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.

What is the Roof Drain & Stormwater Pipe Sizing Calculator?

Applies IPC 1106 and ASCE 7 design rainfall (bundled 100-year 1-hour intensity by US state) to compute required drainage flow in GPM, then sizes the vertical leader and horizontal storm drain from IPC Tables 1106.2 and 1106.3. It runs free in your browser on Gera Tools, with nothing uploaded.

Roof Drain & Stormwater Pipe Sizing Calculator

Name: Roof Drain & Stormwater Pipe Sizing Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

Worked example

A commercial building in Florida has a 5,000 sq ft roof draining to two internal roof drains. Design rainfall intensity for the site is 4.7 in/hr (from NOAA Atlas 14 for the specific location).

Flow per drain = 2,500 sq ft × 4.7 in/hr × 0.0104 = 122 GPM
Adjusted area  = 2,500 × 4.7 = 11,750 sq ft equivalent at 1 in/hr

Using IPC Table 1106.2, a 4-inch vertical leader handles up to about 12,800 sq ft at 1 in/hr, so a 4-inch leader is the minimum for each drain. The horizontal collector piping is sized from Table 1106.3 at 1/8 inch per foot slope for the combined drainage from both leaders.

Always check the secondary overflow drainage separately — IPC Section 1107 requires a secondary system of equal capacity at each drain location.

Understanding the IPC scaling method

The table-scaling approach reflects an important principle: a larger roof area at low rainfall intensity produces the same flow as a smaller roof at high intensity. The IPC normalizes everything to the 1 in/hr table by multiplying the actual roof area by the actual intensity, then looks up that product in the table.

This means that in a high-rainfall climate like Florida or Hawaii (often 6–8 in/hr or more for the 100-year event), even a modest 2,000 sq ft roof might require a 4-inch or 5-inch leader, while the same roof in an arid climate with 2 in/hr design intensity could be served by a 3-inch leader.

Choosing the right design rainfall intensity

The IPC requires the 100-year, 1-hour rainfall intensity for your specific site. The tool includes representative state values as a starting point, but for permit drawings you should:

Look up the intensity from NOAA Atlas 14 (hdsc.nws.noaa.gov) for the precise latitude and longitude.
Alternatively, use the IPC rainfall map in Chapter 11, which provides regional values at a coarser resolution.
Enter the exact intensity in the override field rather than relying on the state default, since intensities vary significantly across a single state — Florida’s coast can differ from its interior by several inches per hour.

Secondary overflow systems

IPC Section 1107 is mandatory: every roof drainage system must have a secondary (overflow) drainage system capable of handling the full design rainfall load independently. The secondary outlets are positioned at a higher elevation than the primary drains so that if the primary system blocks, water pools above the overflow outlet inlet and drains visibly through the secondary path — providing a visual signal of primary blockage before structural overload occurs.

Size the overflow drains and scuppers to the same flow and the same pipe sizes as the primary system. Do not share a common drain body between primary and secondary; they must be separate to function as an independent backup.