4.2 Drainage Slopes, Building Drain & Sewer Rules, Offsets and Backwater Valves

Why this section matters

Grade, inverts and the rules about where drains go show up in Drain Waste and Vents and System Installation questions. Learn the slopes cold and know how the building drain differs from the building sewer.

Minimum slope (704.1)

Horizontal drainage must be installed in uniform alignment at uniform slopes, no less than Table 704.1:

• Piping upstream of a grease interceptor needs at least 1/4 inch per foot, whatever its size, so that grease moves along.
• Table 704.1 offers no exception that lets 2-1/2-inch and smaller pipe run at 1/8 inch per foot.

Why grade matters

Gravity drains are open-channel flow. Too little slope lets solids settle. Engineers often aim for a self-scouring velocity of about 2 feet per second. Very steep slopes can let liquid run ahead of solids. These velocity targets are design guidance, not IPC numbers. The code controls minimum slope, and the table capacities in 710.1(1) assume the slope you install. Use a laser or a slope-marked level, and keep the grade uniform. A sag ("belly") holds water and solids and violates the uniform-slope rule.

Fall and invert calculations

$$\text{Fall (in)} = \text{Run (ft)} \times \text{Slope (in/ft)} \qquad \text{Downstream invert} = \text{Upstream invert} - \frac{\text{Fall}}{12}$$

Example. A 4-inch building sewer runs 120 feet at the 1/8-inch minimum. Fall is $120 \times 0.125 = 15$ inches, or 1.25 feet. The invert leaves the wall at 104.50 feet and reaches the lateral at $104.50 - 1.25 = 103.25$ feet. If the public lateral invert is 101.50 feet and the ground there is at 106.00 feet, the sewer arrives 1.75 feet above the lateral. That is positive fall, so the plumber may steepen the slope (for example to 1/4 inch per foot, giving a 102.00-foot invert) to use the extra drop.

Building drain, building sewer and the connection to the public sewer

• Building drain: the lowest piping of the drainage system that receives discharge from soil, waste and other drainage pipes inside the building. It extends 30 inches in developed length beyond the exterior walls (Section 202).
• Building sewer: the part from the end of the building drain to the public sewer, private sewer, individual disposal system or other point of disposal.
• Materials differ:
  • Above-ground and underground building drains use Tables 702.1 and 702.2 (PVC and ABS Schedule 40 and DR pipe, cast iron, and copper Types K, L, M and DWV).
  • The building sewer uses Table 702.3, which adds thinner sewer pipe such as PVC SDR 35 (ASTM D3034), concrete and vitrified clay. Copper in the building sewer must be Type K or L.
• 703.2: On filled or unstable ground, building sewers and drains must be ABS, cast iron, copper, PVC or polypropylene from Table 702.3.
• 701.2 and 701.3: Every building with plumbing must connect to the public sewer where one is available. Each building on a street with a public sewer needs a separate connection, although buildings on the same lot may share a building sewer. Where no public sewer exists, connect to an approved private system. In Alabama those are regulated by the Department of Public Health.
• 703.3: Separate sanitary and storm sewers may share one trench.
• 703.4: When an existing building's whole drainage system is replaced, existing drains under slabs and existing sewers must be internally examined (camera) for slope, breaks, obstructions and size.
• 703.6 and 1109.1: Where the public sewer is combined, the sanitary and storm sewers connect to it independently.

Separation from the water service (603.2)

Potable water service may never run in, under or above a septic tank, drain field, cesspool or seepage pit (603.2.1).

Stack offsets (711 and 907)

Section 711 applies only to buildings of five stories or more.

• Horizontal pipe vs. vertical pipe: a fitting or pipe that makes an angle of less than 45 degrees with the horizontal is horizontal. One at 45 degrees or more with the horizontal is vertical (Section 202).
• 711.1: If a horizontal branch connects within 2 feet above or below a vertical stack offset, and that offset is more than four branch intervals below the top of the stack, the offset must be vented per 907. The connection itself is allowed; the offset needs venting. 711.1.1: No vent is needed if the stack and offset are sized as a building drain.
• 711.2: A horizontal offset more than four branch intervals below the top of the stack must be vented per 907. It is sized as follows:
  • the stack above the offset, for the dfu above it;
  • the offset, as a building drain (710.1.1);
  • the stack below the offset, as the larger of the offset size or the size needed for the whole stack's dfu.
• 711.2.1: The vents may be omitted if the stack and offset are one size larger than a building drain and meet the cross-section rule in that section.
• 711.3: A horizontal offset below the lowest branch is sized as a building drain.
• 907.1–907.3: Where five or more branch intervals are above a horizontal offset:
  • the upper section is vented as a separate stack, with the offset treated as its base;
  • the lower section is vented by a yoke vent between the offset and the next lower branch, at least the size of the stack's vent stack.

The rule that forbids fixture connections within 2 feet above or below a horizontal offset belongs to the single-stack vent system (917.7). It is not a general rule for every stack.

Backwater valves (714)

• 714.1: Fixtures on a floor whose finished elevation is below the manhole cover of the next upstream public-sewer manhole must be protected by a backwater valve in the building drain or the branch serving them.
• Fixtures above that elevation must not discharge through the backwater valve. That way a sewer surcharge backs up only into the protected lower fixtures, which the valve closes off. Existing buildings are exempt from this second rule.
• Backwater valves must conform to ASME A112.14.1 (or CSA B181.1 or B181.2) and give access to their working parts (714.2, 714.3).
• Storm systems need backwater valves on the same basis (1101.9).

Exam traps

• Slope by size: 1/4 in/ft for 2-1/2 inches and under, 1/8 in/ft for 3 to 6 inches, and 1/16 in/ft for 8 inches and up. Anything upstream of a grease interceptor needs 1/4 in/ft.
• 30 inches outside the wall ends the building drain.
• 711 offsets need venting. They are not a "no-connection zone" outside a single-stack system.
• Backwater valve: only the fixtures below the upstream manhole cover drain through it.