DESIGN SPECIFICATION No. 5
March 1, 2011
SECTION 1: DESCRIPTION
Vegetated roofs (also known as green roofs, living roofs or ecoroofs) are alternative roof surfaces that typically consist of waterproofing and drainage materials and an engineered growing media that is designed to support plant growth. Vegetated roofs capture and temporarily store stormwater runoff in the 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 vegetated roof systems: intensive vegetated roofs and extensive vegetated roofs. Intensive systems have a deeper growing media layer that ranges from 6 inches to 4 feet thick, which is planted with a wider variety of plants, including trees. By contrast, extensive systems typically have much shallower growing media (2 to 6 inches), which is planted with carefully selected drought tolerant vegetation. Extensive vegetated roofs are much lighter and less expensive than intensive vegetated roofs and are recommended for use on most development and redevelopment sites.
NOTE: This specification is intended for situations where the primary design objective of the vegetated roof is stormwater management and, unless specified otherwise, addresses extensive roof systems.
Designers may wish to pursue other design objectives for vegetated roofs, such as energy efficiency, green building or LEED points, architectural considerations, visual amenities and landscaping features, which are often maximized with intensive vegetated roof systems. However, these design objectives are beyond the scope of this specification.
Vegetated 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 waterproofing layer towards the outlet. Extensive vegetated roofs are designed to have minimal maintenance requirements. Plant species are selected so that the roof does not need supplemental irrigation or fertilization after vegetation is initially established.
SECTION 2: PERFORMANCE
The overall stormwater functions of vegetated roofs are summarized in Table 5.1.
5.1: Summary of Stormwater Functions Provided by
Vegetated Roofs 1
Level 1 Design
Level 2 Design
|Annual Runoff Reduction (RR)||
|Total Phosphorus (TP) Removal 2||
|Total Nitrogen (TN) Removal 2||
|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 the 100 year storm = 75
Sources: CWP and CSN (2008) and CWP (2007).
2 Moran et al (2004) and Clark et al (2008) indicate no nutrient reduction or even negative nutrient reduction (due to leaching from the media) in early stages of vegetated roof development.
3 See Miller (2008), NVRC (2007) and MDE (2008)
SECTION 3: DESIGN TABLE
The major design goal for Vegetated Roofs is to maximize nutrient removal and runoff volume reduction. To this end, designers may choose the baseline design (Level 1) or choose an enhanced (Level 2) design that maximizes nutrient and runoff reduction. In general, most intensive vegetated roof designs will automatically qualify as being Level 2. Table 5.2 (next page) lists the design criteria for Level 1 and 2 designs.
Table 5.2. 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: Must be in conformance to ASTM (2005) International Green (Vegetated) Roof Stds.|
|1Rv represents the runoff coefficient for a conventional roof, which will usually be 0.95. The runoff reduction rate applied to the vegetated roof is for "capturing" the Treatment Volume (Tv) compared to what a conventional roof would produce as runoff.|
SECTION 4: TYPICAL DETAILS
Figure 5.1. Photos of Vegetated Roof Cross-Sections (source: B. Hunt, NCSU)
Typical Section - Extensive Vegetated Roof
(Source: Northern VA Regional Commission)
Typical Section - Intensive Vegetated Roof
(Source: Northern VA Regional Commission)
SECTION 5: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS
5.1. Typical applications
Vegetated 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. Vegetated roofs can be used on a variety of rooftops, including the following:
Local regulations may also permit the use of vegetated roofs on single family residential roofs.
5.2. Common Site Constraints
Structural Capacity of the Roof. When designing a vegetated roof, designers must not only consider the stormwater storage capacity of the vegetated roof, but also its structural capacity to support the weight of the additional water. A conventional rooftop typically must be designed to support an additional 15 to 30 pounds per square foot (psf) for an extensive vegetated roof. As a result, a structural engineer, architect or other qualified professional should be involved with all vegetated roof designs to ensure that the building has enough structural capacity to support a vegetated roof.
Roof Pitch. Treatment volume (Tv) is maximized on relatively flat roofs (a pitch of 1 to 2%). Some pitch is needed to promote positive drainage and prevent ponding and/or saturation of the growing media. Vegetated roofs can be installed on rooftops with slopes up to 25% if baffles, grids, or strips are used to prevent slippage of the media. The effective treatment volume (Tv), however, diminishes on rooftops with steep pitches (Van Woert et al, 2005).
Roof Access. Adequate access to the roof must be available to deliver construction materials and perform routine maintenance. Roof access can be 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 the confined space.
Roof Type. Vegetated 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 vegetated rooftops due to pollutant leaching through the media (Clark et al, 2008).
Setbacks. Vegetated roofs should not be located near rooftop electrical and HVAC systems. A 2-foot wide vegetation-free zone is recommended along the perimeter of the roof, with a 1-foot vegetation-free zone around all roof penetrations, to act as a firebreak. The 2-foot setback may be relaxed to 1 foot for very small vegetated 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 building's 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 vegetated 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 stormwater treatment systems, vegetated roofs can cost between $12 and $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 vegetated roof may make it a more attractive investment. In addition, several communities may offer subsidies or financial incentives for installing vegetated 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 vegetated roof installations, see Chapter 9 in Weiler and Scholz-Barth (2009).
