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Roof Drain Sizing Calculator Engineering & Industrial
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Roof Drain Sizing Calculator

Determine primary roof drain leader pipe diameter, stormwater runoff GPM, and secondary emergency overflow scupper width according to International Plumbing Code (IPC).

in/hr
Total Stormwater Runoff Flow
468 GPM
117 GPM per primary drain
Min Vertical Leader Size
4" Conductor
IPC cap: 192 GPM
Overflow Scupper Width
24 Inches
12" each (2 scuppers)
Horizontal Leader Pipe (¼"/ft): 4" Pipe (157 GPM capacity)
Max Water Ponding Load: 10.4 psf (2" depth)
Total Ponded Water Weight: 78.0 Tons across roof
Drain and overflow systems comply with IPC 1106 and IBC 1503.4 secondary drainage mandates.

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Roof Stormwater Drainage Engineering & The International Plumbing Code (IPC)

Commercial roof drainage design requires two entirely independent drainage networks: a primary roof drain system sized for the local 100-year 1-hour rainfall storm, and a secondary emergency overflow system (independent overflow drains or through-wall perimeter scuppers) positioned 2 inches above low points to prevent catastrophic roof structural collapse if primary drains become clogged with debris.

Key Sizing Governing Equations

  1. Stormwater Flow Rate (GPM):
    Q (GPM) = 0.0104 × Catchment Area (sq ft) × Rainfall Intensity (in/hr)
  2. Francis Broad-Crested Weir Equation for Scuppers:
    Q_scupper (GPM) = 3.33 × Width (ft) × [ Head (ft) ]^1.5 × 448.83
  3. Water Ponding Structural Weight:
    Each inch of standing water creates 5.20 pounds per square foot (psf) of structural dead load.

Frequently Asked Questions

Why does building code require secondary emergency roof overflow drains?

The International Building Code (IBC 1503.4) requires an entirely independent secondary drainage system so that if primary drains plug during a severe downpour, water cannot pond to a depth that causes structural roof collapse.

What is the formula for calculating stormwater runoff in GPM?

The IPC stormwater equation is Q = 0.0104 * A * I, where A is catchment roof area in square feet and I is local 100-year 1-hour rainfall intensity in inches per hour.

How heavy is standing water on a flat roof?

Water weighs 62.4 pounds per cubic foot, which equates to exactly 5.20 pounds per square foot (psf) for every 1 inch of water depth.