IPC Roof Drain Calculator
For plumbing engineers sizing roof drainage under the IPC, where the whole drain-and-leader selection hinges on one number — IPC 1106 states that a rainfall of 1 inch per hour on 96 square feet of roof produces about 1 gallon per minute, so this page turns area and rainfall rate into that GPM figure directly.
Calculate Design Flow Rate (GPM)
Note: Select the drain and leader size from IPC Table 1106.2 for this flow rate; secondary drains are sized for the same rate.
How To Use This Calculator
- Enter the roof's horizontal projected area in square feet — the footprint the rain actually falls on, not the sloped surface area.
- Enter the design rainfall rate for the project in inches per hour.
- Read the design flow in GPM and the formula line beneath it to confirm the arithmetic: (area × rain rate) ÷ 96.
- Check the secondary readouts for gallons per hour and the square footage one gallon per minute will drain at that rainfall rate.
- Take the GPM figure to IPC Table 1106.2 to select the drain and leader size, sizing secondary drains for the same rate.
FAQ
Q: How do I calculate roof drain flow in GPM under IPC 1106?
A: Multiply the roof area in square feet by the rainfall rate in inches per hour, then divide by 96 — the IPC 1106 relationship where 1 inch per hour on 96 square feet yields about 1 gallon per minute.
Q: How many square feet of roof does 1 GPM drain at a 4 in/hr rainfall rate?
A: 24 square feet — 96 divided by 4, since the 96 sq ft per 1 GPM figure holds at 1 inch per hour and scales inversely with the rainfall rate.
Q: How do I pick the roof drain and leader size once I have the GPM?
A: Select the drain and leader size from IPC Table 1106.2 for that flow rate; the calculator gives you the design flow, and the table converts it into pipe and drain sizes.
Q: How are secondary roof drains sized compared with the primary?
A: Secondary drains are sized for the same flow rate as the primary drain, so you size the secondary from the same GPM figure this calculator produces.