Green Roof Runoff Reduction Calculator
See how much stormwater runoff a green roof can keep out of the drainage system compared to a standard roof for a given rainfall event.
How green roofs reduce stormwater runoff
Green roofs use a layer of soil-like growing medium and vegetation to absorb and temporarily store rainwater, releasing much of it later through evaporation and plant transpiration instead of sending it straight into storm drains. This reduces peak runoff volume and flow rate compared to a conventional impermeable roof, helping lower flood risk and combined-sewer overflows.
How this calculator works
Enter your roof area, the rainfall depth for the storm event you want to model, your green roof’s expected retention rate (the percentage of rainfall it holds back — typically 40-60% for extensive green roofs, depending on substrate depth and season), and the runoff coefficient of a conventional roof (commonly ~0.90-0.95, since a hard roof sheds nearly all the rain that falls on it). Since 1 mm of rain over 1 m² equals 1 liter of water, the calculator finds:
- Conventional Roof Runoff (L) = Rainfall (mm) × Area (m²) × Runoff Coefficient
- Green Roof Runoff (L) = Rainfall (mm) × Area (m²) × (1 – Retention Rate)
- Volume Reduced = Conventional Runoff – Green Roof Runoff
Actual retention varies with storm intensity, antecedent soil moisture, substrate depth, and plant type, so treat these figures as general planning estimates rather than an engineering design calculation.