Quick Answer
A pole barn roof is an excellent rainwater collection surface. A 40x60 pole barn (2,400 sq ft) can harvest approximately 1,500 gallons of water from just 1 inch of rainfall. A complete system includes: roof collection area, gutters and downspouts, leaf/debris filtration, first-flush diverter (diverts first runoff), storage tank(s), additional filtration, and pump/distribution. For most applications, size storage for 1-2 months of water supply. Rainwater is ideal for irrigation, livestock, equipment washing, and with proper treatment, can be potable.
Why Collect Rainwater?
- Water Independence: Reduce reliance on wells or municipal water
- Cost Savings: Lower water bills, reduce well pump wear
- Better for Plants: No chlorine, naturally soft water
- Livestock Preferred: Animals often prefer rainwater
- Erosion Control: Reduces runoff from roof
- Emergency Supply: Backup water source
System Sizing
Calculate Collection Potential
Formula: Roof Area (sq ft) × Rainfall (inches) × 0.623 = Gallons Collected
- 30x40 Barn: 750 gallons per inch of rain
- 40x60 Barn: 1,500 gallons per inch of rain
- 50x80 Barn: 2,500 gallons per inch of rain
Determine Your Needs
- Irrigation: Calculate garden/lawn water needs
- Livestock: 10-20 gallons per day per horse/cow
- Equipment Washing: Estimate frequency and volume
- Household: ~50-100 gallons per person per day (if treating to potable)
Storage Sizing
- Small System: 1,000-2,500 gallons (supplemental irrigation)
- Medium System: 3,000-5,000 gallons (irrigation, livestock)
- Large System: 10,000+ gallons (whole-site supply)
Roof Collection
Metal Roofing (Ideal)
- Smooth Surface: Efficient water flow
- Inert: Doesn't leach chemicals
- Durable: Long-lasting collection surface
- Low Debris: Less contamination than shingles
Other Roofing
- Asphalt Shingles: Collectible but water may contain chemicals
- Treated Wood: Not recommended for potable use
- Coatings: Ensure roof coatings are water-safe
Gutter and Downspout System
Gutter Sizing
- 5-inch Gutters: Standard residential, adequate for most
- 6-inch Gutters: Larger roofs, heavy rainfall areas
- Commercial Box Gutters: Maximum capacity
Installation Tips
- Slope: 1/16 inch per foot toward downspouts
- Downspout Placement: Every 20-40 feet
- Leaf Guards: Screen or foam inserts reduce debris
- Material: Aluminum or PVC (avoid copper with metal roofing)
Filtration System
Pre-Storage Filtration
- Leaf Strainers: At gutter downspouts
- First-Flush Diverter: Diverts first 10-20 gallons (contains most contaminants)
- Inlet Filter: 300-500 micron screen at tank inlet
Post-Storage Filtration
- Sediment Filter: 50-100 micron cartridge
- Carbon Filter: Removes odors, organic compounds
- UV Sterilizer: For potable water (kills bacteria)
- Reverse Osmosis: Optional for highest quality potable water
Storage Tanks
Above-Ground Tanks
- Plastic/Polyethylene: Most common, cost-effective
- Steel: Durable, can be painted
- Fiberglass: Lightweight, corrosion-resistant
- Concrete: Permanent, can be underground partially
- Placement: Level, solid foundation, protected from damage
Underground Tanks
- Advantages: Invisible, space-saving, cooler water
- Disadvantages: More expensive installation, harder to inspect
- Cisterns: Traditional underground storage
- Considerations: Groundwater level, soil conditions, access
Multiple Tanks
- Modular System: Multiple tanks connected for total capacity
- Flexibility: Add capacity as needed
- Redundancy: One tank can be isolated for maintenance
- Gravity Feed: Tanks at different elevations enable gravity flow
Water Distribution
Pump Systems
- Surface Pump: For above-ground tanks
- Submersible Pump: For underground or gravity tanks
- Pressure Tank: Maintains consistent pressure (20-40 psi typical)
- Pressure Switch: Automatically activates pump
Gravity Systems
- Tank Elevation: 1 psi per 2.3 feet of elevation
- Tank on Platform: Create pressure without pump
- Overhead Tank: Site building on slope with tank uphill
Uses for Collected Rainwater
Irrigation
