12.4 Putting a Vent System Together: A Worked Building

The design sequence

Work these seven steps in order and the venting method for any building falls out.

1. Count the branch intervals on each drainage stack. Five or more triggers a vent stack (904.2).
2. Confirm the outdoor vent. At least one dry vent to the outdoors per building drain (904.1), and at least one stack vent or vent stack to open air in the system (918.7).
3. Pick a venting method for every trap. Individual, common, wet, waste stack, circuit, combination waste and vent, island, single-stack, or air admittance valve. Every trap must fall under a named method (901.2.1).
4. Check every trap arm against Table 909.1 and the fall rule in 909.2, and confirm no vent is within two pipe diameters of a weir (909.3).
5. Size each vent — 906.1 for stack vents and vent stacks, 906.2 for everything else, plus the 40-foot increase and the two floors.
6. Verify connection elevations — above the centerline of horizontal drains (905.3), 6 inches above the flood level rim for vertical rises (905.4) and for vent-to-vent connections and horizontal branch, relief and loop vents (905.5), and gravity grade back to the drain (905.2).
7. Place the terminal — 903.1 method, 903.5 location, 903.2 frost closure (model code), 903.3 flashing.

Worked building A: a four-story apartment building

Given: four stories, one branch interval per floor. Each floor has two apartments; each apartment has a bathroom group (water closet 3 dfu, lavatory 1 dfu, tub-shower 2 dfu = 5 dfu as a group) and a kitchen sink (2 dfu). One 4-inch soil stack serves all eight apartments. Floor-to-floor is 10 feet; the stack runs 46 feet from the building drain to the roof terminal.

Step 1 — branch intervals. Four floors at 10 feet each = four branch intervals. That is fewer than five, so no vent stack is required under 904.2. The stack vent alone may serve.

Change the building to five stories and the answer flips: a vent stack becomes mandatory, connecting at the base of the drainage stack at or below the lowest horizontal branch (904.4).

Step 2 — outdoor vent. The stack vent — the extension of the soil stack above the highest horizontal branch — carries the system to open air. That satisfies both 904.1 and 918.7.

Step 3 — drainage load and stack size. Total = 8 x (5 + 2) = 56 dfu. Per branch interval: 2 x 7 = 14 dfu. From Table 710.1(2):

• Stack greater than three branch intervals: 3-inch = 72 dfu. 56 dfu fits a 3-inch stack.
• Single branch interval: 3-inch allows 20 dfu; 14 dfu fits.
• But the stack serves water closets, and Table 710.1(1) footnote a requires the building drain serving a water closet to be at least 3 inches. A 3-inch stack is workable; a 4-inch stack gives margin and is the common choice.

Step 4 — venting methods per bathroom. Each bathroom group is a candidate for horizontal wet venting under 912.1 (any combination of fixtures within two bathroom groups on the same floor level). The kitchen sink is not a bathroom group fixture, so it must discharge downstream of the horizontal wet vent (912.1) and be separately vented — an individual vent, or common vented with another fixture on the same floor.

Step 5 — trap arms. Lavatory 1-1/4-inch trap: 5 feet maximum (Table 909.1). Tub 1-1/2-inch trap: 6 feet. Kitchen sink 1-1/2-inch trap: 6 feet. Water closet: unlimited (909.1 exception).

Step 6 — vent sizing.

• Kitchen sink individual vent: the sink drain is 1-1/2 inches; half is 3/4 inch, below the floor, so 1-1/4 inches.
• The branch vent collecting the kitchen sink vents on all four floors runs roughly 42 feet to the stack vent. Over 40 feet — increase one nominal size for the entire length: 1-1/2 inches.
• Stack vent: sized from Table 906.1 at 4-inch stack, 56 dfu, 46 feet developed length; never less than half of 4 inches = 2 inches.

Step 7 — connections and terminal.

• Every dry vent rises to 6 inches above the flood level rim of the highest fixture it vents before offsetting (905.4).
• Branch vents run horizontally 6 inches above the flood level rim of the highest fixture served (905.5).
• All vent piping grades back to the drain (905.2).
• The terminal goes through the roof at the jurisdiction's height (903.1.1), not within 10 feet horizontally of any openable window unless 3 feet above it (903.5), flashed watertight (903.3), and — under the model code — not less than 3 inches in diameter if the 97.5 percent design temperature is 0 degrees F or below (903.2).

Worked building B: a single-family house

Given: two full bathrooms upstairs, a powder room and a kitchen downstairs, a laundry, and a basement floor drain. One 3-inch stack.

Two hard checks on this house:

1. The island vent may not be the required outdoor vent. IPC 904.1.1 states the required vent shall not be an island fixture vent. The 3-inch stack vent through the roof is the required outdoor vent.
2. The island vent needs cleanouts. IPC 916.3 requires cleanouts in the island fixture vent to permit rodding of all vent piping located below the flood level rim, with rodding in both directions permitted through a cleanout.

Common failures an inspector writes up