Maker Label Studio

Roof Drain & Scupper Sizing Calculator

Calculate IPC-style storm drainage flow, per-drain load, primary drain and leader size, and secondary overflow capacity for a roof area.

Inputs

Cited standard: IPC Ch. 11 storm drainage

Square feet of roof draining to this primary system.

Inches per hour, typically the adopted 100-year, 1-hour value.

Whole-number count sharing the roof area.

Used for the horizontal storm leader and combined main lookup.

Results

Flow formula: Q = area x rainfall / 96.23

Input Summary

Roof area -
Design rainfall -
Primary drain count -
Horizontal slope -

Drainage Sizing

Total design flow -
Flow per primary drain -
Area per primary drain -
Required roof drain / vertical leader -
Required horizontal leader at selected slope -
Combined horizontal main if drains join -
Minimum drains at selected vertical size -
Secondary overflow design flow -

Notes

IPC-style flow is calculated as Q (gpm) = roof area (sf) x rainfall (in/hr) / 96.23.

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IPC Lookup Reference

Construction, Trades & MEP
Flow-versus-size capacities used by this calculator
Pipe size Vertical drain/leader gpm Horizontal 1/8 in./ft. gpm Horizontal 1/4 in./ft. gpm Horizontal 1/2 in./ft. gpm

Self-Tests

Golden checks for formula and lookup
Self-tests not run.

About the Roof Drain & Scupper Sizing Calculator

Plumbing engineers, architects, and contractors use a roof drain sizing calculation to estimate stormwater flow from roof area and rainfall intensity, then select drains, leaders, gutters, and overflow paths. The calculator applies the common area-times-rainfall conversion to support IPC-based design checks before final tables and local rainfall data are confirmed.

How it works

  1. Enter roof area draining to the roof drain, leader, gutter, or scupper.
  2. Add the rainfall intensity required by the adopted code or local authority.
  3. Review the calculated flow in gallons per minute.
  4. Compare the flow with IPC sizing tables for horizontal and vertical conductors.
  5. Check overflow drainage separately from the primary drainage system.

Frequently asked questions

How is roof drainage flow calculated from area and rainfall?

A common IPC worksheet conversion is gallons per minute equals roof area times rainfall intensity divided by 96.23, when area is in square feet and rainfall is in inches per hour.

Do roof drains and overflow scuppers use the same design check?

They are related but separate. Primary drains handle normal design rainfall, while overflow drains or scuppers protect the roof if primary drainage is blocked or overwhelmed.

Where does rainfall intensity come from for roof drain sizing?

Use the rainfall rate required by the adopted plumbing code, local building department, or project design criteria. Do not substitute a generic city average for code design rainfall.

Can the calculator choose the final pipe size by itself?

No. It estimates flow. Final sizing must account for IPC tables, pipe slope, conductor orientation, drain rating, roof geometry, and local code amendments.

References