VEGETATED ROOF

Table 1:  Summary of Stormwater Functions Provided by Green Roofs 1
Stormwater Function	Level 1 Design	Level 2 Design
Annual Runoff Reduction	45%	60%
Total Phosphorus Removal 2	0	0
Total Nitrogen Removal 2	0	0
Channel Protection & Flood Mitigation 3	Use the following Curve Numbers (CN) for Design Storm events: 1-year storm= 64; 2-year storm= 66; 10-year storm= 72; and 100 year storm= 75
1 Sources: CWP and CSN (2008) and CWP (2007). 2 Moran et al (2004) and Clark et al (2008) indicate no or even negative nutrient reduction in early stages of green roof development due to leaching of media. 3 See Miller (2008), NVRC (2007) and MDE (2008)

Table 2:  Green Roof Design Guidance
Level 1 Design (RR:45; TP:0; TN:0)	Level 2 Design (RR: 60; TP:0; TN:0)
Tv = 1.0 (Rv)1 (A)/12	Tv = 1.1 (Rv) 1 (A)/12
Depth of media up to 4 inches	Media depth 4 to 8 inches
Drainage mats	2 inch stone drainage layer
No more than 20% organic matter in media	No more than 10% organic matter in media
All Designs: Be in conformance to ASTM (2005) International Green Roof Standards
1Rv represents the runoff coefficient for a conventional roof, which will usually be 0.95.  The runoff reduction rate applied the green roof is for “capturing” the Tv compared to what a conventional roof would produce as runoff.

SECTION 4:  PHYSICAL FEASIBILITY & DESIGN APPLICATIONS

4.1. Typical applications.

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.

4.2. Common Site Constraints

• Structural Capacity of the Roof: When designing a green roof, designers must not only consider the stormwater storage capacity of the green roof, but also its structural capacity to support it.  A conventional rooftop typically must be designed to support an additional 15-30 pounds per square foot (psf) for an extensive green roof. As a result, 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 to support a green roof.

• Roof Pitch:  Treatment volume (Tv) is maximized on relatively flat roofs (pitch of 1 to 2%). Some pitch is needed to promote positive drainage and prevent ponding and/or saturation of the growing media.  Green roofs can be installed on roofs with slopes up to 25% if baffles, grids, or strips are used to prevent slippage of media. The effective treatment volume (Tv), however, diminishes on rooftops with steep pitches (Van Woert et al, 2005).

• Roof Access:  Adequate access must be available to the roof to deliver construction materials and perform routine maintenance. Roof access can achieved either by an interior stairway through a penthouse or by an alternating tread device with a roof hatch or trap door not less than 16 square feet in area and with a minimum dimension of 24 inches (NVRC, 2007). Designers should also consider how they will get construction materials up to the roof (e.g., by elevator or crane), and how construction materials will be stockpiled in confined space.  

• Roof Type:  Green roofs can be applied to most roof surfaces, although concrete roof decks are preferred. Certain roof materials, such as exposed treated wood and uncoated galvanized metal, may not be appropriate for green rooftops due to pollutant leaching through the media (Clark et al, 2008).

• Setbacks:  Green roofs should not be located near rooftop electrical and HVAC systems. A 2 foot vegetation free zone is recommended along the perimeter of the roof and one foot vegetation free zone around all roof penetrations to act as a firebreak. This setback may be relaxed to a foot for very small green roof applications.

• Retrofitting Green Roofs:  Key feasibility factors to consider when evaluating a retrofit include the area, age and accessibility of the existing roof, and the capability of the buildings owners to maintain it. Options for green roof retrofits are 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.

• Local Building Codes:  Building codes often differ in each municipality, and local planning and zoning authorities should be consulted to obtain proper permits. In addition, the green roof design should comply with the Virginia Uniform Statewide Building Code (VUSBC) with respect to roof drains and emergency overflow devices.

• Construction Cost: When viewed strictly as a stormwater treatment systems, green roofs can cost between $12 to 25 per square foot, ranking them among the most costly stormwater practices available (Moran et al, 2005, Schueler et al 2007). These cost analyses, however, do not include life cycle cost savings relating to increased energy efficiency, higher rents due to green building scores, and increased roof longevity. These benefits over the life cycle of a green roof may make it a more attractive investment. In addition, several communities may offer subsidies or financial incentives for installing green roofs.

