9.4 Stacks, Branch Intervals, and Offsets in Tall Buildings

Branch intervals

IPC 202 defines a branch interval as 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.

Three consequences follow:

1. A stack in a building with 9-foot floors has one branch interval per floor.
2. A stack in a building with an 18-foot two-story atrium where branches connect only at the top has one branch interval spanning that height, not two.
3. A mezzanine 6 feet above the floor below, with branches connecting at both levels, still yields one branch interval, because the 8-foot minimum is not met.

Branch intervals matter because Table 710.1(2) sizes stacks in three columns keyed to them.

Reading the stack columns

A 4-inch stack, for example, permits 90 dfu into any one branch interval, 240 dfu total if the stack spans three intervals or fewer, and 500 dfu total if it spans more.

Why does a taller stack carry more? Because falling water reaches terminal velocity within roughly one to two stories and then spreads into an annular sheet along the pipe wall, leaving a stable air core. Load entering at widely separated intervals has time to re-establish that pattern. Load dumped into a single interval does not — hence the much lower single-interval column.

Both tests must pass

A 12-story stack serves 20 dfu at every floor.

• Whole-stack test: 12 x 20 = 240 dfu, greater than three branch intervals. Table 710.1(2) gives 3-inch = 72 (fails), 4-inch = 500 (passes).
• Single-interval test: 20 dfu at one interval. For 4-inch pipe the limit is 90 (passes). For 3-inch it would be 20 — exactly at the limit — but the whole-stack test already ruled out 3 inches.

Now change the case: a 2-story stack receiving 100 dfu on the first floor only.

• Whole-stack test: 100 dfu, three intervals or less: 4-inch = 240 (passes).
• Single-interval test: 100 dfu into one interval: 4-inch allows 90 — fails. A 5-inch stack is required, since it allows 200 into one interval.

The single-interval column is the one candidates forget.

Reducing stack size — footnote b

Stacks shall be sized based on the total accumulated connected load at each story or branch interval. As the total accumulated connected load decreases, stacks are permitted to be reduced in size. Stack diameters shall not be reduced to less than one-half of the diameter of the largest stack size required.

So a stack that must be 6 inches at its base may be reduced as it rises, but never below 3 inches. This is not a violation of IPC 704.2 (no reduction in the direction of flow) because in a stack, flow travels downward — reducing the size going up is reducing it against the direction of flow, which is permitted.

Offsets in buildings of five stories or more — IPC Section 711

Section 711's title limits it to buildings of five stories or more. Within that scope, the trigger throughout is "more than four branch intervals below the top of the stack."

711.1 Horizontal branch connections near a vertical stack offset

If a horizontal branch connects to the stack within 2 feet above or below a vertical stack offset, and the offset is located more than four branch intervals below the top of the stack, the offset shall be vented in accordance with Section 907.

711.1.1 Omission of vents: not required where the stack and its offset are sized as a building drain per Table 710.1(1).

711.2 Horizontal stack offsets

A stack with a horizontal offset located more than four branch intervals below the top of the stack shall be vented per Section 907 and sized in three parts:

1. Above the offset: sized as a vertical stack based on the total dfu above the offset.
2. The offset itself: sized per 710.1.1 — that is, as a building drain per Table 710.1(1).
3. Below the offset: sized as for the offset, or based on the total dfu on the entire stack, whichever is larger (Table 710.1(2), the "greater than three branch intervals" column).

711.2.1 Omission of vents: not required where the stack and its offset are one pipe size larger than required for a building drain, and the entire stack and offset are not less in cross-sectional area than a straight stack plus the area of the offset vent required by Section 907.

711.3 Offsets below the lowest branch

Below the lowest branch, nothing discharges into the stack there, so pressure transients from the offset cannot reach a trap. That is why the venting requirements of 711.1 and 711.2 do not apply and only the sizing rule survives.

Worked example: an eight-story office building

An 8-story stack, one branch interval per floor, carries 36 dfu per floor and must offset horizontally at the sixth floor — that is, two branch intervals below the top, counting down from floor 8.

Step 1 — is 711.2 triggered? The offset is located two branch intervals below the top of the stack, which is not more than four. Section 711.2's venting and three-part sizing requirements are not triggered. The stack is sized straight from Table 710.1(2).

Change the case: the offset is at the third floor, which is five branch intervals below the top. Now 711.2 applies.

Step 2 — size the portion above the offset. Floors 4 through 8 contribute 5 x 36 = 180 dfu. Greater than three branch intervals: 3-inch = 72 (fails), 4-inch = 500 (passes). Use 4 inches.

Step 3 — size the offset. Per 710.1.1, use Table 710.1(1) as a building drain carrying 180 dfu. At 1/4 inch per foot, 4-inch carries 216 — passes. Use 4 inches at 1/4 inch per foot.

Step 4 — size the stack below the offset. The larger of (a) the offset size, or (b) the total dfu on the entire stack, which is 8 x 36 = 288 dfu. Table 710.1(2), greater than three branch intervals: 4-inch = 500 — passes. Use 4 inches.

Step 5 — vent the offset per Section 907, unless the 711.2.1 relief applies — the stack and offset one pipe size larger than required for a building drain, with the combined cross-sectional area not less than a straight stack plus the offset vent area.

Step 6 — check the single-interval column. 36 dfu into one branch interval; a 4-inch stack permits 90. Passes.