber (This Old House).

Common signs:

• The smell is strongest right around one toilet.
• The toilet rocks or shifts when you sit on it.
• There's water, staining or soft flooring around the base.

3. A clogged or dirty drain

Hair, soap, grease and food build up inside drains and start to smell as they break down. This Old House lists a clogged sink or tub drain as a source of the smell (This Old House). If the smell comes from one sink or shower and that drain is also slow, this is a likely cause.

4. A blocked plumbing vent

Your drain system has vent pipes that run up through the roof. They let air into the pipes so water drains smoothly and trap seals stay in place. When a vent is blocked by ice, snow, leaves or even an animal, draining water can pull the water out of traps, letting sewer gas in (This Old House). Wisconsin's health department also lists a leaking or blocked roof vent as a way sewer gas gets indoors (Wisconsin DHS).

Signs of a vent problem often include gurgling drains, a toilet bowl whose water level drops on its own, or slow drains across the house. Don't climb onto the roof to check a vent yourself. A plumber can inspect it safely.

5. A main sewer line clog

When the main line that carries everything out of the house gets clogged, the smell often comes with other signs. Fixr lists a foul odor as one of the most common symptoms of a main line clog, along with (Fixr):

• Water backing up into a tub or sink
• Gurgling from the toilet
• Water bubbling in the shower or tub when the washer or dishwasher runs
• Several drains running slowly at once
• Soggy patches in the yard

If you're seeing several of these at once, read our guide to gurgling drains and main line clogs.

6. A cracked pipe or foundation gap

Sewer gas can also get in through a cracked waste pipe under or beside the house, or through foundation cracks where gas has built up in the soil (Wisconsin DHS). This is less common, and it usually takes a plumber with a camera to confirm.

7. A septic tank, sump pit or water heater

A few other sources can smell similar:

• A full septic tank. This Old House lists a full septic tank among the causes of sewer odors in homes on septic (This Old House).
• A sump pit. Fixr notes that a dried-out sump pit can give off a sulfur smell (Fixr).
• The water heater. If only your hot water smells like rotten eggs, bacteria in the water heater may be producing hydrogen sulfide. This Old House says this is most common with untreated well water (This Old House). That's a water heater issue rather than a sewer one.

How do you find where the smell is coming from?

Following your nose works better than you might expect. Wisconsin's health department suggests tracing the smell to likely entry points, such as a basement floor drain (Wisconsin DHS). Try this:

1. Go room by room. Note where the smell is strongest. Basements, laundry rooms and bathrooms are the usual places.
2. Check every drain in that area. Put your nose near each sink, tub, shower and floor drain.
3. Check the base of each toilet. A smell that's strongest at floor level around a toilet points toward the seal.
4. Notice when it happens. A smell that shows up after the washer drains or a toilet flushes can point to a venting or main line problem. A smell that's always there in one room points to a dry trap or a seal.
5. Run hot and cold water separately. If only the hot water smells, look at the water heater rather than the drains.

What can you safely check yourself?

These steps are safe for most homeowners and often solve the problem:

• Refill every trap. Run water for a minute in each sink, tub and shower, and pour a bucket of water into each floor drain. Flush any toilets that rarely get used.
• Look under the sinks. Check that the curved pipe under each sink is there, connected and not leaking. A loose connection can let gas out.
• Clean the sink overflow. The small opening near the top of a bathroom sink can collect gunk and smell. A bottle brush and soapy water usually clear it.
• Clean slow drains. Remove and rinse drain stoppers and clear hair from shower drains.
• Check the toilet. Gently see whether it rocks. Don't try to reset or remove it unless you're comfortable doing that work.
• Wait a day. If refilling the traps was the fix, the smell should fade within a day or so.

Skip anything that puts you at risk. Don't climb onto the roof, and don't open a sewer cleanout if you suspect a backup, since sewage can come out under pressure.

When should you call a plumber?

Call a plumber if:

• The smell comes back after you've refilled every trap.
• A toilet rocks, leaks at the base or smells at the floor.
• Several drains are slow, gurgle or back up into each other.
• Sewage or dirty water is coming up through a drain or tub.
• You see soggy, smelly patches in the yard over the sewer line.
• The smell is inside a wall or ceiling with no drain nearby.

