5.1 Fixture Traps, Trap Seals, Prohibited Traps & Trap Seal Protection
Key Takeaways
Each fixture must be separately trapped, with no more than 24 inches vertically and 30 inches horizontally from the fixture outlet to the trap, and a fixture may not be double trapped (IPC 1002.1).
A combination fixture may share 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 (IPC 1002.1, exception).
IPC 1002.3 prohibits traps that depend on moving parts, bell traps, crown-vented traps, non-integral traps with interior partitions (unless made of approved corrosion-resistant material), S traps and drum traps. Drum traps used as solids interceptors or in chemical waste systems are allowed.
Each trap must hold a liquid seal of 2 to 4 inches, deeper only for special designs relating to accessible fixtures (IPC 1002.4), and building (house) traps are prohibited (IPC 1002.6).
Emergency floor drains and seals subject to evaporation need protection: an ASSE 1018 primer valve, an ASSE 1044 wastewater-supplied primer, an ASSE 1072 barrier device, or a drain from a nearby lavatory, hand sink or drinking fountain (IPC 1002.4.1).
Why this section matters
The trap seal is the only barrier between the sewer atmosphere and occupied space. Questions in Drain Waste and Vents and Fixtures and Fittings test the trap rules in Chapter 10 of the 2021 IPC, especially the numbers 24, 30, 2 to 4 inches and 6 inches.
Trap anatomy
- Inlet: where the fixture tailpiece enters.
- Dip: the lowest inside point of the trap bend.
- Weir (crown weir): the lowest point of the outlet passage. It sets the water level.
- Trap seal: the vertical distance between the weir and the top of the dip (Section 202).
- Fixture drain (trap arm): the drain from the trap to its junction with any other drain. Its fall and length to the vent are limited by Table 909.1 (section 6.2).
Basic trap rules (1002.1)
- Each plumbing fixture must be separately trapped by a liquid-seal trap, unless the code says otherwise.
- The vertical distance from the fixture outlet to the trap weir may not exceed 24 inches. A long tailpiece builds momentum and can siphon the trap.
- The horizontal distance may not exceed 30 inches, measured from the centerline of the fixture outlet to the centerline of the trap inlet.
- No fixture may be double trapped. Two seals in series trap air between them and slow drainage.
- A clothes washer standpipe's height above its trap follows 802.4.3 (18 to 42 inches above the weir).
Exceptions to separate trapping:
- Fixtures with integral traps, such as water closets and most urinals.
- A combination plumbing fixture 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.
- Floor drains in multilevel parking structures that discharge to a storm sewer. If they must discharge to a combined sewer, they may connect through a main trap per 1103.1.
- A pot or utensil sink served by a hydromechanical grease interceptor installed per the manufacturer, under the conditions in the exception. These include an emergency floor drain downstream and limits of 30 inches vertical and 60 inches developed length to the interceptor.
Trap design (1002.2) and prohibited traps (1002.3)
Fixture traps must be self-scouring. Traps may not have interior partitions unless they are integral with the fixture or made of approved corrosion-resistant material. Slip joints may be used only at the trap inlet, trap outlet and within the trap seal, with approved elastomeric gaskets.
| Prohibited trap | Problem |
|---|---|
| Trap that depends on moving parts to keep its seal | Parts foul and stick open |
| Bell trap | Seal is easily lost and hard to clean |
| Crown-vented trap (vent at the crown) | Vent opening fouls with waste |
| Non-integral trap with interior partitions (unless approved corrosion-resistant material) | Hidden partition can fail without anyone seeing |
| "S" trap | Outlet drops vertically and self-siphons |
| Drum trap | Not self-scouring. Exception: drum traps used as solids interceptors or serving chemical waste systems |
1002.6: Building (house) traps are prohibited. A building trap in the building drain would block air circulation between the public sewer and the building vents. (Section 1103.1 does require traps on storm leaders that connect to a combined sewer. That rule is in section 8.2.)
Trap seal depth (1002.4)
Each fixture trap must have a liquid seal of at least 2 inches and no more than 4 inches, deeper only for special designs relating to accessible fixtures.
- Vent systems must keep the pressure at any trap within 1 inch of water column (901.2), so a 2-inch seal has a safety margin.
- A seal deeper than 4 inches is harder to scour and collects solids.
1002.7: Traps must be set level with respect to their seal and protected from freezing where necessary.
