1. What is Roof Rainfall Runoff?

Roof rainfall runoff refers to the volume of water that flows off a roof during a rain event. This calculation is important for designing rainwater harvesting systems, drainage systems, and understanding stormwater management.

2. How Does the Calculator Work?

The calculator uses the roof runoff equation:

Where:

• \( Rainfall_{in} \) — Rainfall amount in inches
• \( RoofArea_{sqft} \) — Roof area in square feet
• 12 — Conversion factor (inches to feet)
• 7.48 — Gallons per cubic foot conversion factor

Explanation: The equation converts rainfall depth over an area into volume (cubic feet) then converts to gallons.

3. Importance of Runoff Calculation

Details: Accurate runoff calculation helps in sizing rainwater collection tanks, designing proper drainage systems, and preventing water damage to buildings.

4. Using the Calculator

Tips: Enter rainfall amount in inches and roof area in square feet. Both values must be positive numbers for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: How accurate is this calculation?
A: It provides a theoretical maximum. Actual runoff may be less due to evaporation, absorption, and roof slope.

Q2: Does roof material affect runoff?
A: Most materials have minimal effect on total volume, but may affect flow rate. Metal roofs typically have highest runoff coefficients.

Q3: How do I measure my roof area?
A: Measure building footprint and multiply by roof pitch factor (1.0 for flat roofs up to 1.4 for steep pitches).

Q4: What about gutter efficiency?
A: This calculation assumes 100% collection efficiency. Real systems typically capture 75-95% of theoretical runoff.

Q5: Can I use this for irrigation planning?
A: Yes, it helps estimate water available for irrigation during rainy periods.