What This Solves
Calculates green roof stormwater retention performance, including water holding capacity, runoff reduction, peak flow attenuation, and structural loading.
Best Used When
- You are evaluating a green roof for stormwater credit in a site plan
- You need to estimate the runoff volume reduction and peak flow delay from a green roof
- You want to calculate the saturated weight (structural load) of an extensive or intensive green roof system
Do NOT Use When
- You are designing a ground-level rain garden or bioretention facility — Use Rain Garden Calculator
- You need to size cisterns to capture roof runoff for reuse — Use Cistern Sizing Calculator
Key Assumptions
- Media water holding capacity is based on FLL guidelines for the specified media depth
- Runoff reduction is calculated for a single design storm assuming the media starts at field capacity
- Structural loads use saturated media weight plus drainage layer and vegetation
- Evapotranspiration restores storage capacity between storms
- Drainage layer provides adequate lateral conveyance to roof drains
Input Quality Notes
Media water holding capacity varies by substrate type and depth. Use manufacturer specifications or FLL test data. Structural load calculations should be reviewed by a structural engineer.
Calculate Green Roof Performance
For educational purposes only. Not a substitute for professional engineering judgment.
Green Roof Stormwater Overview
Green roofs provide stormwater management through retention (storing water in substrate and drainage layers) and detention (delaying runoff). Water is removed through evapotranspiration between storm events.
- Retention - Water stored in substrate pore spaces
- Detention - Delay in peak runoff timing
- Evapotranspiration - Water returned to atmosphere
- Runoff Reduction - Percentage of rainfall captured
Green Roof Classifications
| Type | Depth (in) | Maintenance | Irrigation |
|---|---|---|---|
| Extensive | 2-6 | Low | Not Required |
| Semi intensive | 6-12 | Medium | Required |
| Intensive | 12-48 | High | Required |
Source: FLL Guidelines (2018), ASTM E2397
Substrate Water Holding Capacity
| Substrate Type | Min WHC | Typical WHC | Max WHC | Dry Density (pcf) |
|---|---|---|---|---|
| Lightweight mineral aggregate (expanded clay/shale) | 0.25 | 0.35 | 0.45 | 55 |
| Engineered mineral media per FLL specs | 0.3 | 0.4 | 0.5 | 65 |
| Organic-mineral blend (higher WHC, lower density) | 0.4 | 0.5 | 0.6 | 45 |
| Expanded shale/slate aggregate | 0.2 | 0.3 | 0.4 | 50 |
Source: FLL Guidelines (2018), ASTM E2399
Annual Evapotranspiration by Climate
| Climate Zone | Min ET (in/yr) | Typical ET (in/yr) | Max ET (in/yr) |
|---|---|---|---|
| Arid | 60 | 72 | 84 |
| Semi Arid | 40 | 50 | 60 |
| Humid Continental | 24 | 32 | 40 |
| Humid Subtropical | 36 | 44 | 52 |
| Marine | 20 | 28 | 36 |
Source: USGS, NOAA Climate Data
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Last verified: February 2026