4.11 Green Roofs Description: Green 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. 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 remaining stormwaterbefore being conveyed back into the storm drain system. This allows green roofs to provide measurable reductions in post-construction stormwater runoff rates, volumes and pollutant loads on development sites. (Source: http://www.greenroofs.com) Formatted: Font: 14 pt LID/GI Considerations: Green roofs are an excellent method for reducing site impervious area, stormwater runoff volume and pollutant loads, and thermal impacts of development. Green roofs provide outdoor areas and species habitat in developed and highly urbanized areas. The use of green roofs reduces the amount of ground surface area required to treat stormwater runoff, maximizing development space. Formatted: Font: Bold KEY CONSIDERATIONS DESIGN CRITERIA: The use of extensive green roof systems (2-6 deep) should be considered prior to the use of more complex and expensive intensive green roof systems Engineered growing media should be a light-weight mix and should contain less than 10% organic material Waterproofing materials should be protected from root penetration by an impermeable root barrier Green roofs may be installed on rooftops with slopes of up to 25%, but are not recommended for use on rooftops with slopes greater than 10%. The use of extensive green roof systems (2-6 deep) should be considered prior to the use of more complex and expensive intensive green roof systems A landscaping plan should be prepared for all green roofs. The landscaping plan should be reviewed and approved by the local development review authority prior to construction. ADVANTAGES / BENEFITS: Helps reduce post-construction stormwater runoff rates, volumes and pollutant loads without consuming valuable land Particularly well suited for use on urban development and redevelopment sites Use of green roofs allows for more development space on a project site STORMWATER MANAGEMENT CREDITS Runoff Reduction Water Quality Protection Aquatic Resource Protection Overbank Flood Protection Extreme Flood Protection = suitable for this practice = may provide partial benefits = practice has been assigned quantifiable stormwater management credits that can be used to address this SWM Criteria L H L IMPLEMENTATION CONSIDERATIONS Land Requirement Capital Cost Maintenance Burden Residential Subdivision Use: Yes High Density/Ultra-Urban: Yes Roadway Projects: No, Bold Formatted: Font: Not Bold, No underline
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. For redevelopment projects and existing buildings, additional measures (e.g., trusses, joists, columns) may be needed for support.green roofs can be difficult to install on rooftops with slopes of 10% or greater ROUTINE MAINTENANCE REQUIREMENTS: Remove debris from inlet and outlet structures Maintain side slopes and outlet structure Remove invasive vegetation Monitor sediment accumulation and remove periodically 80% 50%50% N/A N/A Rural Use Suburban Use Urban Use POLLUTANT REMOVAL Total Suspended Solids Nutrients - Total Phosphorus / Total Nitrogen removal Metals - Cadmium, Copper, Lead, and Zinc removal Pathogens Fecal Coliform SITE APPLICABILITY Discussion 4.11.1 General Description H M L Construction Cost Maintenance Area Required Soils: Planting media should meet design recommendations Other Considerations: Building drainage (e.g., gutters, deck drains, scuppers) must be capable of managing large rainfall events without inundating the roof. L=Low M=Moderate H=High Runoff Reduction Credit: 60% of the runoff reduction volume provided.stormwater MANAGEMENT PRACTICE PERFORMANCE Runoff Reduction 50% - Annual Runoff Volume 60% - Runoff Reduction Volume Pollutant Removal 1 80% - Total Suspended Solids 50% - Total Phosphorus 50% - Total Nitrogen N/A - Metals N/A - Pathogens 1 = expected annual pollutant load removal Commented [JLS2]: MD SWM Manual Commented [JLS1]: MD SWM Manual, 12 pt, Bold, 10 pt, Underline, Font color: Red Formatted: List Paragraph, Indent: Left: 0", Bulleted + Level: 1 + Aligned at: 0.25" + Indent at: 0.5" Formatted: Normal, Indent: Left: 0.51", No bullets or numbering, Adjust space between Latin and Asian text, Adjust space between Asian text and numbers, 10 pt, Bold, 10 pt Formatted: Adjust space between Latin and Asian text, Adjust space between Asian text and numbers Formatted: Font: 10 pt Formatted: Indent: Left: 0.51", No bullets or numbering Formatted: Indent: Left: 0.26", No bullets or numbering, Bold
