9.2 Traps, Trap Arms, Table 1002.2 Distances, & Prohibited Traps (UPC 1001–1004)

Separate Traps (UPC 1001.2)

• Each plumbing fixture is separately trapped by an approved liquid-seal trap. Fixtures with integral traps, such as water closets, are excepted.
• Not more than one trap is permitted on a trap arm.
• Food waste disposers on restaurant, commercial, or industrial sinks connect to a separate trap.
• Each domestic clothes washer and each laundry tub has its own trap. A laundry tub's trap may also receive an adjacent clothes washer.
• The distance from the fixture outlet down to the trap weir is as short as practicable. A fixture tailpiece may not exceed 24 inches.
• One trap may serve a set of up to three single-compartment sinks or laundry tubs of the same depth, or three lavatories that are immediately adjacent in the same room. The waste outlets must be no more than 30 inches apart, and with three compartments the trap is centrally located.

Trap Arms and Vent Distance (UPC 1002 and Table 1002.2)

A trap arm is the portion of the fixture drain between the trap and the vent. Section 1002.2 requires each fixture trap to have a protecting vent located so the developed length from the trap weir to the inner edge of the vent falls within Table 1002.2, and never less than two times the trap arm diameter.

Table notes:

1. Maintain a 1/4 inch per foot slope on the trap arm.
2. Water closets and similar fixtures: the developed length from the top of the closet flange to the inner edge of the vent may not exceed 6 feet.

Why the maximum? On a long, sloped trap arm, the pipe's end drops below the trap weir. Once the fall exceeds the pipe diameter, the arm can run full and self-siphon the trap. Why the minimum? A vent too close to the weir can let the trap siphon, or act like the prohibited crown vent.

Trap arm fall check: A 1-1/2-inch arm at its 42-inch maximum, sloped 1/4 inch per foot, falls 3.5 × 0.25 = 0.875 inch. That is less than the pipe diameter, so the vent opening stays above the weir level of the trap outlet.

Change of direction (UPC 1002.3)

• A trap arm may change direction up to 90 degrees without a cleanout.
• Exception: Trap arms 3 inches and larger may change up to 135 degrees without a cleanout.
• Horizontal changes follow 706.3 (wyes or combination wye and 1/8 bends).
• Trap arm cleanouts follow 1002.3 (707.14).

Vent opening (UPC 1002.4)

The vent pipe opening from a soil or waste pipe, except for water closets and similar fixtures, may not be below the trap weir.

Trap Construction and Size (UPC 1003)

• 1003.1: Traps are self-cleaning, except those inside an interceptor. Traps for tubs, showers, lavatories, sinks, laundry tubs, floor drains, urinals, and drinking fountains are of standard design, made of ABS, cast brass, cast iron, lead, PP, PVC, or other approved material. An exposed, accessible drawn copper-alloy tubing trap of at least No. 17 B&S gauge is allowed on fixtures discharging domestic sewage.
• 1003.2: No more than one slip joint on the outlet side of a trap, and no tubing trap without a listed tubing trap adapter.
• 1003.3: Trap size is at least the Table 702.1 size and no more than one pipe size larger. The trap is the same size as its trap arm.

Trap Seal (UPC 1005)

Each fixture trap has a liquid seal of at least 2 inches and not more than 4 inches, unless the AHJ requires a deeper seal. Traps are set level with respect to their seals and protected from freezing where necessary. The trap seal is the vertical distance between the crown weir and the top dip of the trap.

Prohibited Traps (UPC 1004)

Floor Drain Traps and Building Traps

• 1006.1: Floor drains connect to a trap that is readily cleanable and sized for its purpose. The drain inlet must be in full view. Where subject to reverse flow of sewage, the drain gets a backwater valve.
• 1008.1: Building traps are not installed unless the AHJ requires them. Where installed, they need a cleanout and a relieving vent or fresh-air intake on the inlet side, not larger than half the drain diameter, ending above grade in a screened outlet outside the building.