The information in this section is not intended to be a comprehensive green roof design manual. The primary reason for not providing design specifications is that each green roof is unique and needs to be designed considering the factors discussed in this article. Poor design can lead to structural failure of a green roof. The main goals of this article are therefore to provide a detailed list of design considerations and examples of issues to consider when designing a green roof, as well as factors that will affect stormwater treatment performance.
In addition to the information provided on this page, we recommend the following references, which address green roof design.
Readers can also consult with a professional skilled in green roof design for design guidance.
A progression for design of a typical green roof consists of the following 12 steps.
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.
Project budget will be crucial to inform project feasibility and design. Design decisions that can be greatly affected by the project budget include
This initial project budget should be updated at strategic points during the design process.
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 be needed.
Potential roles in green roof design.
Link to this table
|Client||An individual or company (i.e. building owners, developers, consortiums, or government entities) that requests the service of professionals to create a green roof / green building. The owner is the ultimate authority on any project. Their participation in the design process can help to ensure green roof installation.||
|Building architect||An architect is a person licensed in the art of planning, designing and overseeing the construction of buildings. They are the designer of a scheme or plan.||
|Landscape architect||A landscape architect is a person licensed in the art of planning, designing and management of the built or natural environment. Landscape Architects are responsible for the design of the natural environment similar to the way architects are responsible for the design of a building or structure. Some landscape architects are skilled in the integration of building requirements and ecological and environmental design issues||
|Structural engineer||Structural engineers determine the loading requirements of a roof to resist live loads, forces such as snow, rainwater, rooftop equipment, green roofs, and maintenance crews, as well as dead loads, the basic roofing structure and materials.||
|Civil engineer||Civil engineers work closely with architects and landscape architects to design and oversee site utilities including, storm drainage, sewer, electric, water, gas and communications supply.||
|Mechanical engineer||Mechanical engineers design the heating, cooling, and ventilation systems for a building.||
|Electrical engineer||Electrical engineers deal with electrical power transmission.||Provides electricity needed on the green roof for irrigation as well as anything else that required electricity on the roof.|
|Roofing consultant||An independent roofing professional who provides information pertaining to the roof membrane, waterproofing, and determines what system would be best suited to the roof.||
|Cost estimator||A cost estimator calculates how much the green roof will cost during construction and during a determined maintenance period.||
|Irrigation specialist||Specialists retained to design irrigation.||Design and construction observation of irrigation systems.|
|Regulatory bodies||Public entities responsible for reviewing a project for compliance with local, state/provincial and federal codes and ordinances. Because green roof technology is new to the North American market, city officials may not be familiar with the systems requirements and considerations. When applying for a permit you may have to educate the reviewer about green roofs.||
|Green Roof Professional||An individual who has achieved a specific knowledge level regarding green roof design, project management, installation and maintenance through a Green Roof Professional accreditation program.||Individuals passing the accreditation program requirements is trained in the following areas and therefore has a comprehensive understanding of all aspects of green roofs.
Project goals can include
See the section on Cost-benefit considerations for green roofs.
See the section on Cost-benefit considerations for green roofs.
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 90o 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.
Examples include stormwater utility fee credits or grants. For example, Philadelphia businesses can apply for a Green Roof Tax Credit that will provide a rebate for 25 percent of green roof costs up to $100,000. New York City provides a similar credit for green roofs.
Evaluate factors that affect roofing design, such as
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.
General characteristics of extensive and intensive green roofs (adapted from Green Roofs for Healthy Cities and the Cardinal Group, 2006).
Link to this table.
|Growing medium depth||6 inches or less||Portions of the green roof above and below 6 inches, with a minimum of 25% of green roof area above or below 6 inches||More than 6 inches|
|Accessibility||Often inaccessible||May be partially accessible||Usually accessible|
|Fully saturated weight||Low: 10 to 35 lb/ft2 (48.8 to 170.9 kg/m2)||Varies: 35 to 50 lb/ft2 (170.9 to 244.1 kg/m2)||High: 35 to 300 lb/ft2 (170.9 to 1464.7 kg/m2)|
|Maintenance||Varies, but generally lower than for intensive green roofs||Varies||Varies, but generally higher than for extensive green roofs|
|Stormwater management||Best cost-benefit balance||More growing medium and more vigorous plant growth provides marginally greater stormwater volume benefits||More growing medium and more vigorous plant growth provides marginally greater stormwater volume benefits|
This section provides a discussion of issues to consider in green roof design.
