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DRAFT VA DCR STORMWATER DESIGN SPECIFICATIONS No .5: GREEN ROOF

DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 5

GREEN ROOFVersion 1.0

Note to Reviewers of the Stormwater Design Specifications

The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of non-proprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations. These standards and specifications were developed with assistance from the Chesapeake Stormwater Network (CSN), Center for Watershed Protection (CWP), Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia. These standards and specifications are based on both the traditional BMPs and Low Impact Development (LID) practices. The advancements in these standards and specifications are a result of extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal Performance Database (NPRPD). In addition, we have borrowed from BMP standards and specifications from other states and research universities in the region. Table 1 describes the overall organization and status of the proposed design specifications under development by DCR.

Table 1: Organization and Status of Proposed DCR Stormwater Design Specifications:Status as of 9/24/2008

# Practice Notes Status 1

1 Rooftop Disconnection Includes front-yard bioretention 22 Filter Strips Includes grass and conservation filter strips 23 Grass Channels 24 Soil Compost

Amendments3

5 Green Roofs 16 Rain Tanks Includes cisterns 27 Permeable Pavement 18 Infiltration Includes micro- small scale and conventional

infiltration techniques 2

9 Bioretention Includes urban bioretention 310 Dry Swales 211 OPEN12 Filtering Practices 213 Constructed Wetlands Includes wet swales 214 Wet Ponds 215 ED Ponds 21 Codes: 1= partial draft of design spec; 2 = complete draft of design spec; 3 = Design specification has undergone one round of external peer review as of 9/24/08

Reviewers should be aware that these draft standards and specifications are just the beginning of the process. Over the coming months, they will be extensively peer-reviewed to develop standards and specifications that can boost performance, increase longevity, reduce the maintenance burden, create attractive amenities, and drive down the unit cost of the treatment provided.

Green Roof of 11 9/18/081


Timeline for review and adoption of specifications and Role of the Virginia’s Stormwater BMP Clearinghouse Committee:

The CSN will be soliciting input and comment on each standard and specification until the end of 2008 from the research, design and plan review community. This feedback will ensure that these design standards strike the right balance between prescription and flexibility, and that they work effectively in each physiographic region. The collective feedback will be presented to the BMP Clearinghouse Committee to help complement their review efforts. The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR.

The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized until late 2009. The DCR intends that these stormwater BMP standards and specifications will be finalized by the time the regulations become final.

The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR, which is vested by the Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to manage stormwater runoff.

As with any draft, there are several key caveats, as outlined below:

Many of the proposed design standards and specifications lack graphics. Graphics will be produced in the coming months, and some of graphics will be imported from the DCR 1999 Virginia Stormwater Management (SWM) Handbook. The design graphics shown in this current version are meant to be illustrative. Where there are differences between the schematic and the text, the text should be considered the recommended approach.

There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric between ASTM, VDOT and the DCR 1999 SWM Handbook. These inconsistencies will be rectified in subsequent versions.

While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and specifications, additional side-by-side comparison will be conducted to ensure continuity.

Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic systems, water supply wells and public infrastructure. These setbacks can be extremely important, and local plan reviewers should provide input to ensure that they strike the appropriate balance between risk aversion and practice feasibility.

These practice specifications will be posted in Wikipedia fashion for comment on the Chesapeake Stormwater Network’s web site at http://www.chesapeakestormwater.net, with instructions regarding how to submit comments, answers to remaining questions about the practice, useful graphics, etc. DCR requests that you provide an email copy of your comments, etc., to Scott Crafton ([email protected]). The final version will provide appropriate credit and attribution on the sources from which photos, schematics, figures, and text were derived.

Thank you for your help in producing the best stormwater design specifications for the Commonwealth.


mailto:[email protected]

http://www.chesapeakestormwater.net/


DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 5

GREEN ROOFVersion 1.0

SECTION 1: DESCRIPTION OF PRACTICE

Green roofs (also known as vegetated roofs or eco roofs) are alternative roof surfaces that typically consist of waterproofing and drainage materials and an engineered growing media that is designed to support plant growth. Green roofs capture and temporarily store stormwater runoff in the engineered growing media before it is conveyed into the storm drain system. A portion of the captured stormwater evaporates or is taken up by plants, which helps reduce runoff volumes, peak runoff rates, and pollutant loads on development sites.

