Design criteria for green roofs

Estimate design, installation, and maintenance costs

The information in this section is not intended to be a comprehensive green roof design manual. The main goals of this section are to provide examples of issues to consider when designing a green roof, as well as factors that will affect stormwater treatment performance.

References that address green roof design include the following.

• ANSI/SPRI RP-14 Wind Design Standard for Vegetated Roofing Systems
• ANSI/SPRI VF-1 External Fire Design Standard for Vegetative Roofs
• ANSI/GRHC/SPRI VR-1 Procedure for Investigating Resistance to Root Penetration on Vegetative Green Roofs
• Cantor, S. L. 2008. Green Roofs in Sustainable Landscape Design. W.W.Norton, NY.
• Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (FLL) Richlinien für die Planung, Ausführung and Plege von Dachbegrünung (Guideline for the Planning, Construction and Maintenance of Green Roof –Green Roofing Guideline: developed by the German Research Society for Landscape Development and Landscape Design), latest English Edition
• Greenroofs.com
• Green Roofs Tree of Knowledge (TOK): database on research and policy related to green roof infrastructure at
• Green Roofs for Healthy Cities and The Cardinal Group. 2006. Green Roof Design 101: Introductory Course. Second Edition Participant’s Manual.
• Green Roofs for Healthy Cities. No publication year given. Green Roof Infrastructure: Design and Installation 201.
• [ http://livingarchitecturemonitor.com/ Living Architecture Monitor, A Quarterly Green Roofs for Healthy Cities Publication].
• Mandel, L., 2013, EAT UP; The Inside Scoop on Rooftop Agriculture
• National Institute of Building Sciences. Whole Building Design Guide
• Snodgrass, E.C., and L.L. Snodgrass. 2006. Green Roof Plants. Timber Press, Portland OR.
• Snodgrass, E.C., McIntyre, L, 2010, The Green Roof Manual
• Weiler, S.K., Scholz-Barth, K., Green Roof Systems
• Dunnett, N., and N. Kingsbury. 2004. Planting Green Roofs and Living Walls. Timber Press, Portland OR.

Readers can also consult with a professional skilled in green roof design for design guidance.

Design steps

A typical progression for design of a typical green roof consists of the following 12 steps.

1. Determine project budget
2. Assemble design team
3. Establish goals
4. Estimate design, installation and maintenance goals
5. Conduct lifecycle cost analysis
6. Determine project timeline
7. Determine green roof financial incentives available for projet site, such as stormwater utility fee credits or grants
8. Site analysis
9. Determine type of green roof desired based on goals, constraints and budget
10. Design green roof
11. Refine project design, installation and maintenance, and lifecyle costs based on green roof plan, specifications and detail
12. Adjust design to fit budget if estimated capital or maintenance costs exceed budget.

These steps are explained in greater detail below. Adjust these steps as needed to suit your project. Some projects will not need all these steps, some projects may need additional steps, and the order may need to be changed for some projects.

Determine project budget

Project budget will be crucial to inform project feasibility and design. Design decisions that can be greatly affected by the project budget, for example, are

• how deep growing medium will be,
• whether or not additional structural support is financially feasible if needed, and
• whether or not green roof will be accessible to the building occupants or the public. This initial project budget should be updated at strategic points during the design process.

Assemble a design team

The following table shows roles of various players that can be involved in green roof design and construction. Assemble a team to fit project budget and goals, and level of complexity. Depending on the project, additional roles not shown in the table may also be needed.

Establish goals

Project goals can include, for example:

• Specific stormwater management goals
• Aesthetic goals
• Research and monitoring goals
• Specific uses, such as, for example, food production, gathering spaces
• Minimize building energy usage for heating and cooling
• Wildlife habitat goals
• Marketing and branding as part of an overall green building strategy
• Green building certification, eg. LEED or B3

Determine project timeline

The ideal window for planting green roofs in Minnesota is from after last frost until four weeks before first frost. Planting during extremely hot weather, above 90 F degrees or so, generally has long term negative impacts on plant health and should be avoided. Other issues to consider are discussed in the section on construction sequencing.

