Every Inch of Rain Counts

One inch of rain falling on 1,000 sq ft of roof is 623 gallons of water — enough to run a garden for weeks. The annual harvest equation is simple: roof area × rainfall depth × 7.48 gallons per cubic foot × runoff efficiency. A 1,500 sq ft roof in 35 inches of rain with an 85% capture rate delivers about 27,800 gallons a year.

The Roof Is a Reservoir

Roof material sets the ceiling: standing-seam metal captures 90–95% of rainfall, asphalt shingles 75–90%, and tile somewhere between. First-flush diverters sacrifice the first 10–20 gallons of each storm — carrying dust, pollen, and bird droppings — which the capture-efficiency figure accounts for. The rest of the loss is evaporation and overshoot during intense downpours.

Sizing Storage to the Drought

The natural instinct is to size tanks to the annual harvest; the practical instinct is to size them to the dry spell. A 1,000-gallon tank at 30 gallons/day of garden use provides 33 days of supply — enough to cover most summer gaps between storms. Overflow during wet months is a feature, not waste: it protects the system and the foundation.