9.1 Storm Drainage Sizing

9.1 Storm Drainage Sizing

Design Rainfall and Sizing Fundamentals

All components of a commercial storm drainage system—including roof drains, vertical conductors, leaders, horizontal branches, and building storm sewers—must be sized to handle the runoff from the design storm. According to IPC Section 1106.1, the design rainfall rate must be based on the 100-year, 1-hour rainfall rate established for the local jurisdiction. These rates are obtained from IPC Figure 1106.1 or from local climatological data.

A common exam mistake is sizing storm systems using average annual rainfall. The code specifically demands the peak 100-year, 1-hour rate to prevent structural failure due to water weight (ponding) during extreme weather events.

Projected Roof Area Sizing

The sizing of any roof drain or conductor is based on the projected horizontal roof area in square feet. For flat roofs, this is simply the length multiplied by the width. However, if a roof is sloped, the horizontal projection (not the actual sloped surface area) is used.

Handling Vertical Wall Runoff

Wind-driven rain hitting vertical walls that face a roof surface will run down and add to the roof's storm water load. IPC Section 1106.4 requires the design roof area to be adjusted to include these wall areas using the following rules:

• One Wall: Add 50% of the wall area.
• Two Walls: Add 50% of the area of the largest wall, plus 25% of the area of the second wall.
• Three or More Walls: Add 50% of the area of the largest wall, plus 25% of the area of the next largest wall, and 0% for the remaining walls.

Worked Example: Wall Runoff Calculation

Scenario: A flat roof measures 50 ft by 100 ft. A vertical penthouse wall facing the roof measures 20 ft wide by 10 ft high.

1. Primary roof area = 50 ft × 100 ft = 5,000 sq ft.
2. Penthouse wall area = 20 ft × 10 ft = 200 sq ft.
3. Added area (single wall) = 50% of 200 sq ft = 100 sq ft.
4. Total design roof area = 5,000 sq ft + 100 sq ft = 5,100 sq ft.

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Sizing Vertical Conductors and Leaders (IPC Section 1106.2)

Vertical pipes that carry storm water down through the inside of a building are called conductors. Pipes that run on the exterior of a building are called leaders. Both are sized using IPC Table 1106.2.

Table 1106.2 provides the maximum allowable projected roof area for various pipe sizes based on a baseline rainfall rate of 1 inch per hour. To size a leader or conductor for a different rainfall rate, divide the area values in the table by the local rainfall rate.

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Sizing Horizontal Storm Drains and Building Sewers (IPC Section 1106.3)

Horizontal storm drainage piping and building storm sewers are sized based on IPC Table 1106.3. Sizing depends on three variables:

1. Pipe Diameter
2. Slope (Pitch): 1/16, 1/8, 1/4, or 1/2 inch per foot.
3. Local Rainfall Rate: Sizing columns correspond to different rainfall rates in inches per hour.

As the slope increases, the velocity of the water increases, allowing a smaller pipe to carry more area. For example, under a 4-inch-per-hour rainfall rate:

• A 6-inch horizontal drain at 1/8 inch per foot slope can handle 3,280 sq ft.
• A 6-inch horizontal drain at 1/4 inch per foot slope can handle 4,640 sq ft.
• A 6-inch horizontal drain at 1/2 inch per foot slope can handle 6,560 sq ft.

Always verify the slope and rainfall rate columns when answering sizing questions on the P2 exam.

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Secondary (Emergency Overflow) Drainage (IPC Section 1107)

Where the roof perimeter construction (such as parapet walls) extends above the roof surface, water will be entrapped if the primary roof drains become blocked. Accumulating water creates a severe structural hazard due to weight. Therefore, IPC Section 1107.1 mandates a secondary (emergency overflow) drainage system.

Key Rules for Secondary Systems:

1. Complete Separation: The secondary system must have its own independent piping network, separate from the primary system. The two systems must never interconnect inside the building.
2. Sizing: The secondary system must be sized identically to the primary system, using the same 100-year, 1-hour design rainfall rate.
3. Discharge Location: The secondary system must discharge to an approved, visible location (e.g., discharging through a side wall above grade via a nozzle or scupper) to alert occupants or maintenance personnel that the primary system is blocked and requires service.
4. Inlet Elevation: Per IPC Section 1107.2, secondary roof drains must have their inlet flow line set at least 2 inches and not more than 4 inches above the low point of the roof surface, unless otherwise specified by the roof design.

Overflow Scuppers (IPC Section 1108)

As an alternative to secondary roof drains, overflow scuppers can be installed in parapet walls.

• Inlet Elevation: The bottom of the scupper opening must be set at an elevation of 2 to 4 inches above the roof low point.
• Minimum Opening Height: Not less than 4 inches (102 mm).
• Minimum Opening Width: Equal to or greater than the circumference of a roof drain sized for the same roof area.
• Structural Safety: Scuppers must be located and sized such that the maximum designed ponding head of water on the roof is not exceeded.