13.1 Horizontal and Vertical Wet Venting

What wet venting is

A wet vent is a vent that also carries waste from other fixtures. The drain from one fixture serves as the vent for another. Wet venting saves an enormous amount of pipe in bathroom groups, and the IPC permits it within tightly drawn limits because bathroom fixtures discharge in a predictable, non-simultaneous pattern.

IPC 912.1:

Any combination of fixtures within two bathroom groups located on the same floor level is permitted to be vented by a horizontal wet vent. The wet vent shall be considered to be the vent for the fixtures and shall extend from the connection of the dry vent along the direction of the flow in the drain pipe to the most downstream fixture drain connection to the horizontal branch drain. Each wet-vented fixture drain shall connect independently to the horizontal wet vent. Only the fixtures within the bathroom groups shall connect to the wet-vented horizontal branch drain. Any additional fixtures shall discharge downstream of the horizontal wet vent.

Five separate requirements:

1. Two bathroom groups maximum, and the same floor level.
2. The extent of the wet vent is defined: from the dry vent connection to the most downstream fixture drain connection.
3. Each wet-vented fixture drain connects independently — no ganging two fixtures onto one connection.
4. Only bathroom group fixtures may connect to the wet-vented branch.
5. Additional fixtures discharge downstream of the wet vent.

Colorado field note (not exam-scored): Board Rule 1.2(E)(1)(tt) adds an exception permitting fixtures other than bathroom group fixtures, of equivalent drainage fixture units, to be included in the wet-vented section, provided the total dfu does not exceed the total included in two bathroom groups and the non-bathroom fixtures are valued at one drainage fixture unit or less. The model IPC has no such allowance, so a laundry tray or floor drain on a wet vent is a Colorado-only arrangement.

Vertical wet venting — IPC 912.1.1

Any combination of fixtures within two bathroom groups located on the same floor level is permitted to be vented by a vertical wet vent. The vertical wet vent shall be considered to be the vent for the fixtures and shall extend from the connection of the dry vent down to the lowest fixture drain connection. Each wet-vented fixture shall connect independently to the vertical wet vent. Water closet drains shall connect at the same elevation. Other fixture drains shall connect above or at the same elevation as the water closet fixture drains. The dry-vent connection to the vertical wet vent shall be an individual or common vent serving one or two fixtures.

The elevation rules are the distinguishing feature:

• Water closets connect at the same elevation as each other.
• Everything else connects above or at the same elevation as the water closets.

Nothing may connect below a water closet in a vertical wet vent, because the closet's 3-dfu surge would seal the vent path for anything under it.

The dry vent connection — IPC 912.2

Every wet vent system needs a dry vent somewhere, and the code says exactly where.

912.2.1 Horizontal wet vent

The dry-vent connection for a horizontal wet-vent system shall be an individual vent or a common vent for any bathroom group fixture, except an emergency floor drain. Where the dry vent connects to a water closet fixture drain, the drain shall connect horizontally to the horizontal wet-vent system. Not more than one wet-vented fixture drain shall discharge upstream of the dry-vented fixture drain connection.

Three requirements:

1. The dry vent is an individual or common vent for a bathroom group fixture — never an emergency floor drain.
2. A dry vent on a water closet requires the closet drain to connect horizontally to the wet vent.
3. At most one wet-vented fixture drain may be upstream of the dry-vented fixture.

912.2.2 Vertical wet vent

The dry-vent connection for a vertical wet-vent system shall be an individual vent or common vent for the most upstream fixture drain.

In the vertical case the dry vent is not free to attach anywhere — it must serve the most upstream fixture drain.

Sizing — IPC 912.3

The dry vent serving the wet vent shall be sized based on the largest required diameter of pipe within the wet-vent system served by the dry vent. The wet vent shall be of a size not less than that specified in Table 912.3, based on the fixture unit discharge to the wet vent.

Table 912.3 Wet vent size:

Note that the dry vent and the wet vent are sized by completely different rules. The dry vent takes the largest required pipe diameter within the wet-vent system; the wet vent takes its size from Table 912.3 on the dfu discharging to it.

Also note Table 912.3 is close to but not identical with Table 911.3 (common vent sizes: 1 dfu at 1-1/2 in, 4 dfu at 2 in, 6 dfu at 2-1/2 to 3 in). Table 912.3 splits 2-1/2 inch (6 dfu) from 3 inch (12 dfu).

Worked example: a horizontal wet vent

A single bathroom on one floor has a water closet (3 dfu), a lavatory (1 dfu) and a tub-shower (2 dfu). The branch runs from the lavatory, past the tub, past the water closet, to the stack.

Layout, upstream to downstream: lavatory -> tub -> water closet -> stack.

1. Dry vent location. Take the dry vent as an individual vent on the lavatory, the most upstream fixture. That satisfies 912.2.1 — the dry vent is an individual vent for a bathroom group fixture, and zero wet-vented fixture drains are upstream of it.
2. Extent of the wet vent. From the dry vent connection at the lavatory downstream to the water closet fixture drain connection, the most downstream fixture drain connection on the branch (912.1).
3. Wet vent size. The load discharging to the wet vent downstream of the dry vent connection is the tub (2 dfu) plus the water closet (3 dfu), plus the lavatory (1 dfu) passing through — 6 dfu. Table 912.3: 2-1/2 inches. In practice, because the branch serves a water closet, it will be built at 3 inches, which also satisfies the table at up to 12 dfu.
4. Dry vent size. Sized on the largest required diameter of pipe within the wet-vent system — here 3 inches — so half of 3 inches = 1-1/2 inches under 906.2.
5. Independent connections. The lavatory, tub and closet each connect independently to the horizontal wet vent (912.1).
6. The kitchen sink on the same branch? Not permitted. Only bathroom group fixtures may connect to the wet-vented branch; the sink must discharge downstream of the wet vent and be separately vented (912.1).

Moving the dry vent

Suppose instead the dry vent is taken on the tub, with the lavatory upstream of it.

• One wet-vented fixture drain (the lavatory) is upstream of the dry-vented fixture drain connection. 912.2.1 permits not more than one. Compliant.

Add a second lavatory upstream of the tub and the arrangement fails — two wet-vented fixture drains would be upstream of the dry vent connection.

Worked example: a vertical wet vent

Two back-to-back bathrooms share a vertical wet vent: two water closets (3 dfu each), two lavatories (1 dfu each) and two showers (2 dfu each).

1. Two bathroom groups, same floor level. Within the 912.1.1 allowance.
2. Elevations. Both water closet drains connect at the same elevation (912.1.1). The lavatories and showers connect above or at the same elevation as the closets.
3. Dry vent. An individual or common vent for the most upstream fixture drain (912.2.2) — here, common venting the two lavatories at the top.
4. Extent. From the dry vent connection down to the lowest fixture drain connection (912.1.1).
5. Wet vent size. Load to the wet vent is 2 x 3 + 2 x 1 + 2 x 2 = 12 dfu. Table 912.3: 3 inches.
6. Independent connections. Each wet-vented fixture connects independently (912.1.1).

What disqualifies a wet vent