2.3 Horizontal Branch & Vertical Stack Sizing

Horizontal Branch & Vertical Stack Sizing

Once the Drainage Fixture Unit (DFU) values for all fixtures have been calculated, the plumber must translate those values into nominal pipe diameters. Sizing horizontal fixture branches and vertical soil and waste stacks requires consulting Florida Plumbing Code Table 710.1(2).

Horizontal branches and vertical stacks operate under radically different hydraulic regimes. In a horizontal branch, wastewater flows as an open-channel stream along the bottom of the pipe. In a vertical stack, wastewater falls rapidly as an annular film clinging to the pipe walls around a central core of air. Consequently, code limits vary significantly between horizontal and vertical piping.

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1. Sizing Horizontal Fixture Branches

Definition of a Horizontal Branch

Under FPC Chapter 2, a horizontal branch is a drain 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 and conveys it to the stack or building drain.

Maximum DFU Capacities (FPC Table 710.1(2))

The horizontal branch column of Table 710.1(2) governs the maximum permissible DFU load on any horizontal branch based on its nominal diameter:

Water Closets: Capacity, Outlet and Minimum-Size Checks

Select fixture units from Table 709.1 for the actual fixture type and use. Private 1.6 gpf water closets are 3 DFU each; public ones are 4 DFU. Do not substitute a fixture-count rule for the current table.

1. Minimum Diameter (3 Inches): for a closet with a 3-inch outlet, and FPC Section 704.2 prohibits reducing drainage pipe size in the direction of flow, so the branch receiving a water closet is 3 inches minimum. Section 704.2 lists the narrow exceptions that are not treated as a reduction: a 4-inch by 3-inch water closet flange, a water closet bend with a 4-inch inlet and 3-inch outlet where the 4-inch leg is upright and below but not connected to the flange, and an offset closet flange. Footnote a to Table 710.1(1) adds that the minimum size of any building drain serving a water closet is 3 inches.
2. DFU capacity is the working limit. A 3-inch horizontal branch is capped at 20 dfu by Table 710.1(2). Private 1.6 gpf water closets are 3 dfu each and public ones are 4 dfu each, so use the actual DFU total and the applicable sizing and connection rules.

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2. Sizing Soil and Waste Stacks

Soil Stack vs. Waste Stack

• Soil Stack: A vertical pipe that carries wastewater containing human fecal matter or urine (i.e., receives discharges from water closets, clinical sinks, or urinals).
• Waste Stack: A vertical pipe that carries wastewater free of fecal matter (i.e., receives discharges from sinks, lavatories, bathtubs, showers, and clothes washers).

The Concept of a Branch Interval

To properly size vertical stacks in multi-story buildings, plumbers must master the definition and application of a Branch Interval:

A branch interval is a vertical measurement of 8 feet or more in developed length between horizontal branch connections to a drainage stack, measured down from the highest connection (FPC Chapter 2). A floor count alone is not a reliable substitute.

Why does the code limit the load per branch interval? When a horizontal branch enters a vertical stack, the incoming water forms a sheet across the stack diameter. If too much water enters at one level, it blocks the downward flow of air through the center of the stack, causing severe pressure spikes that blow out trap seals on floors below.

FPC Table 710.1(2) Stack Sizing Matrix

Choose the total-load column by the number of branch intervals, and separately check the discharge into any one interval.

Apply Both Limits

For every proposed stack size, compare the accumulated load with the applicable total-stack column and each interval load with the single-interval column. A total-load pass cannot excuse an overloaded interval.

Example: A stack has four branch intervals and a single interval receives 24 DFU. A 3-inch stack fails the 20-DFU interval limit even if its total is below 72 DFU. A larger size must satisfy both columns.

Accumulated Load

Table 710.1(2), footnote b, sizes each part of the stack from its accumulated connected load. Size reductions are permitted as that load decreases, but not below half the largest required stack diameter.

Three consequences:

• Size each story from what is connected above it, not from a single whole-building number. A stack may legitimately be 4 inches at the base and 3 inches at the top floors.
• The reduction floor is one-half the largest required diameter. A stack that needs 6 inches at its base may never be reduced below 3 inches anywhere above.
• The columns are the limits. For a 3-inch stack: 20 dfu may discharge into any one branch interval, 48 dfu total for a stack of three branch intervals or less, and 72 dfu total for a stack greater than three branch intervals.

FPC Section 710.1.2 adds that vertical stack offsets are sized as required for straight stacks under Table 710.1(2), except where Section 711.1.1 requires them to be sized as a building drain, and Section 710.1.1 requires horizontal stack offsets to be sized as building drains under Table 710.1(1), except as required by Section 711.3.

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3. Flow Changes at Stack Bases

Wastewater in a stack interacts with air and pipe walls; turning into horizontal drainage changes the flow pattern and can create pressure disturbances. The behavior depends on loading, diameter and geometry. Do not assume a universal terminal velocity or a fixed distance over which it develops.

A hydraulic jump is a change from faster, shallower flow to deeper flow. Stack transitions and nearby connections need the code-prescribed arrangement so local pressure changes do not undermine trap seals.

Proper venting provides air paths; proper connection location keeps branches away from the immediate transition. Neither an assumed velocity nor a schematic sketch replaces the sizing and venting checks.

Stack-Base and Offset Connection Distance

FPC 704.3 places horizontal branch connections at least 10 times the drainage stack diameter downstream of the stack base or of the upper stack at a horizontal offset. For a 4-inch stack, that is 10 × 4 = 40 inches. This is not a universal 2-foot prohibition above every stack base. The separate 2-foot condition in 711.1 concerns branches near a vertical offset, as explained below.

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4. Offsets in Drainage Stacks (FPC Section 711)

In architectural design, structural columns or floor layout changes often prevent a drainage stack from dropping straight down from roof to basement, requiring a horizontal dogleg or offset.

A pipe at least 45 degrees above horizontal is vertical under Chapter 2. Therefore, an offset up to 45 degrees from vertical remains a vertical offset; an offset greater than 45 degrees from vertical is horizontal. State which reference direction an angle uses.

Vertical Offsets

Section 710.1.2 generally uses the straight-stack table. In buildings five stories or more, 711.1 requires Section 907 venting if a horizontal branch connects within 2 feet above or below a vertical offset located more than four branch intervals below the top. Section 711.1.1 permits that venting to be omitted if the stack and offset are sized as a building drain. Thus, angle alone does not establish a venting exemption.

Horizontal Offsets

Section 710.1.1 sizes horizontal offsets by the building-drain table, subject to 711.3. For an offset more than four branch intervals below the top in the scope of 711.2, size the upper stack for the load above; size the offset by Table 710.1(1); and size the lower portion for the larger of the offset requirement or the entire stack load. Section 907 addresses the upper-stack vent and lower yoke vent. The 711.2.1 venting exception requires an offset one pipe size larger than required for a building drain and an area at least equal to the straight-stack area plus the offset-vent area. Below the lowest branch, 711.3 allows a vertical offset without a size change and sizes a horizontal offset by Table 710.1(1). An offset above all drainage connections is a vent, so evaluate it under Chapter 9 instead.