Home Articles New Design Procedure and Criteria used in Vertical Gardens and Green Roof Constructions

Design Procedure and Criteria used in Vertical Gardens and Green Roof Constructions

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vertical garden
Dr. A.N. Sarkar
Ex-Senior Professor (International Business) & Dean (Research), Asia-Pacific Institute of
Management, New Delhi

A vertical garden also known as green wall or living wall is self sufficient vertical garden attached to exterior or interior walls of a building. In vertical gardens, various types of modular panels can be used along with geo-textile fabrics, growing media, irrigation systems, and plants. Living walls are particularly suitable for cities, as they allow good use of available vertical surface areas. The living wall could also function for urban agriculture, urban gardening, or for its beauty as art. Green walls may be indoors or outside, freestanding or attached to an existing wall, and come in a great variety of sizes. A Green roof is a green space created by adding layers of plants on top of a traditional roofing system. There are several important designs and structural differences between ground level landscape development and rooftop developments. Green roofs provide ecosystem facilities in cities, including mitigated urban heat island effect, balancing of building temperature, improved run-off water management, and improved urban biodiversity. Designers try to improve the environmental sustainability of their society by adding more vegetation through green roofs.

Green roofs have been heralded as a “sustainable building practice” in cities throughout the world as one response to mounting environmental stresses. The use of vegetation on walls and roofs of buildings, particularly roofs, is a common practice in the architecture of older buildings in countries like Iceland, Scandinavia, Switzerland, Germany and Tanzania. Today, a thriving green roof industry prevails in many parts of the world, as jurisdictions recognize the myriad environmental benefits associated with the technology. Throughout Europe, particularly Germany, the last thirty years have produced a wide complement of academic green roof research, predictable research funding and a proliferation of green roof installations.

The paper discusses the various design procedures and criteria that are employed in vertical garden and Green roof construction works.

1.0. Design Considerations of Vertical Gardens/ Green Walls

With multiple benefits to improve the urban heat island, aesthetics, biodiversity, sound control, and building energy savings, the market for green walls is growing rapidly. The challenge is to develop wall systems that are durable and cost-effective for commercial applications, residential towers and prefabricated structures. There is a huge potential for inexpensive green facades on big-box retail, industrial buildings, freeways, rapid transit, and blank concrete walls, as well as for rooftops that cannot support the weight of a green roof. Modular living walls, if integrated with the building envelope and intensively maintained, can support higher density plantings of groundcovers, ferns and flowers. By transforming urban environments with green facades and living walls, cities will become more livable, cooler and quieter (https://www.asla.org/uploadedFiles/CMS/Resources/22610_SableJames.pdf).

A green roof can be an oasis in the city landscape. A green roof is a roof surface, flat or pitched, that is planted partially or completely with vegetation and a growing medium over a waterproof membrane. They may be ‘extensive’ and have a thin growing medium (up to 200mm deep) with ‘groundcover’ vegetation, or ‘intensive’ and have soil over 200mm deep supporting vegetation up to the size of trees. Green walls are external or internal vertical building elements that support a cover of vegetation rooted either in stacked pots or growing mats.

Green roofs are an accepted part of modern building in Europe where some city and national governments have mandated their use. The Austrian city of Linz, for example, requires green roofs on all new residential and commercial buildings with rooftops larger than 100m2, and German green roof building has been encouraged by the Federal Nature Protection Act, the Building Code and state-level nature protection statutes. Australian examples are less common but in 2007 a national organization was formed to promote green roofs, and Brisbane City Council included green roofs in its proposed action plan for dealing with climate change.

The growing interest in green roof and wall construction has been encouraged by the increasing availability of technologies that make their construction easier and more economical. Earth-sheltered houses have green roofs by design, and anyone who has grown climbers across a vertical trellis has had some experience in creating green walls. The growing interest in green roof and wall construction has been encouraged by the increasing availability of technologies that make their construction easier and more economical.

Green roofs and walls have become common features in illustrations of modern architectural and urban design proposals but their implementation remains limited by perceptions of high costs and questions over their utility. A changing climate and increasing demands for high levels of environmental performance are likely to see these arguments continue to shift in their favour (http://www.yourhome.gov.au/materials/green-roofs-and-walls).

Design Considerations of Vertical Gardens/ Green Walls include the following:

There are several important design and structural differences between ground-level garden design and vertical garden design. Here are a few special construction requirements and considerations for developing the vertical garden/green-wall.

  • Protection of the integrity of the structural support system and waterproofing protection of that system,
  • Positive drainage throughout the system so that plantings at the bottom of what may be a couple of stories tall of planting will have optimal growing conditions without becoming oversaturated
  • A long-term, lightweight planting medium that is not subject to deterioration through decomposition (normally, this is a synthetic/manufactured product).
  • Irrigation and fertilization for optimum plant growth and sustainability
  • Adaptation of the plantings to the environmental conditions

Design procedures and various design criteria employed for construction of Vertical Gardens/ Green Walls include the following:

Provisions for Maintenance

The single most important element in the construction of a green-wall is to protect the integrity of the structural components supporting the vertical garden. For this reason there must be waterproofing of exceptional longevity to prevent damage and to reduce the possibility of a long term expensive reconstruction, think about these things:

Load Bearing Capacity

You just cannot hang a green-wall on any old wall you have, they can weigh from 10 to 40 pounds per square foot once they are fully saturated with water. Consult with a structural engineer to verify the load bearing capacity of your wall (or whatever structure will support the green-wall.

Waterproofing

There are several types of waterproofing available; however, fluid applied elastomeric materials offer excellent protection. The modular system is best protected by a powder coated protection similar to the racks in a dishwasher. Also, if funding permits the structural members can be constructed of stainless steel verses waterproofing the system.

Planting Provisions

Like any planting the success of the installation depends on providing the correct infrastructure on which the vertical garden is built; therefore, it is imperative to take care in choosing and installing materials of the highest quality and species conductive to the green-wall environment. In an arid region you should use plant that can stand the dry conditions, in an interior green-wall you need to select plants that can take the lighting levels offered by the space.

Drainage Requirements

The system can be designed as a closed drainage system or an open system. In closed drainage systems the water irrigation water is collected and recycled; however, this system needs to have a little water sent to the drain to control the build-up of soluble salts that are left over as the water evaporates. Failure to drain water from the system can result in damage to the green-wall. The closed system is more ecological to operate; however, it does have drawbacks. If any plant disease is introduced into the plantings, it is transmitted throughout the system by the use of recycled water. This drawback can be managed by a process of ultra violet sanitation of the recycled water. In an open system, excess irrigation water is discharged into the buildings drainage system. By using an open system the build-up of salts in the planting medium, and the sanitation issues, is reduced significantly. Excess irrigation water must be captured at the base of the green-wall.

Planting Media Preparation

The critical criteria of a suitable planting media for green-wall planting: It should be light weight; it should have the ability to hold nutrients, and should offer adequate moisture-holding capacity and the capability of developing a firm root zone (for plant stability) – but it must also drain easily. Additionally, if the system is to become an active green-wall/bio-wall air filtration system the media must provide adequate air exchange.

Irrigation System Development

The supply of moisture to the soil mass is critical to the survival of the green-wall. Although it may sound like an elementary process, the supply of moisture to the plants is a rather complex operation. The supply of water, how it will be supplied, and some inherent problems with water types, are all factors to be considered in the design process so the plantings can be successful and economical. The relatively thin, well-drained soilless mixtures used in a green-wall cannot provide the plantings with the subsurface water normally available to ground level plantings. Care must be exercised to prevent the planting media from drying out and causing damage to the plant materials. Normally a sensor controlled drip irrigation system can manage the irrigation system using a minimum of water.