Green 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 4.11-17.25).. Stormwater runoff is captured and temporarily stored in the engineered growing media, where it is subjected to the hydrologic processes of evaporation and transpiration, before beingwith any excess conveyed back into the storm drain system. This allows green roofs to provide measurable reductions in post-construction stormwater runoff rates, volumes and pollutant loads on development sites. 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 to 24 inches) that supports a diverse plant community that may even include trees (Figure 4.11-27.26). Extensive green roof systems typically have a much thinner layer of engineered growing media (i.e., 2 to 6 inches) that supports a plant community that is comprised primarily of drought tolerant vegetation (e.g., sedums, succulent plants) (Figure 4.11-37.27). Extensive green roof systems, which can cost up to twice as much as traditional impervious roof surfaces, are much lighter and are 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 in coastal Georgia. Extensive green roof systems typically contain multiple layers of roofing materials (Figure 4.11-17.25), and are designed to support plant Figure 4.11-17.25: Components of a Green Roof System (Source: Carter et al., 2007) Figure 4.11-27.26: Intensive Green Roof System (Source: City of Portland, OR, 2004) Figure 4.11-37.27: Extensive Green Roof System (Source: City of Portland, OR, 2004)
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 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. 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 must be designed to support an additional 15 to 30 pounds per square foot (psf) of load. Consequently, a structural engineer or other qualified professional should be involved with the design of a green roof to ensure that the rooftop itself has enough structural capacity sufficient to support the green roof system. 4.11.2 Stormwater Management Suitability Stormwater Management Credits The Center for Watershed Protection (Hirschman et al., 2008) recently documented the ability of green roofs to reduce annual stormwater runoff volumes and pollutant loads on development sites. Consequently, this low impact development practice has been assigned quantifiable stormwater management credits that can be used to help satisfy the SWM Criteria presented in this CSS: Runoff Reduction Runoff reduction credit can be applied to the green roof contributing drainage area if properly designed, installed and maintained. As shown in Table 4.1.3-2, the runoff reduction volume (RRv) conveyed to a green roof may be reduced by 60%. Water Quality If installed as per the recommended design criteria and properly maintained, 80% total suspended solids removal will be applied to the water quality volume (WQv) flowing to the green roof. Channel Protection No Channel Protection volume (CP v) 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 BMP that provides storage and treatment of the CP v. Proportionally adjust the post-development runoff curve number (CN) to account for the runoff reduction provided by a green roof when calculating the CP v for the regional BMP. See Section 4.1.6.4 for more information about curve number reduction. Overbank Flood Protection No Overbank Flood Protection volume is provided by a green roof. Proportionally adjust the postdevelopment runoff CN to account for the runoff reduction provided by a green roof when calculating the overbank peak discharge (Qp 25) on a development site. See Section 4.1.6.4 for more information about curve number reduction. Extreme Flood Protection No Extreme Flood Protection volume is provided by a green roof. Proportionally adjust the postdevelopment runoff curve number (CN) to account for the runoff reduction provided by a simple green roof for the contributing drainage area when calculating the extreme peak discharge (Qp f) on a development site. See Section 4.1.6.4 for more information about curve number reduction.
