Green Roof Best Management Practice

Florida Stormwater BMP Training Series  ·  Prepared for stormwater permitting and design professionals  ·  Based on FDEP/SFWMD Applicant’s Handbook criteria and FSEC monitoring data

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1. Green Roof Overview

Source slides 1–4  ·  Fundamentals, system types, and stormwater function

A green roof is a vegetated roof cover system installed over a conventional roofing membrane. Rather than shedding all rainfall as impervious runoff, the growing medium and plant canopy intercept, store, and evapotranspire a meaningful fraction of each storm event before any water ever reaches the drainage layer below. Green roofs are recognized as a stormwater Best Management Practice (BMP) in Florida and are applicable to both new construction and retrofit projects across a wide range of building types.

System Types: Intensive vs. Extensive

Green roofs are broadly classified into two categories based on growing-medium depth, plant palette, and the degree of ongoing maintenance required:

How Green Roofs Reduce Runoff

Compared to a conventional impervious roof, a vegetated roof system intercepts rainfall through three primary mechanisms: canopy interception by leaf surfaces, temporary storage within the growing medium’s pore space, and consumptive loss through evapotranspiration between storm events. The net effect is a significant reduction in both runoff volume and peak discharge rate. For Florida’s permitting purposes, the key metric is runoff volume reduction — quantified either through a runoff volume coefficient or an equivalent curve number — rather than peak-rate attenuation alone.

Co-Benefits Beyond Stormwater

Green roofs deliver a suite of benefits that extend well beyond water quality and quantity management:

• Roof membrane longevity: The growing medium and root barrier shield the waterproofing membrane from UV radiation and thermal cycling, often doubling or tripling membrane service life.
• Building insulation: The growing medium and trapped air space reduce heat transfer through the roof assembly, lowering cooling loads in Florida’s hot climate.
• Urban heat island mitigation: Vegetated surfaces absorb and release heat differently than dark membrane or gravel ballast, reducing localized air temperatures.
• Aesthetics and amenity: Green roofs provide usable green space in dense urban settings, improve views from adjacent buildings, and can support biodiversity.
• Economic value: Studies have documented increased rental rates for office space adjacent to or overlooking green roof installations.

Figure 1 — Green Roof System Cross-Section. Schematic showing the typical layers of an extensive green roof system from top to bottom: plant canopy, growing medium, filter fabric, drainage layer, root barrier, waterproofing membrane, and structural deck. The growing medium pore space provides the primary storage volume for stormwater retention.

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2. Green Roof Examples & Benefits

Source slides 5–10  ·  Florida installations, monitored performance data, and economic findings

Florida has a growing inventory of documented green roof installations spanning university research facilities, county government buildings, commercial office complexes, and residential structures. Long-term monitoring of these installations has produced quantitative performance data that directly informs the BMP credit values used in permitting.

UCF Campus Green Roof — Monitored Research Installation

The University of Central Florida (UCF) campus hosts one of Florida’s most extensively documented green roof research installations. This roof has been continuously monitored for over ten years, generating the long-term performance record that underpins Florida-specific design guidance. The Florida Solar Energy Center (FSEC), located on the UCF campus, conducted the monitoring program and published findings that are widely cited in state permitting documents.

Escambia County — Largest Green Roof in Florida

The Escambia County government complex in Pensacola is home to what is documented as the largest green roof installation in Florida, covering approximately 33,000 square feet of rooftop area. This installation demonstrates the scalability of extensive green roof systems on large institutional and government buildings. The Panhandle location — with its distinct rainfall patterns compared to South Florida — also provides performance data applicable across Florida’s diverse climate zones.

Residential and Commercial Applications

Green roofs have been successfully installed across a range of Florida building types beyond institutional campuses:

• Residential: Single-family and multi-family residential buildings have incorporated extensive green roofs, particularly in urban infill contexts where site-level stormwater management options are limited. The lower structural loads of extensive systems are compatible with most residential roof framing when properly engineered.
• Commercial office: Commercial buildings have adopted green roofs both for stormwater permitting credit and for tenant amenity value. Market studies have documented increased rental rates for office space in buildings with — or with views of — green roof installations, providing a direct economic return on the additional capital cost.
• Runoff reuse: Several installations collect green roof drainage in on-site cisterns for non-potable reuse, including landscape irrigation. This pairing of a green roof with a cistern increases both the volume capture fraction and the site’s overall water efficiency.

Figure 2 — Florida Green Roof Installations. Photo collage showing representative Florida green roof examples including the Escambia County government complex extensive roof, a residential green roof installation in an urban infill setting, and the UCF/FSEC monitored research installation. Images illustrate the range of scale and plant palette across installation types.