There are two different types of green roof systems: intensive green roofs and extensive green roofs. Intensive systems have deep engineered growing media depths (greater than 6 inches), and can be planted with a wide variety of plants, including trees. Extensive roofs typically have much shallower engineered growing media depths (2-6 inches) and are planted with carefully selected drought tolerant vegetation. Extensive green roofs, which can cost up to twice as much as traditional roofs, are much lighter and less expensive than intensive green roofs, and are recommended for use on most development and redevelopment sites.

Green roofs typically contain a layered system of roofing, which is designed to support plant growth and retain water for plant uptake while preventing ponding on the roof surface. The roofs are designed so that water drains vertically through the media and then horizontally along a


www.bae.ncsu.edu/stormwater

3


waterproofing layer towards the outlet. Extensive green roofs are designed to have minimal maintenance requirements. Vegetation should be selected so that the roof does 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 water storage capacity of the green roof, but also the structural capacity of the rooftop. A conventional rooftop typically must be designed to support an additional 15-30 pounds per square foot (psf) for an extensive green roof and an additional 60-200 psf for an intensive green roof. Consequently, a structural engineer, architect or other qualified professional should be involved with all green roof designs to ensure that the building has enough structural capacity sufficient to support a green roof.

SECTION 2: PERFORMANCE CRITERIA

The overall stormwater functions of green roofs are summarized in Table 1.

Table 1: Summary of Stormwater Functions Provided by Permeable PavementsStormwater Function Level 1 Design Level 2 Design

Annual Runoff Reduction 45% 60%Total Phosphorus Removal 1 0 0Total Nitrogen Removal 1 0 0Channel Protection Moderate.

Reduced Curve Numbers and Time of Concentration.Flood Mitigation Partial.

Reduced Curve Numbers and Time of Concentration.1 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate.Sources: CWP and CSN (2008) and CWP (2007).

SECTION 3: PRACTICE APPLICATIONS AND FEASIBILITY

Available Space: Green roofs do not require additional space.

Site Topography: Green roofs may be installed on roofs with slopes of up to 25%, although it can be difficult to install them on roofs with slopes greater than 10%.

Available Hydraulic Head: No restrictions.

Depth to Water Table: No restrictions.

Soils: No restrictions.

Contributing Drainage Area: Green roofs cannot be used to manage stormwater from adjacent contributing drainage areas.

Hotspot Land Uses: Not applicable.



Setbacks: Green roofs should not be located near rooftop electrical systems.

SECTION 4: ENVIRONMENTAL AND COMMUNITY CONSIDERATIONS

Not applicable. (NOTE: Are there any concerns that should be listed here?)

SECTION 5: DESIGN APPLICATIONS AND VARIATIONS

Green roofs are ideal for use on commercial, institutional, municipal and multi-family residential buildings. They are particularly well suited for use on ultra-urban development and redevelopment sites. Green roofs can be used on a variety of rooftops, including the following:

Non-residential buildings (e.g. commercial, industrial, institutional and transportation uses)

Multi-family residential buildings (e.g condominiums or apartments)

Mixed-use buildings

Local regulations may also permit the use of green roofs on single family residential roofs.

Site Constraints and Permits: The key factors to consider when investigating a rooftop retrofit includes its area, age, accessibility, structural capacity, and commitment of ownership.

Structural Capacity of the Roof: A key constraint is whether the existing roof can support the additional weight of the soil and plants. A licensed structural engineer or architect should conduct a structural analysis to determine the appropriate type of green roof system and any needed structural reinforcement.

Access to the Roof: Safe access must be available for workers and materials during both construction and maintenance.

Local Building Codes: Building codes often differ in each municipality, and local planning and zoning authorities should be consulted to obtain proper permits.

Options are available to retrofit green roofs in the urban landscape, as described in Profile Sheet RR-3 of Schueler et al (2007). The structural capacity of the existing rooftop can be a major constraint to a green roof retrofits.

SECTION 6: SIZING AND TESTING GUIDANCE

6.1: Overall Sizing

Green roof areas should be sized to capture a portion of the Treatment Volume (Tv). The required size of a green roof will depend on several factors, including the porosity and hydraulic conductivity of the growing media and underlying drainage materials. Site designers and



planners should consult with green roof manufacturers and material suppliers for specific sizing guidelines.

Often, the depth of green roof media will be limited by the structural capacity of the rooftop, particularly in retrofit situations. Designers should consult with a licensed structural engineer or architect to ensure buildings will be able to support the additional structural load.