Site analysis

Evaluate factors that affect roofing design, such as, for example:

• Climate and microclimate: sun and wind exposure, precipitation, proximity to and reflection from glass curtain walls
• Type and condition of existing waterproofing, deck, and parapet if retrofit
• Roof slope (see 10 A for implications of roof slope)
• Dead and live load structural capacity
• Views of the roof
• Access points
• Roof height
• Roof drain locations and type
• Building HVAC systems; including, for example, nature of intakes and exhausts onto the roof surface and presence of condensate releases; potential impacts on vegetation; impact on potential building heating and cooling energy savings
• Water storage and supply; spigot and tap availability
• Opportunities for rainwater harvesting and storage
• Source of power if needed for irrigation or installation
• Elevations of existing or planned parapets, door thresholds, sills of curtain walls.
• Building code and insurance requirements
• Criteria presented by LEED, Sustainable Sites Initiative, Passive House, Living Building Challenge, etc.

Determine type of green roof desired based on goals, constraints, and budget

The following table describes characteristics of extensive, semi-intensive, and intensive green roofs. In summary, intensive green roofs typically have slightly higher stormwater volume benefits, but also have higher installation and maintenance costs, and require more structural capacity compared to semi-intensive and extensive green roofs.

Design steps

A typical progression for design of a typical green roof consists of the following 12 steps.

1. Determine project budget
2. Assemble design team
3. Establish goals
4. Estimate design, installation and maintenance goals
5. Conduct lifecycle cost analysis
6. Determine project timeline
7. Determine green roof financial incentives available for projet site, such as stormwater utility fee credits or grants
8. Site analysis
9. Determine type of green roof desired based on goals, constraints and budget
10. Design green roof
11. Refine project design, installation and maintenance, and lifecyle costs based on green roof plan, specifications and detail
12. Adjust design to fit budget if estimated capital or maintenance costs exceed budget.

These steps are explained in greater detail below. Adjust these steps as needed to suit your project. Some projects will not need all these steps, some projects may need additional steps, and the order may need to be changed for some projects.

Determine project budget

Project budget will be crucial to inform project feasibility and design. Design decisions that can be greatly affected by the project budget, for example, are

• how deep growing medium will be,
• whether or not additional structural support is financially feasible if needed, and
• whether or not green roof will be accessible to the building occupants or the public. This initial project budget should be updated at strategic points during the design process.

Assemble a design team

The following table shows roles of various players that can be involved in green roof design and construction. Assemble a team to fit project budget and goals, and level of complexity. Depending on the project, additional roles not shown in the table may also be needed.

Establish goals

Project goals can include, for example:

• Specific stormwater management goals
• Aesthetic goals
• Research and monitoring goals
• Specific uses, such as, for example, food production, gathering spaces
• Minimize building energy usage for heating and cooling
• Wildlife habitat goals
• Marketing and branding as part of an overall green building strategy
• Green building certification, eg. LEED or B3

Determine project timeline

The ideal window for planting green roofs in Minnesota is from after last frost until four weeks before first frost. Planting during extremely hot weather, above 90 F degrees or so, generally has long term negative impacts on plant health and should be avoided. Other issues to consider are discussed in the section on construction sequencing.

Site analysis

Evaluate factors that affect roofing design, such as, for example:

• Climate and microclimate: sun and wind exposure, precipitation, proximity to and reflection from glass curtain walls
• Type and condition of existing waterproofing, deck, and parapet if retrofit
• Roof slope (see 10 A for implications of roof slope)
• Dead and live load structural capacity
• Views of the roof
• Access points
• Roof height
• Roof drain locations and type
• Building HVAC systems; including, for example, nature of intakes and exhausts onto the roof surface and presence of condensate releases; potential impacts on vegetation; impact on potential building heating and cooling energy savings
• Water storage and supply; spigot and tap availability
• Opportunities for rainwater harvesting and storage
• Source of power if needed for irrigation or installation
• Elevations of existing or planned parapets, door thresholds, sills of curtain walls.
• Building code and insurance requirements
• Criteria presented by LEED, Sustainable Sites Initiative, Passive House, Living Building Challenge, etc.