• Risks of Leaky Roofs. Although well designed and installed green roofs have less problems with roof leaks than traditional roofs, there is a perception among property managers, insurers and product fabricators that this emerging technology could have a greater risk of problems. For an excellent discussion on how to properly manage risk in green roof installations, see Chapter 9 in Weiler and Scholz-Barth (2009).    

 

SECTION 5:  DESIGN CRITERIA

5.1. 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.

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

Tv =  (RA * D * P)/12

Tv =     Storage Volume (cubic feet)
RA =   Vegetated Roof Area (square feet)
D =      Media depth (inches)
P =       Media porosity, usually 0.3, but consult manufacturer specifications

The resulting Tv can then be compared to the required Tv for the entire rooftop area (including all non-vegetated areas) to determine if it meets or exceeds the required Tv for Level 1 or 2 design, as shown in Table 2.  

Guidance for selecting the appropriate post development CN for the green roof for four different design storms is also provided in Table 2; in general, lower curve numbers are associated with more frequent design storms. In most cases, the maximum design storm is the ten year event.

5.2. Structural Capacity of the Roof:

Green roofs can be limited by the additional weight of the fully saturated soil and plants, in terms of the structural capacity of the roof to bear loads.  Designers should consult with a licensed structural engineer or architect to ensure buildings will be able to support the additional live and dead structural load, determine the maximum depth of the green roof system and any needed structural reinforcement.

In most cases, fully-saturated extensive green roofs have loads of about 15 to 25 lbs/square foot, which is fairly similar to traditional new rooftops (12 to 15 lbs/square foot) with a waterproofing layer anchored with stone ballast. For an excellent discussion on the structural design issues with green roofs, please consult Chapter 9 in Weiler and Scholz-Barth (2009), and ASTM E-2397 Standard Practice for Determination of Dead Loads and Live Loads Associated with Green Roof Systems.

5.3. Functional Elements of a Green Roof System

A green roof is composed of up to eight different systems or layers from bottom to top that are combined together to protect the roof and maintain a vigorous cover. Designers can employ a wide range of materials for each layer, which can differ in cost, performance, and structural load. The entire system as a whole must be assessed to meet design requirements. Some manufacturers offer proprietary green roofing systems, whereas in other cases, the designer or architect must assemble their own system, and are advised to consult Weiler and Scholz-Barth (2009), Snodgrass and Snodgrass (2006) and Dunnett and Kingsbury (2004).    

1.   Deck Layer: The roof deck layer is the foundation of a green roof, and may be composed of concrete, wood, metal, plastic, gypsum or composite. The type of the deck material determines the strength, load bearing capacity, longevity and potential need for insulation in the green roof system. In general, concrete decks are preferred for green roofs, although other materials can be used, as long as the appropriate system components are matched to them.

2.   Waterproofing Layer:  All green roof systems must include an effective and reliable waterproofing layer to prevent water damage through the deck layer.  A wide range of waterproofing materials can be used including built up roofs, modified bitumen, single ply, and liquid applied methods (see Weiler and Scholz-Barth, 2009 and Snodgrass and Snodgrass, 2006).  The waterproofing layer must be 100% waterproof and have an expected life span as long as any other element of the green roof system.

3.   Insulation Layer: Many green rooftops contain an insulation layer, usually located above, but sometimes below, the waterproofing layer. The insulation increases the energy efficiency for the building and/or to protect the roof deck (particularly for metal roofs). According to Snodgrass and Snodgrass (2006), the trend is to install insulation on the outside of the building, in part to avoid mildew problems.

4.   Root Barrier:  The next layer of a green roof system is a root barrier that protects the waterproofing membrane from root penetration.  A wide range of root barrier options are described in Weiler and Scholz-Barth (2009). Chemical root barriers or physical root barriers that have been impregnated with pesticides, metals or other chemicals which could leach into stormwater runoff should be avoided.

5.   Drainage Layer and Drainage System:  A drainage layer is then placed between the root barrier and the growing media to quickly remove excess water from the plant root zone. 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. A wide range of prefabricated water cups or plastic modules can be used, as well as traditional system of protected roof drains, conductors and roof leader. The required depth of the drainage layer is governed by both the required stormwater storage capacity and the structural capacity of the rooftop. ASTM E2396 and E2398 can be used to evaluate alternative material specifications.

6.   Root-Permeable Filter Fabric:  A semi-permeable polypropylene filter fabric is normally placed between the drainage layer and the growing media to prevent the media from migrating into the drainage layer and clogging it.

7.   Growing Media:  The next layer in an extensive green roof is the growing media, which is typically 4-8 inches deep. The depth and composition of the media is described in Section 5.5.

