Why Does My House Smell Like Sewer? What to Check First

The short answer

A sewer smell inside the house usually means sewer gas is getting past one of the barriers that is supposed to keep it in the pipes. North Dakota’s Department of Environmental Quality says sewer gas in a home “is usually caused by leaks in plumbing or a dry trap in a rarely used drain” (North Dakota DEQ).

A dry trap is the easiest of those to check, so start there. Find the drain closest to the smell. If it is one that rarely gets used (a basement floor drain, a guest shower, a utility sink), run water into it for a minute. If the smell fades and stays gone, a dry trap was the likely cause. If the smell keeps coming back, is strong, or is spread through the house, the problem is somewhere else in the plumbing, and it is time to have it looked at.

Not sure it is sewer gas? A rotten-egg smell can also mean a natural gas leak. If you have gas service and cannot tell which it is, treat it as gas: leave the house and call your gas utility or 911 from outside. See what counts as a plumbing emergency for the full gas-odor steps.

What a drain trap does

Every sink, tub, shower, and floor drain has a trap: a curved section of pipe just below the drain that stays full of water. Under a sink it is the U-shaped pipe you can see. Toilets have one built into the bowl.

That small pool of water is the seal. The City of Minneapolis describes traps as pipes that “hold water and prevent gas from backing up from sewer” (City of Minneapolis). Waste water flows through it; sewer gas cannot get past it, as long as the water is there.

When a drain goes unused for weeks, that water slowly evaporates. Once the trap is dry, Minneapolis notes, “gases from the sewer system can enter your building.”

Can a drain that never gets used really cause the smell?

Yes. When a drain sits unused, its trap can dry out and let sewer gas into the room. Drains that tend to go unused:

• Basement floor drains, often near the water heater, furnace, or washer
• A shower or tub in a guest bathroom or a finished basement
• A utility or laundry sink that only gets used a few times a year
• Any drain in a house that sat empty, like after a long trip or between owners

The fix costs nothing. Run water into the drain for a minute or two to refill the trap (North Dakota DEQ). For a floor drain, a bucket of water does it. Minneapolis suggests pouring a bucket down each floor drain once a month and running water in rarely used sinks periodically so the traps do not dry out.

Give it a few hours. If the smell clears and does not come back once you keep that drain wet, you found it.

First checks a homeowner can do

These are all look-and-note checks. Nothing here involves taking anything apart.

1. Find where the smell is strongest. Is it in one room, near one drain, or all over the house? Is it worse at certain times, like after a toilet flush, a load of laundry, or a heavy rain?
2. Refill every rarely used drain. Floor drains, spare showers and tubs, utility sinks. Then wait and see if the smell changes.
3. Look at each toilet base. Is the floor wet around it? Does the toilet rock when you sit on it? Is there staining or soft flooring at the base? A toilet sits on a seal that connects it to the drain pipe. The Delaware Department of Insurance notes that “a telltale sign of toilet seal leaks is water that gathers around the base” (Delaware Department of Insurance). A bad seal can let odor out as well as water, though a smell by itself does not prove that is the source.
4. Look under sinks. Check for wet cabinet floors, drips at the drain pipes, or a loose connection.
5. Listen to the drains. Gurgling from a sink or toilet when another fixture drains is worth writing down for the plumber.
6. Air the room out, if it is safe to stay there: open windows and run the bathroom fan.

Write down what you found. “It is the basement, worst near the floor drain, and it comes back two days after I refill it” is far more useful to a plumber than “the house smells.”

What a smell that keeps coming back can mean

If refilling traps does not fix it, or the smell keeps returning, the odor is likely coming from a part of the plumbing you cannot easily see. Some possibilities a plumber would consider:

• A failed toilet seal. The seal under the toilet can break down or shift, especially if the toilet rocks.
• A leaking or cracked drain pipe inside a wall, under a floor, or in a crawlspace.
• A loose or damaged fitting where drain pipes connect.
• A vent problem. Plumbing systems have vent pipes that run up through the roof and let air move through the drains. Minneapolis notes that a blocked roof vent “can impact the flow in your sewer system.” Leaves, ice, or nesting animals can block one. A vent problem can show up as gurgling drains or slow draining along with the smell. Do not go on the roof to check it yourself.
• A trap that will not hold water, because it is cracked, missing, or was never installed right.