SECTION 6: DESIGN CRITERIA
6.1. Overall Sizing
Vegetated roof areas should be sized to capture a portion of the Treatment Volume (Tv). The required size of a vegetated roof will depend on several factors, including the porosity and hydraulic conductivity of the growing media and the underlying drainage materials. Site designers and planners should consult with vegetated 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 vegetated roof:
Tv = (RA * D * P)/12
Tv = storage volume (cu. ft.)
RA = vegetated roof area (sq. ft.)
D = media depth (in.)
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 Level 2 design, as shown in Table 5.2 above.
Guidance for selecting the appropriate post development CN for the vegetated roof for four different design storms is also provided in Table 5.2; in general, lower curve numbers are associated with more frequent design storms. In most cases, the maximum design storm is the 10-year event.
6.2. Structural Capacity of the Roof
Vegetated roofs can be limited by the additional weight of the fully saturated soil and plants, in terms of the physical capacity of the roof to bear structural loads. The designer should consult with a licensed structural engineer or architect to ensure that the building will be able to support the additional live and dead structural load and determine the maximum depth of the vegetated roof system and any needed structural reinforcement.
In most cases, fully-saturated extensive vegetated roofs have loads of about 15 to 25 lbs./sq. ft., which is fairly similar to traditional new rooftops (12 to 15 lbs./sq. ft.) that have a waterproofing layer anchored with stone ballast. For an excellent discussion of vegetated roof structural design issues, 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 (Vegetated) Roof Systems.
6.3. Functional Elements of a Vegetated Roof System
A vegetated 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 vegetated roofing systems, whereas in other cases, the designer or architect must assemble their own system, in which case they 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 vegetated roof. It and may be composed of concrete, wood, metal, plastic, gypsum or a composite material. The type of deck material determines the strength, load bearing capacity, longevity and potential need for insulation in the vegetated roof system. In general, concrete decks are preferred for vegetated roofs, although other materials can be used as long as the appropriate system components are matched to them.
2. Waterproofing Layer. All vegetated 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 vegetated roof system.
3. Insulation Layer. Many vegetated rooftops contain an insulation layer, usually located above, but sometimes below, the waterproofing layer. The insulation increases the energy efficiency of the building and/or protects 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 vegetated 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 that 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 vegetation root zone. The drainage layer should consist of synthetic or inorganic materials (e.g. gravel, recycled polyethylene, etc.) 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 a 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 vegetated roof is the growing media, which is typically 4 to 8 inches deep. The depth and composition of the media is described in Section 6.5.
8. Plant Cover. The top layer of a vegetated roof consists of non-native, slow-growing, shallow-rooted, perennial, succulent plants that can withstand harsh conditions at the roof surface. Guidance on selecting the appropriate vegetated 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 value of a green roof.
Pretreatment is not needed for green roofs.
6.5. Filter Media Composition
The recommended growing media for extensive vegetated roofs is composed of approximately 80% to 90% lightweight inorganic materials, such as expanded slates, shales or clays, pumice, scoria or other similar materials. The remaining media should contain no more than 20% organic matter, normally well-aged compost (see Stormwater Design Specification No. 4). The percentage of organic matter should be limited, since it can leach nutrients into the runoff from the roof 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 vegetated roofs may be different, and it is often much greater in depth (e.g., 6 to 48 inches). If trees are included in the vegetated 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.
6.6. Conveyance and Overflow
The drainage layer below the growth media should be designed to convey the 10-year storm without backing water up to into the growing media. The drainage layer should convey flow to an outlet or overflow system such as a traditional rooftop drainage system with inlets set slightly above the elevation of the vegetated roof surface. Roof drains immediately adjacent to the growing media should be boxed and protected by flashing extending at least 3 inches above the growing media to prevent clogging.
6.7. Vegetation and Surface Cover
A planting plan must be prepared for a vegetated roof by a landscape architect, botanist or other professional experienced with vegetated roofs, and it must be reviewed and approved by the local development review authority.
Plant selection for vegetated rooftops is an integral design consideration, which is governed by local climate and design objectives. The primary ground cover for most vegetated roof installations is a hardy, low-growing succulent, such as Sedum, Delosperma, Talinum, Semperivum or Hieracium that is 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 southeastern portion of the watershed fall in the slightly warmer Hardiness Zone 8 (AHS, 2003).
A list of some common vegetated roof plant species that work well in the Chesapeake Bay watershed can be found in Table 5.3 below. Designers may also want to directly contact the short list of mid-Atlantic nurseries for vegetated roof plant recommendations and availability (see Table 5.3).
