How is required flow calculated from roof area?

One inch of rain per hour over one square foot is about 0.0104 gallons per minute, so the tool multiplies roof area by rainfall intensity by 0.0104 to get design flow in GPM. A 5,000 sq ft roof at 4 in/hr therefore produces roughly 208 GPM.

Where do the pipe sizes come from?

Vertical leaders and conductors are sized from IPC Table 1106.2 and horizontal storm drains from Table 1106.3 at 1/8 inch per foot slope. The tables are normalized to a 1 in/hr rainfall, so the tool scales the tributary area by the actual intensity before selecting a size.

What rainfall intensity should I use?

The IPC requires the 100-year, 1-hour rainfall rate for the building location. The bundled state values are representative; for a permit you should read the exact intensity from the IPC rainfall map or NOAA Atlas 14 for the specific site, which the override field lets you enter directly.

Do I need a secondary drainage system?

Yes. IPC Section 1107 requires a secondary (overflow) roof-drainage system sized for the same design rainfall, with its outlet positioned above the roof so a blockage of the primary system becomes visible. Size the overflow drains and scuppers separately to equal capacity.

Does vertical wall area count?

It can. Where adjacent vertical walls drain onto the roof, a portion of that wall area is added to the horizontal roof area as tributary area per IPC commentary. Add it to the roof-area input so the leader and drain are not undersized.

Roof Drain & Stormwater Pipe Sizing Calculator

Getting water off the roof

A flat or low-slope roof can collect thousands of gallons during a storm, and the drains and pipes have to move it fast enough to avoid ponding and structural overload. The IPC sets the required capacity from two inputs: the roof area draining to each point and the local design rainfall intensity. This calculator turns those into a required flow and the minimum leader and storm-drain sizes.

How it works

Required flow comes from a simple conversion: one inch of rainfall per hour falling on one square foot equals about 0.0104 GPM. Multiplying roof area by the design rainfall intensity and that constant gives the design flow in gallons per minute.

The IPC pipe-sizing tables are written for a 1 in/hr rainfall, listing how many square feet of roof each pipe size can drain. To use them at any other rate, the tool scales the tributary area by the actual intensity — a 5,000 sq ft roof at 4 in/hr behaves like 20,000 sq ft at 1 in/hr — then selects the smallest leader from Table 1106.2 and the smallest horizontal drain from Table 1106.3 (at 1/8 inch per foot slope) whose capacity covers that adjusted area.

Example and notes

A 5000 sq ft roof in Florida at 4.7 in/hr needs roughly 244 GPM and an adjusted basis near 23,500 sq ft, which selects a 5″ leader and a horizontal drain accordingly. Always add a secondary overflow system of equal capacity per IPC 1107, include vertical walls that shed onto the roof in the tributary area, and confirm the exact 100-year 1-hour rainfall for your site from NOAA Atlas 14 rather than relying on the state average.