Determine type of green roof desired based on goals, constraints, and budget

The following table describes characteristics of extensive, semi-intensive, and intensive green roofs. In summary, intensive green roofs typically have slightly higher stormwater volume benefits, but also have higher installation and maintenance costs, and require more structural capacity compared to semi-intensive and extensive green roofs.

Design green roof

Determine implications of roof slope

Specialized reinforcement is needed to protect green roofs on slopes steeper than 2:12 from sliding. INSERT Figure 5.1: Comparison of roof slope expressed as roof pitch vs. roof slope in degrees. Pitch and degrees on same line express same roof slope. For example, a 1:12 slope is a 4 degree roof slope. Even with reinforcement, slopes should be limited. The German FLL standards, which are widely accepted in the US, recommend that green roofs should not be installed on slopes steeper than 40 degrees. The systems used to stabilize green roof installations on slopes greater than 2:12 depend on the underlying structural capacity and design, and the steepness of the roof. Examples range from geotechnical matting systems like Enkamat, to slope restraint systems, cable grids, and mechanically attached structural grids. An engineered slope stability analysis should be performed for green roofs with slopes above 2:12 (10 degrees). (provide link to http://www.zinco-greenroof.com/EN/greenroof_systems/extensive_green_roofs/pitched_35.php if we want to include an image) Several research studies have been performed on the impacts of roof slope on green roof stormwater performance, with mixed results. See, for example, Berndtsson (2010) for an overview of studies of impact of slope on stormwater performance of green roofs. While some studies found no significant correlation between green roof slope and stormwater runoff (Bengtsson 2005; Mentens et al 2006), others found greater stormwater retention at lower roof slopes (e.g. Getter et al 2007, Van Woert et al 2005.)

Examples of the magnitude of difference found in the studies that did find a correlation between green roof slope and stormwater retention are given below.

Getter et al (2007) studied 12 green roof platforms at varying slopes and varying rain intensities and found mean retention to be greatest (85.6%) at the lowest slope (2%) studied, and least (76.4%) at the greatest slope studied (25%). Retention was also greatest for light rain events (94%) and least for heavy rain events (63%).

Van Woert et al 2005 observed greatest retention (87%) at the lowest slope studied (2% slope), and least retention (65.9%) at the greatest slope studied (6.5%).

Determine what areas of roof can be vegetated and what areas need to remain vegetation free

Green roofs may include vegetation free zones (hyperlink to 10 D m) designed, for example, to: 1) resist wind uplift and scour, 2) reduce fire risk associated with air intakes or proximity to flammable materials and equipment, 3) provide access for roof maintenance related issues 4) provide enhanced flow path toward drains out scuppers for runoff sheeting off walls and parapets and 5) in areas where exhausts onto the roof surface or presence of condensate releases would negatively affect plant growth.

These vegetation free zones are most often located at a minimum around the roof perimeter and around roof drains and other roof penetrations. INSERT Figure 5.2: Vegetation free zones at Target Center Green Roof, Minneapolis, MN, Image courtesy of The Kestrel Design Group, Inc. The ANSI/SPRI VF-1 External Fire Design Standard for Vegetative Roofs, Available at http://www.greenroofs.org/resources/ANSI_SPRI_VF_1_Extrernal_Fire_Design_Standard_for_Vegetative_Roofs_Jan_2010.pdf provides guidance for minimizing the risk of fire on green roofs, including recommendations for location and width of vegetation free zones for fire safety.