Specialized reinforcement is needed to protect green roofs from sliding on slopes steeper than 2:12. 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).
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 slope impact 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). Two examples from the literature are summarized below.
Green roofs may include vegetation free zones designed to
These vegetation free zones are most often located at a minimum around the roof perimeter and around roof drains and other roof penetrations.
The ANSI/SPRI VF-1 External Fire Design Standard for Vegetative Roofs 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 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.
For projects where stormwater goals for volume or pollutant reduction are the primary driver of roof size, use Minimal Impact Design Standards (MIDS) or other credits calculator to determine green roof size needed to meet the goals.
The following components are part of almost all green roofs. Each of these is discussed in greater detail below.
Examples of optional green roof components are listed below. These are also discussed in greater detail below.
Choosing a durable, quality waterproofing assembly is crucial 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.
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.
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 a 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).
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.
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.
Drainage components typically include the following.
“A light-weight, rot-proof material placed over or included as a part of the drainage layer to keep the growing medium in place and thereby prevent fine particles from blocking the drainage system.” (Green Roofs for Healthy Cities and the Cardinal Group, 2006). In most assemblies, a fabric is selected that will freely admit plant roots.
“A combination of organic and inorganic matter than anchors plant roots, drains water from the roof, and sustains plant growth.” ([[References for green roofs|Green Roofs for Healthy Cities and the Cardinal Group, 2006) Growing medium characteristics that affect stormwater performance include the following.
Green roofs need to be protected from erosion during all phases of construction and maintenance. Some techniques that can be used to protect soil from eroding include erosion control blanket, mats, or soil tackifier.Care must be taken not to damage waterproofing membrane when securing erosion control fabric. Once a roof is fully covered with vegetation, the vegetation typically protects soil from erosion.
Plant selection should be informed by
Green roofs with a diverse plant palette are usually more resilient than those with very few species and also generally provide greater stormwater and other ecological benefits. If winter aesthetics are of concern, be sure to include some species with winter interest.
A number of different techniques can be used to install green roof vegetation, each with its own advantages and disadvantages. Choice of technique used to install green roof vegetation will depend on
The following table shows some of the pros and cons of some potential green roof vegetation installation techniques.
Comparison of most common extensive green roof planting methods
Link to this table
|Planting method||Description||Survival rate||Installation labor||Maintenance requirements||Expense||Establishment time||other pros, cons, notes|
|Plugs||Plants in small pots||High||Medium||Low||Low-medium||Quick, 1-2 years depending on species, plug size, and initial planting density||Allows for most flexible and controlled planting design; can be added to green roofs started from cuttings or to pre-grown mats to increase species diversity|
|Cuttings||Small pieces of sedums that are spread across or mixed into growing medium||Good||Low||Low||Low - More than plugs, less than seed||Quick, 1-2 years||Less control over final look than with plugs|
|Seed||Seed||Good||Lowest||Higher||Lowest - Less than plugs or cuttings||Longer than plugs or cuttings – typically 2-5 years depending on species||Less control over final look than with plugs; seed cannot be allowed to dry out until germinated; more bare soil can result in higher weed pressure; need more erosion protection during establishment since soil is bare; only a limited number of species can germinate from seed on a green roof|
|Pregrown Mats||Plants delivered to the site pre-grown into an erosion control mat with growing medium||Good||Low||Low||Low-Medium||Almost instant green||Instant erosion protection if fully vegetated; precise plant composition difficult to predict; less control over final look; less species diversity possible than with plugs, can be combined with plugs to increase species diversity|
|Modular Systems||Plastic, metal, or degradable trays filled with growing medium and delivered to site pre-grown||Good initially||Low||High||High||Almost instant green||Allow for greater precision of design, some may require frequent plant replacement due to edge effect; some trays may retain heat and cause soil to dry out faster, negatively affecting plant health|
A combination of techniques can be used to combine benefits of several techniques as well as to maximize vegetation resilience. Examples include the following.
Leak detection systems allow for pinpointing the exact location of leaks and can also detect small imperfections in the waterproofing. Milestones when leak detection testing is especially valuable include the following.