Stormwater Runoff Reduction: Reduce the runoff reduction volume (RRv) conveyed through a green roof by 60%. Water Quality Protection: Reduce the runoff reduction volume (RRv) conveyed through a green roof by 60%. Aquatic Resource Protection: Proportionally adjust the post-development runoff curve number (CN) to account for the runoff reduction provided by a green roof when calculating the aquatic resource protection volume (ARPv) on a development site. Overbank Flood Protection: Proportionally adjust the post-development runoff curve number (CN) to account for the runoff reduction provided by a green roof when calculating the overbank peak discharge (Qp25) on a development site. Extreme Flood Protection: Proportionally adjust the post-development runoff curve number (CN) to account for the runoff reduction provided by a green roof when calculating the extreme peak discharge (Qp100) on a development site. In order to be eligible for these credits, it is recommended that green roofs satisfy the planning and design criteria outlined below. 4.11.3 Pollutant Removal Capabilities Green roofs are presumed to remove 80% of the total suspended solids (TSS) load in typical urban postdevelopment runoff when sized, designed, constructed, and maintained in accordance with the recommended specifications. Green roofs remove 50% total Phosphorus and 50% total Nitrogen from stormwater. Green roofs are not presumed to remove fecal coliform or metals such as Cadmium, Copper, Lead, and Zinc. In order to provide the most efficient pollutant removal, green roofs should be designed according to the criteria provided in this section. For additional information and data on pollutant removal capabilities for green roofs, see the National Pollutant Removal Performance Database (Version 3) available at www.cwp.org and the National Stormwater Best Management Practices (BMP) Database at www.bmpdatabase.org. 4.11.4 Application and Site Feasibility Criteria Green roofs can be used on a wide variety of development sites in rural, suburban and urban areas. They are especially well suited for use on commercial, institutional, municipal and multifamily residential buildings on urban and suburban development and redevelopment sites. When compared with other low impact development practices, green roofs have a relatively high construction cost, a relatively low maintenance burden and require no additional surface area beyond that which will be covered by the green roof. Although they can be expensive to install, green roofs are often a component of green buildings, such as those that achieve certification in the Leadership in Energy and Environmental Design (LEED) Green Building Rating System. General Feasibility Suitable for Residential Subdivision Usage YES Suitable for High Density/Ultra Urban Areas YES Regional Stormwater Control NO Level: 1 + Aligned at: 0.5" + Indent at: 0.75" Physical Feasibility - Physical Constraints at Project Site
Drainage Area Green roofs should only be used to replace traditional impervious roof surfaces. They should not be used to treat any stormwater runoff generated elsewhere on the development site. Space Required Green roofs require 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 of greater than 10%. Minimum Depth to Water Table Separation from the water table is not applicable to a green roof. Minimum Head - 6 to 12 inches Soils An appropriate engineered growing media, consisting of approximately 80% lightweight inorganic material, 15% organic material and 5% sand, should be used in green roof systems. Other Constraints / Considerations Hot spots May not be used for hot spot runoff 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 must be designed to support an additional 15 to 30 pounds per square foot (psf) of load. Consequently, a structural engineer or other qualified professional should be involved with the design of a green roof to ensure that the rooftop itself has enough structural capacity sufficient to 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 should remain on the property upon which it was generated until it is discharged to downstream receiving waters, a municipal stormwater system, or to the pre-construction point of discharge Trout Stream Green roofs help to treat stormwater for pollutants and to reduce the volume and velocity of runoff. Therefore, green roofs are an effective BMP for use where trout streams or other protected waters may receive stormwater runoff Coastal Areas: Green roofs can be used without restriction in Coastal Georgia, where there is flat terrain, low site slopes and shallow water tables. 4.4.5 Planning and Design Criteria Overall Feasibility The criteria listed in Table 7.10 should be evaluated to determine whether or not a green roof is appropriate for use on a development site. It is important to note that green roofs have few constraints that impede their use on development sites. Table 7.10: Factors to Consider When Evaluating the Overall Feasibility of Using a Green Roof on a Development Site Site Characteristic Criteria Green roofs should only be used to replace traditional impervious roof Drainage Area surfaces. They should not be used to receive any stormwater runoff generated elsewhere on the development site. Area Required Green roofs require 100% of their contributing drainage areas. Although green roofs may be installed on rooftops with slopes of up to Slope 25%, it can be difficult to install them on rooftop with slopes of greater than 10%. Level: 1 + Aligned at: 0.5" + Indent at: 0.75" Formatted: Font: Not Bold Level: 1 + Aligned at: 0.5" + Indent at: 0.75" Formatted: Indent: Left: 0.36", Tab stops: 0.61", List tab + Not at 0.25" + 0.5" Level: 1 + Aligned at: 0.5" + Indent at: 0.75" Formatted: Underline Formatted: Underline Formatted: Underline Formatted: Underline
