P-Trap

The Full Picture

A P-trap is a bend in a drain pipe shaped roughly like the letter P on its side. After each use, some water stays in the low part of the bend, forming a seal. That water plug stops gas from the drain and sewer from rising into the room, while wastewater still passes through. The trap connects the fixture tailpiece to the trap arm, which then runs to the vent and drain. A trap needs a vent nearby so siphoning does not pull the seal out, and plumbing codes control the trap seal depth, the allowed distance to the vent, and the pipe sizes. Those rules are in the adopted plumbing code, either the International Plumbing Code or the Uniform Plumbing Code, as locally amended.

Not every fixture needs a separate trap. Water closets and many urinals have a trap built into the fixture, so no extra trap is priced for them. Lavatories, kitchen sinks, service sinks, bathtubs and floor drains generally take a separate trap, and some use a deep-seal or primer-fed version where the seal could dry out. The fixture schedule and the fixture specification, with the manufacturer's data, tell the estimator which fixtures include an integral trap, so counting should follow the schedule rather than a rule of thumb. Where a fixture is accessible, the ADA Standards section 606 covers lavatories and sinks, and the clearance and exposed piping below them can affect which trap is used and how it is finished.

On plans, a P-trap is rarely drawn as its own item. It is implied by the fixture on the plan and shown on the plumbing riser diagram, isometric and fixture detail, with the drain size labeled. Because the trap is visible on a detail but not on the floor plan, estimators often miss it when they count only what is symbolized. The fixture schedule, the detail sheet and the specification need to be read together. Materials include plastic (ABS, PVC or polypropylene) for most drains, brass or chrome-plated brass where the trap is exposed, and cast iron for larger floor drains and heavy-duty fixtures. Size commonly follows the fixture drain, and it is stated on the plan or in the schedule.

The takeoff unit is each, by size and material, counted with the fixture. Pricing includes the trap, the trap arm connection and any cleanout or slip joint nuts, and pipe from the trap arm to the branch is carried in the DWV pipe quantity. Common mistakes are to count a separate trap for every toilet or urinal, to miss traps for fixtures that rely on one (floor drains, service sinks), and to price a plastic trap where an exposed chrome trap is called out. A trap primer, where required for floor drains, is a separate item.