Wisconsin's health department notes that a plumber may be needed if the source is inside a wall (Wisconsin DHS). If you'd like help sorting out what you're dealing with first, Check my plumbing shows the likely causes and what to ask.

What will a plumber check?

A plumber will usually work from the simplest cause to the hardest:

• Traps and drains. Whether each trap holds water and each drain flows freely.
• Toilet seals. Whether a toilet needs to be pulled and reset on a new seal, and whether the flange underneath is damaged.
• Vents. Whether the roof vents are clear and drains are venting properly.
• The main line. If several drains are involved, they may run a camera down the line to look for a clog, roots or a break. Fixr says a camera inspection can show what's causing a main line clog (Fixr).
• Hidden pipes. If the smell is coming from a wall or the foundation, they'll look for a cracked or loose pipe.

What does it cost to fix a sewer smell?

Costs depend on the cause, and many sewer smells are cheap to fix. These are national ranges as of 2026, read on the pages linked:

• Service call: the initial fee ranges from $50 to $300 (Fixr).
• Toilet seal replacement: about $60 to $200 on average (Fixr).
• Toilet flange replacement: $60 to $200 (Fixr).
• Drain cleaning: standard drain cleaning runs $150 to $300 (Fixr).
• Main sewer line clog: $350 to $650, with a national average of $475 (Fixr).
• Sewer camera inspection: $100 to $800 (Fixr).
• Tree root removal: $100 to $600, with pipe replacement extra if needed (Fixr).
• Replacing a broken sewer line: $1,500 to $3,000 (Fixr).
• Septic tank cleaning: $400 to $1,000 (Fixr).
• Sump pump rotten-egg smell: $100 to $250 (Fixr).

We couldn't find a published cost for clearing a blocked plumbing vent, so ask for that price in writing before work starts. A dry trap often costs nothing at all: running water into the drain is the fix.

For more detail on the bigger jobs, see our guides to drain and sewer line cleaning costs and toilet repair costs.

What changes the price?

• Where the problem is. A trap under a sink is easy to reach. A cracked pipe under a slab or behind a finished wall is not.
• How many fixtures are involved. One drain is a small job. A main line clog affects the whole house.
• Whether a camera is needed. Many main line jobs start with a camera inspection.
• Timing. Fixr notes off-hours emergency charges of $50 to $100 for main line work (Fixr).

What should you do if sewage is backing up?

A backup is more than a smell, and it needs a plumber soon. Until then:

• Stop using sinks, toilets, the dishwasher and the washing machine, since that water has nowhere to go.
• Keep children and pets away from the area.
• If you have to clean up, wear rubber boots, rubber gloves and goggles (CDC).
• If water is near outlets, cords or appliances, stay out of it and don't touch anything electrical (CDC).

What should you ask a plumber?

Good questions help you understand the job and compare quotes:

• What do you think is causing the smell, and how did you confirm it?
• Is there a service-call fee, and does it go toward the repair?
• Will you use a camera to look at the line, and can I see the footage?
• If the toilet needs a new seal, will you check the flange too?
• Can you check the roof vents while you're here?
• Can I get the price in writing before you start?
• Is your work guaranteed, and for how long?
• Are you licensed and insured for this work in my area?

How do you keep the smell from coming back?

A few habits keep trap seals full and drains clean:

• Run water in seldom-used sinks, tubs and showers every week or two.
• Pour a bucket of water into floor drains every so often.
• Keep grease, wipes and food scraps out of drains.
• Fix a rocking toilet early, before the seal fails.
• If you'll be away for a long time, ask a plumber about ways to keep floor drain traps from drying out.

If the smell keeps returning even with those habits, it's worth having a plumber look for a venting problem or a crack, since those won't fix themselves. When you're ready, use Check my plumbing to get a snapshot of the likely cause and typical costs before you talk to a local company. You may also find our guide to what plumbers cost useful when comparing quotes.