ANSI/SPRI RP-14 Wind Design Standard for Vegetative Roofing Systems, available at http://www.greenroofs.org/resources/ANSI_SPRI_RP_14_2010_Wind_Design_Standard_for_Vegetative_Roofing_Systems.pdf provides guidance for minimizing risk of wind uplift on green roofs, including recommendations for location and width of vegetation free zones in areas of the roof particularly vulnerable to wind uplift and scour.

Guidelines for locations and widths are also included in the FLL Green Roofing Guideline.

Currently available guidelines, with the exception of the FLL Green Roofing Guideline, are based on very limited field data. Designers and practitioners should stay abreast of updated recommendations and guidelines as more reliable field information becomes available.

If the green roof will be used to help meet water quantity of water quality goals, determine green roof size needed to meet stormwater and other goals.

The following components are part of almost all green roofs. each of these is discussed in greater detail below.

• Waterproofing assembly
• Root barrier (thermoplastic and thermoset membranes frequently do not require a root-barrier)
• Protection layer
• Drainage components
• Filter layer
• Growing medium
• Wind or surface runoff erosion protection
• Vegetation

Examples of optional green roof components are listed below.

• Leak detection system
• Water retention layer
• Irrigation system
• Edging
• Vegetation free zone
• Railing
• Worker safety anchor systems
• Amenities: for example, walkways, gathering areas, site furniture, water features, lighting, interpretive materials, other structural elements such as trellises and arbors

INSERT Figure 5.3: Typical Green Roof Sections, Images from www.greenroofservice.com INSERT Figure 5.4: Typical native soil vs. Typical Green Roof Profile, Images from ZinCo

Waterproofing assembly

Choosing a durable, quality waterproofing assembly is crucial especially for green roofs, since the waterproofing assembly is buried under the green roof, so repairing or replacing the waterproofing is more costly and more complicated than for a traditional roof. Consult with a roofing consultant or other qualified professional to design the waterproofing assembly for a new roof, or to evaluate an existing roof on which the green roof will be installed. It is highly recommended that the waterproofing membrane is tested for leaks (see l, leak detection system) both after the waterproofing is installed as well as after all construction traffic on the green roof is complete, including, for example, installation of mechanical equipment, or windows on adjacent walls. Testing right after waterproofing is installed allows for correction of any leaks prior to installing the green roof. Testing after all construction traffic on the roof is complete will detect whether or not any leaks developed between the time of the first leak detection test and the completion of all subsequent work on the roof. The Importance of preserving an option for post-construction leak surveys will, however, influence the green roof design. Leak detection of green roof assemblies that incorporate root-barriers is very challenging, if not impossible in most instances.

Workmanship and proper construction sequencing are the factors mostly closely correlated to waterproofing success. Leak testing, while a prudent precaution and check, is not a substitute for craftsmanlike installation of the waterproofing layer. INSERT Figure 5.5: Roof Membrane Installation at Target Center Green Roof, Minneapolis, MN, Image courtesy of The Kestrel Design Group, Inc.

Root barrier

A root barrier prevents plant roots from damaging the waterproofing membrane. When using waterproofing membranes that are root resistant, such as, for example, PVC, TPO and EPDM membranes, a separate root barrier may not be needed. While some waterproofing membranes can resist roots on their own, many will require an additional component to protect the waterproofing membrane from root damage. When plants with vigorous roots are selected, an additional root barrier layer is often installed above root resistant membranes. Common materials used for root barriers include PVC, TPO, and polyethylene. The root barrier is sometimes part of the drainage board.

It is recommended to use al root-barrier that successfully passed the VR-1 test, a standardized method to evaluate root resistance of both waterproofing and root-barrier products: ANSI/GRHC/SPRI VR-1 Procedure for Investigating Resistance to Root Penetration on Vegetative Green Roofs, available at www.spri.org. INSERT Figure 5.6: Root Barrier Welding, Image courtesy of Roofscapes

Protection layer

In most applications a cushioning layer will be installed on top of the waterproofing or root-barrier to resist strains induced by point loads or puncture from sharp protections. This protection layer is a water-permeable, synthetic fiber material with good puncture resistance. It is often part of the drainage panel.

