4.1 Drainage Fixture Units & Sizing Branches, Stacks and Building Drains

Why this section matters

Drain sizing is the core of Sizing and Design and many plan-reading questions. The method is: assign fixture units, add them up in the direction of flow, then choose the correct table. Most mistakes come from reading the wrong table, such as using the building drain table for a horizontal branch.

Drainage fixture units (709)

A drainage fixture unit (dfu) is a load factor that reflects how much a fixture discharges, how fast and how often. It is a probability weighting, not a flow rate. Hunter's original work was based on a lavatory discharging about 1 cubic foot per minute, but on the exam you use only the table values.

Selected values from Table 709.1

Notes to the table:

• For drains and sewers, water closets and urinals may not be given lower values unless testing proves them.
• Fixtures added to a bathroom group add their own dfu.
• 709.2: Fixtures not listed use Table 709.2, based on drain or trap size: 1-1/4 in = 1, 1-1/2 in = 2, 2 in = 3, 2-1/2 in = 4, 3 in = 5, 4 in = 6.
• 709.3: Where only a gpm flow is known, count 1 gpm as 2 dfu. A pump discharging 15 gpm adds 30 dfu.
• 709.4: An indirect waste receptor carries the total dfu of the fixtures that drain into it, but no less than its own table value. 709.4.1: A receptor taking only clear water from display cases, ice bins, coolers and freezers is 1/2 dfu.

Choosing the right table (710)

Table 710.1(2): horizontal branches and stacks

• A stack must meet both its total limit and its per-interval limit.
• A branch interval is at least 8 feet of stack between branch connections (Section 202).
• A stack may get smaller higher up as its load drops, but never smaller than half the largest size required.

Table 710.1(1): building drains and sewers

Note a to the table says any building drain serving a water closet must be at least 3 inches. The 2021 IPC tables do not limit how many water closets a 3-inch branch or stack may carry. The dfu columns govern. (Some older codes had such limits; the single-stack vent system in Section 917 still limits a 3-inch stack to two water closets.)

Other sizing rules:

• 704.2: Drain piping may not be reduced in size in the direction of flow. A 4 × 3-inch closet flange or closet bend is not a reduction.
• 704.3: Horizontal branches connect to the base of a stack at least 10 stack diameters downstream.
• 710.2: Pipe sizes must include the load of any roughed-in future fixtures.

Clothes washer standpipes (406.2 and 802.4.3)

• A washer must discharge through an air break into a standpipe or into a laundry sink.
• The trap and fixture drain must be at least 2 inches, and the fixture drain must connect to a 3-inch or larger branch or stack.
• The standpipe must be individually trapped and extend 18 to 42 inches above the trap weir (802.4.3).
• Gravity-discharge washers may drain to a waste receptor or approved trench drain.

Worked example: four-story office stack

Each floor has 2 public 1.6-gpf water closets (4 dfu each), 2 lavatories (1 each), 1 urinal at 1 gpf or less (2) and 1 service sink (2).

1. Load per floor: $2(4) + 2(1) + 2 + 2 = 14$ dfu.
2. Branch on each floor: water closets need at least 3 inches, and a 3-inch branch carries 20 dfu, so 14 dfu fits. Use 3 inches.
3. Stack: 4 floors means 4 branch intervals, which is more than 3, and $4 \times 14 = 56$ dfu. A 3-inch stack allows 72 dfu total and 20 per interval, and 14 per interval passes. Use a 3-inch stack.
4. Building drain at 1/4 inch per foot: 56 dfu exceeds the 42-dfu limit for 3 inches, so the drain must be 4 inches (216 dfu).

Lesson: the stack can legally be smaller than the building drain it feeds, because the two are sized from different tables.

Exam traps

• Washer dfu: 2 residential, 3 commercial. Both need a 2-inch trap.
• Public 1.6-gpf water closet = 4 dfu. The 6-dfu value is for public water closets flushing more than 1.6 gallons.
• No "two water closets per 3-inch branch" rule in the 2021 IPC Table 710.1(2).
• Branch vs. building drain: a 3-inch horizontal branch carries 20 dfu, but a 3-inch building drain at 1/4 inch per foot carries 42.