2.3 Drain Stacks, Branch Intervals & Stack Offsets
Key Takeaways
A branch interval is a vertical distance of 8 feet or more between horizontal branch connections to a drain stack (SPS 381.01 (35)).
A 4-inch stack may take 90 dfu into one branch interval, 240 dfu total with 3 or fewer intervals, and 500 dfu total with more than 3 intervals.
Every gravity sanitary building sewer needs at least one stack, at least 3 inches in diameter, running from the building drain to a vent terminal or vent header (SPS 382.31 (3) (b)).
Stacks may be reduced as the load drops, to no less than half the base diameter and never smaller than the required stack vent.
Horizontal branches may not connect within 10 pipe diameters of the drain downstream of a stack base fitting, and building drain branches not within 20 diameters (SPS 382.30 (7)).
Branch intervals: the key definition
SPS 381.01 (35) defines a branch interval as "a vertical measurement of distance, 8 feet or more in length, between the connections of horizontal branches to a drainage stack." In practice this is usually one floor. Branches that connect less than 8 feet apart fall in the same interval. The number of intervals decides which stack column you read and whether a vent stack is required (5 or more intervals, see section 3.1).
Stack columns of Table 382.30-2
| Stack diameter (in.) | Total discharge into one branch interval | Stack with 3 branch intervals or less | Stack with more than 3 branch intervals |
|---|---|---|---|
| 1-1/4 | 1 | 2 | 2 |
| 1-1/2 | 2 | 4 | 8 |
| 2 | 6 | 10 | 24 |
| 3 | 20 | 48 | 72 |
| 4 | 90 | 240 | 500 |
| 5 | 200 | 540 | 1,100 |
| 6 | 350 | 960 | 1,900 |
| 8 | 600 | 2,200 | 3,600 |
| 10 | 1,000 | 3,800 | 5,600 |
| 12 | 1,500 | 6,000 | 8,400 |
A stack must pass both checks: the load entering any single branch interval, and the total load on the stack. Footnote a says the check point "includes all of the flow at the design point." Footnote d says the load "into one branch interval" includes the top fitting of the interval but not the bottom fitting that creates it.
Worked example. A six-story office has a 4-inch stack. Each floor's restroom branch carries 33 dfu (see section 2.1). The stack has more than 3 branch intervals, so the total limit is 500 dfu: 6 x 33 = 198 dfu passes. The per-interval limit is 90 dfu, and 33 passes. If one floor instead carried 96 dfu, the 4-inch stack would fail the one-interval column even though the total is fine, so that stack would need to be 5 inches.
The main stack and stack reductions
- Main stack. Each gravity-flow sanitary building sewer must be served by at least one stack extending from a building drain to a vent terminal or vent header. That stack must be at least 3 inches for its full height (SPS 382.31 (3) (b)). A house with only 2-inch stacks fails this rule.
- Reductions. Footnote c of Table 382.30-2 lets drain stacks be reduced as the load decreases. The minimum is one-half the diameter required at the base, and never smaller than the stack vent required by SPS 382.31 (14) (a). Footnote e allows the reduction within a branch interval.
Offsets in vertical drains (SPS 382.30 (6))
| Offset angle from vertical | Sizing rule | Extra conditions |
|---|---|---|
| 45 degrees or less | Size as vertical drain piping | A horizontal branch within 2 ft above or below a 30 to 45 degree offset that has 5 or more branch intervals above it triggers offset venting under SPS 382.31 (5) (a) |
| More than 45 degrees | Above the offset: vertical sizing; the offset itself: building-drain sizing (Table 382.30-3); below the offset: not smaller than the offset | Offset more than 4 branch intervals below the top: no horizontal branch within the offset or within 2 ft above or below it |
An offset of more than 45 degrees below 5 or more branch intervals must be vented under SPS 382.31 (5) (b). The venting is waived when the stack and offset are upsized one pipe size beyond building-drain sizing and the cross-section equals the stack plus the required vent. Exception (6) (c): an offset located 2 or more feet below the lowest branch drain connection needs no offset venting at all.
No-connection zones at the base of a stack (SPS 382.30 (7))
Turbulence and a hydraulic jump occur where a stack turns horizontal. Wisconsin keeps branches out of that zone:
- A horizontal branch drain may not connect downstream of the base fitting of a drain stack 2 inches or larger within 10 pipe diameters of the drain it joins.
- A building drain branch or building subdrain branch may not connect within 20 pipe diameters of the building drain or subdrain.
On a 4-inch building drain, the horizontal-branch zone is 10 x 4 = 40 inches and the building-drain-branch zone is 20 x 4 = 80 inches downstream of the base fitting.
Stack-related rules found elsewhere
- Drain stacks of 5 or more branch intervals need a vent stack and a stack vent (SPS 382.31 (4)).
- Drain stacks of more than 10 branch intervals need a relief vent at each tenth interval (SPS 382.31 (6)).
- A stack may act as a combination drain and vent only for the limited cases in SPS 382.31 (17) (section 3.3).
Exam strategy
Write down three numbers before you open Table 382.30-2: the dfu entering the busiest interval, the total dfu at the stack base, and the number of branch intervals. Then read the correct column. Most wrong answers come from using the "3 intervals or less" column for a tall building, or checking only the total.
A 4-inch drain stack serves a building with more than 3 branch intervals. What is the maximum dfu that may enter any one branch interval?
90 dfu
160 dfu
240 dfu
500 dfu
A drain stack discharges to a 4-inch building drain. How far downstream of the stack's base fitting must a new building drain branch connect?
At least 40 inches
At least 80 inches
At least 10 feet
At least 24 inches
What is the minimum diameter of the stack that every gravity-flow sanitary building sewer must have, extending from the building drain to a vent terminal or vent header?
1-1/2 inches
2 inches
4 inches
3 inches
Sections you finish are checked off in the contents.