Provision for proper Ventilation

Good ventilation is necessary in the photosynthesis and transpiration cycles because it helps dissipate the diffused water vapor molecules resulting from transpiration. Good ventilation is necessary to maintain a normal transpiration rate, which in turn, is necessary for the normal photosynthetic rate. However, placement of HVAC grilles directly next to the planting can result in problems of drying and burned foliage due to excessive moisture loss (i.e. forced transpiration). This is mainly a problem during the winter heating season when hot, dry, blowing air comes in direct contact with the foliage of the plant.

Drafts

Interior green-walls should not be subject to drafts of hot or cold air. Even a rapid change of 15 to 20 degrees has the potential to damage foliage plants. Consistent temperatures are best for interior foliage plants. Heat and cold radiation close to exterior glass and doors can and will damage plants, unless it is modified through air circulation.

Temperature Control

Most interior green-walls are designed using tropical and semi tropical plant materials. This is because those plant materials are adaptable to our human comfort zones. Therefore it is generally recognized that a temperature of 72 degrees Fahrenheit in the winter and 75 degrees Fahrenheit in the summer are acceptable. Night temperatures can – and should – be 10 degrees cooler, or between 62 to 65 degrees F.

Other Design Considerations

While the considerations listed below do not deal directly with green-wall construction requirements and considerations, these factors should be taken into account along with the considerations mentioned above.

Finishing of Surrounding Areas

A green-wall system generates humidity. It is possible through routine operation and maintenance that some dripping or splashing may occur. This should be taken into account when determining the finishes around the green-wall system. Storage Provisions should be made for the storage of gardening materials and supplies. This storage should be readily available to the horticultural staff members who care for the plantings. Additionally, this area can function as the location of the green-walls irrigation controller.

Safety and Serviceability

The difficulty in managing a vertical garden is the safety and serviceability of the plantings. Specifically designed lift systems or ladder systems must be included to facilitate routine maintenance.

Professional Maintenance

Regular professional maintenance of the plantings and facilities should be accounted for in budgeting and planning. Special attention must be given to pruning plant materials in order to maintain balance between plant size and root growth (based on limited availability of rooting area). Regular grooming on the plants to remove dead and dying foliage should be accounted for, and systems should be designed to facilitate this activity.

 

 

2.0. Designing for Deriving Specific Benefits

Our modern society is all about image and nothing beats nature for beauty. A well-designed, flourishing green wall can drastically improve a building’s appearance, adding color and texture that won’t go out of fashion. Each wall is specifically designed, using different varieties of plants which can vary in color, growth and flower to create living art, rather than a lawn on the side of your building (http://www.ambius.com/blog/ultimate-guide-to-living-green-walls/). Designing Vertical gardens/ Green wall and Green roofs are well thought of and planned for with the objectives of deriving some tangible and intangible benefits such as follows (http://iec.edu.in/wp-content/uploads/2016/01/7_Piyush-Sharma_Vertical gardens-pp.42-48.pdf):

Increased Biodiversity: Green walls can help mitigate loss of biodiversity due to the effects of urbanization, help sustain a variety of plants, pollinators and invertebrates, and provide habitat and nesting places for various bird species.

Improved Health and Well-Being: Buildings that feature and promote access to vegetation have been documented as having a greater positive human health impact than those without. Studies have shown that visual access to natural settings leads to increased job satisfaction and productivity and postoperative recovery rates in medical facilities.

Urban Agriculture: Green walls offer the opportunity for urban agriculture, such as vertical gardens of small fruits, vegetables, and herbs.

Onsite Wastewater Treatment: Several water-recycling systems can be applied to green walls. These systems pump grey water through a green wall, which then passes through filters, gravel, and marine plants. Treated water is then sent to a grey water holding tank for household or irrigation use or released into the public water treatment system. Some of these systems also collect storm water, which is filtered for household use or irrigation purposes.

There are several important design and structural differences between ground-level garden design and vertical garden design. Here are a few special construction requirements and considerations for developing the vertical garden/green-wall. Protection of the integrity of the structural support system and waterproofing protection of that system, Positive drainage throughout the system so that plantings at the bottom of what may be a couple of stories tall of planting will have optimal growing conditions without becoming oversaturated A long-term, lightweight planting medium that is not subject to deterioration through decomposition (normally, this is a synthetic/manufactured product).

Provisions for Maintenance: The single most important element in the construction of a green-wall is to protect the integrity of the structural components supporting the vertical garden. For this reason there must be waterproofing of exceptional longevity to prevent damage and to reduce the possibility of a long term expensive reconstruction, think about these things:

Load Bearing Capacity: You just ca not hang a green-wall on any old wall you have, they can weigh from 10 to 40 pounds per square foot once they are fully saturated with water. Consult with a structural engineer to verify the load bearing capacity of your wall (or whatever structure will support the green-wall.

Waterproofing: There are several types of waterproofing available; however, fluid applied elastomeric materials offer excellent protection. The modular system is best protected by a powder coated protection similar to the racks in a dishwasher. Also, if funding permits the structural members can be constructed of stainless steel verses waterproofing the system.

3.0. Design Fundamentals of Vertical Garden/Green Walls

The vertical garden or plant wall, green wall, bio wall is a light framed, mostly self–supporting plant community where the necessary water, light and liquid plant nutrients are provided by a highly automatized system. The system is based on the principles of hydroponics where the plants are rooted in a porous material soaked in fertilizer instead of soil. Adapting its construction to improve the filtration capacity, a vertical garden can be used for greywater treatment as well, permitting to reuse the treated water as shown in Figure 1.

The various design principles and criteria used in construction of Vertical gardens are summed up as follows.

Criteria used to choose the ideal system: Analysis of the building/support structure and site to determine best system for the client.  Analysis results will inform design limitations and opportunities.

Environmental Factors: i.e. sunlight hours, orientation to the sun, whether additional lighting is needed, wind direction and intensity, precipitation patterns (outdoor), shading, temperature range

Structural Factors: i.e. weight capacity, structural integrity, ideal site location, access to green wall install and ongoing maintenance, access to utilities, i.e. water, drainage, electricity Design Intent:

  • Significantly reduce indoor air pollution and improve air quality
  • Improve the health of people, work environment; reduce stress and increase productivity
  • Provide thermal and sound insulation and mitigate uncomfortable extremes
  • Capable of removing volatile organic compounds (VOCs)
  • Significantly reduce urban heat island effect and remove excess atmospheric carbon dioxide

Safety and Building Codes:

New or Retrofit 

  • Can be added to existing structures but advisable to engage other disciplines to determine opportunities and limitations of the building /site
  • Applicable in new building, working with the multidisciplinary design team to execute

Structure, Location, Size 

  • Environmental and structural factors will dictate the size, location and structure
  • Green wall façades and living walls do not sit directly on the wall; they are anchored and hover to the surface or are independent structures
  • Waterproofing to prevent degradation of the existing structure is essential
  • Location’s limited by environmental and structural considerations
  • Maintenance access and frequency will inform location, size and structure

Access, Utilities, Equipment

  • Access for maintenance and replanting
  • Additional lighting requires access to utilities
  • Access to water supply for irrigation for certain design systems
  • Access to plumbing for drainage needed for some systems, albeit not all

