11.2 Sizing Vertical Conductors, Leaders & Horizontal Storm Drains

Sizing Vertical Conductors, Leaders & Horizontal Storm Drains

Undersized storm piping backs water up onto the roof, where its weight can overload the structure. The 2024 IPC sizes storm drainage by flow rate in gallons per minute (gpm). The designer converts roof area and rainfall to gpm (IPC Equation 11-1), checks the roof drain's published flow, and then picks pipe, leader and gutter sizes from Tables 1106.2, 1106.3 and 1106.6. Older editions used tables of roof area at a 1-inch rainfall rate; those tables are gone, so square-foot answers from older study materials do not match the 2024 code.

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Terminology & Materials (IPC Chapter 2 and Section 1102)

• Roof Drain: A drain installed to receive water collecting on a roof and discharge it into a leader or conductor. Roof drains conform to ASME A112.3.1 or A112.6.4, and other than siphonic drains are tested and rated per ASME A112.6.4 or ASPE/IAPMO Z1034 (1102.6). They are installed per the manufacturer's instructions, and the roofing membrane may not obstruct the inside opening (1105.1).
• Conductor: A pipe inside the building that conveys storm water from the roof to a storm drain.
• Leader: An exterior drainage pipe (downspout) that conveys storm water from roof or gutter drains to an approved means of disposal.
• Building Storm Drain: The building drain that conveys storm water only, extending 30 inches beyond the exterior walls to the building storm sewer.
• Building Storm Sewer: The building sewer that conveys storm water only, from the end of the storm drain to the point of disposal.

Materials: Inside conductors above ground use Table 702.1 materials (1102.2), underground building storm drains use Table 702.2 (1102.3), building storm sewers use Table 1102.4 (for example, ABS, cast iron, concrete, copper, PE, PP, PVC including SDR35, and vitrified clay), and subsoil drains use Table 1102.5 (1102.5). Fittings follow Table 1102.7 and may not have ledges or reductions that retard flow.

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Step 1: Convert Roof Area to Flow (IPC 1105.2 and 1106.2.1)

Equation 11-1 converts rainfall on the roof to a flow rate: $Q_{\text{gpm}} = 0.0104 \times A \times R$ where $A$ is the roof area in square feet (including one-half of walls that divert rain onto it, per 1106.4) and $R$ is the 100-year hourly rainfall rate in inches per hour.

Two checks follow:

1. Roof drain flow (1106.2 and 1105.2): The calculated flow is checked against the manufacturer's published flow rate for the specific roof drain model and size, to confirm the drain can pass it. That published rate depends on the head of water above the drain, and piping is sized on the flow at the maximum anticipated ponding at the drain.
2. Pipe capacity: The flow in storm drain piping may not exceed Table 1106.2.

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Step 2: Size Storm Drain Piping (IPC Table 1106.2)

IPC 1106.2 sizes vertical and horizontal storm drain piping on the flow rate through the roof drain:

Reading the table: Horizontal capacity rises with slope. A 6-inch storm drain carries 344 gpm at 1/8 inch per foot but 487 gpm at 1/4 inch per foot. Table 1106.2 includes a 1/16-inch-per-foot column for every size, so storm drains are not limited to the sanitary minimum slopes of Table 704.1; the flatter slope simply carries less. Under IPC 1101.5, storm drains may not be reduced in size in the direction of flow.

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Step 3: Size Vertical Leaders (IPC Table 1106.3)

Vertical leaders are sized on the flow from horizontal gutters or the maximum flow through roof drains, and that flow may not exceed Table 1106.3 (1106.3):

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Step 4: Size Gutters (IPC Table 1106.6)

Horizontal gutters are sized on the flow from the roof surface and may not exceed Table 1106.6 (1106.6). Selected semicircular gutters (single dimension = diameter):

The table also lists rectangular gutters (width by depth). For example, a 3-by-5-inch gutter carries 157 gpm at 1/4 inch per foot.

