Every inch of rain on a thousand square feet of roof is more than six hundred gallons of water. Most of it currently runs off into the ditch, wasted. Capturing it is one of the highest-return projects a homesteader can build.
This guide walks through calculating your roof’s catchment area, sizing tanks for your climate and usage, and laying out first-flush diverters and filtration. You’ll get the formulas, worked examples, and the sizing mistakes that leave tanks dry. Turn your roof into a well.
Every Inch Is Gallons
A 1,000 sq ft roof in a 30-inch rainfall zone catches 18,000 gallons a year — the average person’s indoor use. Rainwater harvesting turns the roof into the well. This guide calculates the catchment, sizes the tank, and builds the system.
1. The Catchment Math: Roof Yield
Formula: roof sq ft × annual inches × 0.623 = gallons/year (the 0.623 — the gallons per sq ft per inch; the constant). Example: 1,000 sq ft × 30″ × 0.623 = 18,690 gallons (the yield; the abundance). Efficiency: × 0.85 (the first-flush + evaporation + the real — the 15% loss). Monthly: ÷ 12 (the 1,300 gal/month — the planning; the dry-month reality).
2. Demand: What You Actually Use
Conservative indoor: 30–50 gal/person/day (the efficient — the low-flow; the mindful). Typical: 60–80 (the American — the average). Garden: the seasonal (the 1,000 sq ft garden — the 600 gal/week in summer; the big). The budget: the water audit (the measured — the month of metering; the honest).
3. Tank Sizing: The Storage Equation
Rule: store the dry-season gap (the longest dry stretch × daily use — the 90-day × 50 gal = 4,500 gal; the sizing). Minimum: 1,000–2,500 gal (the starter — the 2–3 week buffer; the common). Homestead: 5,000–10,000 gal (the serious — the season; the cistern). Types: poly tanks (the cheap — the $0.50/gal; the UV-stable), ferrocement (the permanent — the DIY; the 10,000+ gal), steel (the fire-proof — the wildfire country).
4. The System Components: Gutters to Tap
Gutters: the 6″ (the capacity — the leaf guards; the slope). First-flush: the diverter (the 10-gal — the roof’s first dirt; the automatic). Filtration: the staged (the 50-micron → 5-micron → UV/carbon — the potable; the levels). Pump: the pressure (the on-demand — the 40 PSI; the house pressure). Overflow: the planned (the full tank — the directed; the garden’s bonus).
5. Water Quality: Making It Potable
Roof: the metal (the best — the clean; the asphalt’s caveat). First-flush: the essential (the bird + dust; the diverted). Filtration: the 3-stage (the sediment + carbon + UV — the potable; the maintained). Testing: the annual (the lab — the $50; the verified).
6. Legality: The State Check
The spectrum: the encouraged (Texas — the tax exemptions; the incentivized) → the restricted (Colorado’s history — the prior appropriation; the now-limited-allowed). The check: the rainwater legality guide (the state-by-state; the companion). The trend: the liberalizing (the direction — the sensible; the spreading).
7. Costs: The Harvest Budget
DIY basic: $1,000–2,500 (the gutters + diverter + 1,000-gal tank; the starter). Potable system: $3,000–8,000 (the filtration + pump + 5,000-gal; the complete). Payback: the well’s alternative ($10,000–20,000 well — the rainwater’s fraction; the math).
Final Considerations
Multiply roof by rain by 0.623 (the yield), audit the demand honestly (the budget), size for the dry stretch (the storage), divert the first flush and filter in stages (the potable), check the state’s law (the legal), and price against the well you didn’t drill. The roof is already catching the rain — the system just keeps it.