7.3 Common Vents, Wet Venting & Waste Stack Vents

Common vents (IPC 911)

A common vent is a vent connected at the junction of two fixture drains, or to a fixture branch, that serves both fixtures (IPC Chapter 2).

• 911.1: an individual vent may serve two traps or trapped fixtures as a common vent. The fixtures must be on the same floor level.
• 911.2 same level: where the two fixture drains connect at the same level, the vent connects at the interconnection of the fixture drains, or downstream of it. A typical example is two back-to-back lavatories.
• 911.3 different levels: the vent connects as a vertical extension of the vertical drain. The vertical drain between the two fixture drain connections acts as the vent for the lower fixture drain, and it is sized by Table 911.3. The upper fixture shall not be a water closet.

Table 911.3: common vent sizes

Example: a lavatory (1 dfu) connects above a bathtub on the same vertical drain. The section between them may be 1-1/2 inches, because the upper fixture discharges only 1 dfu. If a kitchen sink (2 dfu) were the upper fixture, the section would need to be 2 inches.

Wet venting (IPC 912)

A wet vent carries waste from some fixtures while also venting others. The code allows it only for bathroom fixtures, where the flows are small and intermittent.

A bathroom group is a water closet, lavatory, and bathtub or shower, with or without a bidet, an emergency floor drain or both, located together 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 connected to the horizontal branch.
• Each wet-vented fixture drain must connect independently to the horizontal wet vent.
• Only fixtures within the bathroom groups may connect to the wet-vented branch. Other fixtures, such as a kitchen sink or clothes washer, must discharge 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 be vented by a vertical wet vent. A vertical wet vent does not span floors.
• The wet vent runs from the dry vent connection down to the lowest fixture drain connection.
• Each fixture connects independently. Water closet drains connect at the same elevation, and other fixture drains connect at or above that elevation.
• The dry vent is an individual or common vent serving one or two fixtures.

Where the dry vent connects (912.2)

• 912.2.1 horizontal: the dry vent is an individual or common vent for any bathroom group fixture except an emergency floor drain. If the dry vent connects to a water closet's 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: the dry vent is an individual or common vent for the most upstream fixture drain.

The 2021 IPC does not require a horizontal wet vent's dry vent to be on the most upstream fixture, and it does not require the water closet to be the last fixture. The lavatory is often the dry-vented fixture because it is convenient, but the code allows other choices within the limits above.

Sizing (912.3)

• The wet vent is sized from Table 912.3, based on the dfu discharging to it.
• The dry vent is sized based on the largest required pipe diameter in the wet-vent system it serves, using the one-half rule of 906.2.

Worked example

Two back-to-back bathroom groups on one floor, each with a lavatory (1 dfu), a bathtub (2 dfu) and a private 1.6 gpf water closet (3 dfu). The lavatories are dry vented.

• The segment carrying one lavatory (1 dfu) may be 1-1/2 inches.
• A segment carrying both lavatories and one tub (4 dfu) needs 2 inches.
• The segments receiving the water closets are 3 inches, because a water closet needs a 3-inch drain and a 3-inch wet vent carries up to 12 dfu. The two groups total 10 dfu using the Table 709.1 bathroom-group value (12 dfu if each fixture is counted separately), so 3 inches is enough either way.
• The dry vent is sized from the largest pipe in the system (3 inches), so it is one-half of that: 1-1/2 inches.

Waste stack vent (IPC 913)

A waste stack may serve as the vent for all fixtures discharging to it (913.1), but only under strict conditions:

• 913.2: the waste stack must be vertical. Horizontal and vertical offsets are prohibited between the lowest and highest fixture drain connections. Each fixture drain connects separately to the stack. The stack shall not receive the discharge of water closets or urinals.
• 913.3: a stack vent is required, at least the size of the waste stack. Offsets in the stack vent are allowed only at least 6 inches above the flood level rim of the highest fixture, and the stack vent must drain back by gravity (905.2). It may join other stack vents or vent stacks under 904.5.
• 913.4: the waste stack is sized from Table 913.4 and must be the same size throughout its length.

Table 913.4: waste stack vent size

The code does not limit the number of stories for a waste stack vent. A drainage stack with five or more branch intervals normally needs a vent stack (904.2), but waste stacks installed under 913 are exempt.

Example: a three-story waste stack serves kitchen sinks (2 dfu each) and laundry trays (2 dfu each), with 8 dfu per floor and 24 dfu total. A 3-inch waste stack works, with no per-interval limit and a 24-dfu total limit. The stack vent must also be 3 inches.