7.3 Horizontal Branch & Building Drain/Sewer Sizing

Horizontal Branch & Building Drain/Sewer Sizing

Once drainage fixture units (DFU) are tabulated, the plans examiner verifies every horizontal drain against IPC Section 710. The 2024 IPC uses two different tables, and choosing the right one is the first test:

• Table 710.1(1), Building Drains and Sewers: Sizes the building drain, the building sewer and branches of the building drain. Capacity depends on slope.
• Table 710.1(2), Horizontal Fixture Branches and Stacks: Sizes horizontal fixture branches (other than branches of the building drain) and soil and waste stacks. The horizontal branch column does not vary with slope.

The terms come from IPC Chapter 2:

1. Horizontal Branch Drain: A drain branch pipe extending laterally from a soil or waste stack or building drain, with or without vertical sections or branches, that receives the discharge from two or more fixture drains or branches and conducts it to the soil or waste stack or the building drain.
2. Building Drain: The lowest piping of the drainage system that receives discharge from soil, waste and other drainage pipes inside the building and extends 30 inches in developed length beyond the exterior walls, conveying the drainage to the building sewer.
3. Building Sewer: The part of the drainage system that extends from the end of the building drain to a public sewer, private sewer, individual sewage disposal system or other point of disposal.

   RESTROOM FIXTURES
   ┌───┐  ┌───┐  ┌───┐
   │WC │  │WC │  │LAV│
   └─┬─┘  └─┬─┘  └─┬─┘
   ──┴──────┴──────┴──────► HORIZONTAL FIXTURE BRANCH
                            Table 710.1(2), branch column (no slope variable)
                                       │
                                       ▼ Stack: Table 710.1(2) stack columns
   ════════════════════════════════════════════════════════════════════
                                       │
   ──────────────────────────────────────────► BUILDING DRAIN (to 30" outside wall)
   Exterior Building Wall        │             Table 710.1(1), by slope
   ══════════════════════════════╪════════════
                                 ▼
   ──────────────────────────────────────────► BUILDING SEWER (to public sewer)
                                               Table 710.1(1), by slope

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IPC Table 710.1(1): Building Drains and Sewers (2024)

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 is 3 inches.

IPC Table 710.1(2): Horizontal Fixture Branches (Branch Column)

The stack columns of the same table are covered in Section 7.4.

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The Critical Difference: Horizontal Branches vs. Building Drains

A common plan review error is sizing a fixture branch from the building drain table. Compare a 2-inch pipe:

• As a horizontal fixture branch (Table 710.1(2)): 6 DFU, regardless of slope.
• As a building drain at 1/4 inch per foot (Table 710.1(1)): 21 DFU.

Why Fixture Branches Are Rated Lower

Fixture branches take discharge directly from traps as sudden surges, often from fixtures close together, so the pipe can momentarily fill and disturb trap seals. By the time flow reaches the building drain, discharges from many fixtures have merged into a steadier stream, and the larger, sloped drain carries more load at the same depth. Building drain capacity also rises with slope, which is why Table 710.1(1) has slope columns and the branch column does not.

Water Closet Rules for Small Drains

The 2024 IPC controls water closets on small drains through minimum sizes, not through a count of water closets:

┌────────────────────────────────────────────────────────────────────────┐
│               WATER CLOSETS ON 3-INCH PIPE (2024 IPC)                  │
├───────────────────────────────────┬────────────────────────────────────┤
│  HORIZONTAL FIXTURE BRANCH        │  BUILDING DRAIN / SEWER            │
│  Table 710.1(2): 20 DFU max       │  Table 710.1(1): 36 / 42 / 50 DFU  │
│  (any slope)                      │  at 1/8, 1/4, 1/2 inch per foot     │
│                                   │  Footnote a: building drain serving│
│  A 3-inch branch with three       │  a water closet is at least 3 in.  │
│  water closets (12 DFU) complies  │  No separate count limit on water  │
│  on DFU                           │  closets in the 2024 tables        │
└───────────────────────────────────┴────────────────────────────────────┘

Rule 1: Size by DFU

A 3-inch horizontal fixture branch may carry up to 20 DFU. Three public 1.6-gpf water closets (3 × 4 = 12 DFU) plus three lavatories (3 DFU) total 15 DFU, which a 3-inch branch carries. Older code editions and other model codes (such as the UPC) limit the number of water closets on 3-inch pipe; that limit does not appear in the 2024 IPC tables. Check local amendments before applying it.

