14.1 Fixture Traps, Trap Seals, and Prohibited Trap Types
Key Takeaways
- IPC 1002.1 limits the vertical distance from the fixture outlet to the trap weir to not more than 24 inches and the horizontal distance to not more than 30 inches measured centerline to centerline.
- IPC 1002.4 requires each fixture trap to have a liquid seal of not less than 2 inches and not more than 4 inches, or deeper for special designs relating to accessible fixtures.
- IPC 1002.3 prohibits traps depending on moving parts, bell traps, crown-vented traps, traps depending on interior partitions, S traps and drum traps.
- IPC 1002.6 prohibits building (house) traps.
- IPC 1002.5 requires a trap to be not less than the size indicated in Table 709.1 and prohibits a trap larger than the drainage pipe into which it discharges.
Why traps exist
A trap holds a plug of water that blocks sewer gas from entering the building. Everything in IPC Chapter 9 exists to protect that plug; everything in Chapter 10 defines it.
IPC 1002.1:
Each plumbing fixture shall be separately trapped by a liquid-seal trap, except as otherwise permitted by this code. The vertical distance from the fixture outlet to the trap weir shall not exceed 24 inches, and the horizontal distance shall not exceed 30 inches measured from the centerline of the fixture outlet to the centerline of the inlet of the trap. The height of a clothes washer standpipe above a trap shall conform to Section 802.4.3. A fixture shall not be double trapped.
Both dimensions matter and they measure different things:
| Dimension | From | To | Maximum |
|---|---|---|---|
| Vertical | fixture outlet | trap weir | 24 inches |
| Horizontal | centerline of the fixture outlet | centerline of the trap inlet | 30 inches |
The vertical limit exists because water falling more than 24 inches arrives with enough velocity to blow the seal out. The horizontal limit keeps the untrapped tailpiece short enough not to become a fouling reservoir.
The three exceptions
- Fixtures with integral traps — a water closet's trap is part of the china.
- A combination plumbing fixture may be installed on one trap, provided one compartment is not more than 6 inches deeper than the other and the waste outlets are not more than 30 inches apart. This is what allows a double-bowl kitchen sink on a single trap.
- Floor drains in multilevel parking structures discharging to a building storm sewer need not be individually trapped. Where they must discharge to a combined building sewer, they need not be individually trapped provided they connect to a main trap per IPC 1103.1.
Double trapping is prohibited. Two traps in series create an air lock between them that neither can clear, and the fixture drains slowly or not at all. A common field error is adding a trap to a fixture that already has an integral one.
Trap seal depth — IPC 1002.4
Each fixture trap shall have a liquid seal of not less than 2 inches and not more than 4 inches, or deeper for special designs relating to accessible fixtures.
Both bounds are real:
- Less than 2 inches is too shallow to survive ordinary pressure fluctuation and evaporates too quickly.
- More than 4 inches is harder to scour, so solids settle in the bend and the trap fouls.
Recall the companion rule from IPC 901.2: the vent system must keep the pressure differential on that seal to not more than 1 inch of water column.
Design of traps — IPC 1002.2
Fixture traps shall be self-scouring. Fixture traps shall not have interior partitions, except where such traps are integral with the fixture or where they are constructed of an approved material that is resistant to corrosion and degradation. Slip joints shall be made with an approved elastomeric gasket and shall be installed only on the trap inlet, trap outlet and within the trap seal.
"Self-scouring" means the trap's own discharge velocity keeps it clean. An interior partition creates a dead zone where solids accumulate and where the partition itself can corrode through, destroying the seal without any visible leak.
The slip-joint rule appears three times in the code — here, in IPC 405.9 for fixtures, and in IPC 705.17 for drainage. Three sections, one requirement: trap inlet, trap outlet, within the trap seal. Nowhere else.
The six prohibited traps — IPC 1002.3
| Prohibited trap | Why |
|---|---|
| 1. Traps that depend on moving parts to maintain the seal | A mechanical seal fails without warning and leaves no water plug |
| 2. Bell traps | A loose bell can be lifted or knocked off, leaving an open drain |
| 3. Crown-vented traps | The vent at the weir floods and seals; see IPC 909.3 |
| 4. Traps not integral with a fixture that depend on interior partitions for the seal | Corrodes through invisibly; excepted where of an approved corrosion- and degradation-resistant material |
| 5. "S" traps | Self-siphons; the vertical leg below the weir pulls the seal out |
| 6. Drum traps | Not self-scouring; solids settle and the trap fouls |
Exception: drum traps used as solids interceptors and drum traps serving chemical waste systems shall not be prohibited.
