Roof Water Collection Calculator
Typical efficiency: 80-90% (accounts for evaporation, guttering losses)
Collection Results
Roof Area: 0.0 m²
Water Collected: 0 liters
Water Collected: 0 gallons (US)
Typical House Examples
Small House
Dimensions: 8m × 10m
Roof Area: 80 m²
Water Collection:
• 10mm rain: 680L
• 25mm rain: 1700L
Medium House
Dimensions: 12m × 15m
Roof Area: 180 m²
Water Collection:
• 10mm rain: 1530L
• 25mm rain: 3825L
Large House
Dimensions: 15m × 20m
Roof Area: 300 m²
Water Collection:
• 10mm rain: 2550L
• 25mm rain: 6375L
Understanding the Math
Key Principle
1 mm of rainfall on 1 m² = 1 liter of water (before efficiency losses)
Calculation Steps:
- Measure roof catchment area (length × width)
- Determine rainfall amount in mm
- Apply collection efficiency (typically 85%)
- Result: Area × Rainfall × Efficiency = Liters
Efficiency Factors:
- Evaporation during collection: 5-10%
- Gutter overflow in heavy rain: 3-8%
- First-flush diverter waste: 2-5%
- Roof material absorption: 1-3%
Maximizing Collection Efficiency
Roof Optimization:
- • Metal roofs: 90-95% efficiency
- • Tile roofs: 80-85% efficiency
- • Proper guttering: Size gutters for peak rainfall
- • Smooth surfaces: Clean gutters regularly
Storage Considerations:
- • Tank sizing: Match to average monthly rainfall
- • First flush: Divert initial roof runoff
- • Overflow design: Handle extreme rainfall events
- • Water quality: Install filters and screens
Related Tools & Guides
Unit Converter
Convert rainfall measurements
Rainfall Intensity
Understand different rain categories
Rain Simulator
Visualize rainfall intensities
References:
• Australian Bureau of Meteorology - Rainfall Statistics
• USGS Water Resources - Precipitation Data
• International Rainwater Harvesting Alliance Standards