6.2.5 Green Roofs

6.2.5 Green RoofsDescription: Green roofs are an alternative to traditional roofs, consisting of plants and engineered soils that manage stormwater before it enters the drainage system. The stormwater is captured and temporarily stored, filtering pollutants through evaporation and transpiration.

LID/GI Consideration: Green roofs are an excellent method for reducing site impervious area, stormwater runoff volumes, pollutant loads, and thermal impacts of development. Green roofs provide multiple benefits, including creating outdoor areas and species habitat in developed and highly urbanized areas, reducing the amount of ground surface area required to treat stormwater runoff, maximizing development space, helping to mitigate runoff temperature by keeping roofs cool, and retaining much of the stormwater runoff, among others. This is considered to be a green infrastructure IMP.

Augusta Stormwater Management Manual

Version: July 20206.2.5- 1

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KEY CONSIDERATIONS STORMWATER MANAGEMENT SUITABILITY

DESIGNCRITERIA: ` Engineered growing media shall be a light-weight mix containing 15% organic material

` Waterproofing materials shall be protected from root penetration by an impermeable root barrier

` Green roofs may be installed on rooftops with slopes of up to 25% with proper engineering, but are not generally recommended for use on rooftops with slopes greater than 10%

` The use of extensive green roof systems (2”-6” deep growing media) should be considered prior to the use of more complex and expensive intensive green roof systems

` A landscaping plan shall be prepared for all green roofs

ADVANTAGES/BENEFITS: ` Helps reduce post-construction stormwater runoff rates, volumes, temperatures and pollutant loads without consuming valuable land

` Particularly well-suited for use in urban development and redevelopment sites

` Use of green roofs allows for more development space on a project site

` Provides a stormwater management alternative for brownfield sites

` Although the cost per square foot of a green roof is notably higher than a regular roof, green roofs have been reported to save costs associated with energy consumption and increasing the lifespan of the roof

DISADVANTAGES/LIMITATIONS: ` Can be difficult to establish vegetation in the harsh growing conditions found on rooftops in coastal Georgia

` The roof structure must be capable of supporting the additional weight (live and dead load) of a green roof. Additional support, such as trusses, may be necessary for redevelopment projects to existing structures

ROUTINEMAINTENANCEREQUIREMENTS: ` Inspect green roof for dead or dying vegetation. Dead vegetation shall be removed along with any woody vegetation and weeds. Plant replacement vegetation as needed

` Inspect waterproof membrane for leaks, and repair as needed

` Remove invasive vegetation

` Monitor sediment accumulation and remove periodically

Residential Subdivision Use: Yes

High Density/Ultra-Urban: Yes

Roadway Projects: No

Soils: Planting media should meet design recommendations.

Other Considerations: Drainage systems for the roof (gutters, downspouts, etc.) must be capable of handling large rainfall events

RUNOFF REDUCTION CREDIT

` 60% of the runoff reduction volume provided

POLLUTANTREMOVAL80% Total Suspended Solids

50%/50%Nutrients - Total Phosphorus/Total Nitrogen removal

NAMetals - Cadmium, Copper, Lead, and Zinc removal

NA Pathogens – Fecal Coliform

Runoff Reduction

TSS Removal

Channel Protection

Overbank Flood Protection

Extreme Flood Protection Retrofit

suitable for this practice

u may provide partial benefits

Low Land Requirement

High Capital Cost

Low Maintenance Burden

IMPLEMENTATION CONSIDERATIONS



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6.2.5.1 GeneralDescriptionandStormwaterManagementSuitabilityGreen roofs (also known as vegetated roofs or eco-roofs) represent an alternative to traditional impervious roof surfaces. They typically consist of underlying waterproofing and drainage materials and an overlying engineered growing media that is designed to support plant growth (Figure 6.2.5-1). Stormwater runoff is captured and temporarily stored in the engineered growing media, where it is subjected to the hydrologic processes of evaporation and transpiration, with any excess runoff conveyed back into the stormwater drainage system. This allows green roofs to provide measurable reductions in post-construction stormwater runoff rates, volumes, and pollutant loads on development sites.

Figure 6.2.5-1. Components of a Green Roof System



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There are two different types of green roof systems: intensive green roof systems and extensive green roof systems. Intensive green roof systems (also known as rooftop gardens) have a thick layer of engineered growing media (i.e., 12-24 inches) that supports a diverse plant community that may include trees (Figure 6.2.5-2). Extensive green roof systems typically have a much thinner layer of engineered growing media (i.e., 2-6 inches) that supports a plant community that is comprised primarily of drought-tolerant vegetation (e.g., sedums, succulent plants) (Figure 6.2.5-3).

