In order to size rain water leaders and gutters there are some simple calculations to follow. First, you must know the maximum 15 minute rainfall intensity in millimetres (mm) for your area; which can be found either through the national building code or your local environmental weather office, and the size of the area in square meters (m2) being served by the drain. Any adjoining vertical walls extending from the roof will be accounted for as well by using half of the square area. The product of these two figures will produce a hydraulic load; in litres, that can be used to find the required pipe size according to your local codes.
A flat roof measuring 3m x 4m with one vertical wall extending from the roof measuring 3m x 4m has a 30mm rainfall intensity.
Area = (3m x 4m= 12m2)+(3m x 4m= 12m2 /2 = 6m2)= 18m2
30mm x 18m2 = 540 litres
The maximum hydraulic load for this example would be 540 litres and would require 2" leader or 3" gutter (1 in 100 slope) as sized according to the NPC of Canada 2005.