1. Use the following formula to determine G.P.M.
G.P.M. = .0104 x R x A
G.P.M. = Gallons per minute
R = Rainfall intensity - inches/hour
A = Roof area - square feet
.0104 = Conversion factor - G.P.M./sq. ft. for one (1) inch/hr. rainfall

2. Example:
A. 4" rainfall in./hr.
B. 135,000 sq. ft. roof area
C. G.P.M. = .0104 x 4" x 135,000 sq. ft. = 5616 G.P.M. (Use 5620)

3. Refer to table 2:
A 4" leader will handle 192 G.P.M. 5616 ÷ 192 = 29.25 or (30) 4" vertical leaders required.
A 6" leader will handle 566 G.P.M. 5616 ÷ 566 = 9.9 or (10) 6" leaders required.

ALLOWABLE FLOW FOR VERTICAL LEADERS AND HORIZONTAL STORM DRAINS

 Related Links:
Selection of Roof Drain Body
Selecting Proper Leader Sizes and Number Required
Vertical Leader Requirements