Reviving the world wonder: why rooftop gardens should cover urban landscapes.

Author:Litichevskaya, Juliya

    Garden rooftops have existed since ancient Babylonian times. (1) Presently, concerns about the environmental and economic costs of energy use in buildings and cities have led to renewed interest in the potential of such gardens by local, state, and federal governments. (2) In the past decade, for example, Chicago has commenced the design or construction of more than 600 public and private green roofs. (3) The city is especially known for the 20,000 square foot garden crowning its eleven-story City Hall. (4) Of the roughly 770 green roof projects currently under way in the United States--which cover approximately 10.3 million square feet in total--two million square feet, or 19%, belong to Chicago. (5)

    Chicago is not the only city embracing the idea that there is finally a "technologically feasible way to have both a high-rise and a sunny park." (6) Washington D.C. and New York City are also actively promoting rooftop gardens. (7) Today, the public sector has served as the main stimulant of green roof development in these cities through tax incentives, grants, direct procurement, research, and regulations. (8) For example, in 2005 Chicago offered twenty $5,000 grants to encourage homeowners and small businesses to install green roofs. (9) The city also granted $400,000 to Schwab Rehab Hospital to convert its roof into a 10,000 square foot garden. (10)

    Moreover, international cities have enacted green-roof mandates for the private sector as well. Toronto, Ontario, Canada became the first city in North America to mandate green roofs on private buildings in May 2009 when city legislature passed a by-law which requires green roofs for all new developments. (11) Berlin and Tokyo have also developed mandatory green roof laws. (12) The proposal that follows in this Note, therefore, has a number of successful international models and precedents. (13)

    This Note will first describe green roof technology, including the varieties of design. Second, the Note will address the environmental and cost-saving benefits of green roofs. A discussion of current public green building initiatives in the United States will follow, including a case study of Newark, New Jersey. Next, the Note will provide examples of green roof initiatives internationally and analyze the legal implications were mandatory green roof legislation adopted in the United States. The author concludes that courts will likely not oppose green roof mandates, and that private developers are likely to comply with such laws. Meaningful environmental and economic impact from urban green roofs can only be accomplished through large-scale implementation, (14) facilitated by progressive zoning regulations.


    "Green roofs are simply roofs bearing vegetation...." (15)

    To grow a roof-top garden, there are essentially two approaches to layering the growing medium, diversifying vegetation, and installing irrigation on a green roof: intensive and extensive. (16) An intensive roof is designed with an aesthetic and recreational purpose in addition to the functional benefits. (17) An intensive green roof has a deep substrate, hosts a variety of flora, and requires significant investment for set up and continued maintenance. (18) By contrast, an extensive garden is designed almost exclusively for its functional benefits, such as thermal insulation, fireproofing, and storm-water management. (19) These benefits can be accomplished with a less elaborate plant core, shallower soil, and low-growing vegetation. (20)

    The basic structure of an extensive garden includes a layer of insulation material and a waterproofing membrane, topped by soil and vegetation. (21) Intensive roof design typically requires the following enhancements to the basic structure: (1) about sixty to two hundred pounds per square foot additional structural support; (2) a minimum of eight inches soil depth, compared to the eight-inch maximum for extensive gardens; (3) widespread plant variety, constrained only by the climate, degree of exposure, substrate depth, and irrigation capacity of the garden, by contrast to the height and weight limits imposed on vegetation in extensive gardens; (4) irrigation systems and maintenance services, compared to the almost no-maintenance nature of extensive gardens. (22) The cost of the two types of gardens also reflects these differences. An extensive garden generally does not cost more than thirty dollars per square foot, while the intensive design starts at twenty dollars per square foot and above. (23)

