Roof Drain Hydraulic Drain Loads

Roof drain hydraulic load calculations

Sizing rain leaders and gutters is a pretty simple process (assuming you know the proper steps to take),
It is also one of the most embarrassing things to get wrong if you don’t know what your doing. Let’s take a brief look at this subject for everyone’s sake, and see what we gain from it First you must determine the maximum fifteen minute rain load for your area in Millimeters (mm). This information can be found either within national code registries, or your local environmental office.

Next, you need to determine the area being served by the drain in square meters (allowing any walls attached to the roof to count as half the square). The product of these two numbers will be a hydraulic load (in litres) that you can use to determine required pipe size.



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.

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