As a general sizing rule, the following equation can be used to determine the storage volume retained by a green roof:

Storage Volume = (vegetated roof area) x (soil depth) x (soil porosity)

SECTION 7: DESIGN CRITERIA

7.1: Level 1 and 2 Green Roof Design Guidelines

The major design goal for the Chesapeake Bay is to maximize nutrient removal and runoff reduction. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced Level 2 that maximizes nutrient and runoff reduction. To qualify for Level 2, the green roof must meet all design criteria shown in the right hand column of Table 1.

Table 2: Green Roof Design GuidanceLevel 1 Design (RR:45; TP:0; TN:0) Level 2 Design (RR: 60; TP:0; TN:0)

Depth of media 4-6 inches Media depth greater than 6 inches Soil media not tested for P-index Soil media with P index less than 10All Designs: be in conformance to ASTM (2005) International Green Roof Standards

7.2: Pretreatment

Pretreatment for green roofs applications is not necessary.

7.3: Conveyance and Overflow

An outlet should be provided to convey stormwater runoff out of the drainage layer and off of the rooftop. Consideration should be given to the stormwater runoff rates and volumes generated by larger storm events (e.g. a 25-year 24-hour storm event) to help ensure that they are able to safely bypass a green roof system. An overflow system, such as a traditional rooftop drainage system with inlets set slightly above the elevation of the green roof surface, should be designed to convey the stormwater runoff generated by these larger storm events safely off of the rooftop.

7.3: Surface Cover and Filter Media

There are several key filter media and cover factors to take into account:

Waterproofing Layer: All green roof systems should include a waterproofing layer to prevent damage to underlying building rooftops. Waterproofing materials typically used in green roof



installations include reinforced thermoplastic and synthetic rubber membranes. The waterproofing layer should be protected from damage by an overlying layer of protective materials.

Root Barrier: Green roofs should contain a root barrier that protects the waterproofing membrane from root penetration. Chemical root barriers or physical root barriers that have been impregnated with pesticides, metals or other chemicals that may leach into stormwater runoff should not be used.

Drainage Layer: 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 retaining water and providing efficient drainage. The required depth of the drainage layer is governed by the required stormwater storage capacity and the structural capacity of the rooftop.

Filter Fabric: A filter fabric should be placed between the drainage layer and the growing media to prevent the growing media from migrating into and clogging the drainage layer.

Media Depth: For extensive green roof systems, the engineered growing media layer should be 4-6 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 can be used. The depth for intensive roofs may range from 6-48 inches. If trees are included in the green roof planting plan, the growing media must be at least 4 feet deep to provide enough soil volume for the root structure of mature trees.

General Filter Media Composition: The recommended green roof soil media is comprised of approximately 80% lightweight inorganic materials and 20% organic matter (e.g. well-aged compost) and should have a maximum water retention capacity of around 30%.

7.4: Landscaping and Planting Plan

A planting plan must be prepared for a green roof. The planting plan must be reviewed and approved by the local development review authority.

When developing a planting plan, the site planning and design team should consult with a botanist, landscape architect or other qualified professional to identify plants that will tolerate the difficult growing conditions found on building rooftops. Planting recommendations for green roofs include:

Drought-tolerant vegetation that requires little or no 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 most green roof installations. Herbs, forbs, grasses and other groundcovers may also be used, but these plants typically have higher watering and maintenance requirements.

To facilitate maintenance tasks, the roof design should include a non-vegetated perimeter walkway.

SECTION 8: REGIONAL AND CLIMATE DESIGN ADAPTATIONS

Plant selection for green rooftops is an integral design consideration, which is governed by local climate and design objectives. A qualified botanist or landscape architect should be consulted when choosing plant material. For extensive systems, plant material should be confined to hardier, indigenous varieties of grass and sedum. Root size and depth should also be considered to ensure that the plant will stabilize the shallow soil media. Plant choices can be much more diverse for intensive systems.

The location of the building plays an important role in the design process. The height of the roof, its exposure to wind, snow loading, orientation to the sun, and shading by surrounding buildings all have an impact on the selection of appropriate plant species.

SECTION 9: TYPICAL GRAPHIC DETAILS

To be provided. (NOTE: Recommendations?)

SECTION 10: MATERIAL SPECIFICATIONS

All green roofs should be designed in accordance with the ASTM International Green Roof Standards (ASTM E2397, ASTM E2398, ASTM E2399, ASTM 2400, and ASTM E2396). General material specifications for green roofs are described in Table 3.

