Green Roof Stormwater Retention Volume Calculator
Estimate stormwater retention volume for a green roof using area, rainfall depth, and a retention rate, with an optional safety factor.
Use this estimate for planning and concept design; confirm final sizing with engineered hydrology models and local code requirements.
Examples
- Roof area 12,000 sq ft, rainfall depth 1.25 in, retention 60%, safety factor 0% ⇒ Retained stormwater volume: 5,610.39 gallons (750.00 cubic feet). Adjusted retention rate: 60.00%.
- Roof area 8,500 sq ft, rainfall depth 0.8 in, retention 70%, safety factor 10% ⇒ Retained stormwater volume: 2,670.55 gallons (357.00 cubic feet). Adjusted retention rate: 63.00%.
FAQ
What retention rate should I use?
Extensive green roofs often retain 40% to 70% of design storms, while intensive systems may retain more. Use measured data when possible.
Does this include overflow drainage?
No. The calculator estimates retained volume only. Any rainfall beyond the retained volume should be routed through drainage layers and overflow structures.
Should I size for the largest storm?
Most jurisdictions size green roof retention for a design storm such as the 1-inch or 2-year event. Align the input with your local requirement.
Additional Information
- Outputs use US customary units: square feet, inches, cubic feet, and gallons.
- Retention rate reflects media depth, vegetation type, and antecedent moisture; validate with field testing if available.
- The safety factor reduces retention to account for clogging, aging, or conservative permitting assumptions.