8.   Plant Cover: The top layer of a green roof consists of non-native, slow-growing, shallow-rooted, perennial, succulent plants that can withstand tough conditions at the roof surface. Guidance on selecting the appropriate green roof plants for hardiness zones in the Chesapeake Bay watershed can be found in Snodgrass and Snodgrass (2006). A mix of base ground covers (usually Sedum species) and accent plants can be used to enhance the visual amenity of a green roof.   

5.4. Pretreatment

Pretreatment is not needed for green roofs.

5.5. Filter Media Composition

The recommended growing media for extensive green roofs is composed of approximately 80 to 90% lightweight inorganic materials such as expanded slates, shales or clays, pumice, scoria or other materials. The remaining media should contain no more than 20% organic matter, normally well-aged compost (see Design Specification No. 4). The percentage of organic matter is limited since it can leach nutrients and clog the permeable filter fabric. The growing media should have a maximum water retention capacity of around 30%. It is advisable to mix the media in a batch facility prior to delivery to the roof. More information on growing media can be found in Weiler and Scholz-Barth (2009) and Snodgrass and Snodgrass (2006).

The composition of growing media for intensive roofs may be different, and is often much greater in depth (e.g., 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.

5.6.  Conveyance and Overflow

The drainage layer below the growth media should be designed to convey the ten-year storm to roof downspouts, conductors and leaders without backing water up to into the growing media.  Roof drains should be boxed and protected by flashing extending at least three inches above the growing media to prevent clogging. An outlet should be provided to convey excess stormwater runoff out of the drainage layer and off of the rooftop. An overflow system, such as a traditional rooftop drainage system with inlets set slightly above the elevation of the green roof surface, is frequently used to convey the stormwater runoff from the ten-year design storm event safely off of the rooftop.

5.7. Vegetation and Surface Cover

A planting plan must be prepared for a green roof by a landscape architect, botanist or other professional experienced with green roofs, and be reviewed and approved by the local development review authority.

Plant selection for green rooftops is an integral design consideration, which is governed by local climate and design objectives.  The primary ground cover for most green roof installations are hardy, low-growing succulents, such as Sedum, Delosperma, Talinum, Semperivum or Hieracium that are matched to the local climate conditions and can tolerate the difficult growing conditions found on building rooftops (Snodgrass and Snodgrass, 2006). Much of the Chesapeake Bay watershed lies within USDA Plant Hardiness Zone 7, although some northern areas of the watershed fall in the colder Hardiness Zone 6, and some areas in the extreme southeast portion of the watershed fall in the slightly warmer Hardiness Zone 8 (AHS, 2003).

A list of some common green roof plant species that work well in the Chesapeake Bay watershed can be found in Table 3. Designers may also want to directly contact the short list of mid-Atlantic nurseries for green roof plant recommendations and availability (see Table 3).  

• Plant choices can be much more diverse for deeper intensive green roof systems. Herbs, forbs, grasses, shrubs and even trees can be used, but designers should understand they have higher watering, weeding and landscape maintenance requirements.
• The species and layout of the planting plan should reflect the location of building, in terms of its height, exposure to wind, snow loading, heat stress, orientation to the sun, and shading by surrounding buildings. In addition, plants should be selected that are fire resistant and able to withstand heat, cold and high winds.

• Designers should also match species to the expected rooting depth of the growing media, that can also provide enough lateral growth to stabilize the growing media surface. The planting plan should usually include several accent plants to provide diversity and seasonal color. For a comprehensive resource on green roof plant selection, consult Snodgrass and Snodgrass (2006).

1. It is also important to note that most green roof plant species will not be native to the Chesapeake Bay watershed (which is contrast to native plant recommendations for other stormwater practices such as bioretention and constructed wetlands).
2. Given the limited number of green roof nurseries in the region, designers should order plants 6 to 12 months prior to expected planting. It is also advisable to have plant materials contract grown (see Table 3 for a current list of mid-Atlantic green roof nurseries).    

• When appropriate species are selected, most green roofs in the Bay watershed will not require supplemental irrigation, except for temporary irrigation during dry months as the green roof is established. The planting window extends from the spring to early fall, although it is important to allow plants to root thoroughly before the first killing frost.
• Plants can be established using cuttings, plugs, mats, and, more rarely, seeding or containers. Several vendors also sell mats, rolls, or proprietary green roof planting modules. For the pros and cons of each method, see Snodgrass and Snodgrass (2006). 
• The goal for green roof systems designed for stormwater management is to establish a full and vigorous cover of low maintenance vegetation that is self-sustaining and requires minimal mowing, trimming and weeding.
• The green roof design should include non-vegetated walkways (e.g., permeable paver blocks) to allow for easy access to the roof for weeding and making spot repairs. 

