Green roofs are one of the most obvious and visible commitments that a green building can make. In addition to providing habitat for plants and animals, a green roof can assist with stormwater management and can provide some additional buffering of the environment, to reduce heating demand in winter and cooling demand in summer. Green roofs are found everywhere in North America, from cold, wet northern places like Toronto and Chicago to hot, dry southern places like Phoenix. The City of Chicago has more than 200 green roof projects underway, including one on the City Hall.
Green roofs come in two varieties, intensive and extensive. An intensive green roof is thicker and can support a wider variety of plants; for example, a LEED Gold apartment project, the 27-story Solaire in Battery Park City, New York, is home to a rose garden on the roof of the 19th floor. However, intensive green roofs add more weight and require more irrigation and maintenance, so most of the projects use extensive treatments, in which the soil layer is thinner (less than four inches) and typically composed of lightweight materials such as perlite.
Green roofs can also be designed as a second-floor amenity over a ground-floor retail podium in office buildings or residential high-rises in the cities. Available to office workers or residents, the green roof provides a park-like space in the midst of a city.
Ford Motor Company River Rouge Plant Green Roof. The rood provides bird habitat as well as stormwater management benefits.
Green roofs are not cheap. Typical costs range from $10 to $20 per square foot. For a 15,000-square-foot green roof, that would be $150,000 to $200,000. However, for a large project with multiple stories, a green roof might represent a significant amenity at a cost of 1% or less of the total project cost. In a project with high-level LEED goals, a green roof can help with open-space goals, thermal comfort, stormwater management and reducing the urban heat-island effect, cutting air-conditioning costs in summer and holding water from small to medium-sized storms for later release.
Green roofs are used in many applications, including commercial, industrial, government and residential buildings. In Europe they are widely used for their stormwater management and energy savings, as well as their aesthetic benefits. Green roof systems may be modular, with drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids, or each component of the system may be installed separately in layers.
A good example is the Ford Motor Company’s River Rouge plant. Lying at the center of the revitalization project, this new assembly plant represents Ford’s efforts to rethink the ecological footprint of a large manufacturing facility. The design emphasizes a safe and healthy workplace with an approach that reduces the impact of the plant on the external environment.
The keystone of the site’s stormwater management system is the plant’s 10-acre (454,000-square-foot) living roof, the largest in the world. This green roof is expected to retain half the annual rainfall that falls on its surface. It will also provide habitat, decrease the building’s energy costs and protect the roof membrane from thermal shock and UV degradation, extending its life. Very quickly, local birds discovered a safe place to lay eggs!
In describing the project’s strong economic justification, architect William McDonough discovered that Ford was prepared to spend $48 million on a conventional stormwater management system to handle the runoff from the roof and parking lots of the facility. His design cost about $13 million ($29 per square foot of roof), including the green roof and a parking lot with gravel filters and bioswales (planted drainage ditches), followed by constructed wetlands treating stormwater runoff, saving Ford $35 million along the way. He said, “It takes three days for water to flow from the plant to the river, and it’s purified naturally along the way…It took [Ford’s] board about a minute and a half to approve the project.”