Equipment for monitoring moisture and automatic irrigation

Lighting 

  • Measurement of available ambient light
  • Light requirements of the plants selected will affect their survival
  • Additional input of lights require access to an adequate power source

Drainage

  • Plumbing access for drainage can decide system type
  • Drainage access is a factor for the site location

Closed looped systems do not require additional drainage or plumbing access

Construction Installation

Pre-planted or planted on site

Pre-planted: plants installed in modules and grown in a greenhouse or warehouse, then modules transported to project site for installation

Planted on site: plants delivered to project site, removed from their growing medium, i.e. soil, and input bare root into the growing medium or set in tray systems with wicks

Sown on site: plants intolerant of transplanting or root disturbance that must be sown into the growing medium after the modules/growing medium have been installed

Green Wall Types

Modular Hydroponics: support frame for large panels or “tiles” with openings for plants, grown hydroponically or semi-hydroponically with drip, wick or spray irrigation/fertigation

Bio filtration: system that integrates hydroponically-grown plants with an air-cleansing system –can be connected to HVAC system

Fixed Hydroponics:

Vertical Display: plants in pots or trays supported by rack or frame, hand-watered or irrigated with drip, wick or spray system – could be used for event or seasonal display with plants rotated for color and pattern

Plant Pockets: plants inserted into fabric pockets that provide support and moisture retention can be used for display but somewhat long lasting for interiors or mild climates

Trellised: Use vining and climbing plant where trellis is the medium for vertical growth

Hybrid Installation: living wall system is integrated with green façade system such as trellises for vining or climbing plants; modular system can also be accented with display pots

Artificial/Replica Plants: high quality, artificial plants can be used where live plants cannot be grown or maintained due to structural, environmental or maintenance limitations

Retention/Retaining Walls: engineered green walls that use durable materials to create permanent stabilization of walls with steep slopes, while incorporating plant aesthetics, reducing erosion and run off

Drainage Availability 

  • Limits the site location and design system type for access to plumbing for external drainage
  • Closed loop systems do not require additional access to plumbing for external drainage.  Size, growing medium and plants selected can demand additional drainage.

Technology

  • Emerging technology includes automatic drain valves, moisture and nutrient sensors, remote monitoring
  • Technology to combat winter conditions i.e. freeze sensors, radiant heat mats, polycarbonate panel covers and polyethylene row crop covers for insulation and wind protection
  • Wind screen covers can be placed over exterior green walls to prevent wind damage

3.1. Design and Construction of Hydroponic System

Site Location

Site location is essential to growing successfully. The ideal location to produce hydroponics is a greenhouse that has been positioned for full exposure to the sun. The site should be for the most part level and be located near a large population (Resh, 1995). The system was set up in greenhouse of the crops unit. This greenhouse is ideal because it is cooled with an evaporative cooler and is located with full exposure of the sun.

Available Space

A well-designed layout can improve the ease of maintenance and cleaning. The space needs to be able to accommodate the nutrient rich reservoir and the troughs that the water will be ran through. For this design space was limited. The tower needed to be designed to fit onto a 74” wide steel bench. The tower design took up 46” x 6’ in space. The length of the design comes from the 6’ length of the troughs selected for use. This design was smaller than what was originally anticipated. The height of the design was limited by the greenhouse lighting and gave a max height available of 7’. The original system that was designed stood on the ground and rose up 6’. This would allow for more troughs to be positioned on the tower allowing for more plants per area.

Reservoirs and Pumps

A reservoir in a hydropoinc system is important as it functions as the source of the nutrient solution that is used to feed the plants. For the NFT system it is important to supply enough water to fill the trough to ensure the plants roots can absorb water. The maintained water level in the reservoir is critical to system maintenance. If the water level drops too low and the pump run dry the pump can burnout and leave the plants un-irrigated. Reservoirs need regular cleaning and refreshing of water to remove algae and harmful pathogens that can develop in the water. The pump selected will need to be able to overcome the tower height and supply each plant with water. The other requirement of the pump is reliability. If the pump goes out and the plants are left without water continuously running to them they will dry out and die.

Nutrient Selection

The nutrient solution that was used for this project was MaxiGro purchased from General Hydroponics in San Luis Obispo. The nutrient contents are shown in Table 1 and these include all primary, secondary, and micronutrients. As instructed on the back of the bag one teaspoon of nutrient solution was added with every gallon of water added. For simplicity five teaspoons were mixed in a five-gallon bucket with water from the greenhouse. Once the bucket was filled it was dumped into the reservoir. Ten gallons were added initially and the pump was turned on. Once the NFT troughs were filled to the selected level and the water was draining from each hose another five gallons was added to the reservoir.

Plant Selection

The plant selection included Bonnie Spinach, Swiss Chard, Head Lettuce, Winterbor Kale, and Romaine Lettuce. These leafy lettuces were all selected to determine which one grew the best with each system, which one grew the best with the provided nutrients, and whether or not you could grow different types of leafy greens together using one hydroponic system.

The Design and Installation of the Systems

The tower and two systems were designed and constructed with a lot of work, time, and effort, but after the installation was complete the system was capable of producing high quality leafy lettuce a rapid pace. From my personal experience the construction of this project can be done by anyone with reasonable tool experience and the right set of tools as well as the time commitment to see the project through. The design and construction phases of the project took the most time. The repairs that needed to be done took up a good amount of time as well.

Maintenance

Other than the problems mentioned earlier the only maintenance that needed to be done was adding water when the water level in the reservoir dropped to low. This was done by mixing up 5 gallons of water with nutrient mix and dumping it into the reservoir. An important thing to note is that algae did not become an issue through the 18 days of growth. This lack of growth could have been due to the weekly reservoir water change outs.

Plant Production

In the beginning of the project the pump was shut off the second and third days after transplantation happened. This happened because the pump was tied to the lights and someone unknowingly turned the lights off which turned the pump off. Once the water was continuously flowing, the plants grew at a rapid rate and turned out really well.

4.0. Green Roof Design Procedure and Criteria

Green roof has been in existence for over 3000 years, the earliest green roofs were seen to be as form of a turf roof. The turf roof contains growth of grasses and plants roots and this kind of roof is still in existence in Norway and Iceland. In warm climatic region green roof was first identified as roof garden which was seen in the ruins of Pompeii after the volcanic eruption of Vesuvius Mountain in AD 79. Green roof was also seen in the famous Hanging Garden of Babylon which was constructed around 500 B.C. During Middle Ages green roof was also found in Guinigi Tower, Lucca, Italy (Stater, 2008). Modern green roof emanated from Germany and Switzerland in 1960s after a lot of researches were taken place about terraced green roof technologies. A German researcher Reinhard Bornkamm published his works in 1961 on green roof which really helped on understanding green roof more. In 1961 Geno Haus was also built and it was functional until 1990 (Metropolis Magazine). In 1970 other important researches were also taken place on green roof especially on its components like, root repelling agents, water proofing membrane growing media, drainage etc.

In 1980 green roof market gained so many acceptances with annual growth of 15% to 20%. In 1989 about 1 million square meters of green roof were used in Germany and by 1996 that number rose to 10 million square meters. This success was achieved as a result of help and encouragement from the German Government which it contributes 35 to 40 Deutsh Marks per square meter of roof. Such kind of gesture was copied in other European countries with some of their large cities and urban areas incorporating roof and vertical greening in their planning regulations and these gave birth to a new green roof industry for supplying plants and materials, installers and maintenance crew and green roof professionals. In countries like Germany, France, Austria, Norway, Switzerland and other European countries green roofs have been accepted widely in their construction industries and urban landscape (Peck et al, 1991). Countries like Germany, Switzerland and Scandinavia have the highest contribution on Europe’s earlier green roof researches and they were not written in English (Dvorak, 2010, Koehler, 2007, Mentens et al., 2006).

