Sewer Smell in the House: Why the Bathroom or Basement Smells Like Sewage, and What to Do

A sewer smell in the house means sewer gas is getting past the barriers that should keep it in the pipes. Every drain has a trap, a bend that holds water, and the City of Kerrville, Texas, explains that the water blocks sewer gas while vent pipes carry the gas up and out through the roof. Kerrville calls a dry trap a frequent cause of indoor odors and says a quart of water may correct it.

If the smell comes only with the hot water, our guide to hot water that smells like rotten eggs covers the water heater. A musty, damp smell is a different problem, covered in musty smell in the basement, and drains that are slow or backing up are in slow or clogged drain.

First, Check: Water or Drain?

The City of Phoenix's test: fill a glass from the cold tap, carry it to another room, and smell it. If the glass doesn't smell, the problem is in the drains. If only the hot water smells, see the hot water guide. Otherwise, follow the odor to where it's strongest, as Wisconsin's Department of Health Services suggests; it says sewer gas gets in through floor drains, leaking or blocked roof vents, and, when the gas is in the soil, foundation cracks.

Most Likely Causes

1. A dry trap. Kerrville explains that a rarely used basement floor drain loses its trap water to evaporation until the seal is gone. Oatey, a plumbing products maker, says a basement sink, tub, shower, or laundry drain can dry out after about 2 to 4 weeks unused, so check guest baths and unused showers too. The Milwaukee Metropolitan Sewerage District (MMSD) adds unused toilets, and notes that a finished basement's floor drain may be hidden under carpet.
2. A toilet's wax seal. The wax ring under a toilet forms what Fluidmaster, a toilet parts maker, calls "a gas-tight, odor-tight and watertight seal." Kerrville's test: grab the bowl and try to slide it side to side. It should resist a few pounds of pressure; if it rocks, Kerrville says the wax ring has failed. Oatey says a flange set too low or unevenly squeezed wax can let gas escape, with signs such as a sewer smell at the base, seepage, or rocking.
3. A loose or missing cleanout plug. Cleanouts are capped openings that give access to the drain line for cleaning, and Oatey says its cleanout cap "prevents sewer gas escapement." The City of Siloam Springs, Arkansas, says the outdoor sewer cleanout is usually within three to five feet of the house and that its cap should be on tight and in good shape.
4. A blocked or leaking vent. Kerrville says bird nests, leaves, and pests clog vents. North Richland Hills, Texas, explains that with a clogged vent, a flushing toilet or a washing machine pump can suck the water out of a nearby tub or sink trap. Kerrville links gurgling and bubbling in fixtures, with a rotten-egg odor, to missing venting. Phoenix and Kerrville both point to cracked pipe or loose joints in vent piping. An S-trap under a sink is a related venting problem; see S-trap plumbing.
5. A slimy drain, a clog, or a sewer backup. Phoenix says hair, dirt, and grime can build into a layer of slime in a drain that gives off a rotten-egg sulfur smell, and Kerrville and Oatey say debris in a sink's overflow channel can smell too. A blocked or damaged sewer line, Oatey says, forces gases into the house, and Minneapolis says a clogged floor drain can bring gurgling in drains or toilets, water on the floor, and sewage backing up.
6. A septic system problem. The EPA's signs of a failing septic system include sewage odors around the tank or drainfield, gurgling plumbing, slow drains, backups, and damp spots or bright green, spongy grass over the drainfield. It also says odor can come from a roof vent that lets the system's pressure equalize, which is normal, or from a vent clogged with leaves, frozen in a long cold spell, or caught in a downdraft.
7. A sump pit or sewage pit lid. Minnesota Extension says a sump pit's pump lifts drainage water outside and that sumps should have an airtight cover. A sewage basin is different: Liberty Pumps, which makes them, sells them for basement bathrooms, with the fixtures' drains piped in and the basin vented to the building vent or outdoors. Its covers bolt down over rubber gaskets, and Liberty says "proper sealing is required to keep sewer gas from leaking from the system."

What You Can Fix Yourself

Refill Every Trap

Pour about a quart of water into each floor drain, sink, tub, and shower you rarely use, as Kerrville and Phoenix advise, and flush any toilet that hasn't been used. Shine a flashlight into a floor drain first: MMSD says you should see a reflection off the water, and that the added water restores the seal and should address the odor. If you add water and still see none sitting in the pipe, MMSD says the drain may have no trap; call a licensed plumber. For a trap that dries out again, Kerrville suggests adding ¼ cup of mineral oil to slow evaporation.

Check the Toilets

Do Kerrville's slide test on each toilet and look for water or stains at the base. If the seal has broken, Kohler's instructions say it needs a new wax seal; our loose toilet guide covers the reset and when to hand it to a plumber.

Snug the Cleanout Caps

Find the capped cleanouts on your drain pipes and the sewer cleanout outside. Each cap should be present, tight, and in good shape, as Siloam Springs asks; replace a cracked one. Leave opening a cleanout to a plumber.

