Rainfall Draining of Roof Areas and Vertical Downspouts

Capacities of vertical downspouts when draining roof areas in rainfall. Based on rainfall intensity of 100 mm per hour (4 inches per hour).

Downspouts Size (mm)	Size (inches)	Roof Area (m²)	Roof Area (ft²)
50	2	65	700
65	2 1/2	120	1300
80	3	205	2200
100	4	430	4600
150	6	1255	13500

Notes

The table is based on rainfall intensity of 100 mm per hour (4 inches per hour)
Rainfall intensity should always be checked with local authorities
Gutters and downspouts are vulnerable to obstructions caused by branches and leaves from surrounding trees
These factors should be considered when designing roof drainage systems