Short definition
A P-trap is the U-shaped fitting under every sink, lavatory, tub, shower, laundry, and floor drain in a modern home. The U holds a column of standing water — the trap seal — that blocks sewer gas from rising into the house. Code requires one P-trap per fixture, with a 2- to 4-inch water seal, vented within a code-prescribed distance.
What it is
Looked at from the side, a P-trap looks like the letter P laid on its back: a vertical inlet drops from the fixture into a U-bend at the bottom, and a horizontal outlet runs from the top of the U toward the wall. The U is always full of water. As soon as you stop running the fixture, the standing water in the U is what blocks sewer gas from coming back up.
UPC 1002.4 and IPC 1002.4 both require the liquid seal to be between 2 and 4 inches deep. Standard residential traps run about 2 inches; deep-seal traps used in floor drains and other high-evaporation locations run 3 to 4 inches.
The horizontal outlet of a P-trap is what makes it work. Compared to an S-trap (banned), which exits vertically, the P-trap’s horizontal exit prevents the continuous-water-column problem that causes self-siphonage. As long as the trap arm downstream is short enough, sloped right, and connects to a vent within code distance, the trap holds its seal.
Why it matters to a homeowner
The P-trap is the single most-touched plumbing fitting in a home. It’s also the most common DIY repair: under-sink leaks usually live at the slip-joint washers, and re-tightening or replacing a washer is a 5-minute fix. A persistent sewer smell from a single fixture usually traces to either a dry trap (pour water in) or a slip-joint that’s lost its seal.
When a contractor’s quote calls for “replacing the P-trap and trap arm,” the trap itself is the cheap part — a $5 PVC P-trap, plus washers. The labor and inspection cost lives in re-cutting the trap arm to the right length and confirming the vent connection.
Common variants / not the same as
- P-trap vs. S-trap. P exits horizontally to a vented branch (correct). S exits vertically downward (banned; self-siphons every flow event).
- P-trap vs. drum trap. P is a U-bend; drum is a canister. Drum traps are mostly banned and common pre-1940 in tubs.
- P-trap vs. running trap (house trap). P is at the fixture; running trap is one big trap at the building’s exit.
- P-trap vs. integral trap. Integral traps are molded into toilets, urinals, and bidets — same function, no separate fitting. Double-trapping is prohibited.
- Threaded slip-joint vs. solvent-weld P-trap. Slip-joint allows servicing. Solvent-weld is permanent and often required for in-wall traps.
Common failure modes
- Slip-joint washer hardens or loosens — small drip under the sink. DIY-fixable in 5 minutes.
- Trap dries out in vacation homes or unused fixtures — sewer gas enters.
- Hair, debris, and grease at the bottom of the U — clogs. Lift the trap, dump it out, reassemble.
- Plastic P-trap cracks from disposer heat — rare but happens with some hot-water disposer setups.
- Frozen and split if installed in unconditioned space (kitchen sink against an exterior wall) — a Washington freeze-event failure mode.
Washington note (when to call a pro)
Plastic P-traps installed against exterior walls can freeze during Arctic outbreaks. If your kitchen sink trap is in a cabinet sharing a wall with the outdoors and you’ve had freezing trouble before, a heat cable or trap relocation is the long-term fix. For one-off cold snaps, leave the cabinet door open and a cold-water trickle running at the sink overnight.
FAQ
How do I know if my P-trap needs replacing?
Most P-traps last 20–30 years. Replace when you see staining or corrosion at the slip joints, repeated leaks even after replacing washers, or visible cracks. A $5 PVC trap with new washers takes 10 minutes to swap.
Why does my P-trap smell after I return from vacation?
The water seal evaporated. A standard 2-inch seal dries fully in about 8–10 days under normal indoor conditions. Pour a quart of water into the trap to refill, and the smell clears in a few hours. For long absences, a tablespoon of mineral oil floats on top of the water and slows evaporation for months.
Can I use a P-trap horizontally?
The trap itself is always installed in its standard orientation — U at the bottom, inlet vertical, outlet horizontal. The “horizontal” part refers to the trap arm that comes after the trap. That arm runs horizontally to the vent connection; UPC limits how long it can be (30″ for 1¼” pipe up to 120″ for 4″+).