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3. Plant Selection & Maintenance

Source slides 11–12  ·  Native plant species, maintenance schedule, and permitting requirements

The long-term hydrologic performance of a green roof depends critically on sustained plant health. A struggling or dead plant community does not intercept or evapotranspire rainfall at the design rate, undermining the stormwater credit. Florida’s native plant palette is well-suited to green roof applications — these species evolved in the state’s hot, drought-stress-and-deluge climate cycle, making them naturally resilient once established.

Native Florida Plants for Green Roofs

Native plants are the most commonly specified species for Florida green roofs, on both extensive (passive) and intensive (active) systems. Key advantages include drought tolerance after establishment, low fertilizer requirements, resistance to Florida’s insect and disease pressures, and suitability for the shallow growing media depths typical of extensive systems. Commonly used native species include:

• Muhly Grass (Muhlenbergia capillaris): A clump-forming native grass with showy pink-purple plumes in autumn. Highly drought-tolerant once established, adaptable to shallow growing media, and provides year-round structural interest. Widely used on both residential and commercial green roofs across Florida.
• Dune Sunflower (Helianthus debilis): A sprawling, salt-tolerant native groundcover with bright yellow flowers produced nearly year-round in South Florida. Thrives in sandy, low-nutrient substrates — conditions that mimic the lightweight aggregate media used in extensive green roofs. Excellent for coastal installations.
• Rosemary (Salvia rosmarinus / coastal rosemary Ceratiola ericoides): Drought-tolerant aromatic shrubs suitable for the drier conditions of an extensive growing medium. Florida rosemary species are adapted to nutrient-poor sandy soils and handle the thermal extremes of a rooftop environment well.

Maintenance Schedule and Requirements

Green roofs require significantly less maintenance than conventional landscaped areas, but they are not maintenance-free. After an establishment period (typically 1–2 growing seasons with supplemental irrigation), extensive native-plant green roofs typically require maintenance visits 2 to 4 times per year. Maintenance activities include:

• Inspection of drainage outlets and overflow structures for blockage
• Removal of invasive or weedy volunteer species
• Replacement of failed or dead plant material to maintain target coverage
• Inspection of root barrier and membrane at accessible penetrations
• Assessment of growing medium depth and condition

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4. Capture Effectiveness & CN Values

Source slides 13–16  ·  Simulation methodology, runoff coefficients, curve numbers, and cistern sizing

The hydrologic performance parameters used for Florida green roof permitting credit were developed through continuous simulation modeling calibrated to 18 monitoring sites across the state. The simulation approach allows performance to be expressed in terms of metrics directly compatible with Florida’s stormwater design framework — specifically, runoff volume coefficients and SCS curve numbers.

Simulation Basis — 18 Florida Sites

Eighteen Florida monitoring sites spanning the state’s range of rainfall regimes, soil conditions, and climatic zones were used as the basis for continuous simulation. Using long-term rainfall records at each site, the green roof system was modeled using parameters derived from field-monitored installations. The multi-site approach ensures that the resulting design parameters are representative of Florida conditions statewide rather than being calibrated to a single location or rainfall record.

Runoff Volume Coefficient (ROC)

The runoff volume coefficient (ROC) expresses the fraction of total annual rainfall that becomes runoff from the green roof surface. A lower ROC indicates better volume reduction performance. For Florida green roof systems modeled across the 18 simulation sites, the ROC ranges from 0.4 to 0.5, meaning that 50 to 60 percent of annual rainfall is retained or evapotranspired by the vegetated system before reaching the drainage outlet. This compares favorably to a conventional impervious roof, which has an ROC approaching 0.9 to 0.95 in Florida’s climate.

Recommended Curve Number (CN = 95)

For event-based stormwater design — including FDEP and water management district treatment volume calculations — a Soil Conservation Service (SCS) Curve Number of 95 is recommended for green roof areas. This CN value reflects the hydrologic behavior of the vegetated roof system for design storm events, accounting for the limited antecedent moisture recovery that occurs between closely spaced events in Florida’s wet season. The CN of 95 is substantially lower than the CN of 98 typically assigned to impervious rooftop surfaces, reflecting the meaningful volume reduction provided even during large design events.

Cistern Storage and Capture Enhancement

Adding a cistern downstream of the green roof drainage layer significantly increases annual volume capture. The cistern captures drainage that passes through the growing medium during and after storm events and holds it for reuse or controlled release. Simulation results show that capture effectiveness increases substantially when cistern storage is included in the system design, particularly for smaller, more frequent storm events that dominate annual runoff volume in Florida.

