Rainfall Draining of Roof Areas and Vertical Downspouts

Capacity of vertical downspouts when draining roof areas in rainfall

Capacities of clean vertical downspouts without obstructions when draining roof areas are indicated below:

Downspouts SizeRoof Area
mminchesm2ft2
50 2 65 700
65 2 1/2 120 1300
80 3 205 2200
100 4 430 4600
150 6 1255 13500

The table is based on rainfall intensity 100 mm per hour (4 inches per hour). Rainfall intensity should always be checked with the local authorities.

Gutters and downspouts are vulnerable for obstructions caused branches and leafs from surrounding trees and similar. This should be considered when connecting gutters and downspouts.

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