5.3. Ground Covers for Vegetated Roofs in Chesapeake Bay Watershed
Plant Hardiness Zone 7
Plant Hardiness Zone 6
|Delosperma 'Tiffendell Magenta'||Delosperma cooperi|
|Hieracium lanatum||Delosperma ecklonis var.latifolia|
|Sedum lineare 'Variegatum'||Hieracium villosum|
|Sedum makinoi||Orostachys boehmeri|
|Sedum tetractinum||Sedum hispanicum|
|Sedum stoloniferum||Sedum pluricaule var. ezawe|
|Note: Landscape architects should choose species based on shade tolerance, ability to sow or not, foliage height, and spreading rate. See Snodgrass and Snodgrass (2006) for definitive list of green roof plants, including accent plants.|
Green Roof Plant Vendors in Mid-Atlantic States
1295 Mt. Elbert Road NW
Riner, VA 24149
|Emery Knolls Farm|
3410 Ady Road
Street. Maryland 21154
159 Bay Shore Drive
Nebo, NC 28761
|North Creek Nurseries, Inc.|
388 North Creek Road
Landenburg, PA 19350
7114 McCallum Street
Philadelphia, PA 19119
6.8. Material Specifications
Standards specifications for North American vegetated 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 vegetated roof standards, which are described and referenced in Table 5.4 below.
Designers and reviewers should also fully understand manufacturer specifications for each system component listed in Section 6.3, particularly if they choose to install proprietary "complete" vegetated roof systems or modules.
Extensive Vegetated Roof Material Specifications
|Roof||Structural Capacity should conform to ASTM E-2397-05, Practice for Determination of Live Loads and Dead Loads Associated with Green (Vegetated) Roof Systems. In addition, use standard test methods ASTM E2398-05 for Water Capture and Media Retention of Geocomposite Drain Layers for Green (Vegetated) Roof Systems, and ASTME 2399-05 for Maximum Media Density for Dead Load Analysis.|
|Waterproof Membrane||See Chapter 6 of Weiler and Scholz-Barth (2009) for waterproofing options that are 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 impedes root penetration of the membrane.|
|Drainage Layer||1 to 2 inch layer of clean, washed granular material, such as ASTM D 448 size No. 8 stone. Roof drains and emergency overflow should be designed in accordance with VUSBC.|
|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. Determine acceptable saturated water permeability using ASTM E2396-05.|
|Plant Materials||Sedum, herbaceous plants, and perennial grasses that are shallow-rooted, self-sustaining, and tolerant of direct sunlight, drought, wind, and frost. See ASTM E2400-06, Guide for Selection, Installation and Maintenance of Plants for Green (Vegetated) Roof Systems.|
SECTION 7: REGIONAL & SPECIAL CASE DESIGN ADAPTATIONS
7.1. Karst Terrain
Vegetated roofs are an ideal stormwater control measure 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.
7.2. Coastal Plain
Vegetated roofs are an 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 materials that can tolerate drought and salt spray.
7.3. Cold Climate and Winter Performance
Several design adaptations may be needed for vegetated 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, vegetated roofs should be designed so the growing media is not subject to freeze-thaw, and provide greater structural capacity to account for winter snow loads.
7.3. Acid Rain
Much of the Bay watershed experiences acid rain, with rainfall pH ranging from 3.9 to 5.1. Research has shown that vegetated 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 8: CONSTRUCTION
8.1. Construction Sequence
Given the diversity of extensive vegetated roof designs, there is no typical step-by-step construction sequence for proper installation. The following general construction considerations are noted:
8.2. Construction Inspection
Inspections during construction are needed to ensure that the vegetated 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 vegetated roof system. The vegetated 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 vegetated roof installation, as follows:
SECTION 9: MAINTENANCE
9.1. Maintenance Inspections and Ongoing Operations
A vegetated roof should be inspected twice a year during the growing season to assess vegetative cover, and to look for leaks, drainage problems and any rooftop structural concerns (see Table 5.5 below). In addition, the vegetated roof should be hand-weeded to remove invasive or volunteer plants, and plants/media should be 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 vegetated 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, and 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 vegetated 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 inspections. An example maintenance inspection checklist for Vegetated Roofs can be accessed in Appendix C of Chapter 9 of the Virginia Stormwater Management Handbook (2010).
5.5. Typical Maintenance Activities Associated with Green Roofs
SECTION 10: COMMUNITY & ENVIRONMENTAL CONCERNS
SECTION 11: REFERENCES
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 (Vegetated) Roof Systems. Standard E2399-05. ASTM, International. West Conshohocken, PA. available online: http://www.astm.org/ Standards/E2399.htm.
2005. Standard Test Method for Saturated Water Permeability of Granular
Drainage Media [Falling-Head Method] for Green
(Vegetated) Roof Systems. Standard E2396-05. ASTM, International.
West Conshohocken, PA. available online:
ASTM International. 2005. Standard Test Method for Water Capture and Media Retention of Geocomposite Drain Layers for Green (Vegetated) 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 (Vegetated) 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 (Vegetated) 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.
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. Green roof research of stormwater runoff quantity and quality in North Carolina. NWQEP Notes. No. 114. North Carolina State University. Raleigh, NC.
State University (NCSU). 2008. Green Roof Research Web Page.
Department of Biological and Agricultural Engineering. http://www.bae.ncsu.edu/
Northern Virginia Regional Commission (NVRC). 2007. Low Impact Development Manual. "Vegetated Roofs." Fairfax, VA.
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.