A roof leader, also called a downspout or roof drain, is a pipe installed to drain water from roof gutters to a storm drain or other disposal outlet. This document provides calculations to determine the appropriate diameter of roof leaders for two different sized roofs based on dividing the roof area between multiple leaders to avoid overloading any single pipe. It determines that a 75mm diameter pipe is suitable for both roofs if divided into two leaders each.

1. In plumbing, a pipe installed to drain water
from the roof gutters or roof catchment to the
storm drain or other means of disposal. Also
called a conductor, downspout, or roof drain.
8.4 Roof Leader

2. Roof gutter

3. Downspout
Roof Drain

4. Roof leader is either Exposed or Concealed Type
Pit
Drain Pipe

5. Illustration

6. Area of Roof A
10m x 20m = 200 m 𝟐
Solution
A
B
Area of Roof B
8m x 20m = 160 m 𝟐

7. Comments
 For 20 meters length of the gutter the rains has to
travel a long way, It might be overloaded and overflow.
The terminal maybe clogged with dirty such as leaves,
etc. Thus, We need two or more terminals for roof
leader.
D = 100mm & D = 90mm
( expensive or oversized ):

8. Find the number of terminals
𝐃 = 𝟏𝟎𝟎𝐦𝐦
 Cross section: A1 = πD2
/4 = 3.14 × 1002
/4 = 7850𝑚m2
Divide into two terminals: 3925𝑚𝑚2
Chooses D = 75mm ; A2 = π ×
752
4
= 4416𝑚m2
> 3925mm2
D = 75mm OK ! 
D = 50mm, A = 1963𝑚m2
< 3925𝑚𝑚2

D = 90mm, A = 6359mm2
> 3925𝑚𝑚2
(Not economic)
𝐃 = 𝟗𝟎𝐦𝐦
 Cross section: A1 = πD2
/4 = 3.14 × 902
/4 = 6359𝑚m2
Divide into two terminals: 3179𝑚𝑚2
Chooses D = 75mm ; A2 = π × 752
/4 = 4415𝑚m2
> 3179𝑚𝑚2
D = 75mm OK ! 

9. Second Solution
Roof A
𝐴 = 200𝑚2
2 terminals = 100𝑚2
Roof B
𝐴 = 160𝑚2
2 terminals = 80𝑚2
Choose 𝐷 = 75𝑚𝑚
Thank You !!!