P-Trap: What It Does, Why Drains Smell and How to Clean It

What It Means

A P-trap is the curved piece of drainpipe under a sink, tub or shower, and behind a washing machine's drain. Oatey, which makes them, explains that the bend holds a small amount of water, and that water forms a seal that stops sewer gas and odors from rising up through the drain. Every time the fixture is used, fresh water flushes through and replaces what was sitting in the curve.

Try It: Is a Dry Trap Behind That Smell?

Tick what's true about the drain you suspect. The dry trap signs come from Oatey and Roto-Rooter.

There's a sewer smell near a drain That drain hasn't been used in weeks It's in a guest bath, basement or laundry area

Likely answer

No smell, nothing to fix

Not sure if it's sewer gas or natural gas? Leave the house and call your gas company or 911 from outside.

How It Sounds at Home

"The guest bathroom smells like sewer."
"Just run the water, the trap probably dried out."
"I dropped my ring down the sink, check the trap."

The Breakdown

Clean it out, following Oatey and Roto-Rooter

Put a bucket under the trap to catch the water and anything in it.
Loosen the slip nuts at each end of the curved section by hand.
Take the trap off and empty it into the bucket.
Clean the inside with a bottle brush.
Put it back, tighten the slip nuts, then run water. Buhata's own suggestion: watch the joints for a minute to be sure nothing drips.

Stop a dry trap from smelling. Oatey and Roto-Rooter say a trap in a drain that sits unused, such as a basement shower, laundry tub or guest sink, can lose its water until sewer gas gets through. Running water into that drain refills the seal.

Situation	What to do
Sewer smell from a drain nobody uses	Run water into it to refill the trap
Something valuable went down the sink	Stop running water and take the trap off to look
Sewer smell from a drain used every day	Call a licensed plumber
Trap leaking after you put it back	Check the slip nuts; call a plumber if it keeps dripping

The first row comes from Oatey and Roto-Rooter; the other three are Buhata's own suggestions.

Short version to try today. Run water into every drain in the house that hasn't been used this month.

Good to Know

Buhata's own safety rule: if you can't tell whether a smell is sewer gas or natural gas, leave the house and call your gas company, or 911, from outside. Don't take a trap apart if chemical drain cleaner has gone down that drain, since the water inside can hold it; call a plumber and say what was used. A cracked trap, a leak you can't stop at the slip nuts, or a smell from a drain that gets used every day is a job for a licensed plumber.

FAQ

What does a P-trap do?

It holds a little water in its bend, and that water blocks sewer gas and odors from coming up through the drain, Oatey explains.

Why does my unused bathroom smell like sewer?

The water in its P-trap has likely evaporated, leaving no seal. Oatey and Roto-Rooter say running water into the drain refills the trap.

How do I clean a P-trap?

Set a bucket underneath, loosen the slip nuts, take the trap off, empty it and scrub the inside with a bottle brush, then put it back and check for drips.

Do It in Buhata

Tasks with a repeat

Add "Run water in the guest bath and basement drains" to Tasks with a monthly repeat so the traps stay full.

Open Buhata

Related Terms and Categories

Clogged drain
A drain blocked by debris built up inside the pipe, so water drains slowly or stops draining altogether.
Drain snake
A long, flexible cable you feed into a drain and turn so its tip breaks up or pulls out a clog that a plunger can't reach.
Garbage disposal cleaning
Keeping a sink garbage disposal clean and odor-free by using it often, scrubbing under the rubber baffle, and grinding only food.
Licensed contractor
A contractor who holds the license your state or local government requires for the kind of work being done, which you can check before you hire.
Bathroom deep clean
A top-to-bottom bathroom cleaning that reaches grout, tub, toilet, floor and fixtures. The ACI's routine puts it every two weeks.
Mold
A fungus that grows indoors wherever moisture lingers. EPA says fix the water problem, then scrub it off hard surfaces with detergent and water.

Home maintenance and repairs

Sources

Oatey, "What is a P-Trap and How Does it Work?"; Oatey, "Why Does My House Smell Like Sewer Gas? (And How to Fix It)"; Oatey, "How Do You Install a P-Trap?"; Roto-Rooter, "What Is a P-Trap?"; Roto-Rooter, "What Happens if I Have an Unused Bathroom Drain?"

Last updated September 30, 2026.