6.2 Fixture Vents, Individual Vents, Common Vents & Wet Venting

Why this section matters

Fixture-level venting is where most field mistakes and many exam questions happen. Common mistakes include a vent too far from the trap, a vent below the weir, and a kitchen sink on a bathroom wet vent. The 2021 IPC rules are in 909 (fixture vents), 910 (individual), 911 (common) and 912 (wet venting). Some older rules repeated in study materials are no longer in the 2021 text, such as "water closet must be downstream of all fixtures" and "larger fixture must enter below the smaller."

Distance from trap to vent (909)

909.1: Each fixture trap must have a protecting vent placed so that the slope and developed length of the fixture drain, from the trap weir to the vent fitting, stay within Table 909.1.

Exception: self-siphoning fixtures such as water closets have no length limit.

909.2: The total fall of the fixture drain from slope may not exceed the diameter of the fixture drain. The vent connection may not be below the trap weir, except for water closets. Check: 6 feet × 1/4 inch per foot = 1.5 inches, exactly the diameter of a 1-1/2-inch drain. That is why the table lengths are what they are. A vertical drop in the fixture drain before the vent would put the vent below the weir and turn the trap into an S trap.

909.3: A vent may not be installed within two pipe diameters of the trap weir. That would be a crown vent, which fouls.

Individual vents (910)

Any trap may have its own individual vent, connected to the fixture drain of the trap it vents. It is sized at least half the drain and no less than 1-1/4 inches (906.2), and it must meet the 905 rise and connection rules.

Common vents (911)

• 911.1: An individual vent may serve two traps or trapped fixtures as a common vent, provided they are on the same floor level.
• 911.2, same level: Where the two fixture drains connect at the same level, the vent connects at or downstream of their interconnection. A typical example is back-to-back lavatories on a double fitting.
• 911.3, different levels: The vent connects as a vertical extension of the vertical drain. The vertical drain between the two fixture connections is the vent for the lower fixture. It is sized by Table 911.3 from the dfu of the upper fixture. The upper fixture may not be a water closet.

Example. A lavatory (1 dfu) drains in above a bathtub (2 dfu) on the same wall. A 1-1/2-inch vertical section between them carries the lavatory's 1 dfu and vents the tub. The arrangement complies as long as the upper fixture is not a water closet.

Wet venting (912)

A wet vent is a vent that also carries drainage. The 2021 IPC lets bathroom-group fixtures share one.

Bathroom group (Section 202): a water closet, a lavatory, a bathtub or shower, with or without a bidet, an emergency floor drain or both, all on the same floor level.

Horizontal wet vent (912.1)

• Any combination of fixtures within two bathroom groups on the same floor level may be vented by a horizontal wet vent.
• The wet vent runs from the dry vent connection, in the direction of flow, to the most downstream fixture drain connection.
• Each wet-vented fixture drain connects independently to the horizontal wet vent.
• Only bathroom-group fixtures may connect to the wet-vented branch. Others, such as kitchen sinks and clothes washers, must connect downstream of the wet vent.

Vertical wet vent (912.1.1)

• Any combination of fixtures within two bathroom groups on the same floor level may use a vertical wet vent. It runs from the dry vent connection down to the lowest fixture drain connection.
• Water closet drains connect at the same elevation. Other fixture drains connect at or above that elevation.
• The dry vent is an individual or common vent serving one or two fixtures.

The dry vent connection (912.2)

• 912.2.1, horizontal wet vents: The dry vent is an individual or common vent for any bathroom-group fixture except an emergency floor drain. If the dry vent is on a water closet fixture drain, that drain must connect horizontally to the wet vent. No more than one wet-vented fixture drain may discharge upstream of the dry-vented fixture drain connection.
• 912.2.2, vertical wet vents: The dry vent is an individual or common vent for the most upstream fixture drain.

Sizing (912.3)

The dry vent is sized from the largest pipe diameter within the wet-vent system it serves (at least half that size). The wet vent is sized by the dfu discharged into it:

Worked example: master bath

A two-lavatory vanity (1 + 1 dfu) has the dry vent. Downstream on a horizontal branch, a bathtub (2) and a shower (2) connect independently, then the water closet (3).

1. All fixtures are in one bathroom group plus an extra lavatory and shower, on one floor, so wet venting is allowed.
2. The dry vent is on the lavatory fixture drain, and only one fixture discharges upstream of it, so 912.2.1 is met.
3. The wet vent load is the dfu flowing through it. Through the tub and shower connections that is 2 + 2 + 2 = 6 dfu, so a 2-1/2-inch wet vent is the minimum. Many plumbers run 3-inch pipe because 2-1/2-inch fittings are hard to find. The water closet drain is 3 inches regardless.
4. The dry vent is at least half the largest pipe in the system: half of 3 inches is 1-1/2 inches.

Exam traps

• Distance to vent: 6 feet for 1-1/2 inches, 8 feet for 2 inches, 12 feet for 3 inches. No limit for water closets.
• Common vent at different levels: the upper fixture can't be a water closet. Size the vertical section with Table 911.3.
• Wet vents: two bathroom groups on the same floor, bathroom-group fixtures only, and each fixture drain connects independently.
• No "water closet last" rule in the 2021 IPC. A water closet may even carry the dry vent if its drain connects horizontally.