- Every inch of rain on 1,000 sq ft of roof yields about 623 gallons. Run this number before buying anything.
- Size storage to bridge your longest dry spell, not your annual rainfall.
- Divert the first 1–2 gallons per 100 sq ft of roof each storm — that's where the bird droppings and dust go.
- For drinking: sediment filter → carbon filter → disinfection (boil, UV, or unscented bleach). Never skip the last stage.
- Check your state's rules first. Most encourage harvesting; a few (like Colorado) cap it.
Water is the shortest siege. You can live weeks without food, but three days without water ends the argument. Yet of the four pillars of self-sufficiency — water, food, heat, power — water is the cheapest to secure and the one most people skip, because the tap has never once failed them. The tap is a subscription. Your roof is an asset. This entry is about converting one into the other.
Everything below assumes a normal house with gutters. No land, no well, no permits for heavy equipment. If you rent, most of it still applies at barrel scale.
Part oneRun the numbers before you buy a single fitting
Rainwater harvesting fails in two ways: people build too small and give up because it never matters, or they build too big and resent the money. Both mistakes come from skipping ten minutes of arithmetic. The core formula:
One inch of rain falling on one square foot is 0.623 gallons — that constant does all the work. Use your roof's footprint (the area it shades at noon), not its sloped surface area. In practice you'll capture 75–90% of the theoretical number after splash, overflow, and first-flush losses, so multiply by 0.8 to stay honest.
| Roof footprint | 20″ rain/yr | 30″ rain/yr | 45″ rain/yr |
|---|---|---|---|
| 800 sq ft | 7,975 gal | 11,960 gal | 17,940 gal |
| 1,500 sq ft | 14,950 gal | 22,430 gal | 33,640 gal |
| 2,200 sq ft | 21,930 gal | 32,890 gal | 49,340 gal |
Now the demand side. The average American household burns 80–100 gallons per person per day, but most of that is lawns, long showers, and old toilets. A deliberate household runs on far less:
- Survival drinking and cooking: 1 gallon per person per day.
- Austere but civilized (drinking, cooking, dishes, sponge bathing): 5–8 gallons.
- Comfortable conservation (short showers, efficient fixtures): 20–35 gallons.
- Garden: roughly 0.6 gallons per sq ft per week in summer — a 400 sq ft garden drinks ~240 gallons a week, more than a family does.
Most people should start with a non-potable system for the garden and emergencies, and add drinking-water treatment later. Irrigation needs no treatment at all, and the garden is usually the biggest consumer anyway.
Part twoSize the tank to the drought, not the year
Annual rainfall is a vanity number. What matters is the gap between rains. A tank's job is to bridge your longest expected dry spell:
Look up your area's typical longest run of dry days (30 is a fair default in temperate climates; 60–90 in Mediterranean or monsoon climates where rain arrives in one season). A household drawing 25 gallons a day across a 30-day dry spell needs about 900 gallons — call it 1,000. That's three IBC totes or one poly tank.
| Vessel | Capacity | Cost | Notes |
|---|---|---|---|
| Rain barrel | 50–80 gal | $50–120 | Gateway drug. Fills in one storm. |
| IBC tote (food-grade, used) | 275–330 gal | $75–180 | Best value. Verify it never held chemicals. Must be shaded or painted — sunlight grows algae. |
| Poly tank | 500–3,000 gal | $0.60–1.00/gal | Buy opaque ("dark green/black"). UV-stabilized outlasts everything. |
| Buried cistern | 1,500–10,000 gal | $1.50–4.00/gal installed | Freeze-proof, cool, invisible. The endgame. |
Water pressure is 0.43 psi per foot of elevation. A tank raised 10 feet gives you ~4.3 psi — enough for drip irrigation and a slow tap, not enough for a shower (you want 20+ psi, i.e. ~46 feet, i.e. a pump). A $60 12-volt RV diaphragm pump on a battery solves this and doubles as your grid-down water pressure.
Part threeThe catchment chain, upstream to downstream
Build in this order. Each piece protects the one after it.
1. The roof
Metal is the gold standard — smooth, inert, sheds fast, and safe for potable collection. Asphalt shingles are fine for irrigation but leach small amounts of grit and hydrocarbons; most people still drink filtered shingle water, but if you're building new, choose metal. Avoid collecting from treated cedar shakes, or any roof with zinc anti-moss strips (zinc kills plants too).