Extensive green roof systems, which can cost up to twice as much as traditional impervious roof surfaces, are much lighter and less expensive than intensive green roof systems. Consequently, it is recommended that the use of extensive green roof systems be considered prior to the use of intensive green roof systems.

Extensive green roof systems typically contain multiple layers of roofing materials (Figure 6.2.5-1), and are designed to support plant growth while preventing stormwater runoff from ponding on the roof surface. Green roof systems are designed to drain stormwater runoff vertically through the engineered growing media, and then horizontally through a drainage layer that is sloped towards an outlet. They are designed to require minimal long-term maintenance and, if the right plants are selected to populate the green roof, should not need supplemental irrigation or fertilization after an initial vegetation establishment period.

Figure 6.2.5-2. Intensive green Roof System (Source: City of Portland, OR, 2004)

Figure 6.2.5-3. Extensive Green Roof System (Source: City of Portland, OR, 2004)



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Stormwater Quality ProtectionGreen roofs provide measurable reductions in post-construction stormwater runoff rates, volumes, and pollutant loads on development sites. Thus, they are particularly well suited to reduce both the runoff reduction volume (RRv) and/or water quality volume (WQv ) for compliance with the 80% TSS removal criteria.

Channel ProtectionNo channel protection volume (CPv) storage is provided by a green roof. Stormwater runoff generated by the contributing impervious rooftop area and pervious green roof should be routed to a downstream regional IMP that provides storage and treatment of the CPv. Proportionally adjust the post-development runoff curve number (CN) to account for the runoff reduction provided by a green roof when calculating the CPv for the regional IMP. See Section 5.2.7 for more information about curve number reduction.

OverbankFloodProtectionNo overbank flood protection volume storage is provided by a green roof. Proportionally adjust the post-development runoff CN to account for the runoff reduction provided by a green roof when calculating the overbank peak discharges (Qp2, Qp10 Qp25, and Qp50) on a development site. See Section 5.2.7 for more information about curve number reduction.

Extreme Flood ProtectionNo extreme flood protection volume storage is provided by a green roof. Proportionally adjust the post-development runoff CN to account for the runoff reduction provided by a simple green roof for the contributing drainage area when calculating the extreme peak discharge (Qp100) on a development site. See Section 5.2.7 for more information about curve number reduction.

6.2.5.2 PlanningThe following criteria shall be considered when evaluating the suitability of green roofs for a development site:

Physical Feasibility Requirements ` Drainage Area – Green roofs can only be used to replace traditional impervious roof surfaces. They cannot be used to treat stormwater runoff generated elsewhere on the development site.

` Space Required – Green roofs comprise 100% of their contributing drainage areas.

` Site Slope – Although green roofs may be installed on rooftops with slopes of up to 25%, they are not recommended for use on rooftops with slopes greater than 10% and may not be approved.

` Minimum Depth to Water Table – Separation from the water table is not applicable to a green roof.

` Minimum Head – The minimum head is 6 to 12 inches.

` Soils – An appropriate engineered growing media, consisting of approximately 80% lightweight inorganic material, 15% organic material, and 5% sand, must be used in green roof systems.

` Hotspots – Green roofs shall not be used for hotspot runoff.



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` Damage to Existing Structures and Facilities – When designing a green roof, site planning and design teams must not only consider the stormwater storage capacity of the green roof, but also the structural capacity of the rooftop itself. To support a green roof, a rooftop shall be designed to support an additional 15 to 30 pounds per square foot (psf) of load. Consequently, a structural engineer or other qualified professional shall be involved with the design of a green roof to ensure that the rooftop itself has enough structural capacity to safely support the green roof system.

` Proximity – Green roofs may be used without restriction near:

` Private water supply wells

` Open water

` Public water supply reservoirs

` Public water supply wells

` Property Lines – Green roofs may be used near property lines; however, ensure that stormwater runoff is not redirected onto an adjacent owner’s property. Stormwater runoff shall remain on the property where it was generated until it is discharged to downstream receiving waters, the municipal stormwater drainage system, or to the pre-construction point of discharge.

The data listed below are necessary for the design of a green roof and shall be included with the SWMP:

` Architectural roof plan with rooftop pitches and downspout locations

` The proposed site design, including, buildings, parking lots, sidewalks, stairs and handicap ramps, and landscaped areas for downspout discharge locations and bypass outfalls

` Information about downstream IMPs and receiving waters

See Appendix B for more information on required elements for the SWMP.

