11.3 Vent Connections, Grade, and the Six-Inch Rules
Key Takeaways
- IPC 905.3 requires every dry vent connecting to a horizontal drain to connect above the centerline of the horizontal drain pipe.
- IPC 905.4 requires every dry vent to rise vertically to a point not less than 6 inches above the flood level rim of the highest trap or trapped fixture being vented, excepting vents for interceptors located outdoors.
- IPC 905.5 requires a connection between a vent pipe and a vent stack or stack vent to be made not less than 6 inches above the flood level rim of the highest fixture served.
- IPC 905.2 requires vent and branch vent pipes to be graded and connected so as to drain back to the drainage pipe by gravity.
- IPC 905.6 requires a vent rough-in for future fixtures, sized not less than one-half the diameter of the rough-in drain served, identified as a vent.
Connect to what
IPC 905.1: individual, branch and circuit vents shall connect to a vent stack, a stack vent, an air admittance valve, or extend to the open air. Those are the only four legal destinations for a vent.
A vent that dead-ends in an attic, terminates in a soffit cavity, or ties into a mechanical chase is none of the four.
Grade — IPC 905.2
Vent and branch vent pipes shall be so graded and connected as to drain back to the drainage pipe by gravity.
Condensate forms constantly inside a vent as warm, moist sewer gas meets cold pipe. Water that cannot drain back accumulates at a low point and eventually seals the vent, at which point the vent is a dead leg and the trap it protects loses its seal. A vent with a belly in it is a vent that will fail in the first cold season.
The above-centerline rule — IPC 905.3
Every dry vent connecting to a horizontal drain shall connect above the centerline of the horizontal drain pipe.
A horizontal drain runs partially full. Water occupies the bottom half of the bore. A vent tapped into the side at or below the centerline will take waste, plug, and stop venting. Connecting above the centerline keeps the vent in the air space.
In practice this means a sanitary tee with the branch rolled to 45 degrees or straight up, never a bottom or low-side tap. IPC 706.4 reinforces the point from the fitting side: a low-heel inlet shall not be used as a wet-vented connection.
The two six-inch rules
These are different rules measuring different things, and exam items exploit the confusion.
905.4 Vertical rise of vent
Every dry vent shall rise vertically to a point not less than 6 inches above the flood level rim of the highest trap or trapped fixture being vented.
Exception: vents for interceptors located outdoors.
This governs a single vent leaving a fixture. The vent must go straight up to 6 inches above the fixture's flood level rim before it may turn horizontally. Turning a lavatory vent horizontally inside the wall at 24 inches above the floor — below the rim of the lavatory it serves — is a violation, because waste backing up in the fixture would flow into the vent.
905.5 Height above fixtures
A connection between a vent pipe and a vent stack or stack vent shall be made at not less than 6 inches above the flood level rim of the highest fixture served by the vent. Horizontal vent pipes forming branch vents, relief vents or loop vents shall be located not less than 6 inches above the flood level rim of the highest fixture served.
This governs the vent network: where individual vents come together, and where horizontal vent piping runs. The reference is the highest fixture served by that vent, which in a multi-fixture branch is often not the fixture nearest the connection.
| Rule | What it governs | Reference point |
|---|---|---|
| 905.4 | A single dry vent rising from its fixture | Flood level rim of the highest trap or trapped fixture being vented |
| 905.5 | Vent-to-vent-stack connections and horizontal branch, relief and loop vents | Flood level rim of the highest fixture served by the vent |
Two related six-inch dimensions elsewhere in the chapter:
- IPC 913.3: offsets in a stack vent serving a waste stack shall be located not less than 6 inches above the flood level of the highest fixture.
- IPC 915.2.3: the vent connection to a combination waste and vent pipe shall extend vertically to not less than 6 inches above the flood level rim of the highest fixture being vented before offsetting horizontally.
- IPC 918.4: a stack-type air admittance valve is located not less than 6 inches above the flood level rim of the highest fixture being vented; an individual or branch-type AAV is not less than 4 inches above the horizontal branch drain or fixture drain being vented.
Crown venting is prohibited — IPC 909.3
A vent shall not be installed within two pipe diameters of the trap weir.
A "crown vent" taps the vent into the top of the trap arm immediately at the weir. It fails because the turbulent flow leaving the weir throws water straight into the vent opening, which then fouls and seals. Moving the connection at least two pipe diameters downstream puts it past the turbulent zone.
For a 1-1/2-inch trap, two pipe diameters is 3 inches. For a 2-inch trap, 4 inches.
Venting of fixture drains — IPC 909.2
The total fall in a fixture drain due to pipe slope shall not exceed the diameter of the fixture drain, nor shall the vent connection to a fixture drain, except for water closets, be below the weir of the trap.
Two constraints:
- Total fall ≤ one pipe diameter. For a 1-1/2-inch trap arm at 1/4 inch per foot, the maximum fall of 1-1/2 inches is reached at 6 feet — which is exactly the Table 909.1 distance for a 1-1/2-inch trap. The two rules are the same rule expressed twice.
- The vent connection may not be below the trap weir, except for water closets. Below the weir, the vent opening is in the wetted portion of the drain.
Vents for future fixtures — IPC 905.6
Where the drainage piping has been roughed-in for future fixtures, a rough-in connection for a vent shall be installed. The vent size shall be not less than one-half the diameter of the rough-in drain to be served. The vent rough-in shall connect to the vent system, or be vented by other means provided in the chapter. The connection shall be identified to indicate that it is a vent.
Pair this with IPC 704.4 (future drainage piping terminates with an approved cap or plug) and IPC 710.2 (future fixtures are considered in determining drain pipe sizes). Roughing in a future basement bathroom means sizing the drain for it, capping the drain, running the vent, and labeling the vent.
A rough-in walkthrough
A second-floor bathroom has a lavatory (1-1/4-inch trap), a bathtub (1-1/2-inch trap) and a water closet, individually vented to a 2-inch branch vent that ties into a 3-inch stack vent.
| Check | Requirement | Section |
|---|---|---|
| Lavatory vent takeoff on the trap arm | Not within 2 pipe diameters (2-1/2 inches) of the trap weir | 909.3 |
| Lavatory vent takeoff elevation | Not below the weir of the trap | 909.2 |
| Total fall in the lavatory trap arm | Not more than 1-1/4 inches | 909.2 |
| Lavatory vent rise before offsetting | Straight up to 6 inches above the lavatory flood level rim | 905.4 |
| Bathtub vent connecting to the horizontal branch drain | Above the centerline of the horizontal drain | 905.3 |
| Horizontal branch vent above the fixtures | 6 inches above the flood level rim of the highest fixture served | 905.5 |
| Branch vent connection to the stack vent | 6 inches above the flood level rim of the highest fixture served | 905.5 |
| Grade of all vent piping | Drains back to the drainage pipe by gravity | 905.2 |
| Branch vent size | Not less than one-half the required diameter of the drain served, never less than 1-1/4 inches | 906.2 |
| Future laundry rough-in on the same branch | Vent rough-in installed, half the drain diameter, connected and identified as a vent | 905.6 |
A dry vent connects to a horizontal drain. Where must the connection be made under IPC 905.3?
How high must a dry vent rise vertically before it may offset horizontally?
A 2-inch trap serves a sink. How far downstream of the trap weir must the vent connection be, at a minimum?
Drainage piping is roughed in for a future fixture. What does IPC 905.6 require for the vent?