Short definition
A trap is a bent drain fitting that holds a small column of water — the trap seal — to block sewer gas from rising into the home. Every fixture in the house has one. Modern code (UPC 1002.4) requires the trap seal to be between 2 and 4 inches deep, with one trap per fixture, no double-trapping. The most common modern trap is the P-trap.
What it is
Drain pipes are open to the public sewer at one end and to your home at the other. Without a barrier, sewer gas would migrate up the pipe and into every room. The trap solves this with a water seal: a U-shape, an S-shape, or a canister geometry that holds standing water against gravity. As long as water is in the bend, gas can’t push past it.
By UPC 1002.4 and IPC 1002.4, the liquid seal must be 2 inches minimum and 4 inches maximum. Standard residential traps are about 2 inches; deep-seal traps are 3 to 4 inches and used where evaporation or pressure events are bigger concerns. Code also bans certain trap types outright: S-traps, bell traps, crown-vented traps, traps with movable parts, and traps with concealed interior partitions (with an exception for plastic and glass). Drum and bottle traps are allowed only under specific conditions.
Why it matters to a homeowner
Every plumbing fixture in the house has a trap, and every “sewer smell” complaint eventually comes back to one of them. Knowing the geometry — P-trap (good), S-trap (banned), drum (mostly banned), house trap (legacy) — turns a confusing smell complaint into a quick diagnosis. A failed trap is also one of the most fixable plumbing problems: pour water in, replace a slip-joint washer, or swap a non-compliant trap during the next remodel.
Common variants / not the same as
- P-trap. Current standard at every sink, lavatory, tub, shower, laundry, and floor drain.
- S-trap. Banned. Self-siphons every flow event.
- Drum trap. Mostly banned. Common pre-1940 under tubs.
- House trap. Legacy. One U-bend at the building drain’s exit.
- Integral trap. Built into the fixture (toilet, urinal, bidet). Same function, no separate fitting.
The trap (DWV) is unrelated to the heat trap in a water heater, which prevents convection in supply lines.