Green roofs can be particularly effective in denser, more urban environments, where they can compensate for the loss of productive landscape at ground level. Examples range from herbs growing on a 2m2 bicycle shed roof in Sheffield, England, to vegetables growing on a 558m2 organic rooftop farm in Brooklyn, New York. ‘Green wall’ techniques can be used on homes in suburban settings as part of aesthetic enhancement, to improve the overall climate responsiveness of individual dwellings, and even to treat wastewater.

The benefits of green roofs include:

  • longer roof lifespan
  • improved sound insulation
  • reduced heating and cooling requirements
  • reduced and slowed storm-water runoff
  • capture of gaseous and particulate pollutants
  • alleviation of urban heat island effects
  • increased biodiversity.

There is also the potential for green roofs to provide carbon sequestration. Many of these benefits also apply to green walls. Green roofs are sometimes referred to as the ‘fifth façade’. Each of the two kinds of green roof – intensive and extensive – is appropriate for different purposes. The intensive roof is typically much heavier, supports more substantial vegetation and is more expensive than ‘extensive’ roofs, which are often light enough to be retrofitted to existing buildings without the need to upgrade their structural capabilities (http://www.yourhome.gov.au/materials/green-roofs-and-walls).

Design procedures and various design criteria employed for construction of Vertical Gardens/ Green Walls include the following:

Site Selection

Green roofs can be installed on commercial or residential buildings as well as on underground structures such as the parking garage. Green roofs may be particularly well suited for ultra urban areas where development is typically lot-line-to-lot-line and garden space is at a premium. Green roofs are particularly valuable when their use extends to a place of enjoyment for those that inhabit the building. Several Colorado examples are provided at the end of this Fact Sheet. For existing buildings, the structural integrity of the building must be verified prior to consideration of retrofitting the building with a green roof. For both existing and new construction, it is essential that the design team be multi-disciplinary. This team may include a structural engineer, storm-water engineer, architect, landscape architect, and horticulturist. It is recommended that all members of the design team be involved early in the process to ensure the building and site conditions are appropriate for green roof installation

Designing for Maintenance

During design, the following should be considered upfront to ensure ease of maintenance for green roofs over the long-term:  Access for equipment and inspections following construction.  The irrigation system, growing media, and plant selection are critical factors determining long-term maintenance requirements and survival of the green roof vegetation under hot, dry conditions; otherwise, vegetation may have to be repeatedly replanted and/or the irrigation system replaced.  If an under drain system is used, provide cleanouts as appropriate for both inspection and maintenance. There is potential over the long term for the roof under drain system to become clogged with soil/media that migrates down beneath the plant root zone. The ability to access the under drain system for cleanout is important.

Design Procedure and Criteria

Green roofs contain a high quality water proofing membrane and root barrier system, drainage system; filter fabric, a lightweight growing media, and plants. Green roofs can be modular, already prepared in trays, including drainage layers, growing media and plants, or each component of the system can be installed separately on top of the structure.

As shown in Figure 2, the basic elements of green roof design include:

Structural Support: Roof structure that supports the growing medium, vegetation, and live loads associated with rainfall, snow, people, and equipment.  Waterproof Membrane: This prevents water from entering the building.

Root Barrier: This protects the waterproof membrane by preventing roots from reaching the membrane.

Drainage Layer: This is sometimes an aggregate layer or a proprietary product.

Filter Membrane: This prevents fine soil and substrate from being washed out into the drainage layer.

Growing Medium: Although the growing medium is typically not “soil,” the terms soil matrix, soil media and growth substrate are sometimes used.

Vegetation: Native/naturalized, drought-tolerant grasses, perennials, and shrubs with relatively shallow root depths are possibilities for roof plantings.

Irrigation: Even vegetation with low water requirements will require supplemental irrigation in Denver, USA.

Design considerations for Green roofs include:

  1. Providing Storm-water
  2. Treatment and Slow Release:

An early version of the USDCM provided guidance on rooftop detention. This was removed because rooftop controls can be easily modified by maintenance personnel unfamiliar with its purpose. In contrast, green roof vegetation benefits from storm-water detained in the growing medium and the volume the system detains should be recognized when designing for the water quality capture volume (WQCV). The WQCV for the Denver area is the runoff resulting from a storm event of approximately 0.6 inches of rainfall. Based on the data that the EPA has collected to date from the Region 8 green roof in Denver, it appears the green roof retains and evapo-transpires 98 to 100% of the WQCV even without a restriction on the outlet for drain time control. This is largely due to wetting and subsequent evapo-transpiration in the growing media. The data show few exceptions to this, which may be attributed to successive rain events. For this reason, UDFCD recognizes green roofs as a volumetric BMP, able to capture the WQCV for the area of the green roof, without constructing a controlled release at the outlet. This is for roofs that meet or exceed the EPA green roof section, which a modular system is using trays that allow for 4 inches of growing medium. An intensive roof should also be considered to capture the WQCV. A green roof can also be designed to accept runoff from a traditional roof. This can be done for additional water quality and/or irrigation benefits or, if designed with a slow controlled release, the green roof can provide the WQCV for an area in excess of the area of the green roof.

The design volume can be calculated as follows:

Equation GR-1

Where:

V= design volume (ft3)

A= the watershed area tributary to the green roof (ft2)

The volume should be provided within the void space of the drainage layer and the growing media. This is a function of the material selected. The outlet can be controlled by an orifice or orifices located at one central location or at each roof drain. This is also a function of the overall drainage design.

Structural Integrity: For the purpose it is better to consult a structural engineer to ensure the load bearing capacity of the existing roof is adequate for the system to be installed. If new construction, the green roof should be part of the building design.

Impermeable Membrane and Waterproofing: Check waterproofing warranty and consult the warranty company to ensure the policy will not be voided by a green roof application. A leak test is recommended following installation of the impermeable membrane.

Drainage System: A filter membrane is required to keep the growing media from clogging the drainage media; however, roots can pass through the filter membrane. Roots are not expected to pass through the waterproof/repellant membrane. Other components of the drainage system must be kept free of debris and plant material in order to convey drainage properly. Figure 3 and Figure 4 show a stainless steel edge that separates growing media from the rock that surrounds the roof drains. This provides both material separation as well as a root barrier. The plate is perforated to allow the growing media to drain. Roof outlets, interior gutters, and emergency overflows should be kept free from obstruction by either providing a drainage barrier (e.g., a gravel barrier between the green roof and the emergency overflows) or they should be equipped with an inspection shaft. A drainage barrier should also be used at the roof border with the parapet wall and for any joints where the roof is penetrated, or joins with vertical structures.

Growing Medium: Growing medium is a key issue with regard to plant health, irrigation needs, and potential storm-water benefits. The growing medium is not the same thing as “soil.” Most extensive green roof substrate is predominantly made up of expanded slate, expanded shale, expanded clay, or another lightweight aggregate such as pumice. However, such lightweight aggregates have some limitations. These materials typically drain very quickly and leave little water or nutrients available to plants. Therefore, additional research is necessary on substrate mixes appropriate for use on extensive green roofs. For intensive green roof applications where weight is explicitly factored into the structural design, the soil matrix can include materials with higher water retention characteristics such as organic matter (e.g., compost), provided the structural design accounts for the saturated load.