Section 5.8. Material Specifications

Standards specifications for North American green roofs continue to evolve, and no universal material specifications exist that cover the wide range of roof types and system components currently available. The American Society for Testing and Materials (ASTM) has recently issued several overarching green roof standards, which are described and referenced in Table 4.  

Designers and reviewers should also fully understand manufacturer specifications for each system component in Section 5.3, particularly if they choose to install proprietary “complete” green roof systems or modules.

SECTION 6:  REGIONAL AND SPECIAL CASE DESIGN ADAPTATIONS

6.1. Karst Terrain

Green roofs are an ideal practice for karst terrain, although it is advisable to direct downspout discharges at least 15 feet away from the building foundation to minimize the risk of sinkhole formation.

6.2. Coastal Plain
Green roofs are acceptable runoff reduction practice for the coastal plain, but they have a limited water quality function since rooftops are not a major loading source for nutrients or bacteria. Designers should also choose plant materialsthat can tolerate drought and salt spray.

6.3. Winter Performance

Several design adaptations may be needed for green roofs. The most important is to match the plant species to the appropriate plant hardiness zone. In parts of the Bay watershed with colder climates, green roofs should be designed so the growing media is not subject to freeze-thaw, and provide greater structural capacity to account for winter snowfall loads.

6.3. Acid Rain

Much of the Bay watershed experiences acid rain, with rainfall pH ranging from 3.9 to 5.1. Research has shown that green roof growing media can neutralize acid rain (Berhage et al, 2007), but it is not clear whether acid rain will impair plant growth or leach minerals from the growing media.

SECTION 7:  GREEN ROOF CONSTRUCTION SEQUENCE AND INSPECTION

7.1. Construction Sequence

Given the diversity of extensive green roof designs, there is no typical step-by-step construction sequence for proper installation. The following general construction considerations are noted:

1. Construct the roof deck with the appropriate slope and material.
2. Install the waterproofing method, according to manufacturer’s specifications
3. Conduct a flood test to ensure system is water tight by adding at least two inches of water over the membrane for 48 hours to confirm the integrity of the waterproofing system   
4. Add additional system components (e.g., insulation, root barrier, drainage layer and interior drainage system, and filter fabric) taking care not to damage the waterproofing. Drain collars and protective flashing should be installed to ensure free flow of excess stormwater.
5. The growing media should be mixed prior to delivery to the site.  Media should be spread evenly over the filter fabric surface. The growing media should be covered until planting to prevent weeds from growing. Sheets of exterior grade plywood can also be laid over the growing media to accommodate foot or wheelbarrow traffic. Foot and equipment traffic over the growing media should be limited to reduce compaction.
6. The growing media should be moistened prior to planting, and then planted with the ground cover and other plant materials, per the planting plan, or in accordance with ASTM E2400. Plants should be watered immediately after installation and routinely during establishment.
7. It generally takes 12 to 18 months to fully establish the green roof. An initial fertilization using slow release fertilizer (e.g., 14-14-14) with adequate minerals is often needed to support growth. Temporary watering may also be needed during the first summer, if drought conditions persist. Hand weeding is also critical in the first two years (see Table 10.1 of Weiler and Scholz-Barth, 2009 for a photo guide of common rooftop weeds). 
8. Most construction contracts should contain a care and replacement warranty that specify a 75% minimum survival after the first growing season of species planted and a minimum effective vegetative ground cover of 75% for flat roofs and 90% for pitched roofs. 
7.2. 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 and confirm that the contractor’s interpretation of the plan is consistent with the intent of the designer and/or manufacturer.

An experienced installer should be retained to construct the green roof system. The green roof should be constructed in sections for easier inspection and maintenance access to the membrane and roof drains.  Careful construction supervision is needed during several steps of green roof installation, as follows:

• During placement of the waterproofing layer to ensure that it is properly installed and watertight
• During placement of the drainage layer and drainage system
• During placement of the growing media to confirm that it meets the specifications and is applied to the correct depth.
• Upon installation of plants, to ensure they conform to the planting plan
• Before issuing use and occupancy approvals
• At the end of the first or second growing season to ensure desired surface cover in the care and replacement warranty has been achieved

 