6.2.5.3 DesignThe following criteria shall be considered minimum standards for the design of a green roof.

General Design ` Green roof systems shall be designed to provide enough storage for the RRv . The required dimensions of a green roof system are governed by several factors, including the hydraulic conductivity and moisture retention capacity of the engineered growing media and the porosity of the underlying drainage layer. Site planning and design teams shall consult with green roof manufacturers and/or materials suppliers to the degree necessary to ensure that designed green roof systems will provide enough storage for the RRv.

` During the design of a green roof system, site planning and design teams shall ensure that structural capacity of the rooftop itself is adequate to support the green roof system. Portions of the SWMP that require structural calculations for green roof design shall be prepared and stamped by a professional engineer competent in structural design and licensed to practice in



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the State of Georgia.

` Green roofs shall not be used to treat any stormwater runoff generated elsewhere on the development site.

` All green roofs shall be designed in accordance with the ASTM International Green Roof Standards (American Society for Testing and Materials (ASTM) International, 2005a; ASTM, 2005b; ASTM, 2005c; ASTM, 2005d; ASTM, 2006).

` Supplemental measures, such as battens, may be needed to ensure stability against sliding on rooftops with slopes of greater than 10%. All green roof systems shall include a waterproofing layer that will prevent stormwater runoff from damaging the underlying rooftop. Waterproofing materials typically used in green roof installations include reinforced thermoplastic and synthetic rubber membranes.

` The waterproofing layer shall be protected from root penetration by an impermeable, physical root barrier. Chemical root barriers or physical root barriers that have been impregnated with pesticides, metals, or chemicals that may leach into post-construction stormwater shall not be used.

` A drainage layer shall be placed between the root barrier and the engineered growing media. The drainage layer should consist of synthetic or inorganic materials (e.g., gravel, recycled polyethylene) that are capable of both retaining water and providing efficient drainage when the layer becomes saturated. The required depth of the drainage layer will be governed by the required storage capacity of the green roof system and by the structural capacity of the rooftop itself.

` An appropriate engineered growing media, consisting of approximately 80% lightweight inorganic materials, 15% organic matter (e.g., well-aged compost), and 5% sand, shall be installed above the drainage layer. The engineered growing media shall have a maximum water retention capacity of approximately 30%.

` To prevent clogging within the drainage layer, the engineered growing media shall be separated from the drainage layer by a layer of permeable filter fabric. The filter fabric shall be a non-woven geotextile with a permeability that is greater than or equal to the hydraulic conductivity of the overlying engineered growing media.

` The engineered growing media shall be between 4 to 6 inches deep, unless synthetic moisture retention materials (e.g., drainage mat with moisture storage “cups”) are placed directly beneath the engineered growing media layer. When synthetic moisture retention materials are used, a 2-inch-deep engineered growing media layer may be used.

` To assist in conveying runoff to the building drainage system, a semi-rigid, plastic geo-composite drain or mat layer shall be included in the design of a green roof. If the roof is flat, a perforated network may be necessary to help rainfall drain properly.

` Extend the roof flashing 6 inches above engineered growing media and protect by counter flashing.



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PhysicalSpecifications/Geometry ` The drainage area of a green roof is comprised of the green roof itself. No additional stormwater shall be allowed to “run on” to the green roof, with the exception of walking paths or vegetation access ways that are incorporated into the green roof design.

Pretreatment/Inlets ` Green roofs are designed to directly receive rainfall. Pretreatment and inlets are not required.

Outlet Structures ` An outlet (e.g., scupper, downspout) should be provided to convey stormwater out of the drainage layer and off of the rooftop when the drainage layer becomes saturated.

Safety Features ` Consideration shall be given to the stormwater runoff rates and volumes generated by the 25-year, 24-hour storm event to design green roofs that are able to safely convey or bypass larger flows. An overflow system, such as a traditional rooftop drainage system with inlets set slightly above the elevation of the surface of the green roof, shall be designed to convey the stormwater generated by the 25-year storm event safely off of the rooftop.

` Stormwater shall flow through and exit green roof systems in a safe and non-erosive manner. Overflow structures should be capable of passing the 2-year, 24-hour design storm without inundating the roof.

Landscaping ` A landscaping plan for the green roof shall be prepared for all green roofs and must be submitted with the SWMP.

` When developing a landscaping plan, site planning and design teams are encouraged to consult with a botanist, landscape architect, or other qualified professional to identify plants that will tolerate the growing conditions found on rooftops in Augusta. Planting recommendations for green roofs include:

` Drought- and full sun-tolerant vegetation that requires minimal irrigation after establishment

` Low-maintenance vegetation that is self-sustaining and does not require mowing, trimming or the use of fertilizers, pesticides, or herbicides

` Vegetation that is fire resistant and able to withstand heat, cold, and high winds

` Since sedum and succulent plants possess many of the characteristics listed above, they are recommended for use on green roof systems installed in Augusta. Herbs, forbs, grasses, and other groundcovers may be used, but these plants typically have higher watering and maintenance requirements.