    Garden infrastructure can also vary between directly planting on the roof surface, using vegetative matting, and other more complex methods. For example, the modular approach involves unit-by-unit installation of patented, custom-made containers made of recyclable, Ultra Violet-Resistant High Density Polyethylene. (24) This approach is different from direct planting because the modules have built-in water reservoirs and drainage systems that help to both irrigate plants and to decrease pressure from rainwater on the existing infrastructure. (25) The units are also raised three quarters of an inch from the roof in order to free air flow, cool off the surface, and drain excess runoff. (26) In addition, the modules can vary in depth depending on the need of different plant species, and are moveable for easy maintenance and flexible landscaping. (27)

    Rooftop landscapers select vegetation by considering the average temperature in the building's locale, the degree of sun exposure over the course of the year, wind patterns, and the architectural design of the roof itself. (28) These factors help determine which kinds of plants can thrive in a given micro-climate and whether the planting should consist of pre-cultivated mats, modular systems, plugs, cuttings, or seeds. (29)

    Even the low-maintenance extensive gardens require some attention in the initial period, including selective weeding and fertilization of the soil for the plants to fully establish themselves. (30) After the first five years, however, the attention paid to an extensive garden can be minimized significantly, while its intensive counterpart will likely require continued maintenance as any other elaborate garden. (31)


    Modern governments have begun to mandate and provide incentives for green roofs because of the compelling evidence of this technology's social and economic benefits. (32) For example, green roofs compensate for externalities such as the bareness of modern architecture and over-consumption of energy. (33) Green roofs also serve as a foil to the current construction methods, which green roof proponents argue are environmentally un-sustainable. (34)

    Forty years ago, Malcolm Wells--the pioneer of rooftop garden architecture--attempted to reverse deleterious effects of building construction on the landscape by developing an alternative building method called "underground architecture." (35) This method extends green rooftops by covering not only the roofs with vegetation, but also the rest of the building except the windows. (36)

    In the meantime, rooftop gardens are a more realistic solution to the long-term environmental risks posed by traditional building techniques. Rooftop gardens offer a range of benefits for their host buildings and for the urban environment: they create space that is aesthetically pleasing and therapeutic; offer the potential of homegrown produce; (37) minimize exchange of noise between the outside and the inside of the building; (38) and even make buildings more fire resistant. (39) In addition, by directly shading the roof and improving insulation, rooftop gardens help reduce the need for, and thus costs associated with, space conditioning, such as heating and cooling. (40) The rooftop garden also helps protect a building's roof membrane from exposure to ultraviolet light and from contraction and expansion caused by fluctuating roof temperatures. (41)

    Further, large-scale implementation of rooftop gardens helps alleviate microclimate problems that plague American cities. (42) Plants and trees help reduce harmful greenhouse gases in the atmosphere by sequestering carbon in the vegetation's biomass. (43) Rooftop gardens improve air quality by cooling and humidifying the air around the building, and by binding dust and filtering out pollution. (44) They have been shown to assuage the urban heat island effect, (45) prevent health problems associated with heat stress, reduce smog, and improve rainwater management. (46) The urban heat island is a phenomenon where air temperatures in urban centers are significantly higher compared to the surrounding areas. The warmer urban climate results from changes in the surface radiation and energy imbalance due to dense human architecture and increased energy use in urban centers. These deviations from the natural norm reduce the rate of cooling in cities, and increase concentration of pollution, problems in human health, and energy use for cooling. (47)

    Chicago's Urban Heat Island Initiative, launched in the 1997 and funded by the Environmental Protection Agency, is aimed at combating these problems. (48) The city strives to replace heat-absorbing surfaces with trees, plants, and shrubs to cool the air through evapotranspiration, (49) and to reverse the warming effects of concrete surfaces and eroded land cover associated with city development. (50) The hallmark project of the Initiative is the City Hall Rooftop Garden, installed in 2001 and intended to demonstrate the benefits of a rooftop garden. (51) The project has succeeded in lowering energy use by 9272 kW/hr peryear--equivalent to approximately $3600/year--and achieving a differential in roof temperature of at least 50[degrees]F between the City Hall roof and the adjacent blacktop roof of the Cook County Building. (52)

    In 2002, the National Research Council of Canada (NRCC) conducted a field study to measure the...

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