Clean a Smelly Sink

If a sink with a full trap still smells, clean the stopper and trap as in our slow drain guide, then the overflow. Oatey's method: ½ cup of baking soda, then 1 cup of vinegar, 15 minutes, then hot water.

Look at the Pit Lids

Seat the sump pit lid firmly, and unplug the pump before touching anything in the pit (see sump pump runs constantly). Don't open a sewage basin. Liberty warns that sewage systems can contain flammable and explosive gases and says no one unqualified should handle the system. If its cover, cord seals, or vent pipe looks loose or damaged, call a licensed plumber.

What Sewer Gas Can Do to Your Health

Hydrogen sulfide is the rotten-egg part of sewer gas. The federal Agency for Toxic Substances and Disease Registry (ATSDR) says it is often called sewer gas and that people usually can smell it at low concentrations. OSHA says low concentrations irritate the eyes, nose, throat, and airways, and that people with asthma may have trouble breathing at lower levels than others; its chart links prolonged exposure at 2 to 5 parts per million to nausea, tearing eyes, and headaches.

Don't use your nose as a gauge. ATSDR says people lose the ability to smell the gas after about 2 to 15 minutes at around 100 ppm, and OSHA says not to rely on your sense of smell to warn of hazardous levels. OSHA adds that hydrogen sulfide is heavier than air, collects in low, enclosed spaces such as basements, and is highly flammable. If you think sewer gas is building up in an enclosed space, Wisconsin's health department says to evacuate, call the fire department, and avoid sparks from appliances, matches, or lighters. The CPSC lists headache, dizziness, and nausea among the symptoms of carbon monoxide, which has no smell, so if anyone has them, get everyone outside to fresh air and call 911 from there.

If Sewage Backs Up

Stop running water and call a plumber, who Portland's sewer utility says should investigate the cause first. Portland suggests a sewage cleanup company, and the EPA says water damage caused by sewage calls for a professional experienced with contaminated water. If you clean up a small backup yourself:

• Keep children and pets away until the cleanup is done, and wear rubber boots, rubber gloves, and goggles (CDC).
• Keep sewage off your face and eyes, and wash any cut that touches it right away (Portland).
• Wash, then disinfect. The CDC says to clean wet surfaces with hot water and detergent; Portland then says to disinfect with one part household bleach to ten parts water. Open windows first, and never mix bleach with ammonia or any other cleaner (CDC).
• Throw out what you can't clean. The CDC says to discard things you can't clean or dry quickly, such as carpet and padding, and drywall and insulation contaminated with sewage.
• Wash up. Use soap and water, and wash cleanup clothes in hot water, apart from other laundry (CDC).

When to Call a Pro

• The smell comes back with the traps full. Wisconsin says a leak behind a wall may need a plumber, and Kerrville and North Richland Hills say a plumber can smoke-test the drains to find it.
• A floor drain won't hold water, or a toilet rocks or seeps at the base.
• A vent needs clearing. Kerrville says a plumber may need to clean the vents all the way through the roof; leave roof work to them. On a septic system, the EPA says a vent may need to be cleaned or raised, and charcoal filters for roof vents may help.
• Several drains are slow, gurgle, or back up. See when it's the main line, not the toilet.
• Septic warning signs. The EPA says an odor can't be diagnosed remotely; call a septic service provider or plumber.

Prevent It

• Run water. Minneapolis says to pour a bucket of water down each floor drain once a month and to run water periodically in bathroom and utility sinks.
• Protect a trap that dries out. Sioux Chief, which makes trap primers, says they add a small amount of water to the trap automatically and that barrier-type devices slow evaporation and block sewer gas; codes often require primers for floor drains in remote areas.
• Watch the vents. Minneapolis says to check that roof vents aren't blocked by leaves, snow, or ice; look from the ground.
• Maintain a septic system. The EPA says to have it inspected at least every three years; tanks are typically pumped every three to five.

Frequently Asked Questions

Why does my house smell like sewer after a vacation?

Usually dry traps. Oatey says a basement sink, tub, shower, or laundry drain can lose its trap water after about 2 to 4 weeks unused. Pour a quart of water into each drain and flush the toilets.

Why does the sewer smell come and go?

Phoenix says sewer odor is more common at night and early morning, when heavy flows push gas ahead of them toward a dry trap. Oatey says intermittent odors often point to a partly blocked vent, a nearly dry trap, or a small leak.

Is it sewer gas or a gas leak?

Don't try to tell by smell. Indiana's utility regulators say to leave if you notice even a faint gas odor. Leave the house without switching anything on or off or using a phone inside, and call 911 and the gas utility once you're a safe distance away.

Should I call the city or a plumber?

Check your traps first. MMSD asks residents to report odors that persist with working traps, and Kerrville asks residents to call about a suspected city sewer problem. Inside the house, call a licensed plumber.

The Bottom Line

Rule out gas first. Then refill every rarely used drain, check each toilet for rocking, and snug the cleanout caps and pit lids. If the smell returns, a drain won't hold water, or sewage backs up, call a licensed plumber, or a septic service provider on a septic system.

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