Drainage components

While green roofs are designed to retain and detain stormwater and supply vegetation with the water they need, drainage components are also needed to remove excess water. Inadequate drainage can result, for example, in structural loading problems, major damage to the building, as well as problems with plant health. Drainage capacity must also account for vertical sheet flow from adjacent facades or tall parapets.

It is highly recommended that the waterproofing membrane is tested for leaks (see l, leak detection system) both after the waterproofing is installed as well as after all construction traffic on the green roof is complete, including, for example, installation of mechanical equipment, or windows on adjacent walls. Testing right after waterproofing is installed allows for correction of any leaks prior to installing the green roof. Testing after all construction traffic on the roof is complete will detect whether or not any leaks developed between the time of the first leak detection test and the completion of all subsequent work on the roof. The Importance of preserving an option for post-construction leak surveys will, however, influence the green roof design. Leak detection of green roof assemblies that incorporate root-barriers is very challenging, if not impossible in most instances.

Workmanship and proper construction sequencing are the factors mostly closely correlated to waterproofing success. Leak testing, while a prudent precaution and check, is not a substitute for craftsmanlike installation of the waterproofing layer. INSERT Figure 5.5: Roof Membrane Installation at Target Center Green Roof, Minneapolis, MN, Image courtesy of The Kestrel Design Group, Inc.

Root barrier

A root barrier prevents plant roots from damaging the waterproofing membrane. When using waterproofing membranes that are root resistant, such as, for example, PVC, TPO and EPDM membranes, a separate root barrier may not be needed. While some waterproofing membranes can resist roots on their own, many will require an additional component to protect the waterproofing membrane from root damage. When plants with vigorous roots are selected, an additional root barrier layer is often installed above root resistant membranes. Common materials used for root barriers include PVC, TPO, and polyethylene. The root barrier is sometimes part of the drainage board.

It is recommended to use al root-barrier that successfully passed the VR-1 test, a standardized method to evaluate root resistance of both waterproofing and root-barrier products: ANSI/GRHC/SPRI VR-1 Procedure for Investigating Resistance to Root Penetration on Vegetative Green Roofs, available at www.spri.org. INSERT Figure 5.6: Root Barrier Welding, Image courtesy of Roofscapes

Protection layer

In most applications a cushioning layer will be installed on top of the waterproofing or root-barrier to resist strains induced by point loads or puncture from sharp protections. This protection layer is a water-permeable, synthetic fiber material with good puncture resistance. It is often part of the drainage panel.

Drainage components

While green roofs are designed to retain and detain stormwater and supply vegetation with the water they need, drainage components are also needed to remove excess water. Inadequate drainage can result, for example, in structural loading problems, major damage to the building, as well as problems with plant health. Drainage capacity must also account for vertical sheet flow from adjacent facades or tall parapets.

If the green roof will be used to help meet water quantity of water quality goals, determine green roof size needed to meet stormwater and other goals.

The following components are part of almost all green roofs. each of these is discussed in greater detail below.

• Waterproofing assembly
• Root barrier (thermoplastic and thermoset membranes frequently do not require a root-barrier)
• Protection layer
• Drainage components
• Filter layer
• Growing medium
• Wind or surface runoff erosion protection
• Vegetation

Examples of optional green roof components are listed below.

• Leak detection system
• Water retention layer
• Irrigation system
• Edging
• Vegetation free zone
• Railing
• Worker safety anchor systems
• Amenities: for example, walkways, gathering areas, site furniture, water features, lighting, interpretive materials, other structural elements such as trellises and arbors

INSERT Figure 5.3: Typical Green Roof Sections, Images from www.greenroofservice.com INSERT Figure 5.4: Typical native soil vs. Typical Green Roof Profile, Images from ZinCo