PSI publishes a sample question on precisely this exception: "A drum trap can be used for?" — with the answer keyed to a solids interceptor, citing IPC 1002.3.
Why an S trap self-siphons
In a properly vented P-trap, the trap arm slopes gently to the vent and air enters at the vent to break any siphon. In an S trap, the outlet leg drops vertically immediately after the weir. Water falling down that leg behaves like the long arm of a siphon and pulls the seal out behind it. The fixture drains, then the trap gurgles and empties, and sewer gas enters the room. The same failure is what Table 909.1 prevents in a P-trap by capping the trap arm length.
Trap size — IPC 1002.5
Fixture trap size shall be sufficient to drain the fixture rapidly and not less than the size indicated in Table 709.1. A trap shall not be larger than the drainage pipe into which the trap discharges.
Two bounds again:
- Floor: Table 709.1 (1-1/4 inch for a lavatory, 1-1/2 inch for a bathtub, sink or shower, 2 inch for a floor drain or clothes washer).
- Ceiling: never larger than the drain it discharges into. An oversized trap slows velocity below the self-scouring threshold.
For fixtures not listed in Table 709.1, IPC 709.2 sets the minimum trap size at the size of the drainage outlet but not less than 1-1/4 inches.
Building traps prohibited — IPC 1002.6
Building (house) traps shall be prohibited.
A building trap is a single large trap in the building drain intended to block sewer gas from the entire building. Modern codes prohibit them because they block the building's drainage system from the sewer's air, defeating the circulation the vent system depends on, and because they are a permanent obstruction that collects solids.
Setting, protection and special conditions
| Section | Requirement |
|---|---|
| 1002.7 | Traps shall be set level with respect to the trap seal and, where necessary, protected from freezing |
| 1002.8 | A recess for an underground trap connection, such as one serving a bathtub in slab construction, shall have sides and a bottom of corrosion-resistant, insect- and verminproof construction |
| 1002.9 | A vitrified clay or other brittleware acid-resisting trap installed underground shall be embedded in concrete extending 6 inches beyond the bottom and sides of the trap |
| 1002.10 | In mental health centers, pipes and traps shall not be exposed |
A trap set out of level loses effective seal depth: tilt a 2-inch seal far enough and the shallow side governs.
Applying the rules to a rough-in
A double-compartment kitchen sink is proposed on a single 1-1/2-inch trap, with a 30-inch-deep base cabinet.
| Check | Requirement | Result |
|---|---|---|
| One trap for two compartments | 1002.1 Exception 2: compartments not more than 6 inches different in depth, outlets not more than 30 inches apart | Permitted if both conditions hold |
| Vertical drop to the trap weir | Not more than 24 inches | Measure from the sink outlet |
| Horizontal distance to the trap inlet | Not more than 30 inches centerline to centerline | Measure to the trap |
| Trap size | Not less than 1-1/2 inches (Table 709.1), not larger than the drain it enters | 1-1/2 inch into a 2-inch branch is correct |
| Trap seal | 2 to 4 inches | Standard P-trap |
| Slip joints | Only at the trap inlet, trap outlet and within the trap seal | No slip joint in the concealed wall arm |
| Disposer on one compartment | Disposer outlet not less than 1-1/2 inches (416.2) | |
| Trap arm length | Table 909.1: 6 feet for a 1-1/2 inch trap | |
| Vent takeoff | Not within 3 inches (two pipe diameters) of the weir, not below the weir | 909.3, 909.2 |
What is the maximum vertical distance from a fixture outlet to the trap weir under IPC 1002.1?
Which trap type is prohibited by IPC 1002.3 EXCEPT when used as a solids interceptor or serving a chemical waste system?
What liquid seal depth does IPC 1002.4 require for a fixture trap?
A double-compartment sink is to be served by a single trap. What two conditions does IPC 1002.1 Exception 2 impose?