One Millimetre for Every Roof? First-Flush Diversion Depth Set by Wash-Off Behaviour

Rainwater harvesting guides commonly instruct householders to divert the first millimetre of rain from the roof, as if one depth suited every roof. This paper tests that fixed rule against the standard exponential wash-off model for three stated roof conditions. For a typical suburban roof with wash-off coefficient 0.22 per mm, one millimetre diverts only 19.7 percent of the accumulated surface load; capturing 90 percent requires 10.47 mm and 95 percent requires 13.62 mm. For a roof under trees, where material continues to wash off slowly, 90 percent capture requires 19.19 mm. Only the dusty rural case, at 6.58 mm for 90 percent, is close to the one-millimetre rule. The cost of correcting the depth is modest: diverting 2 mm at every event on a 120 square metre roof in the stated climate sacrifices 14.6 percent of annual rainfall. A single fixed diversion depth is disproved as a general rule; depth should be set from the roof's wash-off behaviour. All results are simulated, not measured.