What is a P-trap?
Updated August 2026
A P-trap is the U-shaped bend of pipe under every sink, tub, and shower that holds a small pool of standing water to seal sewer gas out of the room. The name describes its profile: a U turned on its side, with a horizontal arm running back into the wall. That geometry holds 2 to 4 inches of water at all times, and plumbing code requires one on every fixture connected to a drain.
Where it is
Open the cabinet under a sink and it is the first thing in view, a chrome or white plastic U hanging off the tailpiece and held together by two hand-tight slip nuts. Bathroom lavatories use 1-1/4 inch pipe; kitchen sinks use 1-1/2 inch. Tub and shower traps sit out of sight below the floor, reachable only through the ceiling underneath or an access panel behind the faucet wall. A basement floor drain has its trap cast into the body, and a washing machine standpipe hides one inside the laundry box.
What it does
The water in the bend works as a plug. Sewer gas, which carries hydrogen sulfide and methane along with the drain flies that breed in the line, cannot push past it, while waste water still flows straight through. Break that seal and the room smells like a sewer within a day. Seals break three ways: a fixture goes unused for 3 to 4 weeks and the trap evaporates dry, a blocked vent siphons it empty (listen for a gurgle right after the sink drains), or a slip washer hardens and drips. The bend catches solids too. Grease and hair settle at the bottom of the U, which is why one slow sink is nearly always a trap problem rather than a main line problem, and why a plumber's first move is a bucket and two nuts.
Maintenance
Pull and rinse the trap when a sink starts to drain slowly, roughly once a year in a kitchen and every 2 to 3 years in a bathroom. Both the cleaning interval and the step-by-step come down to 15 minutes with a bucket and channel locks. Pour a quart of water into any drain you rarely use once a month, and add two tablespoons of mineral oil in a guest bath or floor drain to slow evaporation. If it drips after reassembly, read why P-traps leak before reaching for pipe dope. Letting a slow drain ride carries its own bill: ignoring slow drains.
What it costs to replace
A replacement P-trap runs $8–20 for a PVC kit from any hardware store, or $125–250 if a plumber comes out for it.
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Frequently asked questions
Why does my bathroom smell like sewer gas?
Nine times out of ten a trap in that room has gone dry, and the guest shower or the floor drain is the usual suspect. Run every fixture for 30 seconds, flush the toilet, pour a quart of water down anything you cannot run, then wait a day. If the odor holds, look for a cracked trap body, a failed toilet wax ring, or a plugged vent stack on the roof. A plumber can smoke-test the system for $150–350 and find it in one visit.
Can I get a ring back out of a P-trap?
Usually, and this is exactly what a removable trap is for. Set a bucket under the U, unscrew both slip nuts by hand (channel locks with a rag around them if they resist), and tip the trap into the bucket. Anything heavy that went down the drain in the last few uses is sitting in that bend. Do not run water first: flushing hard can push a ring past the trap into the drain line, where it is gone for good.
Why are S-traps not allowed anymore?
An S-trap sends its outlet straight down rather than sideways into a vented wall line, so a full column of draining water can siphon the trap dry behind itself and leave nothing between the room and the sewer. Codes have banned them in new work for decades, though plenty survive under old pedestal sinks. Converting to a properly vented P-trap costs $150–400 when the wall vent is close, and more when a new vent line has to be run up through the framing.