As a general guide, in non cyclonic areas, for every 1m² of roof area drained, 70mm² of downpipe is required. This calculation is based on the assumption that the maximum rainfall does not exceed 125mm per hour.

It is considered the best practice to drain to different points, using smaller downpipes rather than to one large outlet. Where possible, downpipes should be spaced at not more than 12000mm centres.

For a more accurate guide, consult the Installation Code for Metal Roofing and Wall Cladding SAA HB39-1997.

Material Specification and Sizes
Profile	BMT (mm)	Tensile (MPa)	Size (mm)	Roof Area Drained (m²)
	0.40 and 0.50	G300	50	30
			65	45
			75	65
			90	90
			100	115
			125	170
			150	250
			175*	345
			200*	450
			225*	570
*1800mm lengths only