Method 1: Gutter and Downpipe Capacity Determined by Graph

When using the simplified capacity calculation graphs, the following assumptions have been made.

• roof pitches are 3° – 10° (for greater pitch see Roof Pitch Adjustments);
• the roof area is 50m² – 300m²;
• the minimum cross-sectional area of gutter is 4000mm²;
• flat gutter or spouting (for design purposes only);
• there are no restrictions — no spouting, gutter or downpipe angles;
• free discharge — weir into a sump or rainwater head with overflow;
• the rainfall intensity is 100mm/h (for greater rainfall see Cross-sectional Area per m2 for rainfall of 100 mm/h); and
• external vertical downpipes.

Given these assumptions, the design capacity of gutters and downpipes is given in the following graphs.