Planting Method: In general, the planting method will be either “modular” (tray approach) or “continuous” (planted in situ).  Modular systems are self-contained trays, which can vary in size, and have relatively shallow depth (2 to 8 inches deep). When modular trays are planted with groundcover and placed close together, the roof often has the appearance of a continuous system once the vegetation is established. Due to the variations in green roof designs, it is important to consult with a multidisciplinary team to determine the type of roof design most appropriate for the short-term and long-term conditions expected at the site. Continuous systems are “built in place” on the roof with layers designed to work together to provide a healthy environment for plants. Continuous roof approaches range from rolled sedum mats to hand-planted buffalo grass plugs.

Plant Selection: General categories of potentially viable plants for Colorado green roofs include native, alpine (grows in shallow rocky soils), and xeric plants (e.g., sedum). Plants must meet certain criteria to optimize their chance of survival on a green roof. Due to the shallow, well-drained materials in extensive green roof systems, plants must be drought resistant. However, not all drought resistant plants are well-suited for green roofs. For example, some plants avoid drought by rooting deeply to access a more stable supply of water. Such plants would not be suitable for a shallow green roof. Grasses with strong rhizome growth such as bamboo and varieties of Chinese reeds should be avoided, as these have the potential to compromise the roof membrane. While there are several species that could potentially adapt to extensive green roof systems, the most commonly used species are stonecrops or sedums because of their prostrate growth form, shallow root systems, and drought tolerance. Another favorable attribute of sedums is that the foliage tends to remain greener than grasses throughout the entire year, even in northern climates. However, drawbacks to a monoculture for green roofs are the same as for a monoculture in agricultural applications – risk of widespread vegetation loss if conditions (e.g., drought, disease, temperature, etc.) change from the anticipated range. Characteristics of plants, which tend to work well on green roofs in a semi-arid climate include:  Self seeding,  Perennial,  Low or compact growth format,  Diffuse or fibrous root system,  Low water use, and  Cressulacean Acid Metabolism (CAM), which is common in sedums (stonecrops) where plant stomata are closed during the day to conserve water.

Irrigation Management: Irrigation is needed for successful green roofs in Colorado. The decision to use drip or overhead spray irrigation is determined based on growing media characteristics and plant needs. Drip irrigation is more efficient when installed below the vegetation layer to avoid heating of the drip line and to get a more effective watering of the roots. Overhead irrigation should be considered for shallow depth applications because drip irrigation may not spread laterally when applied over a rapidly draining media. Current CSU experiments are determining the extent of irrigation requirements for various plants.

Wind: Select growing media and install material layers in a manner to withstand expected average and storm wind conditions.

Roof Microclimates: Consider the effect of roof microclimates on the vegetation, including factors such as shading, localized strong winds, and reflected solar radiation from surrounding buildings. Solar panels can provide partial shade to vegetation that may not perform well when exposed to the typical green roof environment.

Roof Gradient: Green roofs may be installed on flat or steep roofs. For flat roofs (e.g., roof slopes less than 2%) a deeper drainage course is recommended to avoid water logging. For steep roofs (e.g., slopes greater than 30%), structural antishear protection will normally be needed to prevent sloughing of materials.

Protection of Roof Drainage Features: Drainage features on the roof such as area drains, scuppers, downspouts, etc. must be kept free of debris and plant material in order to convey drainage properly. Roof outlets, interior gutters, and emergency overflows should be kept free from obstruction by either providing a drainage barrier (e.g., a gravel barrier between the green roof and the emergency overflows) or they should be equipped with an inspection shaft. A drainage barrier should also be used at the roof border with the parapet wall and for any joints where the roof is penetrated or where the roof joins with vertical structures.

Roof Membrane:  Inspect the roof membrane (the most crucial element of a green roof) and conduct a leak test prior to installing the remaining layers of the roof.

Installation Safety: Most landscapers are accustomed to working on the ground, so safety training is important. If the green roof will be accessible to the public, safety at roof edges should be of paramount concern.

Success of Green roofs depends not only on a good design and maintenance, but also on construction practices that enable the BMP to function as designed. Construction considerations include:  Permit Requirements, General Coordination, and Warranties: Investigate permitting requirements for green roofs in the local jurisdiction. Significant coordination between architects, engineers, roofers, and landscapers is needed. Contractually, it is common to have the roofer warranty the impermeable membrane, whereas the landscaper would be responsible for the growing media, vegetation, and other landscaping. Typically, irrigation systems have warranties, but plants do not, with the exception of situations where a maintenance contract is in place. Where a maintenance contract is in place, some landscapers or greenhouses will provide plant warranties.

4.1. The Basics of Green Roof Garden Design

A Green roof is a green space created by adding layers of plants on top of a traditional roofing system. The layers of a contemporary green roof system, from the top down, include: The plants, often specially selected for particular applications, an integrated irrigation system and controls an engineered growing medium, which generally will not include soil, a landscape or filter cloth to contain the roots and the growing medium, while allowing for water penetration, a specialized drainage layer, sometimes with built-in water reservoirs, the waterproofing/roofing membrane, with an integral root repellent, and the roof structure, with traditional insulation either above or below.

Design Considerations

There are several important design and structural differences between ground level landscape development and rooftop developments. The following are the special construction requirements and considerations when developing a roof garden. Protection of the integrity of the roof and structure

Positive Drainage

  • A long-term, lightweight planting medium
  • Irrigation for optimum plant growth and sustainability
  • Adaptation to the climatic conditions
  • Selection of hard-scape materials (paving, structural materials, site furnishings and water as a design element) and their special use and requirements as part of a roof garden system
  • Provisions for utilities Public safety and security

Protection of the Roof and Structure

The single most important element in rooftop garden construction is protecting the integrity of the roof and the structural components under the garden. For this reason there must be waterproofing of exceptional longevity to prevent damage and to reduce the possibility of long term expensive reconstruction. For this reason it is recommended a completely new waterproofing layer be added to the existing structure to insure the longevity and integrity of the waterproofing system. Load Bearing Capacity The structural engineer should verify the maximum load bearing capacity of the existing structure. These figures should be available from the records of the previous construction of the helipad. Typically, a minimum additional dead load limit of 150 psf between columns is needed to accommodate the construction of a roof garden. Loads above columns and at the roof’s edge can be considerably higher; however a structural engineer should be consulted to establish the load bearing capacity of those areas. These higher load bearing areas should be used to accommodate larger specimen plantings and trees.

Waterproofing

As mention before, a completely new waterproofing system should be installed to protect the building’s structure. There are several types of waterproofing available; however, elastomeric materials offer the greatest protection. Bituminous waterproofing should be avoided. Over time the organic components in bituminous waterproofing interact with the soils and the plant materials and therefore increase the likelihood of system failure. A properly installed waterproofing system can last the lifetime of the building, however a single small leak may require the removal of the entire garden to find and repair the damage. Therefore, in order to insure the integrity of the waterproofing it is recommended a protective topping coat of concrete be applied, as soon as possible, following the installation of the new waterproofing.

Key points of Protection of the Structure

The single most important consideration regarding roof and deck garden construction is protecting the roof and structure from damage due to excessive loading or leaks. A structural engineer should always be consulted prior to roof garden landscape design and construction. Rooftop structures must typically be able to support a dead load of 150psf to accommodate the construction of a garden. The roof must be completely covered by an elastomeric material and protected by a concrete topping slab.