Several types of leak detection systems are available, including high and low voltage surface surveys and built-in time-domain reflectometer (TDR) sensors. High voltage methods cannot be used in wet environments and therefore are useful only as constructon-phase quality control approach. Low voltage and TDR methods rely on the facts that: 1) the waterproofing membrane is an electrical insulator, and 2) water is an electrically conductive medium. The low voltage method is a survey technique that can be applied to green roof that are designed to enable this approach. For this reason there are few, if any, initial capital costs. TDR sensor arrays must be built into the roofing system. Unlike the low voltage method, however, these systems can provide real-time on-demand information about the waterproofing status and alarm owners if a problem is detected. Descriptions of these techniques are provided in ASTM Standard Methods D6747 and D7007.
Low voltage systems are currently the most commonly used leak detection system.
If leak detection is desired, ensure green roof system is designed to be compatible with leak detection, as leak detection of green roof assemblies that incorporate root-barriers is very challenging, if not impossible, in most instances.
Typically a water holding fabric or a plastic sheet with cup-like depressions, the water retention layer holds water for later use by plants. Water retention layers are available in a range of water holding capacities, typically between 0.06 gal/ft2 and 0.16 gal/ft2
While not all extensive green roofs require permanent irrigation, almost all green roofs require irrigation during the establishment period (unless adequate rainfall occurs), often several times a day. Overhead watering is usually needed immediately after installing plugs, seeds, or cuttings. Even green roofs with underground drip irrigation systems will need overhead watering until the roots have grown enough to reach water from the irrigation driplines. It is therefore essential to ensure access to water will be available during the plant establishment period.
Many different types of irrigation systems exist, including manual or automated spray systems, drip, and flood irrigation systems.
While a simple manual overhead system is less expensive, drip systems are typically more water efficient than overhead systems and provide more uniform coverage. Once vegetation is mature, introducing water from as low as feasible in the growing medium typically also results in the most resilient plants, as it draws plant roots to grow deeper.
A variety of controllers and sensors are available that can be used to maximize water efficiency and stormwater holding capacity. For example: Soil moisture sensors can be used to program irrigation to only be activated when soil is dry and plants need water controllers are available that time irrigation based on weather forecast and predicted evaporation rates, e.g. can be programmed to not irrigate for set length of time before rain is predicted
Potential irrigation water sources include:
While almost all green roofs will need water during the plant establishment period, extensive green roofs can be designed without permanent irrigation. Intensive green roofs almost always need a permanent irrigation system, depending on factors such as project goals and plant palette.
Efficient irrigation is not expected to decrease stormwater benefits of green roofs, since lusher vegetation and moister soils provide greater evapotranspiration.
Advantages of irrigating extensive green roofs include
Advantages of not irrigating extensive green roofs include the following.
Edging, curbs, or borders are often included to separate vegetated areas from non-vegetated areas. Curbs or borders are also sometimes used to provide a firebreak or protection from wind uplift (Green Roofs for Healthy Cities and The Cardinal Group, 2006).
Vegetated roofs generally also include vegetation free zones, for example, in areas prone to high wind uplift, where firebreaks are needed, for protection in areas where icicles are likely to fall, for easier access to roof flashings, or for other maintenance related issues. These vegetation free zones are most often located at a minimum around the roof perimeter and around roof drains and other penetrations. The surface of the vegetation free zones can consist, for example, of roof ballast or pavers. Under the roof ballast or pavers, the assembly is typically the same as for the green roof.
Green roofs dominated by succulent plant varieties and installed with media containing low organic matter content will qualify as Class A fire resistant surfaces based on ASTM E108. Consequently, Sedum-based extensive profiles may qualify as ‘fire breaks’ on otherwise intensive green roof projects.
Railings are often required by code.
Worker safety anchoring systems may also be desired and/or required.
On accessible roofs, amenities include walkways, gathering areas, site furniture, water features, lighting, other structural elements such as trellises and arborwalkways.
Because many different green roof systems are often available that meet project goals, performance specifications can often result in more competitive pricing than descriptive specifications (insert hyperlink to definition), since performance specifications allow for more systems to meet the specifications than descriptive specifications. Performance specifications also allow for the most innovation. Performance specifications typically include required physical and chemical properties of green roof components and the green roof system as a whole, as well as required performance goals. Examples of performance goals are listed below.
Other key elements of green roof specifications typically include, but are not limited to the following: