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 A x B (mm)	Roof Area Drained (m²)
	0.40 and 0.50	G300	50 x 50*	35
			75 x 50	55
			75 x 75	80
			95 x 45	60
			100 x 50	70
			100 x 75	105
			100 x 100	140
			125 x 100	180
			150 x 100	210
			150 x 150	320
			200 x 200	570
*1800mm lengths only