Table 7.10: Factors to Consider When Evaluating the Overall Feasibility of Using a Green Roof on a Development Site Site Characteristic Criteria Minimum Head 6 to 12 inches Minimum Depth to N/A Water Table An appropriate engineered growing media, consisting of Soils approximately 80% lightweight inorganic material, 15% organic material and 5% sand, should be used in green roof systems. Site Applicability Green roofs can be used on a wide variety of development sites in rural, suburban and urban areas. They are especially well suited for use on commercial, institutional, municipal and multifamily residential buildings on urban and suburban development and redevelopment sites. When compared with other low impact development practices, green roofs have a relatively high construction cost, a relatively low maintenance burden and require no additional surface area beyond that which will be covered by the green roof. Although they can be expensive to install, green roofs are often a component of green buildings, such as those that achieve certification in the Leadership in Energy and Environmental Design (LEED) Green Building Rating System. 4.11.5 Planning and Design CriteriaPlanning and Design Criteria It is recommended that green roofs meet all of the following criteria. to be eligible for the stormwater management credits described above: Formatted: Font: Not Italic Before designing the green roof, the following data is necessary: Architectural roof plan for rooftop pitches and downspout locations The proposed site design, including, buildings, parking lots, sidewalks, stairs and handicapped ramps, and landscaped areas for downspout discharge locations and bypass outfalls Information about downstream BMPs and receiving waters The following criteria are to be considered minimum standards for the design of a green roof. Consult with the local review authority to determine if there are any variations to these criteria or additional standards that must be followed. 4.11.5.1. LOCATION AND LAYOUT Green roof systems should be designed to provide enough storage for the stormwater runoff volume generated by the target runoff reduction rainfall event (e.g., 85 th percentile rainfall event). 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 are encouraged to consult with green roof manufacturers and/or materials suppliers to design green roof systems that provide enough storage for the stormwater runoff volume generated by the target runoff reduction rainfall event (e.g., 85 th percentile rainfall event). During the design of a green roof system, site planning and design teams should consider not only the storage capacity of the green roof, but also the structural capacity of the rooftop itself. A structural engineer or other qualified professional should be involved with the design of a green roof to ensure that the rooftop itself has enough structural capacity to support the green roof system. Green roofs should not be used to treat any stormwater runoff generated elsewhere on the development site. Formatted: Adjust space between Latin and Asian text, Adjust space between Asian text and numbers, Bold, No underline
4.11.5.2. GENERAL DESIGN General Planning and Design All green roofs should be designed in accordance with the ASTM International Green Roof Standards (ASTM, 2005a, ASTM, 2005b, ASTM, 2005c, ASTM, 2005d, ASTM, 2006). Green roofs should only be used to replace traditional impervious roof surfaces. They should not be used to receive any stormwater runoff generated elsewhere on the development site. Although green roofs may be installed on rooftops with slopes of up to 25%, it can be difficult to install them on rooftops with slopes of greater than 10%. Supplemental measures, such as battens, may be needed to ensure stability against sliding on rooftops with slopes of greater than 10%. Green roof systems should be designed to provide enough storage for the stormwater runoff volume generated by the target runoff reduction rainfall event (e.g., 85 th percentile rainfall event). 