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Plumbingaka: trapaka: fixture trapaka: S-trap

P-Trap

In Plain English

The curved U-shaped pipe under every sink that holds a small amount of water to block sewer gas from coming up through the drain.

Definition

A curved section of drain pipe shaped like the letter P that retains a small amount of water, creating a seal that prevents sewer gases from entering the building through the drain opening. Every plumbing fixture must have a trap of some type. The water seal in a P-trap can evaporate in unused fixtures, which is why trap primers are used for floor drains.

Why It Matters in Bidding

P-traps are a per-fixture cost item, so the plumbing takeoff ties trap count directly to fixture count, with material and labor for each connection. Estimators also flag where code or design requires trap primers, deep-seal traps, or accessible traps under ADA fixtures, because missing those details in the bid leads to inspection failures and rework that eat into the plumbing margin.

Example

Pricing a restroom rough-in, the plumber's estimator counts 12 lavatories and adds a P-trap and trap arm for each, then notes the three floor drains require trap primers, which he carries as a separate line item.

Related Terms

Drain-Waste-Vent (DWV)Trap PrimerLavatoryCleanout

Frequently Asked Questions

Floor drains require a trap like any fixture, but because they are seldom used the water seal can evaporate and let sewer gas in. Code commonly requires a trap primer to periodically refill the seal. Estimators should carry both the trap and the primer line, plus its water supply connection.
Generally one trap per fixture, since every fixture must be trapped. Estimators count fixtures from the plans and apply a trap, trap arm, and connection labor unit to each, noting exceptions like accessible traps under ADA lavatories or deep-seal traps that carry higher material and insulation cost.
Under ADA lavatories the trap and supply lines must be insulated or shielded to protect users, and the trap configuration must keep knee clearance. That adds protective wrap kits and sometimes offset traps, so estimators price accessible fixture traps higher than standard ones rather than using a single blanket trap unit.

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