Roof Drain and Leader Sizing
Storm flow in GPM from roof area and design rainfall.
Example
You enter
- Roof area served (ft², horizontally projected) 5000
- Design rainfall, 100-yr / 1-hr (in/hr) 4
- Horizontal storm-drain slope 1/4
- IPC edition 2021
You get
- Storm flow 208 GPM
- Vertical leader 5
- Horizontal storm drain 5
Checked against published worked examples from: IPC 2021 Section 1106 (Tables 1106.2 / 1106.3 / 1106.6).
Details, formula, and sources
ScopeStorm flow in GPM from roof area and design rainfall, then the vertical leader and sloped horizontal storm-drain size from IPC capacity tables. IPC 2027 rates the horizontal drain by pipe material (PVC, new or aged cast iron); aged cast iron carries about two thirds of the 2021 single-table figure, so the same roof can need the next pipe size. The 2021 table is an edition option.
Formula or tablegpm = 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 horizontal table for the pipe material and slope with capacity >= gpm (IPC 2027 Tables 1106.2.3.1(1)-(3)), or in the IPC 2021 single Table 1106.2.
Source and editionIPC 2027 Section 1106 (Table 1106.3 vertical leaders, with Table 1106.2.4 vertical storm drain values for 10-15 in; Tables 1106.2.3.1(1) PVC, (2) cast iron aged and (3) cast iron new for horizontal storm drains by slope) by name, with IPC 2021 Table 1106.2 as an edition option; the capacity tables carry the published values as editable breakpoints.
Free accessIPC 2027 and 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 carry the published values as editable breakpoints.
What governsEstimate. AHJ and licensed professional govern.
API field namesroof_area, rainfall_rate, drain_slope, code_edition, gpm, leader_in, horiz_in
- Storm-flow constant 0.0104 GPM per ft^2 per in/hr of design rainfallIPC Section 1106 basis (2021 and 2027)
- Capacity tables editable [size, GPM] breakpoints for the vertical leader (Table 1106.3) and the horizontal storm drain at each slope (Tables 1106.2.3.1(1)-(3) by material, or the 2021 Table 1106.2), at the published valuesIPC 2027 Section 1106