Planting Provisions

Like the roof on which the garden is to be built, a roof garden is constructed in layers. Just as failure in the roof components can cause significant damage to the building, so too can failure of the planting components causes significant and costly damage. Therefore, it is imperative to take care in choosing and installing materials of the highest quality and species conductive to a rooftop’s environment.

The Roof Drains

The existing roof drains are appropriate for use within the roof garden. Some minor modifications may be required to accommodate the new waterproofing and topping slab. Nevertheless, the four roof drains and their size are adequate to support the Roof garden’s needs. Special care should be taken and accommodations made to allow access to those roof drains should there ever be the need to access them for cleaning.

The Drainage Layer

The drainage layer, directly above the concrete protective slab, should be very porous to permit water to pass easily through it. It must be permanent and continuous over the entire roof surface and strong enough to support the weight of the plant materials and hardscape above it. This layer must be kept free of any materials that could prevent the free flow of water to the drains. Because of its lightweight and integrated filter fabric, McCaren Designs recommends the use of Enkadrain for this drainage layer. Further, its .75 inch thickness allows for more planting media in areas next to paved areas.

Filter Fabric

To prevent the planting media from going into solution and being lost in or clogging the drainage layer and roof drains, a water-permeable barrier of filter fabric is needed. As mentioned previously we recommend Enkadrain because the filter fabric is integral with the drainage course.

Planting Media

The critical criteria in the formulation of a suitable planting media for roof gardens include: lightweight; the ability to hold nutrients; adequate moisture holding capacity; and the capability of developing a firm (for plant stability) yet easily drained soil structure. There are several ready mixed media available that meet these requirements. However, care must be taken in selecting these lightweight soils to be sure there are adequate non-organic components incorporated into the mix. Soil mixes consisting of solely organic material will decompose, losing nearly 30% of its mass every year, thereby requiring frequent topdressing to maintain the soil mass. Therefore one should incorporate sand and expanded shale into these prepared planting media. When this mixture is properly moist it will weigh approximately 60 pounds per cubic foot.

Irrigation

The relatively thin, well-drained soil mixtures used in roof garden construction cannot provide the plantings with the subsurface water normally available to ground level plantings. Care must be exercised to prevent the soil mass from drying out and causing damage to the plant materials. Hand watering is too labor intensive and is not cost effective. Therefore, we recommend the installation of a sensor controlled drip irrigation system. Drip irrigation is preferred in roof garden applications because the effects of wind can cause above ground systems to perform inconsistently. Mulch Drying and overheating of the soil can be prevented by the application of 2-3 inches of shredded hardwood mulch. Besides providing protection of the plant materials this mulch serves to hide the drip irrigation lines and emitters.

Key points of Planting Provisions

An important consideration regarding roof and deck garden construction is the substrate supporting the plantings. The existing roof drainage system is adequate to support the installation of a roof garden. Planting media and the drainage course should provide for fast percolation of water and be free of fine silts that can clog the filter blanket and block drainage. Planting media should contain sufficient mineral content to stabilize the plantings and maintain soil mass. On a regular basis soils require topdressing to replenishment decayed organic material. Drip irrigation is the prefer method of providing moisture to the planting.

Climatic Considerations

Climate and exposure can be prime contributing factors in the success or failure of any outdoor space. This is both a consideration in the selection of plant materials but also a factor in human use and comfort. Wind, sun and shade, and extremes of temperature, as well as long dry or wet periods, snow loads and frost are much greater problems for roof gardens than for other landscapes.

Climate: The Minnesota climate is one of extremes and therefore care must be exercised to make provisions for these extremes. As much as possible, increasing the depth of the soils will mitigate some of these extremes. A minimum of 12” of soils is required for sod, groundcovers and annual planting areas. For perennials and small shrubs a 7 minimum of 16’’ of soil is required. For small trees and large shrubs 24” is needed and shade trees require a minimum of 30” of planting medium.

Designing Parameters

Wind: Trees and vertical structures (such as fences, walls, gazebos, trellises and light standards) and other similar elements must be designed or selected to resist wind damage due to overturning or breaking. Plants are also subject to flagging (lopsided growth) due to strong persistent winds, which are typical in a roof garden application. Further, even normal wind flow can cause excessive drying of plant materials and soils and high evaporation of water used in water features. Special guying and support of trees is required to offset the effects on trees of persistent winds. Automatic water fills are required on water features to compensate for evaporation and to protect pumping equipment. And irrigation is required to replenish the soil moisture. Wind direction should also be considered in the design of the barrier system, designing such systems to mitigate the effects of wind on the garden’s visitors.

Sun and Shade: Heat and glare can make a roof garden quite uncomfortable. Except for the confirmed sun worshiper, few people prefer to be in the sun for more than a few minutes on a hot day. Shade relief, usually found under trees, may be at a premium. Trees should be located where they cast the greatest shadow. Artificial shade should be provided in areas where trees cannot adequately shade the area. Providing adequate shade may be the single most important design consideration in relation to the use of the roof garden. If adequate shade is not provided, the garden will receive little or very limited use. Glare is also a significant problem even in areas where heat is not an issue. Avoid using light colored paving or paving that has a high reflection value.

Key points of Climatic Consideration:

Climatic Conditions: The effects of wind, heat, cold and precipitation are amplified on roof gardens. Provide adequate soil mass to support the desired plantings. Make appropriate provisions to replenish soil moisture and water in features to combat the effects of evaporation. Use windscreens to mitigate the effects of wind on the users. Plant trees in areas that will cast the maximum amount of shade. Avoid using paving that increases the amount of glare.

Selection of Hard-scape Materials: The selection and construction of light standards, walls, fences, wind screens, pergolas, curbs and other structural elements should all be considered in relationship to the structural limitations of the roof and its supports below. The omnipresent factor of weight has a strong effect on which materials are used and their placement in the landscape. Lightweight materials should be used wherever and whenever possible.

Paving: As previously mentioned, the consideration of the paving material’s reflectance should be of primary consideration. Secondly, the type and pattern of paving materials chosen are as important to the viewers from surrounding buildings as they are to the actual user of the space. The color, tone and contrast of these materials can create a strong visual interest. Paving materials should be selected for lightweight qualities and durability. Paving if at all possible should allow for the permeation of water to aid in the drainage and removal of heavy rainfall amounts.

Methods of Anchoring: The structural elements, including lighting fixtures need to be carefully anchored when used on rooftops. Special care and method must be employed to avoid penetrating the waterproofing systems. There are many such methods for anchoring and these will be fully developed and detailed during the project’s design phase.

Furniture: Furniture and site amenities are a critical component in the roof garden’s success. Frequently, adequate and abundant seating is often overlooked and is one of the most important elements in the comfort of the user, as well as, in the actual overall project usage. Furniture should be heavy enough to not require anchorage. Wood furniture or heavy poly-resin furniture is more comfortable to the user than metal or concrete seating.

Pools and Fountains: The use of water and fountains add greatly to the enjoyment and use of roof deck gardens. There are several factors to consider when using water as a design element. The weight of water in roof top gardens in most reference materials is widely misunderstood. Most, if not all, reference materials assume the weight of water to be greater than that of the plantings. If a rigid system is used there will be cracking and eventual failure of the system. Paving materials should be selected for lightweight qualities and durability. Paving if at all possible should allow for the permeation of water to aid in the drainage and removal of heavy rainfall amounts. Most, if not all, reference materials assume the weight of water to be greater than that of the plantings. This is in error and for the most part a water feature will weigh less than the planted areas.