- Garden Beds: Drip irrigation from tank
- Lawn: Sprinkler or spray system
- Greenhouse: Ideal water source
- Greenhouse/Aquaponics: Perfect for controlled growing
Livestock
- Watering Troughs: Fill from storage tank
- Automatic Waterers: Float valve maintains level
- Benefits: Animals prefer taste, no chlorine
Equipment Washdown
- Vehicles: Wash tractors, trucks
- Equipment: Clean tools, machinery
- Outdoor Surfaces: Clean pads, driveways
Potable Use (With Treatment)
- Drinking: Requires UV or chemical treatment
- Cooking: Same treatment as drinking
- Bathing: Less treatment required
- Laundry: Minimal treatment needed
Overlow and Overflow Management
- Overflow Pipe: Direct excess away from foundation
- Drainage: Route to garden, rain garden, or French drain
- Diversion: Direct first flush away from storage
- Secondary Storage: Multiple tanks capture more of larger events
Winter Considerations
- Frozen Pipes: Insulate, heat tape, drain system in winter
- Tank Placement: Bury or insulate for year-round use
- Bypass: Direct downspouts away during winter if not using
- Heated Enclosure: For pump and pressure tank in cold climates
Water Quality
Testing
- Basic Test: pH, turbidity, coliform bacteria
- Comprehensive: Full panel for potable consideration
- Frequency: Annually for potable systems
Common Contaminants
- Bird Droppings: Bacteria source—divert first flush
- Debris: Leaves, twings—filter at gutter and inlet
- Dust/Pollen: Normal, filtered out
- Chemicals: From roofing material—choose roof wisely
Maintenance
- Gutter Cleaning: 2-4 times per year
- Filter Replacement: As needed based on use
- Tank Inspection: Annually for sediment, debris
- First Flush Diverter: Empty regularly
- System Flush: Annually before peak use season
Cost Estimates
- Basic System (DIY): $1,000-3,000 (1,500-3,000 gallons)
- Complete System: $3,000-8,000 (3,000-5,000 gallons, pumped)
- Potable System: $8,000-15,000+ (filtration, treatment included)
Expert Tips
After installing dozens of rainwater systems, we've learned that you can never have too much storage. A 2,500-gallon tank fills up surprisingly fast in a heavy rain, and you'll wish you had more capacity. Plan for 1-2 months of supply, and if budget allows, go larger.
Also, don't skip the first-flush diverter. The first water off the roof contains the majority of contaminants—bird droppings, dust, debris. A simple diverter dramatically improves water quality.
Common Questions
Q: Is rainwater from a metal roof safe to drink?
A: Metal roofs produce the cleanest rainwater, but it still needs treatment for potable use. UV sterilization or chemical treatment kills bacteria. For irrigation or livestock, minimal treatment is needed.
Q: How big should my rainwater storage tank be?
A: Size for 1-2 months of your typical water use. For irrigation, estimate garden needs during dry months. For livestock, calculate daily consumption. A 40x60 barn in areas with 30 inches of annual rain could collect 45,000 gallons annually—size storage accordingly.
Q: Can I use rainwater for my entire house?
A: Yes, with proper treatment. Systems include filtration, UV sterilization, and often additional treatment. It's more common to use rainwater for irrigation and non-potable uses, supplementing with well or municipal water for drinking.
Q: Does a rainwater system freeze in winter?
A: Above-ground tanks can freeze. Options: drain system for winter, insulate tank, heat tape on pipes, or bury tank. Below frost line, groundwater stays around 50-55°F year-round.
Sources & References
- American Rainwater Catchment Systems Association (ARCSA), "Rainwater Harvesting Manual"
- U.S. Environmental Protection Agency (EPA), "Rainwater Harvesting Guidelines"
- USDA Natural Resources Conservation Service, "Water Harvesting Technical Guide"
- International Code Council (ICC), "International Plumbing Code" - Rainwater Harvesting
- National Sanitation Foundation (NSF), "Rainwater System Standards"
Last updated: February 10, 2026 | Difficulty: Intermediate | Reading time: 13 minutes