Method sheet
Roof Drainage Flow Screening
Design flow from contributing area and an entered rainfall intensity; per-outlet share; comparison with an entered manufacturer capacity.
Method
roof-drainage 1.0
Engine
1.1.0
Output status
Preliminary - review required
Review
Awaiting review
Formulas
- Q (gal/min) = 0.0103896 x A (ft2) x I (in/h) [= (1/12) x (1728/231) / 60]
Conventions
- Rainfall intensity, duration, return period and source are entered by the user; nothing is looked up.
Limits - what it will not do
- No gutter, scupper, drain or pipe sizing; no code compliance; secondary drainage and ponding are not assessed.
Who must review it
- Plumbing / mechanical engineer or roof drainage specialist
Worked example
Inputs 10,000 ft2 at 4 in/h, 4 outlets
Expected 415.58 gal/min (26.22 L/s); 103.90 gal/min per outlet
Checked by an automated test on every change.
Sources
- U.S. liquid gallon = 231 in3 (statutory definition) - U.S. customary units, read 2026-10-16
1 ft3 = 1728/231 gal = 7.48052 gal; 1 in/h on 1 ft2 = 0.0103896 gal/min.
- NOAA Atlas 14 Precipitation Frequency Data Server - NOAA National Weather Service, read 2026-10-16
Source for site rainfall intensity by duration and return period. Entered manually; never fetched by this tool.
- 2021 International Plumbing Code, Chapter 11 Storm Drainage - International Code Council, read 2026-10-16
Illustrates edition-specific roof drainage requirements (sizing, secondary drainage). Adopted provisions vary by jurisdiction.