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 are encouraged to consult with green roof manufacturers and/or materials suppliers to design green roof systems that provide enough storage for the stormwater runoff volume generated by the target runoff reduction rainfall event (e.g., 85 th percentile rainfall event). During the design of a green roof system, site planning and design teams should consider not only the storage capacity of the green roof, but also the structural capacity of the rooftop itself. A structural engineer or other qualified professional should be involved with the design of a green roof to ensure that the rooftop itself has enough structural capacity to support the green roof system. All green roof systems should 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 should 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 other chemicals that may leach into post-construction stormwater runoff should not be used. A drainage layer should 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 outlet (e.g., scupper and downspout) should be provided to convey stormwater runoff out of the drainage layer and off of the rooftop when the drainage layer becomes saturated. An appropriate engineered growing media, consisting of approximately 80% lightweight inorganic materials, 15% organic matter (e.g., well-aged compost) and 5% sand, should be installed above the drainage layer. The engineered growing media should have a maximum water retention capacity of approximately 30%. To prevent clogging within the drainage layer, the engineered growing media should be separated from the drainage layer by a layer of permeable filter fabric. The filter fabric should 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 should be between 4 and 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. A semi-rigid, plastic geocomposite drain or mat layer should be included to convey runoff to the building drainage system. Flat roof applications may require a perforated internal network to facilitate drainage of rainfall. Additionally, roof flashing should extend six inches above the media surface and be protected by counter-flashing. Level: 1 + Aligned at: 0.5" + Indent at: 0.75", Tab stops: Not at 0.5" Formatted: Indent: Left: 0", Hanging: 0.37" Level: 1 + Aligned at: 0.5" + Indent at: 0.75", Tab stops: Not at 0.5" Commented [JLS3]: MD SWM Manual
Consideration should be given to the stormwater runoff rates and volumes generated by larger storm events (e.g., 25-year, 24-hour storm event) to help ensure that these larger storm events are able to safely bypass the green roof system. An overflow system, such as a traditional rooftop drainage system with inlets set slightly above the elevation of the surface of the green roof, should be designed to convey the stormwater runoff generated by these larger storm events safely off of the rooftop. 4.11.5.3. Physical Specifications / Geometry The drainage area of a green roof is comprised of the green roof itself. No additional stormwater runoff should be allowed to run on to the green roof with the exception of walking paths or vegetation access ways incorporated into the green roof design. 4.11.5.4. Pretreatment / Inlets Green roofs are designed to directly receive rainfall. Pretreatment and inlets are not required. 4.11.5.5. Outlet Structures An outlet (e.g., scupper and downspout) should be provided to convey stormwater runoff out of the drainage layer and off of the rooftop when the drainage layer becomes saturated. 4.11.5.6. Safety Features Consideration should be given to the stormwater runoff rates and volumes generated by larger storm events (e.g., 25-year, 24-hour storm event) to help ensure that these larger storm events are able to safely bypass the green roof system. An overflow system, such as a traditional rooftop drainage system with inlets set slightly above the elevation of the surface of the green roof, should be designed to convey the stormwater runoff generated by these larger storm events safely off of the rooftop. Runoff 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. 4.11.5.7. Landscaping Landscaping A landscaping plan should be prepared for all green roofs. The landscaping plan should be reviewed and approved by the local development review authority prior to construction. 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 harsh growing conditions found on rooftops in coastal Georgia. Planting recommendations for green roofs include: o Drought- and full sun-tolerant vegetation that requires minimal irrigation after establishment. o Low maintenance vegetation that is self-sustaining and does not require mowing, trimming or the use of fertilizers, pesticides or herbicides. o 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 Georgia. Herbs, forbs, grasses and other groundcovers may also 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 percent vegetative cover one year after installation., Not All caps Formatted: Font: (Default) Century Gothic, 10 pt Formatted: List Paragraph, Indent: Left: 0", Hanging: 0.37", Bulleted + Level: 1 + Aligned at: 0.5" + Indent at: 0.75", Tab stops: Not at 0.5" Formatted: Indent: Left: 0", Hanging: 0.37" Formatted: Indent: Left: 0.37", No bullets or numbering Level: 1 + Aligned at: 0.5" + Indent at: 0.75" Formatted: Indent: Left: 0.5", Bulleted + Level: 3 + Aligned at: 1.5" + Indent at: 1.75" Level: 1 + Aligned at: 0.5" + Indent at: 0.75", 12 pt
