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Seeing green up top

January 1st, 2008 / By: / Continuing Education

Green roof technology has come a long way from sod houses and grassed, meadow-like roofs grazed by goats or sheep. It is rapidly moving forward with all expectations of transforming the way urban ecology is legislated, designed, and maintained across the country. Architects and engineers are running toward an understanding and mastery of this “green” design tool, a multi-disciplinary field centered within the realm of landscape architecture. With a basic understanding of this emerging culture, designers not only can participate in this rising market, they can help shape it and take the lead.

Green roof technology

The phrase “green roof technology” is a term broad in meaning that includes many types of greened roofs, the products used to create them, and design techniques available to construct them. Broad categories place blurred lines between three types of greened roof systems, which include extensive, intensive, and semi-intensive systems. The extensive system falls at one end of the spectrum and is most frequently referred to as a green roof, living roof, or “eco-roof.” These systems are thin in profile, between 50 to 102mm thick, weigh 49 to 122 kg/m2, are least diverse in vegetation, and are not likely to demand much design time. Though they can do much to absorb rainwater, they rarely require much direct design effort from landscape architects, other than a competent understanding of their vegetative needs and ecological benefits. Even if they lie low in profile, they can provide significant contributions to the design of sustainable projects and can effectively address improvements to urban ecology and wildlife corridor creation. It would be the landscape architect’s role to integrate these indirect design applications.

At the other end of the spectrum lies the intensive green roof system. The intensive system includes public or private roof gardens. The roof garden is where landscape architects can make significant direct contributions to buildings by the transformation of bare roofs into beautiful and functional space. Where the green roof is a thin profile, roof gardens can be lushly planted with trees, shrubs, and perennials in as little as 152 to 457mm of growing medium, at only 381kg/m2. In comparison, 457mm of topsoil might weigh 586kg/m2. Though green roofs require little maintenance and no irrigation, roof gardens need as much care as gardens at grade. The selection of vegetation and source of irrigation water can make significant differences in the degree that they are self sustaining. Intensive systems are typically placed on flat roof decks. Extensive and semi-intensive systems, however, can be placed on either flat roofs or on roofs with slopes up to 30 degrees.

Urban ecology and natural processes

Green roofs can do much for urban ecology, in part because they replicate natural processes. Green roofs mimic the natural water cycle as moisture falls, is slowed and intercepted by vegetation, absorbed into the growing medium, used by plants, and transpired back into the atmosphere, as it moves slowly through the sub-grade and drainage system. On an annual basis, green roofs can reduce stormwater runoff from rooftops up to 70 percent. Communities across the globe take advantage of these benefits, give credit for reductions in stormwater detention, and save money on downsized conveyance systems.

Just as plants intercept solar radiation on the ground, so also do green roofs intercept solar radiation and cool the air at the surface of the green roof. Green roofs can cool the surface by up to 50 percent and actually cool the air temperature just above plants between two to three degrees. Heating and ventilating (HVAC) systems can be scaled back to accommodate the lower air temperature at intake valves.

Green roofs also score points within the Leadership in Energy & Environmental Design (LEED) rating system. They are included because of their stormwater benefits and insulating nature, which help keep heating costs down and allow for the downsizing of cooling systems. Green roofs also provide for lush views, extend the life of a roof, and provide bird and butterfly habitat. Additional benefits include an increase in property values and a reduction in the urban heat island effect.

Model green roof design process

A green roof system may be designed as a retrofit to an existing building or a new installation to a planned building. Both options require nearly the same principal design processes.

The model design process begins with the selection of and inclusion of an interdisciplinary design team. All green roof system benefits, whether ecological, aesthetic, or economic, can be applied effectively only if the design team starts working and thinking about the installation from the beginning of the project. Treating the green roof as an afterthought translates into lost opportunities and increased costs.

The design team typically consists of architects, landscape architects, and structural and mechanical engineers. Architects and mechanical engineers are required to design and certify drawings that consist of design elements or modifications to waterproofing, parapets, and utility routing. Structural engineers design and stamp structural plans and improvements.

Certify architects should lead and coordinate the design process. They will need to furnish information on required height of the parapet, best location for the roof drain, needs and locations of utilities, and other details. In addition they should design the green roof to function like a natural system within the region.

Treated as an integral part of the building and site design, the green roof should provide numerous benefits, as outlined above. To maximize these benefits, several principal design steps are required. The design team has to gather base information without which the green roof system, whether extensive, intensive, or semi-intensive, can not be designed. Items that must be included in that information set are:

  1. The planned/existing dead and live load capacity of the roof, which should lead to a decision as to which additional reinforcement in a retrofit scenario is needed and if it is economical.
  2. Analysis of the planned/existing drainage points, their location, size, and distribution.
  3. Analysis of planned/existing utility needs, location, and their routing.
  4. Identification of probable irrigation needs and water storage opportunities for irrigation.
  5. Identification of aspect, exposure, and wind impact to the site/roof in question.
  6. Energy objectives (ambient air temperature reduction and insulation factor of different systems).
  7. Review of the local stormwater ordinance and analysis of how it may affect the green roof system design.
  8. Review of land use needs and restrictions:
    • Determine if the green roof should provide retention to maximize the available square footage at grade for other uses.
    • Determine if restrictions at grade require additional garden space or landscape that should be located on the roof.

