5.3 Soil & Waste Stacks, Stack Offsets, Branch Connections & Relief Vents

How stacks behave

Water entering a vertical stack clings to the pipe wall as an annular sheet around a central air core. It accelerates to a terminal velocity within a short fall, which is why a vertical stack can carry more load than a horizontal pipe of the same diameter. Trouble comes where the flow changes direction: at an offset or at the base of the stack, the fast-moving water slows suddenly (a hydraulic jump). The pipe can flow full there and compress the air ahead of it. The code addresses this with offset sizing, offset venting, the 10-diameter rule for branch connections, and relief vents.

Definitions (IPC Chapter 2)

• Stack: any vertical line of soil, waste, vent or inside conductor piping that extends through at least one story, with or without offsets.
• Soil pipe: carries sewage containing fecal matter. Waste pipe: carries only waste (no fecal matter).
• Branch interval: a vertical measurement of distance, 8 feet or more in developed length, between the connections of horizontal branches to a drainage stack. Measurements are taken down the stack from the highest horizontal branch connection.
• Stack vent: the extension of a soil or waste stack above the highest horizontal drain connected to it.
• Vent stack: a vertical vent pipe installed primarily to circulate air to and from the drainage system.
• Offset: a combination of approved bends that makes two changes in direction, bringing one section of pipe out of line but parallel to the other.

Stack sizing: Table 710.1(2)

Check both columns that apply: the load from any one branch interval, and the total on the stack.

Note b: stacks are sized on the total accumulated load at each story or branch interval. As that load decreases higher up, the stack may be reduced in size, but never to less than one-half the diameter of the largest stack size required.

The 2021 table has no "two water closets per interval, six per 3-inch stack" footnote. That rule came from older editions. In the 2021 IPC, 3-inch stacks are limited by their dfu values. (The single-stack vent system in 917.2 does limit a 3-inch single stack to two water closets.)

Worked example

An eight-story building has a waste and soil stack with 32 dfu on each floor (8 branch intervals) and 256 dfu total.

• Per interval: 32 dfu exceeds 20 for a 3-inch stack, so at least 4 inches (90 dfu per interval).
• Total: 256 dfu with more than 3 intervals; a 4-inch stack allows 500. Use a 4-inch stack.

Offsets in stacks (IPC 710.1.1, 710.1.2 and 711)

• 710.1.1 Horizontal stack offsets are sized as building drains under Table 710.1(1), except as 711.3 requires.
• 710.1.2 Vertical stack offsets are sized as straight stacks under Table 710.1(2), unless 711.1.1 requires sizing as a building drain.

Section 711 applies to buildings of five stories or more:

• 711.1: if a horizontal branch connects to the stack within 2 feet above or below a vertical stack offset, and the offset is more than four branch intervals below the top of the stack, the offset must be vented under Section 907. Under 711.1.1, these vents are not needed if the stack and offset are sized as a building drain.
• 711.2: a stack with a horizontal offset more than four branch intervals below the top must be vented under 907 and sized as follows:
  1. The part above the offset is sized as a vertical stack for the dfu above the offset.
  2. The offset is sized under 710.1.1, as a building drain.
  3. The part below the offset is sized as the offset, or for the total dfu on the whole stack, whichever is larger.
• 711.2.1: the offset vents may be omitted if the stack and offset are one pipe size larger than required for a building drain, and the whole stack and offset are no smaller in cross-sectional area than a straight stack plus an offset vent.
• 711.3: a vertical offset below the lowest horizontal branch does not require a size change. A horizontal offset below the lowest branch is sized, together with the stack below it, as a building drain.

Venting a horizontal offset (IPC 907)

Under 907.1, horizontal offsets are vented where five or more branch intervals are above the offset. Vent the upper section as a separate stack, with a vent stack connection per 904.4 and the offset treated as the stack base (907.2). Vent the lower section with a yoke vent between the offset and the next lower horizontal branch, sized at least as large as the vent stack required for the drainage stack (907.3).

Connections near offsets and stack bases (IPC 704.3)

Horizontal branches must connect to the base of a stack at least 10 times the stack diameter downstream of the stack. They must connect to a horizontal stack offset at least 10 times the stack diameter downstream of the upper stack. For a 4-inch stack, that is 40 inches. This keeps branches out of the turbulent, pressurized zone where the falling water turns horizontal.

Relief vents for tall stacks (IPC 908)

• 908.1: soil and waste stacks in buildings with more than 10 branch intervals need a relief vent at each tenth interval, counting from the top floor.
• 908.2: the relief vent is the same size as the vent stack it connects to. Its lower end connects to the soil or waste stack through a wye below the horizontal branch serving that floor. Its upper end connects to the vent stack through a wye at least 3 feet above the floor.

A relief vent that connects a stack upward to a vent stack is called a yoke vent: "a pipe connecting upward from a soil or waste stack to a vent stack for the purpose of preventing pressure changes in the stacks."

Vent stacks for drainage stacks (IPC 904.2 and 904.4)

A vent stack is required for every drainage stack with five or more branch intervals, except a waste stack vented under 913. The vent stack connects to the base of the drainage stack, at or below the lowest horizontal branch. If it connects to the building drain, the connection must be downstream of the drainage stack and within 10 stack diameters of it.