Table 3: Permeable Pavement Material SpecificationsMaterial Specification Notes

Waterproof Membrane

Reinforced thermal plastic or synthetic rubber membranes.

Designed to convey water horizontally across the roof surface to drains or gutter. This layer may sometimes act as a root barrier.

Root Barrier Impermeable liner that will impede root penetration of the membrane.

Drainage Layer 1-2 inch layer of clean, washed granular material, such as ASTM D 448 size No. 8 stone.

Thickness should be 4 inches for intensive green roof designs.



Table 3: Permeable Pavement Material SpecificationsMaterial Specification Notes

Filter Fabric Needled, non-woven, polypropylene geotextile.

Density (ASTM D3776) > 16 oz/sq. yd. or approved equivalent.Puncture resistance (ASTM D4833) > 220 lbs or approved equivalent.

Growth Media 80% lightweight inorganic materials and 20% organic matter (e.g. well-aged compost). Media should have a maximum water retention capacity of around 30%.

Media should provide sufficient nutrients and water holding capacity to support the proposed plant materials.

Plants Extensive systems: sedums, mosses, herbaceous plants, and grasses.

Intensive systems: ground covers can be expanded to include shrubs, and small trees.

Selected plants should be shallow-rooted, self-sustaining, and tolerant of direct sunlight, drought, wind, and frost.

SECTION 11: CONSTRUCTION SEQUENCE AND INSPECTION

11.1: Construction Sequence for Green Roof Installation

The following is a typical construction sequence to properly install an extensive green roof practice. These steps may be modified to reflect expected site conditions:

Step 1: Install the waterproofing membrane, root barrier, drainage layer, and filter fabric according to manufacturer’s specifications.

Step 2: The growing media should be mixed prior to delivery to the site. Media should be spread evenly over the filter fabric surface, and should then be moistened prior to planting. During construction, care should be taken not to compact the soil media. Foot and equipment traffic to the area should be limited.

Step 3: Plants should be watered immediately after installation and routinely during establishment. If plants are installed during the summer, additional watering will be required to insure plant health. Plant coverage should be such that 90% coverage is achieved within two years of planting.

11.2: Construction Inspection

Inspections during construction are needed to ensure that the green roof is built in accordance with these specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is consistent with the designer’s intent.



An experienced installer should be used to avoid conflicts and maintain accountability for green roof construction. The green roof should be constructed in sections for easier inspection and maintenance access to the membrane and roof drains. Some common pitfalls can be avoided by careful construction supervision that focuses on the following key aspects of green roof installation:

Inspect the water proofing membrane to ensure there are no cracks or areas where water can leak through to the rooftop.

Check the media material to confirm that it meets the specifications and is appplied to the correct depth.

Make sure the desired surface cover has been planted.

12. SECTION 12: OPERATION AND MAINTENANCE

12.1: Maintenance OperationsA green roof should be inspected after construction for plant establishment, leaks, and other functional or structural concerns. Maintenance may include watering and weeding, for which the greatest need occurs in the first two years, as plants become established. The use of herbicides, insecticides, fungicides, and fertilizers should be avoided, since their presence could hasten degredation of the waterproof membrane. The use of native vegetation is recommended to reduce plant maintenance. Irrigation and fertilization is only required during the first year before plants are established. After the first year, maintenance consists of two visits a year for weeding of invasive species, and membrane inspections. A penthouse or roof hatch access should be provided for maintenance and inspection of the green roof.

Care must also be taken with certain activities near the green roof. Activities such as powerwashing or use of cleaning agents, detergents, or other chemicals that may drift onto the green roof may harm the roof’s plant communities.

Table 4: Typical Maintenance Activities Associated with Permeable Pavement SystemsActivity Schedule

Water to promote plant growth and survival. Inspect the green roof and replace any dead or dying

vegetation.

As Needed(Following Construction)

Inspect the waterproof membrane for leaking or cracks. Inspect outflow areas for sediment accumulation.

Remove any accumulated sediment or debris. Inspect the green roof for dead, dying, or invasive

vegetation. Plant replacement vegetation as needed.

Semi-Annually

12.2: Maintenance Inspections

It is highly recommended that a spring maintenance inspection be conducted at green roof sites.



13. SECTION 13: REFERENCES

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.


http://www.astm.org/Standards/%20E2400.htm

http://www.astm.org/%20Standards/E2397.htm

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