Roof Drain and Leader Sizing
Storm flow in GPM from roof area and design rainfall, then the vertical leader and sloped horizontal storm-drain size from IPC capacity tables.
Formula and source
gpm = roof_area x rainfall_rate x 0.0104 (GPM per ft^2 per in/hr); leader_in = smallest pipe in the vertical-leader table with capacity >= gpm; horiz_in = smallest pipe in the slope table with capacity >= gpm.
IPC 2021 Section 1106 (Tables 1106.2 vertical conductors, 1106.3 horizontal storm drains by slope, 1106.6 roof drains) by name; the capacity tables ship as editable conservative breakpoints, not a transcribed table.
Audience
This tile is built for plumbers and the adjacent professions in the Plumbing and Gas group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tiles
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.