Enter roof area and location to get recommended gutter and downpipe sizes based on design rainfall intensity.
Flat=1.0, 1:3=1.05, 1:2=1.12, 1:1.5=1.20
For an 80 m² roof in Auckland (130 mm/hr design rainfall intensity), the design flow is approximately 2.9 L/s. A 150 mm quad gutter rated at 3.0 L/s is the minimum suitable size. Enter your exact area in the calculator for a precise recommendation.
NZS 3500.3 recommends a maximum gutter length of 12 m per downpipe outlet for residential gutters. Gutters exceeding this should have additional outlets. This calculator recommends the required number of downpipes based on the design flow.
NZS 3500.3 specifies a minimum fall of 1:500 (2 mm per metre) for gutters to ensure drainage. The gutter capacities in this calculator are based on this standard fall. A steeper fall improves capacity but may look visually uneven on long runs.