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Rainwater Harvesting

Rainwater Harvesting: What Your Roof Actually Yields

One inch of rain on 1,000 square feet is 623 gallons. Here is how to turn that into a tank size, a first-flush setup and water you would drink.

11 min read Intermediate
Galvanized rainwater tanks collecting from a metal roof beside a farmhouse

Most people badly underestimate their roof. A modest 1,200 square foot catchment in a 42-inch rainfall year sheds about 31,000 gallons — far more than a household garden and emergency supply needs.

The constraint is never annual rainfall. It is the longest dry gap, and that is what your tank size has to cover.

Materials

  • Metal or tile roof catchment
  • Gutters and leaf screens
  • First-flush diverter
  • Food-grade storage tanks
  • Overflow piping to a swale or drain
  • Sediment and carbon filtration (for potable use)

Tools

  • Level
  • Hole saw
  • PVC cement
  • Tape measure
  • Tank level gauge

Estimated cost: $0.60–$1.40 per gallon of storage. Difficulty: Intermediate.

1.Calculate real yield

Gallons = roof square feet × rainfall inches × 0.623 × efficiency. Use 0.85 efficiency for metal and 0.75 for asphalt shingle. Our 1,400 sq ft metal roof yields about 742 gallons per inch of rain.

Metal roof and gutter feeding a downspout into a collection tank

2.Size against the dry gap, not the annual total

Pull 20 years of local monthly rainfall and find the longest stretch below half an inch. Ours is 34 days in late summer. Daily use × dry-gap days = minimum storage. For 60 gallons a day of garden and household backup use, that is 2,040 gallons.

3.Build the first flush

The first 10 gallons off the roof carries the dust, pollen and bird droppings. A vertical standpipe that fills and self-drains between storms removes almost all of it, and costs about $40 in PVC.

4.Keep light and insects out

Opaque tanks, screened inlets and a sealed overflow. Every algae and mosquito problem we have troubleshot traced back to one of those three being open.

5.Match treatment to the use

Garden and livestock need screening and settling only. Household non-potable adds sediment filtration. Drinking water adds 5-micron sediment, carbon block and UV or a certified gravity filter — and a lab test twice a year.

Sediment and carbon filter housings plumbed on a tank outlet

Common questions

How much rainwater can I collect from my roof?
Multiply roof square footage by rainfall inches by 0.623, then by about 0.85 for a metal roof. A 1,400 sq ft metal roof yields roughly 742 gallons per inch of rain.
What size rainwater tank do I need?
Size for your longest dry gap, not annual rainfall. Multiply daily use by the number of days in your typical dry stretch — 60 gallons a day across a 34-day gap needs about 2,000 gallons of storage.
Is harvested rainwater safe to drink?
Only after treatment. A first-flush diverter, 5-micron sediment filter, carbon block and UV or a certified gravity filter make it potable, and you should lab-test twice a year.
Do I need a pump for rainwater irrigation?
Not if the tank sits above the beds. Every 2.3 feet of elevation produces 1 psi, and low-pressure drip emitters run happily on 4–10 psi of gravity head.
Ellis Hartwell

Written by

Ellis Hartwell

Twelve years building and rebuilding systems on 28 mountainside acres. Everything published here has run on this property or been documented on site with the owners.

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