Design Parameters

Also, as previously mentioned, automatic fill and management of the water levels is an important consideration in the feature design. Evaporation will easily reduce the water level by 3-4” per day in hot windy conditions. An auto-fill feature will protect the expensive pumping and filtering systems. Planting of water plants will greatly improve the quality of the water and prevent algae from becoming a problem. Additionally, the use of a biomass filtering system will provide the necessary filtering without the use of chemical agents.

Key points for Selection of Hardscape Elements

The omnipresent factor of weight has a strong effect on which materials are used and their placement in the landscape. Consideration of the reflectance of the paving materials should be of primary consideration. Provide adequate seating, the comfort of the user impacts the overall usage of the project. Furniture should be heavy enough to not require anchorage. Wood furniture is preferred. Make appropriate provisions to replenish water in features to combat the effects of evaporation. Use a flexible water retainage system to mitigate the effects of the Minnesota climate on the pool system.

Provisions for Utilities: Electrical Standard 110-120 Volt ac electrical supply is sufficient for most roof garden uses, such as lighting, appliances, fountains and irrigation controllers. All electrical requirements should be met in accordance with the electrical engineer’s recommendations.

Water: A supply of clean water is needed for irrigation, pools and fountains and the cleaning of paved surfaces and furniture. Water pressure of the irrigation system should be provided from a minimum of 35 psi to a maximum of 70 psi.

Storage: Provisions should be made for the storage of gardening materials and supplies. This storage should be water tight and readily available to the horticultural staff caring for the garden. Additionally this area can function as the location of controllers and electrical panels for the garden.

Insulation: Green roofs may or may not include an insulating layer in addition to the soil and vegetation, but even without such a layer they provide significant thermal insulation and shading for the building. Overall insulation values depend on the type and thickness of growing medium, and the type and extent of vegetation. There is little available documentation for R-values; they would, in any case, vary according to the degree of saturation of the growing medium (http://www.yourhome.gov.au/materials/green-roofs-and-walls).

Green roofs provide significant thermal insulation and shading for a building. In Australia, the energy benefits of green roofs are most pronounced in their ability to reduce summer cooling demands. Their contribution to insulating and shading buildings can help significantly in reducing energy consumption and carbon pollution. However, it is difficult to obtain accredited insulation values for green roof construction. For specifying and code compliance purposes, thermal insulation standards should be met by conventional means with the additional insulation value of a green roof regarded as a bonus (an energy assessor may be able to give some credit for a green roof).

Green walls can be retrofitted to existing homes to reduce the heat load on façades. The simplest kind is a trellis set with a gap between it and its supporting wall to create shade from vegetation with passive cooling from transpiration of the vegetation as well as convection of heat passing up through the gap.

In warmer weather, green walls act like green roofs by reducing the surface temperature of a conventional wall through evapo-transpiration and shading. Walls that use irrigation and hydroponic techniques provide additional cooling through evaporation.

The direct solar exposure of windows and walls can be reduced by shading from vegetation which might grow directly on wall surfaces, or be free-standing or supported on trellises. Deciduous vegetation (bio-shading) reduces cooling demands by limiting solar gain in the summer while allowing daylight in during winter. The insulating and low thermal absorption properties of green roofs reduce the urban heat island effect.

Sound insulation: In busy urban settings the acoustically absorbent nature of soil and vegetation of green roofs can insulate against the noise of heavy vehicles like trains, trams, buses and trucks. One office building under the flight path of San Francisco’s International Airport, planted with a mixture of indigenous grasses and wildflowers, helped to achieve reductions in noise levels of up to 50 decibels. An extensive thin green roof just 100mm deep reduces noise transmission from that of a conventional roof by at least 5 decibels (http://www.yourhome.gov.au/materials/green-roofs-and-walls).

Green Roof Membrane Protection and Life Extension: Green roofs help to protect roofing membranes from extreme temperature fluctuations, the negative impact of ultraviolet radiation, and accidental damage from pedestrian traffic. European evidence indicates that green roofs will easily double the life span of a conventional roof, and thus decrease the need for re-roofing and the amount of waste material bound for landfill. These are direct operational cost savings for the building owner. Life cycle costing data which includes the cost of deferred maintenance and replacement suggests that green roofs cost the same or less than conventional roof systems (file:///C:/Users/Gollum/Downloads/Design%20Guidelines%20for%20Green%20Roofs.PDF).

Fire Protection: There is evidence from European manufacturers suggesting that green roofs can help slow the spread of fire to and from the building through the roof, particularly where the growing medium is saturated. However, the plants themselves, if dry, can present a fire hazard. Similar to preventing grass fires at grade, the integration of “fire breaks” at regular intervals across the roof, at the roof perimeter, and around all roof penetrations is recommended. These breaks would be made of a non-combustible material such as gravel or concrete pavers, 60 cm (24”) wide, and located every 40 m (130 feet) in all directions. Other options would be the use of “fire retardant plants”, such as sedums, which have high water content, or a sprinkler irrigation system connected to the fire alarm (file:///C:/Users/Gollum/Downloads/Design%20Guidelines%20for%20Green%20Roofs.PDF).

Waterproofing: One of the most important components of the green roof system is the waterproofing/roof membrane. For an existing building, the membrane should be carefully inspected to determine if it needs to be repaired or replaced before the installation. Many manufacturers of green roof systems will not provide a warranty on the green roof system if new membranes are not applied. The normal 10-15 year reroofing cycle provides a window of opportunity to investigate the potential of applying a longer lasting green roof. Green roofs can be applied on inverted or traditional roofing systems. If the existing system is inverted, then one needs to determine whether the insulation can be replaced by an equivalent R- value of growing medium. If the insulation is to remain, then good drainage must be ensured to prevent continuous contact with water, and subsequent damage.

If the membrane, existing or new, contains bitumen or any other organic material, it is crucial to maintain a continuous separation between the membrane and the plant layer, since the membrane will be susceptible to root penetration and micro-organic activity. Some of the new membranes developed specifically for green roof applications, although still bituminous, now contain a root-deterring chemical or metal foil between the membrane layers and at the joint/seam lines to prevent root damage. The chemical makeup of the membrane must also be compatible with the system components with which it will be in direct contact. Although the green roof will retain much of the rain that falls on it, maintaining proper drainage on the roof is still very important. Parapets, edges, flashing, and roof penetrations made by skylights, mechanical systems, vents, and chimneys must be well protected with a gravel skirt and sometimes a weeping drain pipe. If the drainage layer is too thin or if the routes to the roof drains become blocked, leakage of the membrane may occur, due to continuous contact with water or wet medium. The growing medium itself may sour, causing the plants to drown or rot. On a roof slope greater than 20 degrees, the green roof installer needs to ensure that the sod or plant layer does not slip or slump through its own weight, especially when it becomes wet. This can be prevented through the use of horizontal strapping, wood, plastic, or metal, placed either under the membrane, or loose-laid on top. Support grid systems for green roofs have been designed by some green roof manufacturing companies specifically for this application (file:///C:/Users/Gollum/Downloads/Design%20Guidelines%20for%20Green%20Roofs.PDF).).

