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.

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.

IPC 2021 free read-only at codes.iccsafe.org; the 0.0104 GPM-per-(ft^2 x in/hr) constant is public. The bundled capacity tables are editable approximations to tune to the published edition.

Estimate. AHJ and licensed professional govern.

Field names used by the API: roof_area, rainfall_rate, drain_slope, gpm, leader_in, horiz_in

• Storm-flow constant 0.0104 GPM per ft^2 per in/hr of design rainfallIPC 2021 Section 1106 basis
• Capacity tables editable [size, GPM] breakpoints for the vertical leader and the horizontal storm drain at each slope; tune to IPC Tables 1106.2 / 1106.3 / 1106.6IPC 2021 Section 1106