` Methods used to establish vegetative cover on a green roof should achieve at least 75% vegetative cover one year after installation.



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Design ProceduresStep 1. Determine if the development site and conditions are appropriate for the use of a green roof.

Consider the application and site feasibility criteria in this Chapter. In addition, determine if site conditions are suitable for a green roof. Create a rough layout of the green roof dimensions taking into consideration existing trees, utility lines, and other obstructions.

Step 2. Determine the goals and primary function of the green roof.Consider whether the green roof is intended to:

` Meet a runoff reduction target or pollutant removal target. For information on the sizing of an IMP utilizing the runoff reduction approach, see Step 3A. For information on the sizing of the IMP utilizing the TSS removal approach, see Step 4.

` Be “oversized” to include partial credit for storage capacity for other stormwater requirements (channel protection volume [CPv ])

` Provide a possible solution to a drainage problem

` Enhance landscape and provide aesthetic qualities

The design of the IMP should be centered on the restrictions/requirements, goals, targets, and primary function(s) of the IMP described in this Section. By considering the primary function, the design elements of the practice can be determined (e.g., planting media, underdrain, inlet/outlet, overflow, etc.).

CompleteSteps3A,3B,and3Cforarunoffreductionapproach,orskipStep3andcompleteStep4foraTSSremoval(WQv) approach. Refer to Chapter 5 for detailed information on compliance calculations. Step 3A. Calculate the RRv.

Calculate the RRv using the following formula:RRv=(P)(Rv)(A)/12

whereRRv = runoff reduction volume (ft3)P = runoff reduction rainfall (1.0 inches)Rv = volumetric runoff coefficient which can be found by Rv = 0.05+0.009(I)

and where I = new impervious area of the contributing drainage area (%)A = area draining to this practice (ft2)

12= unit conversion factor (in/ft)

Using Table 6-4 IMP runoff reduction credits, lookup the appropriate runoff reduction percentage (or credit) provided by the practice:



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Using the RRv calculated above, determine the minimum volume of the practice (VP) (VPMIN)≥RRv/(RR%)

whereRR% = runoff reduction percentage, or credit, assigned to the specific practiceVPMIN = minimum storage volume required to provide runoff reduction volume (ft3)RRv = runoff reduction volume (ft3)

Step 3B. Determine the storage volume of the practice and the pretreatment volume.

To determine the actual volume provided in the green roof, use the following equation:VP=(PV+VES[N])

where VP = volume provided (temporary storage) PV = ponding Volume VES = volume of engineered soils N = porosity

To determine the porosity, a qualified licensed professional should be consulted to determine the proper porosity based on the engineered soils used. Most soil media has a porosity of 0.25 and gravel a value of 0.40.

Step 3C. Determine whether the minimum storage volume was met.

When the VP ≥ VPMIN, then the runoff reduction requirements are met for this practice. Proceed to Step 5.

When the VP < VPMIN, then the IMP must be sized according to the WQv treatment method (see Step 4).

Step 4. Calculate the WQv.*

Calculate the WQv using the following formula:WQv=(1.2)(Rv)(A)/12

whereWQv = water quality volume (ft3)1.2 = target rainfall amount to be treated (inches)Rv = volumetric runoff coefficient which can be found by

Rv = 0.05+0.009(I)and where

I = new impervious area of the contributing drainage area (%)A = area draining to this practice (ft2)12 = unit conversion factor (in/ft)

*A green roof is used as an impervious surface area reduction technique. Therefore, the area of the green roof is not included in the new impervious surface area added (I).



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Using the WQvcalculatedabove,determinetheactualsizeandvolumeof thepractice(VP)asshown in Step 3B. Note:VP,calculatedusingtheformulashowninStep3B,shouldbegreaterthan or equal to WQv.

Step 5. Calculate the adjusted curve numbers for CPv(2-year,24-hourstorm),Qp2, Qp10, Qp25, Qp50, and Qp100).

See Section 5.2.7 for more information.

Step 6. Prepare site vegetation plan.

Vegetation is critical to the function and appearance of any green roof. Therefore landscaping plans shall be provided according to the guidance in Subsection 6.2.5.3 (Landscaping) and Appendix D.

