9.3 Sizing Building Drains, Building Sewers, and Horizontal Branches

Two tables, two jobs

This is where candidates lose points. IPC 710.1 assigns each table a specific class of pipe:

The phrase "branch of the building drain" means a horizontal branch that connects to the building drain itself — below the lowest floor, typically under the slab. A "horizontal fixture branch" is a horizontal drain above that, receiving fixtures and carrying them to a stack.

Table 710.1(1) is organized by slope because the pipe runs horizontally its entire length. Table 710.1(2) has no slope column for stacks because a stack is vertical.

Table 710.1(1) — building drains and sewers

Maximum dfu connected to any portion of the building drain or building sewer, including branches of the building drain:

Footnote a: the minimum size of any building drain serving a water closet shall be 3 inches. That floor overrides the dfu math. A single private water closet is 3 dfu, which a 2-inch line would carry numerically — but a water closet needs 3 inches.

Notice the enormous jump from 3 inches to 4 inches: 42 dfu to 216 dfu at 1/4 inch per foot, more than a fivefold increase for a one-size step. Flow capacity in a partially full gravity pipe scales roughly with diameter to the 8/3 power, which is why the 3-to-4 step is so dramatic and why dropping a 4-inch drain to 3 inches to clear an obstruction is almost never workable.

Notice also the blank cells. The 1/16 inch per foot column is empty below 8 inches because Table 704.1 does not allow that slope on smaller pipe.

Table 710.1(2) — horizontal fixture branches and stacks

Footnote a: the table does not include branches of the building drain — refer to Table 710.1(1).

Compare a 3-inch pipe across the two tables: 20 dfu as a horizontal fixture branch, but 36 to 50 dfu as a building drain depending on slope. The horizontal branch value is lower because a branch discharges into a stack, where surging matters, while a building drain runs to open sewer.

Worked sizing: a 24-unit apartment building

Each of 24 units has one bathroom group (1.6 gpf water closet), one kitchen sink and one residential clothes washer. There is also a common laundry with two commercial clothes washers and one service sink, and three floor drains in the mechanical room.

Step 1 — total dfu:

Step 2 — building drain. Enter Table 710.1(1) at 230 dfu:

• At 1/8 inch per foot: 4-inch carries 180 (too small); 5-inch carries 390 — sufficient.
• At 1/4 inch per foot: 4-inch carries 216 — still too small; 5-inch carries 480 — sufficient.
• At 1/2 inch per foot: 4-inch carries 250 — sufficient, if that slope is achievable.

So a 5-inch building drain at 1/8 or 1/4 inch per foot, or a 4-inch drain at 1/2 inch per foot if the site has the fall. In practice the 5-inch at 1/4 inch per foot is the ordinary answer; 1/2 inch per foot over a long run demands a great deal of depth.

Step 3 — check the water closet floor. Footnote a requires at least 3 inches for any building drain serving a water closet. Satisfied.

Step 4 — the stack. If the building has three stacks each serving 8 units, each stack carries roughly (8 x 5) + (8 x 2) + (8 x 2) = 72 dfu and spans four stories, which is more than three branch intervals. From Table 710.1(2), "stack greater than 3 branch intervals": a 3-inch stack carries 72 dfu — exactly at the limit — and a 4-inch stack carries 500. Because there is no margin at 3 inches and every added fixture would bust it, 4 inches is the sound choice.

Step 5 — horizontal branches. A branch serving two bathroom groups plus two kitchen sinks is (2 x 5) + (2 x 2) = 14 dfu. Table 710.1(2) horizontal branch column: 2-1/2 inch carries 12 (too small), 3-inch carries 20 — a 3-inch branch works, and it also satisfies the requirement that a branch serving a water closet be at least 3 inches.

Offsets

• IPC 710.1.1 Horizontal stack offsets: sized as required for building drains per Table 710.1(1), except as required by 711.3.
• IPC 710.1.2 Vertical stack offsets: sized as required for straight stacks per Table 710.1(2), except where required to be sized as a building drain per 711.1.1.

The logic is physical: a horizontal offset behaves like a building drain — flow decelerates and runs partially full along a slope. A vertical offset is still a stack.

Section 711 adds special rules for buildings of five stories or more, covered in the next section of this guide.