Rainwater Harvest Calculator

Rain falls vertically, so only the horizontal shadow of the roof intercepts it. The sloped surface of a steep roof covers more area, but each square foot of slope intercepts the same vertical rain column as one square foot of flat ground directly below it. Using the sloped surface area overstates catchment - the footprint is always the correct number.

Yes, with care. Apply it at the root zone rather than directly on edible leaves or fruit surfaces. Roof runoff can carry bird droppings, leaf litter decomposition products, and trace metals depending on roof type. A first-flush diverter removes the most contaminated portion. For root vegetables, no special precaution beyond the root-zone application is typically needed.

A common rule of thumb is 1 gallon of first-flush volume per 100 sq ft of roof served by that downspout. A 1,200 sq ft roof with 3 downspouts serving 400 sq ft each would need a 4-gallon diverter per downspout. Larger diverters are better in areas with heavy dust, bird activity, or tree debris over the roof.

In most US states, yes - residential rainwater collection for outdoor irrigation is allowed without a permit for tanks under a few thousand gallons. Texas and Colorado have incentive programs. A handful of western states have historically limited collection due to prior appropriation water law, though most have updated their rules. Check with your state water board or county extension office to confirm current rules for your location.