Counted, Not Placed for Symmetry: How Many Downpipes a Roof Actually Needs

Houses are routinely built with downpipes wherever the elevation looks balanced, and gutters overflow in extreme rain is then blamed on the gutter. This paper instead counts downpipes from design flow. Using the rational method with a runoff coefficient of one and stated vertical-stack capacities, a 200 square metre roof under a 150 mm/h design storm produces 8.33 L/s and requires 3 downpipes of 82 mm or 2 of 110 mm, whereas the single pair that symmetry usually provides is hydraulically sufficient only up to the 100 square metre case at the same intensity (4.17 L/s against a combined capacity of 7.6 L/s for two 82 mm stacks). At the severe end, a 400 square metre roof at 250 mm/h yields 27.78 L/s and needs 4 downpipes of 110 mm. The paper tabulates required counts across roof areas and intensities and states the counting rule, including the separate redundancy argument for never relying on one outlet. All results are simulated, not measured.