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.
Estimate the stormwater retention and detention performance of a green roof. Enter the roof area, growing-media type and depth, and a design storm to get the total water holding capacity, runoff reduction, peak-flow delay and saturated structural load, using FLL Guidelines and ASCE stormwater methodology.
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
How the green roof calculation works
A green roof manages stormwater by retention (storing rainfall in the pore space of the growing media and the voids of the drainage/retention layers) and detention (delaying and flattening the peak of the runoff). Between storms, stored water is returned to the atmosphere by evapotranspiration, resetting the available storage. The calculator builds up the total storage layer by layer, then compares it to the design-storm rainfall volume.
1. Layer storage
Each layer stores a depth of water equal to its physical depth times a storage fraction, applied over the roof area A:
- Substrate (growing media): Vsubstrate = dsubstrate × WHC × A — where WHC is the volumetric water holding capacity at field capacity.
- Drainage layer: Vdrain = ddrain × n × A — where n is the void ratio of the dimpled mat, granular or geocomposite layer.
- Retention mat (optional): Vmat = dmat × A — the mat's rated storage depth.
2. Total holding capacity and runoff
The layers are summed and the storm is applied:
- Total storage: Vtotal = Vsubstrate + Vdrain + Vmat
- Rainfall volume: Vrain = P × A — where P is the design rainfall depth.
- Volume retained: Vretained = min(Vtotal, Vrain)
- Runoff reduction: Vretained ÷ Vrain × 100%
- Effective runoff coefficient: Ceff = Vrunoff ÷ Vrain, where Vrunoff = max(0, Vrain − Vtotal). The runoff threshold (rainfall depth before any runoff begins) equals the total storage depth.
3. Structural dead load
Saturated load is the dry media weight plus the weight of water held in the media and drainage layers, using a water density of 62.4 pcf:
wsat = ρdry × dsubstrate + ρw × (dsubstrate × WHC + ddrain × n)
Peak-flow delay and reduction are empirical estimates derived from green roof monitoring studies (delay increases with substrate depth and decreases with slope). Method follows FLL Guidelines (2018), ASTM E2397/E2399 and ASCE stormwater design practice.
Substrate water holding capacity & density
Volumetric water holding capacity (WHC, volume of water per volume of substrate at field capacity) and dry density drive both the storage and the structural load. Typical values used by this calculator:
| Substrate | Min WHC | Typical WHC | Max WHC | Dry density |
|---|---|---|---|---|
| Lightweight mineral (expanded clay/shale) | 0.25 | 0.35 | 0.45 | 55 pcf |
| Engineered mineral (FLL spec) | 0.30 | 0.40 | 0.50 | 65 pcf |
| Organic-mineral blend | 0.40 | 0.50 | 0.60 | 45 pcf |
| Expanded shale/slate | 0.20 | 0.30 | 0.40 | 50 pcf |
Source: FLL Guidelines (2018), ASTM E2399. Drainage-layer void ratios used for storage: dimpled mat 0.40, granular 0.35, geocomposite 0.45, retention mat 0.50.
Extensive, semi-intensive & intensive
Extensive
2–6 in of substrate, sedum/moss planting. Low maintenance, no irrigation, lightest dead load — the most common stormwater-credit roof.
Semi-intensive
6–12 in of substrate with grasses and perennials. Medium maintenance, usually irrigated, more storage than extensive.
Intensive
12–48 in of deep media supporting shrubs and small trees. High maintenance and irrigation, highest storage and dead load.
Source: FLL Guidelines (2018), ASTM E2397.
Worked example
A 5,000 sf extensive roof with 4 in of lightweight mineral substrate (typical WHC = 0.35) over a 0.5 in dimpled drainage mat (void ratio 0.40), for a 1.0 in design storm:
- Substrate storage depth = 4 in × 0.35 = 1.40 in
- Drainage storage depth = 0.5 in × 0.40 = 0.20 in
- Total storage depth = 1.40 + 0.20 = 1.60 in (the runoff threshold)
- Storage volume = (1.60 in ÷ 12) × 5,000 sf = ≈ 667 cf
- Storm volume = (1.0 in ÷ 12) × 5,000 sf = ≈ 417 cf
- Storage (1.60 in) exceeds the 1.0 in storm, so it is fully absorbed: 100% runoff reduction for this event
A larger storm than the 1.60 in threshold would begin to produce runoff; the effective runoff coefficient then rises above zero.
Frequently asked questions
How much stormwater can a green roof retain?
It depends on the depth and water holding capacity (WHC) of the growing media plus any drainage or retention layers. A typical 4-inch extensive sedum roof on a lightweight mineral substrate (WHC ≈ 0.35) holds roughly 1.4 inches of water in the media alone, so it can fully absorb a 1-inch storm and meaningfully reduce larger ones. Annual volume retention for extensive roofs is commonly reported in the 45–65% range, and deeper intensive roofs retain more. The calculator sizes the total storage from your specific layers and design storm.
How is green roof water holding capacity calculated?
Storage in each layer is depth × storage fraction × roof area. For the growing media the storage fraction is its volumetric water holding capacity (WHC); for a drainage or retention layer it is the void ratio. The layers are summed: V_total = (d_substrate × WHC + d_drain × n_void + d_mat) × A. Runoff reduction is then the retained volume divided by the storm rainfall volume, capped at the available storage.
What is the difference between extensive and intensive green roofs?
Extensive roofs use a shallow substrate (about 2–6 in) planted with drought-tolerant sedum and moss; they are lightweight and low-maintenance with no irrigation. Intensive roofs use deep media (12–48 in) that can support shrubs and small trees but carry far higher dead loads and require irrigation and regular maintenance. Semi-intensive roofs (about 6–12 in of grasses and perennials) sit between the two. Deeper assemblies store more water but weigh more.
How much does a saturated green roof weigh?
Saturated dead load is the substrate dry weight plus the weight of water held in the media and drainage layers. Using a 62.4 pcf water density and typical substrate dry densities of 45–65 pcf, an extensive 4-inch roof is often around 18–30 psf saturated, while deep intensive assemblies can exceed 100 psf. Always confirm the saturated load against the structural capacity per ASCE 7 / ASTM E2397 before designing — this tool gives an estimate, not a structural sign-off.
Was this calculator helpful?
Last verified: February 2026