A smell alone cannot tell you which of these it is. That takes someone looking at the actual pipes, and sometimes a camera or a smoke test to find where the gas is getting out.

When to stop troubleshooting and call a plumber

Call a licensed plumber if:

• the smell comes back even after you keep the traps refilled,
• the smell is strong or spread through more than one room,
• a toilet rocks, leaks at the base, or has damaged flooring around it,
• you see leaks, wet spots, or cracked pipe at any drain, or
• drains are gurgling or slow along with the smell.

The North Dakota DEQ’s guidance is direct: if the sewer gas is strong and widespread, contact a plumber immediately; if the odor keeps coming back, a professional inspection is likely necessary to find the root cause (North Dakota DEQ).

If the odor is overwhelming, or anyone in the house feels unwell, get everyone out into fresh air first. Open windows on the way out only if you can do it quickly. Call your doctor or 911 for anyone with symptoms, then call a plumber. And again, if you cannot rule out natural gas, leave and call the gas utility or 911 from outside before anything else.

What not to do: do not pour drain chemicals down the line to try to “kill” the smell. A smell is not a clog, and chemicals will not fix a dry trap, a bad seal, or a cracked pipe.

How this differs from a clog or a sewer backup

These problems can overlap, but they are not the same thing, and they call for different fixes.

• A sewer smell means gas is getting out. The drains may be working fine.
• A clog means water is not getting out. If the same drain keeps backing up, that is a different problem; see why the same drain keeps clogging.
• A sewer backup means waste water is coming up out of a drain. That is an emergency. Stop using water and call for help; see what counts as a plumbing emergency.

If you have a smell and slow drains, gurgling, or water coming up from a floor drain, tell the plumber all of it. Together those point toward a drain problem that deserves a sewer and drain diagnosis, not just a smell check.

Frequently asked questions

Why does my house smell like sewer all of a sudden?

Sewer gas in a home is usually caused by a plumbing leak or a dry trap in a rarely used drain (North Dakota DEQ). A dry trap is the easiest to check: without water in the trap, sewer gas can come up through the drain (City of Minneapolis). Run water into that drain. If the smell does not clear, or keeps returning, have a plumber find the source.

Can a dry drain trap cause a sewer smell?

Yes. The water in a trap is what blocks sewer gas. When a rarely used drain dries out, gas can come up into the room. Running water into the drain for a minute or pouring in a bucket of water refills the seal (North Dakota DEQ).

Why does my bathroom smell like sewer when the drains work fine?

Working drains do not rule out a gas leak. Check whether a rarely used shower or tub in that bathroom has a dry trap, and whether the toilet rocks or is wet at the base. If neither explains it, a plumber can check the toilet seal, the drain connections, and the venting.

Is a sewer smell in the house dangerous?

You cannot judge the source or the risk from how strong the smell is. Take any sewer smell seriously: get fresh air, and call a plumber promptly if it is strong, spreading, or keeps coming back. If anyone feels unwell, get them out of the house and get medical help. If you cannot tell sewer gas from natural gas, leave and call your gas utility or 911.

Should I use a drain cleaner to get rid of the smell?

No. A sewer smell is usually a gas problem, not a clog. Drain chemicals will not refill a trap, reseal a toilet, or fix a cracked pipe.

Smell still coming back?

If the odor keeps returning, is strong or spread through the house, or you have found a leak or a toilet that rocks, contact Spangler Plumbing for an evaluation. Tell us where the smell is strongest, which drains you refilled, and whether it changed. We will find where the gas is getting out and give you honest advice on the fix. Spangler serves homeowners in Middletown and across New Castle County. See all plumbing services, request service, or call 302-293-8008. Delaware Master Plumber #UT1488.