The Austin storm
There’s a homeowner in central Austin who installed five 55-gallon rain barrels along her gutters in spring 2023. Total storage: 275 gallons. They sat half-empty most of April. Then in late May, a single storm dropped 2.3 inches of rain over four hours. Her roof is about 1,400 square feet. The math (covered below) says she could have collected about 2,000 gallons. She collected 275 (her storage limit) and watched the rest overflow into her yard. For the next three weeks, she didn’t run a hose. Her garden grew on rainwater she’d captured from a single storm. Her water bill that month dropped from $87 to $34.
Rainwater harvesting is the most boring miracle in residential sustainability: it costs almost nothing, requires no permits in most places (for outdoor use), and produces tangible savings within months. Here’s how to set up a system from a single barrel up to a serious multi-thousand-gallon cistern.
The math
The collection formula is straightforward:
Gallons collected = roof area (sq ft) × rainfall (inches) × 0.623
Examples:
- 1,000 sq ft roof, 1 inch rain: 623 gallons
- 1,500 sq ft roof, 2 inches rain: 1,869 gallons
- 2,000 sq ft roof, 0.5 inches rain: 623 gallons
Most US regions get 30–50 inches of annual rainfall. A 1,500 sq ft roof in a 35-inch rainfall area would yield roughly 32,700 gallons per year if you captured all of it. Storage is usually the limiting factor, not collection.
Sizing your system
Three approaches based on goals:
Casual outdoor use (lawn / garden watering): 50–200 gallons. One or two rain barrels at downspouts. Cost: $100–300. Handles watering for a week or two between storms.
Serious garden / livestock watering: 500–2,000 gallons. Cisterns or interconnected barrel systems. Cost: $500–2,500. Covers most of a typical residential outdoor water need.
Whole-house non-potable use (toilets, laundry): 2,500–10,000 gallons + first-flush diverters, sediment filters, pump. Cost: $5,000–15,000 installed. Requires building permits in most jurisdictions.
Whole-house potable use: 5,000+ gallons + UV sterilization + multi-stage filtration. Cost: $10,000–30,000+. Requires permits, water testing, and ongoing maintenance.
Most homeowners start at level 1 (rain barrels) and upgrade if needed.
The basic barrel setup
The simplest system you can install in an afternoon:
Components:
- One 50–55 gallon barrel (food-grade plastic; never an old chemical drum)
- Downspout diverter (Oatey, Earthminded, Fiskars all sell them for ~$30)
- Spigot near the bottom of barrel
- Overflow connection at the top of barrel
- Mesh screen on inlet (keeps debris and mosquitoes out)
Process:
- Cut a section of your downspout near the barrel height.
- Install the diverter so rain flows into the barrel until full, then resumes flowing down the downspout.
- Connect a hose or watering can to the spigot.
Total cost: $80–180 per barrel installed DIY. The barrel itself is $40–80; the rest is fittings and diverter.
The multi-barrel upgrade
Once you have one barrel, adding more is straightforward:
- Series connection: Connect barrels at the top via overflow pipes. When the first barrel fills, water flows to the next, and so on. All drain from a single spigot at the lowest point.
- Parallel connection: Connect at the bottom via Y-fittings or manifold. Water levels stay equal across all barrels. More expensive but more flexible.
Either way, raising barrels on cinder blocks or a wooden platform creates head pressure for hose use without needing a pump.
Storage upgrade: cisterns
Above 500 gallons, cisterns (single large tanks) are usually more efficient than chains of barrels:
- Polyethylene tanks (1,000–5,000 gallons): $400–2,500 per tank. Common brands: Norwesco, Bushman.
- Concrete cisterns (custom): $3,000–15,000. Most durable but expensive.
- Modular interconnected systems (Rainwater HOG, Aquascape): $1,500–5,000 for 1,000–2,000 gallons. Stackable, fit narrow spaces.
Cisterns can be above-ground (cheaper, easier to install, prone to algae if not opaque) or below-ground (more expensive, requires excavation, cleaner water).
Filtration for indoor use
If you’re scaling up to toilet or laundry use:
- First-flush diverter: Discards the first few gallons of each rain event (which carry roof dust and debris). Costs ~$60–150.
- Sediment filter: 5–20 micron, removes particles. $50–200.
- Carbon filter: Removes taste, odor, some chemicals. $80–300.
- UV sterilization (only required for potable use): $300–1,500.
Legal considerations
Rainwater harvesting is legal in nearly every US state for outdoor use. Some states regulate or restrict large-scale collection:
- Colorado: Allows up to 110 gallons in two rain barrels per residence (recently changed from previously restrictive rules).
- Nevada and Utah: Allow but with size limits in some areas.
- Some western states: Quirky water-rights laws can affect large collection.
Check your state’s specific rules. Most residential outdoor use (under 1,000 gallons) is unregulated. Indoor use typically requires permits.
Maintenance
- Clean gutters twice yearly — debris affects water quality and clogs diverters.
- Inspect barrels for mosquitoes — mesh screens prevent breeding. If a screen tears, you’ve got a mosquito factory in days.
- Drain and clean barrels annually — algae and sediment accumulate.
- Insulate or drain in winter if your climate freezes — ice will crack barrels.
FAQs
How long can stored rainwater sit safely?
Outdoor use: months (algae develops but doesn’t harm plants). Drinking: needs UV sterilization within days of storage in warm climates. Cool, dark, sealed cisterns extend storage time significantly.
Can I water vegetables with rainwater?
Yes, with one caveat: if your roof has lead-painted trim or asbestos shingles, the runoff might carry contaminants. Modern asphalt shingles are generally safe; older roofs warrant testing.
What’s the payback period?
A $200 barrel setup pays back in 2–4 years on water savings in typical climates. Larger systems take 5–10 years.
Will rain barrels overflow during heavy storms?
Yes — that’s normal. The overflow goes back into your downspout. The system collects what it can hold and lets the rest pass through.
Do I need to elevate barrels?
Yes, for hose use. The hose pressure depends on the height of water above the spigot. Cinder blocks (12–16 inches) provide enough pressure for normal garden watering.
Can I use rainwater for pets and livestock?
Yes for most situations. The roof-runoff first-flush should be diverted away. Most pets and farm animals tolerate clean rainwater without issues; some sensitive species (rabbits, certain birds) might warrant filtered water.
The landing
The Austin homeowner’s $750 setup (five barrels and fittings) collected enough rainwater in one storm to handle three weeks of garden watering. Her water bill dropped 60% during the growing season. She didn’t need permits, didn’t need an engineer, didn’t need a contractor. Rain falls on roofs everywhere. Most of it currently flows into storm sewers and away. Capturing even a fraction of what your roof receives is the most boring high-leverage residential project available. Start with one barrel. See what you collect. Add more if it works. The rain is already paid for.