4.1 Fixture Traps, Trap Seals & Trap Seal Protection

Why traps matter

A trap keeps a water seal between the occupied space and the sewer. The vent system (Chapter 7 of this guide) exists mainly to protect that seal. Interceptors and Traps is a 4-item area, but trap rules also show up in DWV questions.

Trap terms

• Trap seal (IPC Chapter 2): the vertical distance between the weir and the top of the dip of the trap.
• Weir: the overflow edge where water leaves the trap and enters the trap arm.
• Dip: the lowest point of the trap's crown on the inlet side; the top of the dip is the lower limit of the seal.
• Fixture drain (trap arm): the drain from the trap to its junction with another drain pipe.

Where traps are required (IPC 1002.1)

Each plumbing fixture shall be separately trapped by a liquid-seal trap, except as the code allows.

• Vertical limit: the vertical distance from the fixture outlet to the trap weir may not exceed 24 inches.
• Horizontal limit: the horizontal distance may not exceed 30 inches, measured from the centerline of the fixture outlet to the centerline of the trap inlet.
• The height of a clothes washer standpipe above its trap follows 802.4.3: between 18 and 42 inches.
• No double trapping. A fixture must not be served by two traps in series.

Exceptions to 1002.1:

1. Fixtures with integral traps (water closets and many urinals) do not get another trap.
2. A combination fixture may be installed on one trap if one compartment is no more than 6 inches deeper than the other and the waste outlets are no more than 30 inches apart.
3. Floor drains in multilevel parking structures that discharge to a building storm sewer need not be individually trapped. If they discharge to a combined sewer, they may connect through a main trap under 1103.1.
4. A pot, pan and dish sink may connect directly to a hydromechanical grease interceptor following the manufacturer's instructions. The branch serving the interceptor must have an emergency floor drain downstream of the interceptor connection. For a combination sink of up to three compartments, the vertical distance from the fixture outlet to the interceptor inlet may not exceed 30 inches, and the developed length from the most upstream outlet to the interceptor inlet may not exceed 60 inches.

Trap design (IPC 1002.2, 1002.5 to 1002.9)

• 1002.2: traps must be self-scouring. Traps must not have interior partitions unless the trap is integral with the fixture or made of an approved material resistant to corrosion and degradation. Slip joints must have approved elastomeric gaskets and are allowed only on the trap inlet, trap outlet and within the trap seal.
• 1002.5: the trap must be large enough to drain the fixture rapidly and not smaller than the size in Table 709.1. A trap must not be larger than the drainage pipe it discharges into.
• 1002.6: building (house) traps are prohibited.
• 1002.7: traps must be set level with respect to the trap seal and protected from freezing where necessary.
• 1002.8: a recess for an underground trap, such as a bathtub trap in a slab, must have corrosion-resistant, insect-proof and vermin-proof sides and bottom.
• 1002.9: an underground acid-resisting trap of vitrified clay or other brittleware is embedded in concrete extending 6 inches beyond its bottom and sides.

Prohibited traps (IPC 1002.3)

The trap seal (IPC 1002.4)

Each fixture trap must have a liquid seal of at least 2 inches and no more than 4 inches. A deeper seal is allowed only for special designs relating to accessible fixtures. A seal shallower than 2 inches is easily lost to small pressure changes. A seal deeper than 4 inches slows flow and makes the trap collect solids. The vent system is designed so the seal never sees a pressure difference greater than 1 inch of water column (901.2).

How seals are lost

1. Self-siphonage: a full-bore slug leaving the fixture pulls the seal out behind it. This is controlled by trap arm length and slope limits (909.1 and 909.2) and by venting.
2. Induced siphonage: discharge from another fixture past the branch creates suction on the trap. Proper venting prevents this.
3. Back pressure: compressed air at stack bases or offsets pushes the seal into the room.
4. Evaporation: in rarely used drains, the seal dries out. Trap seal protection under 1002.4.1 prevents this.
5. Capillary action: lint or hair over the weir wicks the water out.
6. Wind effects at vent terminals can rock the seal.

Trap seal protection (IPC 1002.4.1)

Trap seals of emergency floor drains and traps subject to evaporation must be protected by one of these methods:

Installation details:

• The discharge pipe from a primer valve or device connects to the trap above the trap seal, on the inlet side of the trap (1002.4.1.1 to 1002.4.1.3).
• A fixture drain used for priming must not run on or above the floor surface. It connects to the floor drain, trench drain or floor sink below the flood level rim and above the trap inlet (1002.4.1.5).
• A barrier-type device stops evaporation and sewer gas without a water supply, using a one-way seal in the drain throat.

Exam reminders

• 24 inches vertical and 30 inches horizontal from fixture outlet to trap.
• Seal: 2 to 4 inches.
• No double traps, no building traps, no S traps.
• A drum trap is allowed only as a solids interceptor or in a chemical waste system.
• Water closets are self-siphoning fixtures with integral traps, so the trap-to-vent distance in Table 909.1 does not limit them (909.1 exception).