4.11.5.8. Construction Considerations Construction Considerations To help ensure that green roofs are properly installed on a development site, site planning and design teams should consider the following recommendations: Measures shall be taken to prevent membrane damage during green roof installation. Any flaws, irregularities, or conditions that may cause leaks or roof damage shall be identified and repaired. The waterproofing membrane should be visually inspected and tested for water tightness prior to installation of the planting mix. Where required, slope stabilization measures should be placed prior to green roof installation. In some situations, slope stabilization may be integrated into the roof structure. Green roof systems should be installed according to the manufacturer s instructions. Generally, root-barrier layers, walkways, and irrigation systems should be installed first. To help prevent compaction of the engineered growing media, heavy foot traffic should 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. Green roofs require annual maintenance to ensure optimum performance. Typically, eighteen months are needed to establish adequate initial plant growth. Periodic irrigation may be needed during this time and basic weeding, fertilizing, and in-fill planting may be required as well. After plants are established, the roof should be inspected and light weeding performed once or twice per year. 4.11.5.9 Construction and Maintenance Costs Extensive green roofs range in price from approximately 5 dollars per square foot to 20 dollars per square foot. Intensive green roofs can be considerably more expensive than extensive green roofs. Estimates range from 20 dollars to 80 dollars per square foot. Cost savings associated with reducing energy consumption and longer roof lifespan have been reported with the use of green roofs. Annualized costs should be lowered considerably by the roof's increased lifespan. Other benefits should be taken into account, however, such as recreational space provided and costs relative to the price of land in an area. 4.11.6 Design Procedures Step 1. Determine the goals and primary function of the green roof Consider whether the green roof is intended to: Meet a runoff reduction target or water quality (treatment) target. Provide a possible solution to a drainage problem Enhance landscape and provide aesthetic qualities Check with local officials and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply. In addition, consider if the green roof has any special site-specific design conditions or criteria. List any restrictions or other requirements that may apply or affect the design. Step 2. 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 the slope of the roof, gutter and downspout locations, roof drains, building mechanical equipment locations, and maintenance access to the green roof area. Level: 1 + Aligned at: 0.25" + Tab after: 0.5" + Indent at: 0.5" Commented [JLS4]: MD SWM Manual Commented [JLS5]: MD SWM Manual Formatted: Indent: Left: 0.5", No bullets or numbering Formatted Formatted: Default, Justified, Indent: Left: 0", Hanging: 0.37", Space After: 0 pt, Bulleted + Level: 1 + Aligned at: 0.25" + Tab after: 0.5" + Indent at: 0.5", Widow/Orphan control, Tab stops: Not at 0.36" + 0.7" + 1.18" + 3.5" + 6.9" Commented [JLS6]: US EPA Formatted: Default Paragraph Font, Font: (Default) Arial, 10 pt, Font color: Black Formatted Formatted: Default, Justified, Indent: Left: 0", Hanging: 0.37", Space After: 0 pt, Bulleted + Level: 1 + Aligned at: 0.25" + Tab after: 0.5" + Indent at: 0.5", Widow/Orphan control, Tab stops: Not at 0.36" + 0.7" + 1.18" + 3.5" + 6.9" Formatted
Complete Steps 3A & 3B for a runoff reduction approach, or skip Step 3 and complete Steps 4A & 4B for a water quality (treatment) approach. Refer to your local community s guidelines for any additional information or specific requirements regarding either method. In cases where RRv can be partially met, the remaining amount of WQv must be traditionally treated, refer to Step 5 for an additional discussion. Step 3A. Calculate the Stormwater Runoff Reduction Target Volume Calculate the Runoff Reduction Volume using the following formula: RR v = (P) (R v) (A) / 12 RR v = Runoff Reduction Target Volume (cubic-feet) P = Target runoff reduction rainfall (inches) R v = Volumetric runoff coefficient which can be found by: R v = 0.05+0.009(I) I = new impervious area of the contributing drainage area (%) A = Site area (square feet) 12 = Unit conversion factor (in/ft) Using Table 4.1.3-2 - BMP Runoff Reduction Credits, lookup the appropriate runoff reduction percentage (or credit) provided by the practice: Calculate the Runoff Reduction Volume provided by the selected practice RR v (provided) = (RR%) (RR v) RR v (provided) = Runoff Reduction Volume provided (cubic-feet) by a specific BMP RR% = Runoff Reduction percentage, or credit, assigned to the specific practice RR v = Runoff Reduction Target Volume (square-feet), as calculated at the beginning of Step 3A When RR v(provided) = Target RR v, Water Quality requirements are met. If RR v (provided) is less than the Target RR v, then continue to Step 5. To size the BMP based on RR v(provided), proceed to step 3B to calculate VP. Step 3B. If using the practice for Runoff Reduction, determine the storage volume of the practice To determine the actual volume provided in the green roof, use the following equation: VP = (VES (N)) VP = Volume provided (temporary storage) VES = Volume of Engineered Soils/Planting Media 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 4A. Calculate the Target Water Quality Volume Calculate the Water Quality Volume using the following formula: WQ v = (1.2) (R v) (A) / 12 WQ v = Water Quality Volume (cubic-feet)