6.2.5.4 Inspection,ProtectionandMaintenanceAll IMPs require proper construction, protection, and long-term maintenance or they will not function as designed, and may cease to function altogether. The design of all IMPs includes considerations for maintenance and maintenance access. A legally binding IMP Maintenance Agreement shall be completed. For Augusta policies, additional information, guidance and forms pertaining to IMP protection, inspection and maintenance requirements, see Chapter 8 of this Manual.

Requirements DURING ConstructionTo help ensure that green roofs are properly installed on a development site, site planning and design teams should consider the following recommendations:

` Care shall be given to avoid damage to the waterproofing membrane during installation of the green roof. If the integrity of the membrane is compromised in a manner that may cause leaks or roof damage, the area should be identified and repaired. Visually inspect for damage and test the membrane for water tightness prior to installation of the engineered growing media.

` If the roof is sloped, stabilization measures may be required before installing the green roof to prevent soil from sliding down the roof. Some situations may allow the stabilization measures to be incorporated into the roof structure.

` Install the green roof according to the manufacturer’s instructions. Usually the root barrier layer, walkway, and irrigation system are installed first.

` To help prevent compaction of the engineered growing media, heavy foot traffic shall be kept off of green roof surfaces during and after construction.

` Construction contracts should contain a replacement warranty that covers at least three growing seasons to help ensure adequate growth and survival of the vegetation planted on a green roof.



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Protection Requirements ` Providing signage for the IMP will:

` Allow for easy identification and location of the IMP

` Serve as a general education tool, making those responsible for property, landscape or IMP maintenance and the general public aware of the water quality features of the IMP and to avoid encroachment

Inspection RequirementsWhile AED requires an annual inspection, more frequent inspections help identify problems earlier and keep maintenance costs lower. The following are recommended frequent inspection items:

Green Roof Typical Routine Maintenance Activities and ScheduleActivity Schedule

` Water to promote plant growth and survival.

` Inspect green roof for dead or dying vegetation. Dead vegetation should be removed along with any woody vegetation. Plant replacement vegetation as needed.

` Mow and remove grass clippings.

` Remove trash, debris, and other pollutants from the rooftop.

` Observe infiltration rates after rain events; the roof should drain in 24 hours of rain event.

As needed

` Inspect waterproof membrane for leaks. Repair as needed.

` Inspect outflow and overflow areas for trash, debris, and sediment accumulation. Remove any accumulated sediment or debris.

` Inspect vegetation for signs of stress. If vegetation begins showing signs of stress, including drought, flooding, disease, nutrient deficiency or insect attack, treat the problem or replace the vegetation.

` Weed and prune vegetation.

Semi-annually (Quarterly during first year)

` Test the planting soils for pH levels. Consult with a qualified licensed professional to determine and maintain the proper pH levels.

Annually

For green roofs, inspections of the following elements are important for proper function of the IMP, and inspection frequency and diligence are critical:

` Ensure access to the site is adequate for inspection and maintenance.

` Ensure area is clean (trash, debris, grass clippings, weeds, etc., removed).

` Inspect green roof for dead or dying vegetation.

` Ensure inlet and outlet pipes are free of trash, debris, etc. Inspect for ponding that may signify clogging at inflow points.

` Inspect waterproof membrane.

` Ensure there are no signs of structural deficiency or settling.



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Maintenance RequirementsGreen roofs represent an alternative to traditional impervious roof surfaces. They typically consist of underlying waterproofing and drainage materials and an overlying engineered growing media that is designed to support plant growth. Runoff is captured and temporarily stored in the engineered growing media, where it is subjected to evaporation and transpiration before being conveyed back into the storm drain system. This allows green roofs to provide measurable reductions in post-construction runoff rates, volumes and pollutant loads on development sites. There are two different types of green roof systems: intensive and extensive. Intensive green roofs have a thick layer of soil, can support a diverse plant community, and may include trees. Extensive green roofs have a much thinner layer of soil that is comprised primarily of drought-tolerant vegetation. Plants chosen for a green roof should be compatible for warmer temperatures found on rooftops.

There are some common problems to be aware of when maintaining a green roof. They include, but are not limited to, the following:

` Clogging in the outlet structure

` Establishing vegetation within the green roof

` Clogging the drainage layer

` Maintaining the proper pH levels for plants

` Pruning and weeding to maintain appearance and prevent roots from potentially compromising the waterproof membrane

Specifically, the following maintenance actions are of critical importance to the long-term performance of the IMP:

` Dead vegetation should be removed along with any woody vegetation

` Plant replacement vegetation as needed

` Remove trash, debris, and other pollutants from the rooftop

` Remove any accumulated sediment or debris

6.2.5 Green Roofs

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