6.1 Drainage Fixture Units (DFUs) and Sizing

Sizing Sanitary Drainage Systems with DFUs

To ensure that sanitary drainage systems operate efficiently under gravity flow, the International Plumbing Code (IPC) uses the Drainage Fixture Unit (DFU) method. A DFU is a relative load factor that represents the waste-producing effect of a plumbing fixture. It accounts for flow rate, discharge duration, and the average time between operations. Sizing based on DFUs prevents hydraulic overload, which can fill drainage pipes completely, disrupt pneumatic pressure, and siphon fixture trap seals.

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Fixture DFU Values (IPC Table 709.1)

Fixture values are divided based on their application: private (residential dwellings, private hotel bathrooms) and public (offices, schools, commercial spaces, public restrooms). For the ICC P2 exam, memorizing key commercial DFU values is essential.

[!IMPORTANT] Exam Tip: Pay close attention to water closet flush rates. A standard commercial water closet flushing at 1.6 gpf carries 4 DFUs. A high-volume or older water closet flushing at greater than 1.6 gpf carries 6 DFUs. This difference changes sizing calculations dramatically.

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Sizing Building Drains and Sewers (IPC Table 710.1(1))

The building drain is the lowest horizontal piping of a drainage system inside the building that conveys waste to the building sewer, which begins 5 feet (1524 mm) outside the foundation wall. Building drains and sewers are sized based on the total DFU load and the slope (pitch) of the pipe.

To prevent solids from settling, the flow must maintain a self-cleansing velocity of at least 2 feet per second (fps). Smaller pipes require steeper slopes to overcome friction. Below is a summary of the maximum DFU capacities for building drains and sewers:

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Sizing Horizontal Branches and Stacks (IPC Table 710.1(2))

Horizontal branches connect fixtures on a single floor to a vertical stack. Stacks carry waste vertically to the building drain. Sizing these elements involves matching the cumulative DFU load against Table 710.1(2) while adhering to strict physical fixture limits.

The 3-Inch Water Closet Limit:

A 3-inch horizontal branch is permitted to carry a maximum of 20 DFUs. However, it is restricted to a maximum of 2 water closets. If a branch serves 3 or more water closets, the pipe size must be increased to 4 inches, regardless of whether the total DFU load is under 20.

Stack Sizing Limits:

• A 3-inch stack with 3 branch intervals or fewer is permitted to carry up to 48 DFUs.
• A 3-inch stack with more than 3 branch intervals is permitted to carry up to 72 DFUs.
• No more than 2 water closets can connect to any single branch interval of a 3-inch stack, and the entire 3-inch stack is limited to a maximum of 6 water closets.
• The diameter of the drainage piping must never decrease in the direction of flow. If a 4-inch branch connects to a stack, the stack must be at least 4 inches.

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Worked Sizing Examples

Example 1: Sizing a Horizontal Branch for a Commercial Restroom Battery

A commercial restroom battery consists of:

• 4 Public Water Closets (1.6 gpf)
• 2 Public Urinals
• 3 Public Lavatories

Step 1: Calculate Total DFU Load

• 4 public water closets: 4 WCs \times 4 DFUs = 16 DFUs
• 2 public urinals: 2 urinals \times 2 DFUs = 4 DFUs
• 3 public lavatories: 3 lavatories \times 1 DFU = 3 DFUs
• Total DFU Load = 16 + 4 + 3 = 23 DFUs

Step 2: Size the Branch

• Looking at Table 710.1(2), a 3-inch horizontal branch can carry up to 20 DFUs. Since our load is 23 DFUs, a 3-inch pipe is hydraulically undersized.
• Furthermore, we have 4 water closets. A 3-inch branch has a strict limit of 2 water closets.
• Therefore, we must use a 4-inch horizontal branch (which carries up to 160 DFUs and has no water closet limit).

Step 3: Size the Building Drain at 1/4 in/ft slope

• The total load is 23 DFUs. According to Table 710.1(1), a 3-inch building drain at 1/4 in/ft slope is rated for up to 42 DFUs, so on capacity alone a 3-inch drain would carry the 23-DFU load.
• However, because the horizontal branch entering the building drain is 4 inches, the no-decrease-in-direction-of-flow rule governs: the building drain cannot be smaller than the pipe feeding it.
• Therefore, the building drain must also be 4 inches.