Water
How much rainwater comes off my roof?
About 0.62 gallons per square foot per inch of rain, less roughly 15 percent lost to splash and first flush.
How this works
An inch of rain drops 0.623 gallons on every square foot; gutters and first-flush lose about 15%, already subtracted. Measure the roof's flat footprint, not the sloped surface.
Questions people ask
How much rain fills a 55-gallon barrel?
About a tenth of an inch off a 1,000 sq ft roof. Most people are surprised how fast a barrel fills, and how quickly it then overflows.
Does the roof material matter?
For the garden, not much - metal, asphalt and tile all work. For drinking it matters enormously, and treated shingles are not suitable.
Can I water vegetables with roof runoff?
Water the soil rather than the leaves and it is fine for most gardens. Extension guidance is more cautious about overhead watering of ready-to-eat greens.