• Sizing guidance: A cistern sized at 1 to 2 gallons per square foot of contributing green roof area provides a practical range that balances capital cost against incremental capture benefit.
• Reuse demand: Cistern effectiveness depends on demand-side drawdown — a cistern that is full at the start of each storm provides no additional capture benefit. Pairing the cistern with landscape irrigation or other non-potable reuse maximizes the system’s performance.
• Permitting treatment: When a cistern is included, the combination system is modeled as green roof + cistern, and the capture benefit of both components is quantified separately in the simulation framework.

Figure 3 — Capture Effectiveness vs. Cistern Size. Graph showing simulated annual volume capture fraction as a function of cistern volume (gallons per square foot of roof area) for a representative Florida green roof system. The curve illustrates the diminishing-returns relationship between cistern size and incremental capture benefit, informing the 1–2 gal/SF practical sizing range.

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5. Green Roof Permit & Loading Data

Source slide 17  ·  BMP classification, EMC values, particulate fractions, and conventional roof loading

Understanding the regulatory classification and nutrient loading characteristics of green roof systems is essential for correctly applying BMP credit in Florida stormwater permit applications. The Applicant’s Handbook and supporting technical documentation provide specific guidance on how green roofs are treated in pollutant load calculations.

BMP Classification — Applicant’s Handbook Vol. 1

Green roofs are formally classified as a Best Management Practice (BMP) in the FDEP/Water Management District Applicant’s Handbook, Volume 1. This classification establishes the regulatory basis for applying green roof performance credit within the standard stormwater permitting framework. The classification as an impervious area BMP means that the green roof area continues to be counted as impervious in site calculations, but the BMP credit adjusts the effective pollutant load and/or runoff volume attributed to that area.

Event Mean Concentration (EMC) Loading

For nutrient loading calculations, the EMC values applied to green roof runoff are based on the adjacent land use rather than a fixed green roof-specific EMC. This approach recognizes that green roof runoff quality is influenced by atmospheric deposition and the surrounding land use context, and that the primary pollutant load reduction benefit of a green roof comes from volume reduction rather than concentration reduction.

Particulate Fractions in Green Roof Runoff

Green roof runoff has been characterized as having low particulate fractions for the primary nutrients of concern (total nitrogen and total phosphorus). This means that a relatively small portion of the total nutrient load in green roof drainage is in a particle-bound form that would be removed by settling or filtration. Most of the nutrient load is in dissolved form. This characteristic limits the effectiveness of downstream particulate-removal BMPs (such as dry retention or wet ponds) when applied to green roof drainage, and is an important consideration when designing treatment trains.

Conventional Roof Loading — Reference Values

The baseline for evaluating green roof water quality performance is the nutrient loading from a conventional (impervious, non-vegetated) roof surface. The reference EMC values for conventional roofing used in Florida permitting calculations are:

Parameter	Conventional Roof EMC	Notes
Total Nitrogen (TN)	1.050 mg/L	Reference value for conventional impervious roof surface
Total Phosphorus (TP)	0.12 mg/L	Reference value for conventional impervious roof surface

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Topic 6: BMPFast Project Example

Source slides: 18–22 · Practical walkthrough of a two-catchment BMPFast model

The following example traces a complete BMPFast modeling exercise for a 2-acre redevelopment site converting from commercial to light industrial use. The site incorporates two green infrastructure practices — a green roof and a ground-level retention basin — modeled as parallel catchments within BMPFast. A 30,000-gallon cistern collects green roof runoff for irrigation reuse, further reducing net discharge.

Site Overview and Design Parameters

The redevelopment converts approximately 2 acres of previously commercial impervious cover to light industrial use. The stormwater management strategy is built around two primary practices: a green roof blanketing the entire building footprint and a ground-level retention basin receiving runoff from the remaining site area. Key site parameters are summarized in the provision blocks below.

Parallel Catchment Configuration in BMPFast

BMPFast models the site using a parallel catchment configuration — the two drainage areas (green roof catchment and ground-level catchment) are each assigned their own BMP and analyzed independently. BMPFast then aggregates results across both catchments to produce a combined site-level water balance. This approach accurately reflects how the two practices operate in the field: each intercepts its own contributing area with no inter-catchment routing.

Catchment 1 — Green Roof with Cistern

The first catchment encompasses the 30,180 SF building footprint. In BMPFast, the user defines the green roof as the BMP type and enters the cistern volume (30,000 gallons / 1.6 inches depth) as the storage parameter. The model applies the CN 95 pre-BMP runoff curve to the catchment and then credits the green roof’s retention capacity and the cistern’s harvest volume against that runoff.