An evaluation of the base information allows for a selection of an appropriate green roof system and development of a program that is compatible with existing restrictions and the project objectives. To select the most suitable system, an integrated analysis of multiple factors is necessary:

  1. Determine detailed stormwater objectives and which system would best meet them:
    • Design of growing and drainage media thickness and appropriate material selection.
    • Design of water harvesting, storage, and reuse for irrigation.
    • Coordination of sizing of conveyance systems and at-grade stormwater treatment.
  2. Determine the energy objectives and which system would provide the best heating/cooling benefits.
  3. Determine how aspect exposure and wind impact of the given site would affect/limit vegetation cover, irrigation needs, and ultimately the green roof system required.

Following this analysis and the selection of a green roof system, landscape architects can select and design effectively the desired garden style and vegetation that best mimic natural systems. The analysis allows them further to determine access and design the appropriate amenities for the given user patterns. Landscape architects also need to finalize utility needs and locations and to coordinate their routing.

Case studies

The suggested model for the design of green roofs and roof gardens is explored with two case studies: the Chicago City Hall Urban Heat Island Initiative and the Illinois Environmental Protection Agency (IEPA) Conservation Design Forum (CDF) Green Roof Monitoring Project. Both of these projects are retrofits that follow the model process.

Chicago City Hall urban heat island initiative

The Chicago City Hall green roof project was initiated for two purposes: 1) to study the heat reduction effects of green roofs in urban environments and 2) to experiment with a diversity of vegetation that is adaptable to growing on rooftops in Chicago’s climate. As a retrofit green roof, the design process followed the above outlined model.

The project began with an analysis of the roof structure and its loading capacity. Originally designed for another floor, the 11th-floor roof deck was meant to support an additional load of 146.5 kg/m2. Wear and re-roofing reduced the additional capacity available for the green roof. The design process included two design iterations due to the complexity of the existing roof deck and design program. The first design stemmed from the existing loads, without additional structural support. Extensive gardens comprised a majority of the total area. Semi-intensive gardens were small in area and placed on the deck through excavation of old roof layers. Two intensive areas were 1.83m in diameter and situated over interior columns.

Inasmuch as the roof excavation process proved to be expensive after the initial bidding, the demonstration garden was completely redesigned. The new design required structural reinforcement of abandoned skylights to include semi-intensive gardens. This change allowed the semi-intensive areas to be greatly increased, which added to the diversity of vegetation and value of the garden.

With this structural analysis and green roof system design in place, the program and layout of the garden followed. Simple symmetrical pathways linked maintenance paths to access points and allowed for access to all three systems. More than 150 species of plants were selected and planted within the three zones.

IEPA CDF green roof monitoring project

This project was funded through a grant by the IEPA to test two variables: 1) the effects of growing media thickness on runoff characteristics and 2) the runoff characteristics of different drainage management systems.

The project site is located at Conservation Design Forum’s offices in Elmhurst, Ill. The building has three roofs, all with different structural characteristics. One roof accommodates an additional 83 to 97.6kg/m2. Another roof was rebuilt to accommodate a roof garden at 488.2kg/m2. The third roof couldn’t accommodate additional loading but was replaced with an ultra-light green roof system at 44kg/m2.

The design followed the structural loading. Panels were created which hydrologically separate segments of the green roof, each of which has a different drainage system, including drainage boards, retention boards, lightweight gravel, and modular systems. These drainage designs were tested at 51mm and 76.2mm thicknesses of growing media. Monitoring equipment records both the volume and rate of rainwater runoff. Vegetation was held consistent among the extensive panels. A roof garden was designed with a 203.2mm- to 304.8mm profile to monitor runoff characteristics of thicker systems.

Conclusion

Green roof technology is finding its way into the growing market for “green” and sustainable design. Its benefits such as improved stormwater management, urban ecological enhancement, urban heat island reduction, and energy conservation, make it an attractive tool for designers, developers, and legislators. Landscape architects could shape and lead this rising market with an understanding of this technology and the design process. It is their role to assure that a green roof system is an integral part of the design process from the very beginning, and not added as an afterthought. Landscape architects should maximize the green roof system benefits and associated cost savings through a thorough design process that leads to full integration into the building and site design. Following the proper design process, landscape architects can assure successful green roof system design and be successful participants and leaders of the emerging culture of green roof technology.

Bruce Dvorak, ASLA, is assisstant professor of landscape architecture at Texas A&M University with expertise in green roof design and is a former associate of Conservation Design Forum, under whose aegis this report was produced.
Marcus de la fleur is an associate with Conservation Design Forum.
Editor’s note: This paper was first published in FA in the July/Aug 2004 issue.

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