1.2 = Target rainfall amount to be treated (inches) R v = Volumetric runoff coefficient which can be found by: R v = 0.05+0.009(I) I = new impervious area of the contributing drainage area (%) A = Site area (square feet) 12 = Unit conversion factor (in/ft) Step 4B If using the practice for Water Quality treatment, determine water quality treatment provided by the green roof practice To determine the actual water quality volume provided by the green roof, use the following equation: WQ vp = (WQ v (WQ / TSS%)) WQ vp = Water Quality Volume provided (temporary storage) WQ v = Target Water Quality Volume (calculate in Step 4A) WQ / TSS% = Water Quality/Total Suspended Solids Removal Rate (from Table 4.1.3-2) 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 5. For a hybrid approach that would provide partial runoff reduction and the remaining amount of volume through traditional water quality treatment, calculate the following parameters: Calculate Target RR v (refer to step 3A) Calculate % Achieved Ratio (% of Overall Reduction provided by RR v): [RR v (provided by practice) / Target RR v] (100) = RR % Achieved RR v provided = Volume of Runoff Reduction that the practice actually provides Target RR v = Volume of Runoff Reduction that is required per local requirements RR % Achieved = The portion of overall water quality requirement volume provided by RRv Calculate WQ v (refer to step 4A) Calculate % Remaining to treat for WQ (% of Overall Treatment provided by WQ v): 100 - (RR % Achieved) = Remaining WQ to treat, as a percentage WQ % Remaining = The remaining portion of volume that must be treated to reach the overall water quality goal. Calculate the remaining volume to be traditionally treated by WQ v (WQ v) [(WQ % Remaining) / 100] = WQ v remaining Step 8. Design bypass / overflow for larger storm events. Overflow structures should be capable of passing the 2-year 24-hour design storm without inundating the roof. See section 3.4 Outlet Structures for additional guidance on designing and installing outlet structures. Formatted: Font: Italic
Step 7. Prepare Vegetation and Landscaping Plan Vegetation is critical to the function and appearance of any green roof. Therefore landscaping plans should be provided according to the guidance in section 4.11.5.7 (Landscaping) and Appendix D. Maintenance Requirements Maintenance is very important for green roofs, particularly in terms of ensuring that they continue to provide measurable stormwater management benefits over time. Consequently, a legally binding inspection and maintenance agreement and plan should be created to help ensure that they are properly maintained after construction is complete. Table 4.7.11-1 provides a list of the routine maintenance activities typically associated with green roofs. Table 47.11-1: Routine Maintenance Activities Typically Associated with Green Roofs Activity Schedule Water to promote plant growth and survival. As Needed Inspect green roof and replace any dead or dying (Following Construction) vegetation. Green roofs may need to be watered periodically if there is As Needed not sufficient precipitation. (Regularly Upon Inspection) Occasional weeding may be required. Inspect waterproof membrane for leaks. Repair as needed. Inspect outflow and overflow areas for sediment accumulation. Remove any accumulated sediment or Semi-Annually debris. (Quarterly During First Year) Inspect green roof for dead or dying vegetation. Plant replacement vegetation as needed. 4.11.8 Additional Resources ASTM International. 2005. Standard Practice for Determination of Dead Loads and Live Loads Associated with Green Roof Systems. Standard E2397-05. ASTM International. West Conshohocken, PA. Available Online: http://www.astm.org/standards/e2397.htm. ASTM International. 2006. Standard Guide for Selection, Installation and Maintenance of Plants for Green Roof Systems. Standard E2400-06. ASTM International. West Conshohocken, PA. Available Online: http://www.astm.org/standards/e2400.htm. City of Portland, OR. 2008. Ecoroof. Portland Stormwater Management Manual. Section 2.3.3. City of Portland, OR. Bureau of Environmental Services. Available Online: http://www.portlandonline.com/bes/index.cfm?c=47952. Pennsylvania Department of Environmental Protection (PA DEP). 2006. BMP 6.5.1: Vegetated Roof. Pennsylvania Stormwater Best Management Practices Manual. Section 6.5.1. Pennsylvania Department of Environmental Protection. Bureau of Watershed Management. Available Online: http://www.depweb.state.pa.us/watershedmgmt/site/default.asp. Formatted: Left, Space After: 6 pt, No widow/orphan control, Tab stops: 0.36", Left + 0.7", Left + 1.18", Left + 3.5", Left + 6.9", Left Formatted: Not Highlight Formatted: Not Highlight, 10 pt, Font color: Black, English (United States) Formatted: Font: 11 pt, 11 pt