Quick Answer
A basic rainwater collection system requires a catchment surface (your roof), gutters and downspouts to channel water, a first-flush diverter to discard the initial dirty runoff, a storage tank (food-grade polyethylene, 500-2,500 gallons), and a filtration system if the water will be used for drinking. A 1,000 sq ft metal roof collects about 600 gallons per inch of rainfall. Install a mesh screen at the tank inlet to keep debris and insects out, use an opaque tank to prevent algae growth, and elevate the tank for gravity-fed pressure. Budget $500-2,000 for a complete residential system.
Building a Rainwater Collection System
How Much Water Can You Collect?
The formula is simple: roof area (sq ft) × rainfall (inches) × 0.623 = gallons collected
A 1,000 sq ft roof in an area receiving 30 inches of rain per year collects approximately 18,700 gallons annually. That’s enough for a family of four’s non-potable needs (garden, livestock, toilet flushing) and a significant portion of drinking water needs with proper filtration.
Metal roofs are the best catchment surface — they shed water cleanly and don’t leach chemicals. Avoid collecting from asphalt shingle roofs for drinking water unless you have a robust filtration system, as they can leach petroleum compounds.
System Components
1. Gutters and Downspouts Standard residential gutters work fine. Use seamless aluminum or vinyl. Clean them regularly to prevent debris buildup. Install gutter guards to keep leaves out.
2. First-Flush Diverter This is critical. The first water off your roof carries bird droppings, dust, pollen, and accumulated debris. A first-flush diverter captures the initial 1-2 gallons per 100 sq ft of roof and diverts it away from your tank. Once the dirty water is flushed, clean water flows to storage.
3. Inlet Screen A fine mesh screen (1mm or smaller) at the tank inlet prevents insects, leaves, and debris from entering the storage tank. This dramatically reduces maintenance and keeps water cleaner.
4. Storage Tank
- Food-grade polyethylene — Most common. Available from 50 to 10,000+ gallons. Use dark colors (black, dark green) to block light and prevent algae.
- Corrugated steel with liner — Durable, large capacity, more expensive
- IBC totes (275 gallon) — Cheap and stackable but must be opaque (wrap in UV-resistant material)
- Underground cistern — Invisible, temperature-stable, but expensive to install
5. Overflow Install an overflow pipe that routes excess water away from the tank foundation. This prevents flooding during heavy rain events.
Filtration for Drinking Water
If you plan to drink collected rainwater, install a multi-stage filtration system after the tank:
- 5-micron sediment filter — Catches particles
- 1-micron carbon block filter — Removes chemicals and improves taste
- UV sterilizer — Kills bacteria and viruses
- Optional: 0.2-micron ceramic filter — Additional pathogen protection
Legal Considerations
Rainwater collection is legal in most states but some have regulations on how much you can collect or require permits for large systems. Check your state and local regulations before building. Colorado, for example, limits residential collection to two 55-gallon barrels. Most other states have no restrictions.
Maintenance Schedule
- Monthly: Check gutters and screens for debris, inspect tank for leaks
- Quarterly: Clean first-flush diverter, check overflow pipe
- Annually: Flush and sanitize tank, replace sediment and carbon filters
- Every 2 years: Replace UV bulb, inspect all plumbing connections
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