4.2. Green Roof Design and Technology

Most often green roofs sit atop nearly flat roofs of commercial or public buildings. Occasionally they can be found on sloping or residential roofs though they are most likely to be part of new building project when extra weight loading can be considered and accounted for in structural design. Retrofitting the structure of existing building is a difficult and expensive proposition. Weight limitations become paramount because green roofs capture and hold a portion of the precipitation that falls on them. Based on the substrate depth, green roofs are classified as extensive (10 and 4 and 15 cm) (6 in.) (GRHC, 2006a). This general nomenclature applied above to depth classifications actually refers to the amount of maintenance expected for shallow, moderate, and deep substrate. Deeper substrate means that a wider array of plantings that include herbaceous perennials, shrubs and trees could be grown creating more of a rooftop garden, whereas, shallow substrate depths support fewer and lower-growing plant types. Most roof decks allow only minimal weight loads and so limit adoption of even extensive green roofs with shallower substrate depths. Where more weight loading can be supported, a semi-intensive or intensive roof can be used. The term extensive comes from a German to English translation of these concepts in the 2002 English translation of the FLL Guidelines for Green Roofing. It is a green roof system that “involves cultivation of vegetation in forms which create a ‘Virtual Nature’ landscape and requires little if any external input for either maintenance or development” (FLL 2002). Its intention is to be extensive or wide spread in its application because of low cost, low-maintenance and ease in population with local flora (FLL, 2002).

A typical green roof cross-section begins at the bottom with the building’s structural system, moving up through its decking, insulation, waterproof membrane, root barrier, drainage layer, drain filter, growing substrate, and finally a living layer of plants (Figure 5). Each layer plays a role in protecting the membrane, buffering and filtering rainfall and, with plant coverage, guarding against wind and rain-caused erosion of the growing substrate. Because substrate ballasts the building’s membrane and insulation, it must possess some weight, yet it must be well drained with large pore spaces to quickly allow percolation of excessive rain and lessen weight loading. Plants must be selected to withstand drought, wind, heat, and cold. If plantings fail then the substrate, becomes exposed to loss due to wind scour. Three key factors must always be kept in mind when designing and maintaining green roofs: (1) stay within structural loading limit, (2) protect the integrity of the waterproofing membrane, and (3) keep plants alive to protect and hold substrate in place.

A green roof design must account for horizontal as well as vertical forces. Daily wind pressure and especially wind action during storm events can cause scouring of substrate and dislodging of plantings. Placement of membrane ballasting and scour protection may be dictated by local building codes (https://link.springer.com/chapter/10.1007/978-3-319-14983-7_1/fulltext.html). The height and location of parapet and building walls can create turbulent, chaotic, unpredictable, and increased speeds for wind flows (Suaris and Irwin, 2010) but also reduce wind speeds. Wall and parapet height and location can also affect sun and shade patterns that should be acknowledged in layout of any designed planting. Substrate can be layered and embedded in several different ways. The simplest is monolithic placement in a bed at the specified depth. Placement could also be built-up with layers of two or three substrates with differing drainage characteristics. The next method consists of modular tray systems either with pre-grown plants or filled with substrate and planted after placement. Trays can be made of plastic or a degradable material. One advantage of plastic material is that the tray can be picked up and moved for roof repair. A third method involves a thin, integrated, pre-planted, flexible, rubberized or plastic rug-like structure embedded with substrate and plants. Where green roofs have been designed for physical access other landscape amenities can be added such as paving, decking, seating, water features, arbors, and trellises. Green roof landscape design per se is beyond the scope of this book. It is suggested that readers wishing to know more about the design and construction process of Vertical gardens and Green roofs may consult books by Osmundson (1999), Weiler and Scholz-Barth (2009), Luckett (2009), Snodgrass and MacIntyre (2010), and Daykin et al. (2013).

4.3. Challenges of Green roof construction

The main design, installation and management challenges of green roofs include:

  • Ensuring the building can support a green roof
  • Quality installation and leak prevention
  • Maintenance requirements
  • Potential plant loss due to environmental conditions or mismanagement, among other items Designers can maximize the benefits of green roofs by properly selecting plants, growth medium, drainage layers and other features tailored to the local climate and the building’s surroundings.

4.4. Future R&D Needs

Additional research in several issue areas is critical to better understanding the costs, benefits, challenges and opportunities of green roofs. These issues include:

  • A thorough comparison of green and white roofs
  • Storm-water and storm dynamics, field monitoring and computer simulation
  • Validating storm-water runoff and delayed peak runoffs for storm-water regulation
  • The interaction between green roofs and solar panels
  • Long-term storm-water and energy performance
  • The process of establishing native plants and created habitat for endangered fauna on green roofs
  • Green roofs’ influence on building energy use
  • A thorough review of irrigated and non-irrigated roofs
  • The economics of rooftop agriculture
  • Air quality improvements associated with green roofs
  • Employment analyses

References

  • Daykin K, Benjamin L, Pantiel, M. (2013). The professional design guide to green roofs. Timber Press, Portland.
  • Dover, J. W. (2015). Green Infrastructure: Incorporating Plants and Enhancing Biodiversity in Buildings and Urban Environments. Routledge.
  • Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (Landscape, Research, Development and Construction Society) (2002). Guideline for the planning, execution, and Upkeep of Green-roof Sites.
  • Green Roofs for Healthy Cities (GRHC) (2006a). Green roof design 101 introductory course, 2nd edn. Cardinal Group, Toronto.
  • Koehler, M. (2006). Long-term Vegetation Research on Two Extensive Green roofs in Berlin. J. Urban Habitats 4 (1), 3–26.
  • Luckett K (2009). Green roof construction and maintenance. McGraw-Hill, New York.
  • Mentens, J., Raes, D., & Hermy, M. (2006). Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscape and urban planning, 77(3), 217-226.
  • Osmundson T (1999). Roof gardens: history, design, and construction. W. W. Norton and Company, New York.
  • Peck, S.W., Callaghan, C., Kuhn, M., & Bass, B. (1999). Greenbacks from Green Roofs: Forging a New Industry in Canada. Canada Mortgage and Housing Corporation, 11-12.
  • Resh, M. Howard Ph.D. (1995). Hydroponic Food Production: a definitive guidebook of soilless food- growing methods – Fifth Edition. Chapter 2, pp.52-58, Chapter 3, pp.59-121, Chapter 6, pp.155-238. Woodbridge Press Publishing Company, Post Office Box 209, Santa Barbara, California 93102.
  • Snodgrass EL, MacIntyre L (2010). The green roof manual. Timber Press, Portland.
  • Stater, D. (2008). Green roofs: Sustainability from top down. Retrieved 3rd December 2015 from http://lda.ucdavis.edu/people/2008/DStater.pdf.
  • Suaris W, Irwin P. (2010). Effect of roof-edge parapets on mitigating extreme roof suctions. J Wind Eng Ind Aerodyn 98(10):483–491http://www.fll.de/shop/english-publications/green-roofing-guideline-2008-file-download.html. Accessed 19 March 2015.
  • Weiler S, Schloz-Barth, K. (2009) Green roof systems: a guide to the planning, design, and construction of landscapes over structure. Wiley, New York
  • Websites:
  • https://www.asla.org/uploadedFiles/CMS/Resources/22610_SableJames.pdf.
  • http://www.yourhome.gov.au/materials/green-roofs-and-walls.
  • http://www.ambius.com/blog/ultimate-guide-to-living-green-walls/
  • http://iec.edu.in/wp-content/uploads/2016/01/7_Piyush-Sharma_Vertical gardens-pp.42-48.pdf.
  • http://www.yourhome.gov.au/materials/green-roofs-and-walls.
  • http://www.yourhome.gov.au/materials/green-roofs-and-walls
  • https://link.springer.com/chapter/10.1007/978-3-319-14983-7_1/fulltext.html
  • file:///C:/Users/Gollum/Downloads/Design%20Guidelines%20for%20Green%20Roofs.PDF

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