1002.5: Traps must be large enough to drain the fixture quickly, no smaller than Table 709.1, and never larger than the drain they discharge into.
How trap seals are lost
| Cause | What happens | Prevention |
|---|---|---|
| Self-siphonage | The fixture's own discharge fills the fixture drain and siphons the trap | Keep within Table 909.1 lengths; no S traps; 24-inch tailpiece limit |
| Induced siphonage | Discharge elsewhere pulls a vacuum on the fixture drain | Proper venting |
| Back pressure | Air compressed ahead of water at the base of stacks or offsets pushes through the seal | Vent stacks, relief and yoke vents |
| Evaporation | Seal dries out in rarely used drains | Trap seal protection per 1002.4.1 |
| Capillary action | Lint, hair or string over the weir wicks the water out | Strainers and cleaning |
| Wind effect | Gusts across vent terminals make the seal oscillate | Proper terminal location |
Trap seal protection (1002.4.1)
Traps of emergency floor drains, and other trap seals subject to evaporation, must be protected by one of these methods:
| Method | Standard or rule |
|---|---|
| Potable water-supplied trap seal primer valve | ASSE 1018. Discharge enters the trap above the seal on the inlet side |
| Reclaimed or graywater-supplied primer valve | ASSE 1018, with water quality per the primer maker |
| Wastewater-supplied trap primer device | ASSE 1044, discharging above the seal on the inlet side |
| Barrier-type floor drain protection device | ASSE 1072, installed per the manufacturer |
| Fixture drain from a lavatory or hand sink in the same room, or a drinking fountain in the same or an adjacent room | Run below the floor (not on or above it). Connect to the floor drain, trench drain or floor sink below its flood rim and above its trap inlet |
Example. A generator room floor drain has no water piping nearby. An ASSE 1072 barrier insert in the drain body satisfies 1002.4.1.4 with no new water line.
Fixture trap sizes (Table 709.1)
| Fixture | Minimum trap |
|---|---|
| Lavatory, bidet, drinking fountain | 1-1/4 in |
| Bathtub, sink, laundry tray, domestic kitchen sink, service sink, domestic dishwasher | 1-1/2 in |
| Shower, 5.7 gpm or less total flow | 1-1/2 in |
| Shower, more than 5.7 to 12.3 gpm | 2 in |
| Clothes washer standpipe, floor drain, emergency floor drain | 2 in |
| Urinal, water closet | Integral or match the fixture outlet |
| Unlisted fixture | Size of its outlet, at least 1-1/4 in (709.2) |
Other trap rules:
- A recess for an underground trap, such as under a slab-on-grade bathtub, must have corrosion-resistant, insect-proof and vermin-proof sides and bottom (1002.8).
- An underground acid-resisting brittleware trap must be encased in concrete extending 6 inches around it (1002.9).
- In mental health centers, traps may not be exposed (1002.10).
Field example: combination sink
A residential double-bowl kitchen sink has outlets 15 inches apart, bowls of equal depth, and an end-outlet continuous waste to one 1-1/2-inch P-trap 16 inches below the outlets. It is a combination fixture (one bowl no more than 6 inches deeper, outlets no more than 30 inches apart), the vertical distance is under 24 inches, and there is one trap. It complies. A restaurant prep sink is different. Section 802.1.1 requires each compartment to discharge independently through an air gap to a receptor.
Exam traps
- 24 inches vertical, 30 inches horizontal from the fixture outlet to the trap.
- 2 to 4 inches of seal. Deeper only for accessible-fixture designs.
- Drum traps are allowed only as solids interceptors or in chemical waste systems.
- Building traps are prohibited, but storm leaders to a combined sewer must be trapped.
Under IPC 1002.4, what liquid seal must a standard fixture trap hold?
At least 1 inch and no more than 4 inches
At least 2-1/2 inches and no more than 5 inches
At least 1-1/2 inches and no more than 3 inches
At least 2 inches and no more than 4 inches
What is the maximum vertical distance from a fixture outlet to its trap weir under IPC 1002.1?
24 inches
36 inches
30 inches
18 inches
Which trap is still permitted under the exception in IPC 1002.3?
A crown-vented P-trap serving a lavatory
A bell trap serving a commercial shower
An S trap serving a residential laundry tray
A drum trap serving a chemical waste system
Sections you finish are checked off in the contents.