 SECTION 8: GREEN ROOF MAINTENANCE

8.1. Maintenance Inspections and Ongoing Operations
A green roof should be inspected twice a year during the growing season to assess vegetative cover, and to look for leaks, drainage problems and any structural rooftop concerns (see Table 5). In addition, the green roof should be hand weeded to remove invasive or volunteer plants, and plants/media added to repair bare areas (Refer to ASTM E2400). Many practitioners also recommend an annual application of slow release fertilizer in the first five years after the green roof is installed. 
If a roof leak is suspected, it is advisable to perform an electric leak survey (i.e., Electrical Field Vector Mapping) to pinpoint the exact location, make localized repairs, and then reestablish system components and ground cover. 
The use of herbicides, insecticides, fungicides should be avoided, since their presence could hasten degradation of the waterproof membrane.   Also, power washing and other exterior maintenance operations should be avoided so that cleaning agents and other chemicals do not harm the green roof plant communities.
Written documentation between the local inspection authority and the property owner or manager should be required in order to ensure adequate notification or authorization for access to conduct inspection responsibilities.

Table 5:  Typical Maintenance Activities Associated with Green Roofs
Activity	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 Annual fertilization (first five years) Weeding to remove invasive plants. Inspect roof drains, scuppers and gutters to ensure they are not overgrown or have organic matter deposits.   Remove any accumulated organic matter or debris. Inspect the green roof for dead, dying, or invasive vegetation.  Plant replacement vegetation as needed.	Semi-Annually

SECTION 9: REFERENCES

The following references and sources were consulted to develop this specification.

American Horticultural Society (AHS). 2003. United States Department of Agriculture Plant Hardiness Zone Map. Alexandria, VA

ASTM International 2005. Standard Test Method for Maximum Media Density for Dead Load Analysis of Green Roof Systems. Standard E2399 – 05 ASTM International. West Conshohocken, PA, available Online: http://www.astm.org/ Standards/E2399.htm.

ASTM International 2005. Standard Test Method for Saturated Water Permeability of Granular Drainage Media [Falling-Head Method] for Green Roof Systems. Standard E2396 – 05 ASTM International. West Conshohocken, PA, available Online: http://www.astm.org/ Standards/E2396.htm.

ASTM International 2005. Standard Test Method for Water Capture and Media Retention of Geocomposite Drain Layers for Green Roof Systems. Standard E2398 – 05 ASTM International. West Conshohocken, PA, available Online: http://www.astm.org/ Standards/E2398.htm.

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.

Berhage, R., A. Jarrett, D. Beattie and others. 2007. Quantifying evaporation and transpiration water losses from green roofs and green roof media capacity for neutralizing acid rain. Final Report. National Decentralized Water Resource Capacity Development Project Research Project. Pennsylvania State University.

Clark, S., B. Long, C. Siu, J. Spicher and K. Steele. 2008. Early-life runoff quality: green versus traditional roofs. Low Impact Development 2008. Seattle, WA. American Society of Civil Engineers.

Dunnett, N. and N. Kingsbury. 2004. Planting Green Roofs and Living Walls. Timber Press. Portland, Oregon.

Maryland Department of Environment. (MDE). 2008. Chapter 5. Environmental Site Design. Green Roofs. Baltimore, MD. 

Miller, C. 2008. Green roofs as stormwater best management practices: Preliminary computation of runoff coefficients: sample analysis in the Mid-Atlantic states. Roofscapes, Inc. Philadelphia, PA.

Moran, A., W. Hunt and G. Jennings. 2004. Greenroof research of stormwater runoff quantity and quality in North Carolina. NWQEP Notes. No. 114. North Carolina State University. Raleigh, NC.

North Carolina State University (NCSU). 2008. Green Roof Research Web Page. Department of Biological and Agricultural Engineering. http://www.bae.ncsu.edu/greenroofs.

Northern Virginia Regional Commission (NVRC). 2007. Low Impact Development Manual. Vegetated Roofs. Fairfax, VA.

Schueler et al  2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD.

Snodgrass, E. and L. Snodgrass. 2006. Green Roof Plants: a resource and planting guide. Timber Press. Portland, OR.

Van Woert, N., D. Rowe, A. Andersen, C. Rugh, T. Fernandez and L. Xiao. 2005. Green roof stormwater retention: effects of roof surface, slope, and media depth. Journal of Environmental Quality. 34: 1036-1044.

Weiler, S. and K. Scholz-Barth 2009. Green Roof Systems: A Guide to the Planning, Design, and Construction of Landscapes over Structure. Wiley Press. New York, NY.