2. Gutters and leaf screens
Standard 5-inch K-style gutters handle most roofs; slope them 1/16 inch per foot toward the downspout. Add gutter guards or at minimum a downspout leaf strainer. Every hour spent here saves ten hours of tank cleaning.
3. The first-flush diverter
The first water off a roof carries the accumulated dust, pollen, and bird droppings of the dry spell before it. A first-flush diverter is a vertical pipe with a floating ball: the dirty first water fills the pipe, the ball seals it, and clean water then flows to the tank. Size it to divert 1–2 gallons per 100 sq ft of roof — a 1,500 sq ft roof wants a 15–30 gallon diverter (a length of 4-inch PVC holds ~0.65 gal/ft, so people often use a fat barrel instead). Drill a weep hole so it drains slowly and resets itself between storms.
4. The tank inlet and overflow
Screen every opening with insect mesh — mosquitoes will find a gap the width of a pencil lead. Fit the overflow with the same mesh and route it away from your foundation. If you're in a freezing climate, either bury the tank below frost line, drain it for winter, or accept ice and use a top-draw.
Part fourMaking it drinkable: the treatment chain
Rainwater is soft and clean-tasting but not sterile: assume bird and rodent contamination on any roof. Treatment is a chain of stages, and the order matters — disinfection only works on clear water.
| Stage | What it removes | Typical hardware |
|---|---|---|
| 1 · Sediment | Grit, rust, cloudiness | 5-micron spun-poly cartridge, ~$10 |
| 2 · Carbon | Taste, odor, organic chemicals | Activated carbon block, ~$20 |
| 3 · Fine filtration | Bacteria, protozoa (Giardia, Crypto) | 0.2-micron ceramic or 0.1-micron hollow-fiber (Sawyer-type) |
| 4 · Disinfection | Viruses, everything living | Boil, UV, or bleach — see below |
Stage 4 options, all endorsed by the EPA/CDC for emergency water treatment:
- Boiling — a rolling boil for 1 minute (3 minutes above 6,500 ft elevation). Foolproof, fuel-hungry.
- Household bleach (plain, unscented, 6% sodium hypochlorite) — 8 drops per gallon of clear water, 16 drops if cloudy. Stir, wait 30 minutes; it should smell faintly of chlorine. If not, repeat once. Bleach loses potency after about a year on the shelf — date your bottle.
- UV sterilizer — inline units treat water at the tap with zero taste change, but demand clear water (stages 1–3 first) and electricity. Pair with your solar bank.
Rainwater contains essentially no salts, heavy metals, or agricultural runoff — the hard problems of well water. You do not need reverse osmosis for it. A $150 gravity filter (Berkey-style, or a DIY dual-bucket with ceramic candles) covers stages 1–3 in one unit and works with no power and no pressure. It's the single best drought/emergency purchase for a household.
Part fiveIs it legal?
In most of the world and most of the United States, yes — increasingly it's subsidized. A few Western US states with prior-appropriation water law restrict it: Colorado caps residential collection at two barrels totaling 110 gallons; Utah requires simple registration above small volumes; some municipalities have their own rules. Check "rainwater harvesting" plus your state before building big, and check whether your area offers rebates — many cities pay you to install barrels.
Part sixMaintenance: the boring 30 minutes that keeps it safe
- Quarterly: clean gutters and downspout strainers; confirm the first-flush weep hole isn't clogged.
- Quarterly: check every mesh screen for tears (mosquitoes, again).
- Every 6 months: replace sediment and carbon cartridges; clean ceramic elements with a soft brush.
- Yearly: drain the tank fully if practical, hose out sediment, inspect for algae (if you see green, light is getting in — fix that, don't just clean).
- Yearly: replace stored bleach; test-run the whole potable chain and actually drink from it.
AppendixThree builds, three budgets
- $150 — The Renter. One 60-gal barrel on the downspout with a diverter kit, mosquito mesh, and a watering-can tap. Plus a gravity filter indoors for emergencies. Waters the balcony garden; covers a week of drinking water in a pinch.
- $700 — The Homesteader's Start. Two used IBC totes (550 gal) painted dark, leaf screens, a proper 20-gal first-flush, 12V pump, and the four-stage potable chain on one kitchen tap. Bridges a month.
- $4,000 — The Standing Reserve. A 2,500-gal opaque poly tank on a gravel pad, full gutter guards, oversized first-flush, pump and pressure tank, UV at the point of entry. Garden, household, and a season of security.
Start with the barrel this month. The point isn't the gallons — it's that the next time it rains, you'll watch the water level rise and feel the shift: weather stopped being scenery and became income.