7.2 Plan-Based Sizing Practice: Branches, Stacks, Building Drain, Vents & Water

Why this section matters

Sizing questions combine several tables in one problem. Work every problem in the same order, and you will avoid the classic mistakes: the wrong table, the wrong column, or skipping the stack's per-interval limit. The exam allows about 2.3 minutes per question (75 questions in 2.92 hours), so a fixed routine matters.

The routine

1. Read the schedule. Note the fixture types, public or private, flush volume, and flush valve or tank.
2. Assign values. Use Table 709.1 for dfu, and Appendix E Table E103.3(2) for wsfu.
3. Add up the load only for the fixtures upstream of the pipe in question.
4. Choose the table:
   • Horizontal branch or stack: Table 710.1(2).
   • Building drain or sewer: Table 710.1(1) at the slope.
   • Stack vent or vent stack: Table 906.1.
   • Other vents: 906.2 (half the drain size, plus one size over 40 feet).
   • Water: E103.3(3) demand, then the friction charts.
5. Apply minimums: 3 inches for any building drain serving a water closet, trap sizes from Table 709.1, and 3/4 inch for a water service.

The building

Two-story office. Public fixtures, 1.6-gpf water closets on flushometer valves, 1-gpf urinals on 3/4-inch flushometer valves.

Horizontal branches (Table 710.1(2))

The 2021 IPC does not limit how many water closets a 3-inch branch may carry. The 20-dfu limit governs. If the restrooms on each floor joined into a single floor branch before reaching the stack, that branch would carry 28 or 30.5 dfu and would need 4 inches (160 dfu).

The stack (Table 710.1(2))

The stack has two branch intervals.

• Total: 58.5 dfu. A 3-inch stack with three or fewer intervals allows 48, so it fails.
• Per interval: Floor 2 discharges 28 dfu and Floor 1 discharges 30.5 dfu. A 3-inch stack allows 20 per interval, so it fails on that test too.
• 4 inches: 240 total and 90 per interval. Both pass. Use a 4-inch stack.

Variation. Suppose the total were only 38 dfu, all entering from one floor. The total fits a 3-inch stack (48), but one interval at 38 exceeds 20, so the stack would still be 4 inches. Always check both columns.

Building drain (Table 710.1(1))

The drain carries 58.5 dfu.

Remember not to use the horizontal-branch column for a building drain. A 3-inch building drain at 1/4 inch per foot carries 42 dfu, not 20.

Stack vent (Table 906.1)

The 4-inch stack carries 58.5 dfu, and the stack vent's developed length is 45 feet. Use the 4-inch, 140-dfu row, the first row at or above 58.5 dfu.

The minimum is 2-1/2 inches, which is also at least half of 4 inches. Many contractors use 3 inches because 2-1/2-inch DWV fittings are scarce. The building has only two branch intervals, so no separate vent stack is required (904.2).

Water supply (Appendix E)

Floor 2 water load, using public values from Table E103.3(2):

Demand (Table E103.3(3), flushometer column): 60 wsfu = 54 gpm and 70 wsfu = 58 gpm, so about 55.8 gpm. The flush-tank column would give only about 33 gpm for the same wsfu, which shows why using the wrong column undersizes flush-valve systems. Each flush valve still needs a 1-inch fixture supply (Table 604.5) and 35 psi of flow pressure (siphonic, Table 604.3).

Time management

• Answer only what is asked. If the question is about one branch, total only its upstream fixtures.
• Tab the four tables you use most: 709.1, 710.1(1), 710.1(2) and 906.1. Add E103.3(2) and E103.3(3) for water.
• Write the load at each node on scratch paper as you trace the drawing.
• Check both stack columns every time.

Exam traps

• Horizontal branches use Table 710.1(2), not Table 710.1(1).
• A public 1.6-gpf water closet is 4 dfu but 10 wsfu on a flush valve.
• Per-interval stack limits often decide the stack size.
• Read vent rows at or above the actual dfu, then the first length that covers the run.