5.2 Horizontal Branches, Building Drains & Building Sewers: Slope, Sizing & Separation

Definitions

• Horizontal branch drain: a branch extending laterally from a soil or waste stack or the building drain, with or without vertical sections, that receives the discharge of two or more fixture drains or branches.
• Building drain: the lowest piping of a 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 to the building sewer.
• Building sewer: the part of the drainage system from the end of the building drain to the public sewer, private sewer, individual sewage disposal system or other point of disposal.

Slope (IPC 704.1 and Table 704.1)

Horizontal drainage piping must be installed in uniform alignment at uniform slopes, at least:

Exception in the same section: piping upstream of a grease interceptor must slope at least 1/4 inch per foot, regardless of size.

Fall over a run: a 3-inch drain 60 feet long at 1/8 inch per foot falls 60 × 0.125 = 7.5 inches. At 1/4 inch per foot, it falls 15 inches.

Horizontal fixture branches and stacks: Table 710.1(2)

Notes:

• Table 710.1(2) does not include branches of the building drain. Those are sized with Table 710.1(1) (note a).
• The 2021 table has no "two water closets on a 3-inch branch" limit. That footnote was dropped from earlier editions. A 3-inch branch is limited only by its 20-dfu capacity and by the size rules: a water closet needs a 3-inch drain, and a clothes washer standpipe needs a 3-inch or larger branch (406.2).
• Five private 1.6 gpf water closets (5 × 3 = 15 dfu) fit on a 3-inch branch. Six public 1.6 gpf water closets (6 × 4 = 24 dfu) need a 4-inch branch.

Building drains and sewers: Table 710.1(1)

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

Note a: the minimum size of any building drain serving a water closet is 3 inches.

Steeper slope increases capacity. A 4-inch drain goes from 180 dfu at 1/8 inch per foot to 216 dfu at 1/4 inch per foot, a 20% increase, and to 250 dfu at 1/2 inch per foot.

Worked examples

1. 210 dfu building drain. At 1/8 inch per foot, 4 inches carries 180 dfu, which is too small, so a 5-inch drain (390 dfu) is required. If the elevations allow 1/4 inch per foot, 4 inches (216 dfu) works.
2. 45 dfu building sewer. At 1/8 inch per foot (36 dfu) or 1/4 inch per foot (42 dfu), a 3-inch sewer is too small, so use 4 inches. Only at 1/2 inch per foot (50 dfu) would 3 inches be large enough.
3. Branch of the building drain with 30 dfu. It is sized from Table 710.1(1), so at 1/8 inch per foot it needs 3 inches (36 dfu).

The building sewer (IPC 703)

• 703.1: the sewer's proximity to the water service follows 603.2.
• 703.2: on filled or unstable ground, a building sewer or building drain must be ABS, cast iron, copper, PVC or polypropylene pipe listed in Table 702.3.
• 703.3: separate sanitary and storm sewers on the same property may be laid side by side in one trench.
• 703.4: when an existing building's entire drainage system is replaced, the existing under-slab building drains and building sewers that will be reused must be internally examined for slope, breaks, obstructions and adequate size.
• 703.5: cleanouts on building sewers follow Section 708 (see Section 6.2 of this guide).
• 703.6: where the public sewer is a combined sanitary and storm system, the sanitary sewer connects to it independently.
• Virginia 703.7 tracer wire: nonmetallic sanitary sewer piping that discharges to a public system must be locatable. Use insulated copper tracer wire of at least 18 AWG suitable for direct burial, or an equivalent product, in the same trench and within 12 inches of the pipe. Run it from within 5 feet of the building wall to the point where the sewer meets the public system. At least one end must terminate above grade in an accessible, damage-resistant location, such as a cleanout or the building wall.

Separating the sewer from the water service (IPC 603.2)

The 2021 text works this way:

1. A water service may share a trench with the building sewer if the sewer is made of materials listed in Table 702.2 (underground building drainage pipe).
2. If the sewer is not made of Table 702.2 materials, the water service and sewer must be horizontally separated by at least 5 feet of undisturbed or compacted earth.
3. The 5-foot separation does not apply where the water service crosses the sewer, if the water service is sleeved for at least 5 feet horizontally on both sides of the sewer centerline. The sleeve must be pipe listed in Table 605.3, 702.2 or 702.3.
4. The 5-foot separation also does not apply where the bottom of the water service within 5 feet of the sewer is at least 12 inches above the top of the sewer.

Under 603.2.1, a potable water service must not be located in, under or above cesspools, septic tanks, septic drain fields or seepage pits.

Trenching and bedding

The Chapter 3 rules apply (see Section 2.3): continuous support with bell holes; no blocking to grade; rock removed 3 inches below the pipe; backfill in 6-inch layers until there are 12 inches of tamped earth over the crown; and no trench in the 45-degree bearing plane of a footing.