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Controlled-Flow Roof Drain Systems (IPC Section 1110)

In dense urban areas, a building may hold water on its roof to limit the peak flow to the public storm sewer. IPC 1110 governs these systems:

          CONTROLLED-FLOW ROOF DRAIN SYSTEM (IPC SECTION 1110)

                           Design water storage level
   ~~~~~~~~~~~~~~~~~~~~~~~▼~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   Parapet Wall           │                               Parapet Wall
   ┌──────────┐           │                              ┌──────────┐
   │ Secondary│ ◄─────────┼──────────────────────────────┤ Secondary│
   │ drainage │           │     Control device with      │ drainage │
   │ (1108)   │           │     strainer (1110.3)        │ (1108)   │
   └──────────┴───────────┼──────────────┼───────────────┴──────────┘
       ===============================[ROOF DRAIN]=======================
                                          ▼ Metered discharge

1. Engineered design (1110.1): The roof must be designed for the storage of water. The controlled-flow system is an engineered system meeting the design, submittal, approval, inspection and testing requirements of IPC 316.1 (alternative engineered design), based on the rainfall rate of 1106.1.
2. Control devices (1110.2 and 1110.3): Runoff is controlled by control devices, installed so the rate of discharge does not exceed the design continuous-flow values. Control devices are protected by strainers.
3. Minimum number of drains (1110.4): Not less than two roof drains for roof areas of 10,000 square feet or less, and not less than four for roofs over 10,000 square feet.
4. Structural coordination (1101.7 and IBC Section 1611): The roof is designed for the maximum possible depth of ponded water, assuming all primary drains are blocked, including the head needed over the secondary drainage inlet. Water weighs 62.4 pounds per cubic foot, or about 5.2 psf per inch of depth, so 4 inches of ponding adds about 20.8 psf.
5. Secondary drainage (1108): Secondary (emergency overflow) drains or scuppers are still required where parapets would trap water.

The model IPC does not state a required drawdown time; the engineered design documents the storage depth and release rate, and local storm water rules may add limits.

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Worked Sizing Example: Conductors and Building Storm Drain

A warehouse has a flat roof of $36{,}000\text{ sq ft}$ where the 100-year hourly rainfall rate is $R = 4.0\text{ in/hr}$. Four interior roof drains each serve one-quarter of the roof, and their conductors join one horizontal building storm drain sloped at $1/4\text{ inch per foot}$.

Step 1: Flow per Conductor

• Area per drain: $36{,}000 / 4 = 9{,}000\text{ sq ft}$
• Flow: $Q = 0.0104 \times 9{,}000 \times 4.0 = 374.4\text{ gpm}$
• The roof drain's published flow at the design head must be at least 374 gpm.

Step 2: Size Each Vertical Conductor (Table 1106.2, vertical column)

• 4-inch: 180 gpm (too small)
• 5-inch: 311 gpm (too small)
• 6-inch: 538 gpm (adequate)
• Selection: four 6-inch conductors.

Step 3: Size the Building Storm Drain (Table 1106.2, 1/4 inch per foot)

• Total flow: $Q = 0.0104 \times 36{,}000 \times 4.0 = 1{,}497.6\text{ gpm}$
• 8-inch: 1,010 gpm (too small)
• 10-inch: 1,855 gpm (adequate)
• Selection: a 10-inch building storm drain at 1/4 inch per foot. At 1/8 inch per foot, the 10-inch drain (1,311 gpm) would be too small and a 12-inch drain (2,093 gpm) would be needed.

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Plan Reviewer Verification Checklist: Sizing Conductors and Drains

• Flow Calculation: Each drain's flow uses $Q = 0.0104 \times A \times R$ with the correct rainfall rate and wall additions.
• Roof Drain Rating: The manufacturer's published flow at the design head equals or exceeds the calculated flow (1105.2 and 1106.2).
• Pipe Sizes: Vertical and horizontal storm piping does not exceed Table 1106.2 capacities for its size and slope.
• Leaders and Gutters: Leaders do not exceed Table 1106.3, and gutters do not exceed Table 1106.6.
• No Reductions: Storm piping is not reduced in size in the direction of flow (1101.5).
• Controlled-Flow Roofs: Engineered per 316.1, control devices with strainers, at least two drains (four over 10,000 sq ft), and a structural design for ponding.