• Enter catchment area: 0.693 acres (30,180 SF)
• BMP type: Green Roof
• Pre-BMP curve number: 95 (fully impervious, no infiltration credit)
• Cistern storage depth: 1.6 inches over catchment area
• Cistern use: irrigation reuse (demand entered to reflect seasonal drawdown)

Catchment 2 — Ground-Level Retention Basin

The second catchment covers the balance of the 2-acre site not occupied by the building — approximately 1.307 acres of drives, parking, and hardscape. Runoff from this area drains to the ground-level retention basin. The basin’s 0.167 acre-foot storage volume is entered directly as the BMP parameter in BMPFast.

• Enter catchment area: approximately 1.307 acres
• BMP type: Retention Basin (infiltration or evaporation-based, per site soils)
• Storage volume: 0.167 acre-feet
• Pre-BMP curve number: appropriate to land cover and hydrologic soil group

Modeled Results — Runoff Volume Reduction

After both catchments are configured and the model is run, BMPFast produces a combined annual water balance for the site. The results demonstrate a substantial reduction in post-development discharge attributable to the combined green roof, cistern, and retention basin system.

BMPFast Data Entry Summary Table

Parameter	Catchment 1 — Green Roof	Catchment 2 — Retention Basin
Catchment area	0.693 ac (30,180 SF)	~1.307 ac
BMP type	Green Roof + Cistern	Retention Basin
Pre-BMP CN	95	Site-specific (HSG-based)
Storage volume / depth	30,000 gal / 1.6 in	0.167 ac-ft
Cistern reuse	Irrigation demand entered	N/A
Configuration	Parallel (independent)	Parallel (independent)

Saving the Project File

Upon completing the model run, save the BMPFast project file (.bmpf) before closing. This file preserves all catchment definitions, BMP parameters, and results in a format that can be reopened, revised, and submitted with a stormwater permit application. Retaining the project file also facilitates future modifications if site conditions or regulatory requirements change after initial permit approval.

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Topic 7: Key Takeaways

Source slides: 23–24 · Summary of essential concepts and modeling guidance from this module

The following takeaways consolidate the core concepts introduced across this module. Practitioners should carry these principles into every green roof and cistern project modeled in BMPFast, from initial site characterization through permit submission and post-construction record-keeping.

1. Two Green Roof Types: Active and Passive

Green roofs fall into two functional categories that differ in how water is managed after it infiltrates the growing media. Understanding which type is specified is essential before configuring BMPFast inputs.

2. Use CN 95 for Green Roof Modeling

When entering pre-BMP conditions for a green roof catchment in BMPFast, apply a curve number of 95. This value treats the roof surface as essentially impervious, consistent with the physical reality that rooftops — even those with growing media — do not infiltrate runoff into the underlying soil. The green roof’s water retention credit is applied by BMPFast separately through the BMP algorithm, not through a reduced curve number.

3. Cistern Reuse for Irrigation Further Reduces Discharge

A cistern that simply stores water and then overflows during the next storm event provides limited long-term benefit — the storage volume fills and stays full. The key to maximizing cistern effectiveness is active drawdown through irrigation reuse. When stored water is used for landscape irrigation between storms, the cistern empties (or partially empties), restoring available capacity to capture the next rainfall event.

• Enter realistic seasonal irrigation demand in BMPFast to reflect actual drawdown patterns
• Greater irrigation demand → more frequent cistern drawdown → greater annual runoff reduction
• Cisterns with no reuse pathway should be modeled with zero drawdown demand — do not overestimate
• Where irrigation systems are not yet designed, use conservative (low) demand estimates for permitting

4. Parallel Catchment Configuration in BMPFast

When a site has multiple drainage areas each served by its own BMP, model them as parallel catchments in BMPFast. Each catchment is defined and analyzed independently; BMPFast sums the results to produce site-level totals. This is the correct configuration when the green roof and ground-level BMPs drain separate areas with no shared flow path between them.

5. Save the Project File for Permit Filing and Future Use

The BMPFast project file is a living document. Saving it at every stage of design and review ensures that the model can be reopened, audited, and revised without re-entering all parameters from scratch. For permit submissions, include both the project file and a PDF export of the results summary. For post-construction records, retain the file in the project archive so that any future modifications — expansions, tenant changes, BMP retrofits — can be modeled against the original permitted baseline.

• Save after initial setup, after each catchment is defined, and after the final model run
• Export a PDF of results from BMPFast for inclusion in the permit package
• Archive the .bmpf file with the construction drawings and stormwater management plan
• Note the BMPFast version used — model outputs may differ between software versions

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