Rule 2: Minimum Pipe Sizes Tied to Water Closets

• Building drain (Table 710.1(1) footnote a): Any building drain serving a water closet must be at least 3 inches.
• Water closet connections (IPC 425.4 and 704.2): A 4-by-3-inch closet bend is acceptable. Where a 3-inch bend is used, a 4-by-3-inch flange receives the fixture horn, and that is not treated as a reduction in the direction of flow.
• Trap size (Table 709.1 note d): The trap must be consistent with the water closet's outlet size.

Rule 3: Building Sewers and Local Rules

Table 710.1(1) sizes building sewers by DFU and slope, so a 3-inch sewer is acceptable under the IPC when the load fits. Many utilities require a 4-inch minimum lateral for maintenance; that is a local or utility rule. Sewers 8 inches and larger also need manholes rather than cleanouts (IPC 708.1.2 and 708.1.8).

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Worked Sizing Review: Single-Story Office Building

Scenario: A single-story office building has three fixture branches discharging to one building drain. All piping is sloped at 1/4 inch per foot.

   Branch A (Men's Restroom):        Branch B (Women's Restroom):      Branch C (Break Area):
   - 2 public 1.6-gpf WCs (8 DFU)    - 3 public 1.6-gpf WCs (12 DFU)   - 2 sinks (4 DFU)
   - 2 urinals, 1 gpf (4 DFU)        - 3 lavatories (3 DFU)            - 1 service sink (2 DFU)
   - 2 lavatories (2 DFU)            Subtotal = 15 DFU                 - 1 drinking fountain (1/2)
   Subtotal = 14 DFU                                                   Subtotal = 6.5 DFU
           │                                 │                                │
           ▼                                 ▼                                ▼
   ┌─────────────────────────────────────────────────────────────────────────────┐
   │                    BUILDING DRAIN (Sloped at 1/4 inch per foot)             │
   └─────────────────────────────────────────────────────────────────────────────┘

Step-by-Step Plans Examination

1. Branch A (Men's Restroom), shown as 3 inches:

   • Load: $8 + 4 + 2 = 14\text{ DFU}$ (urinals using 1 gpf or less are 2 DFU each on a fixture branch).
   • Table 710.1(2): a 3-inch horizontal branch carries 20 DFU. Approved. A 2-inch branch (6 DFU) would be too small.

2. Branch B (Women's Restroom), shown as 3 inches:

   • Load: $12 + 3 = 15\text{ DFU}$.
   • Table 710.1(2): 15 DFU does not exceed 20 DFU. Approved. Three water closets on a 3-inch branch are acceptable under the 2024 tables.

3. Branch C (Break Area), shown as 2 inches:

   • Load: $4 + 2 + 0.5 = 6.5\text{ DFU}$.
   • Table 710.1(2): a 2-inch horizontal branch carries only 6 DFU. Rejected. Upsize to 2-1/2 inches (12 DFU) or 3 inches.

4. Building Drain, shown as 3 inches at 1/4 inch per foot:

   • Fixture-branch loads total $14 + 15 + 6.5 = 35.5\text{ DFU}$.
   • For building drains and sewers, Table 709.1 note e does not allow the lower urinal rating unless testing confirms it, so the two urinals count at 4 DFU each: $35.5 + 4 = 39.5\text{ DFU}$.
   • Table 710.1(1): a 3-inch building drain at 1/4 inch per foot carries 42 DFU, and footnote a's 3-inch minimum is met. Approved.
   • Check the slope: At 1/8 inch per foot, the same 3-inch drain would carry only 36 DFU and would have to be 4 inches (180 DFU).