A Square Metre of Roof Is Not a Square Metre of Water: Annual Harvest by Covering

Harvesting calculators almost universally ask for roof area and rainfall, and almost never for the roof covering — implying that a square metre of roof is a square metre of water. This paper computes annual yield for seven coverings on the same stated case, a 100 square metre roof under 800 mm of rain. A standing-seam metal roof yields 69920 litres after system losses; clay tile yields 62560 litres; a gravel-surfaced flat roof yields 44160 litres; and an extensive green roof yields 25760 litres — a ratio of 2.71 between the best and worst coverings on identical area and rainfall. Area-only calculators overstate the green roof's yield nearly threefold and understate nothing about the metal roof; the covering is not a refinement of the yield estimate but its largest single factor after rainfall itself. Yield tables by covering are provided so that tank sizing and demand matching can begin from the roof that exists rather than from an abstraction. All results are simulated, not measured.