Buzz
The green roof atop Chicago City Hall effectively cools the structure and reduces water runoff.
Photo courtesy of DOE/NREL I Photographer: Katrin Scholz-BarthA bird's-eye view of urban landscapes often reveals vast expanses of asphalt, black tar, and gravel-covered rooftops. These dark surfaces emit heat, and rainwater swiftly flows over their hard, ideally impermeable tops. However, a growing trend is disrupting this uniformity: the green roof.
Green roofs, long favored in Europe, are now gaining popularity among homeowners, businesses, and even city planners as an appealing solution to conventional roofing issues while promoting eco-friendliness. They enhance traditional greenery without interfering with urban systems, transforming unused spaces into functional areas.
Benefits of Green Roofs
Green roofs have a longer lifespan compared to traditional roofs, lower energy expenses through natural insulation, provide serene havens for both humans and wildlife, and soak up rainwater, which may reduce the necessity for intricate and costly drainage systems.
On a broader scale, green roofs enhance air quality and mitigate the Urban Heat Island Effect, a phenomenon where urban and suburban areas absorb and retain heat. Anyone who has crossed a blistering parking lot on a sweltering summer day has experienced this effect firsthand.
The layers of green roof systems must, like any roof, handle drainage, manage stormwater, and shield the building from weather with a waterproof barrier. Additionally, they need to establish a growing environment and potentially offer support, irrigation, and root protection barriers while remaining as lightweight as possible.
Types of Green Rooftops
There are two main types of green roofs: intensive and extensive.
Intensive green roofs function like elevated parks, capable of supporting shrubs, trees, walkways, and benches due to their intricate structural support, irrigation, drainage, and root protection systems. These roofs require at least 1 foot (0.3 meters) of growing medium, resulting in a weight of 80 to 150 pounds (36-68 kilograms) per square foot (0.1 square meter).
Extensive green roofs are lighter, weighing between 15 to 50 pounds (7 to 23 kilograms) per square foot (0.1 square meter). They are designed to support hardy native ground cover that needs minimal upkeep. Unlike intensive roofs, extensive green roofs are primarily installed for their environmental advantages and are not intended as accessible rooftop gardens.
One of the most renowned green roofs in the United States is Chicago's City Hall, which features a combination of extensive, intensive, and semi-intensive green roof systems on a single retrofitted structure.
Under the Mayor's leadership, the City of Chicago's Department of Environment launched a City Hall pilot program, initiating a citywide effort to promote green rooftop systems through incentives and grants.
Green roofs, such as those found on the Faroe Islands, can endure twice as long as traditional rooftops.
Image used under the GNU Free Documentation LicenseCost and Benefits
The upfront cost of installing green roofs often deters potential clients. Due to the need for a specialized design team, thorough structural analysis, and multiple layers and systems, even basic extensive green roofs typically cost at least $8 per square foot (0.1 square meter), which is considerably higher than the $1.25 per square foot for conventional built-up roofs (BURs) [Source: EPA].
However, benefits and incentives, such as those offered by the City of Chicago, are encouraging new green roof projects. Additionally, as the green-roof industry in the U.S. expands, prices are expected to decrease.
In the meantime, the long-term financial advantages already justify the initial investment. Green roof technology shields the roof membrane from extreme weather and UV radiation, allowing green roofs to last twice as long as traditional roofs.
Green roofs also maintain a relatively stable surface temperature, staying at or below air temperature, whereas conventional rooftops can reach up to 90º F (32º C) above air temperature [Source: EPA]. This results in lower heating and cooling expenses, as there is less need to rely heavily on air conditioning during the summer.
The additional layer of growing medium and plants provides insulation against extreme temperatures and reduces heat absorption. A Canadian study found that even a 6-inch (15-centimeter) extensive green roof can cut summer energy consumption by 75 percent [Source: Professional Roofing].
Environmental Impact and Urban Benefits
The following advantages are motivating environmentally conscious homeowners, businesses, and cities to adopt green rooftops:
- Green roofs reduce water runoff and prevent sewer overflows.
- Plants and soil absorb and filter water, preventing it from rushing through gutters, washing over polluted streets, and overwhelming sewer systems.
- The vegetation on green roofs captures air pollutants, generates oxygen, and offers shade.
- They utilize heat energy during evapotranspiration, a natural cooling process where water evaporates from plant leaves.
- Evapotranspiration and plant shading help counteract the Urban Heat Island Effect caused by excessive reflective and impermeable surfaces in urban and suburban areas.
Urban Heat Islands raise temperatures in cities and suburbs, increasing the need for air conditioning and perpetuating a cycle of energy use that exacerbates global warming. Widespread adoption of green roofs could alleviate the adverse effects of Urban Heat Islands.
Green roofs transform rigid urban structures into efficient, aesthetically pleasing, and functional spaces. They provide office workers with serene rooftop escapes and apartment dwellers with areas to cultivate gardens or unwind. Even non-accessible green roofs enhance the skyline for neighboring buildings and offer wildlife a secure and secluded habitat.
Green Roofs: Then and Now
Green roofs and rooftop gardens have been around for millennia.
The Hanging Gardens of Babylon, one of the Seven Wonders of the World, featured an advanced irrigation system to create a lush terraced garden near present-day Baghdad. Historically, Northern Europeans used sod roofs to insulate their homes.
Today, green roofs are widespread and sometimes mandatory in certain European regions. In Germany, 14 percent of all roof space is green [Source: EnviroZine].
