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For the fourth time, Chicago has topped all other North American cities in the amount of newly vegetated surfaces, according to the Green Roofs for Healthy Cities survey. The organization's founder and president, Steven W. Peck, explains their success: "I can tell you that the seeds of Chicago's tremendous crop of new green roofs came from the mayor's office in the form of political will and vision. Proactive policymaking and green roof and wall incentives from the city's planning department, plus excellent educational tools, continue to act like fertilizer, accelerating and supporting healthy design for a more livable city." The city of Chicago offers grants for the construction of green roofs, and the building code includes urban heat-island reduction requirements.

Other municipalities are introducing innovative incentives. In Portland, Oregon, for instance, where runoff harms local salmon populations, the building code allows extra floor space to developers when they build green roofs. Other cities are giving tax breaks to developers who demonstrate reduced runoff. Even without legal incentives, savvy building owners can appreciate the benefits of lowered energy consumption and the advantages of rooftop amenities for occupants. Altruistic goals, such as providing wildlife habitat, may need supporting legislation. However, some environmentally focused institutions, as shown below, see habitat creation as part of their mission.

 

The city of Chicago initiated a green-roof pilot project, beginning with its own City Hall. The trial run has facilitated research and educational outreach. Photo McDonough + Partners

 

The plants on the roof of Chicago City Hall are organized in a sunburst pattern by color and bloom. Over 33 buildings in the area offer a delightful view of this abundant color among the other, more conventional building tops. Photo McDonough + Partners

 

 

Two Kinds of Roofs

Green roofs are generally classified as either "extensive" or "intensive." Extensive roofs are characterized by a shallow growing medium (less than six inches), modest roof loads, limited plant diversity, and minimal maintenance. Often planted with grass, sedum, or moss, such roofs are not necessarily accessible or even visible by occupants, but they offer ecological and energy-saving advantages. If planted with drought-resistant native species, they might require little irrigation besides rainwater.

 

Shallow Assembly Diagram 1 Growth media 2 Moisture retention mat 3 Drainage board 4 Protection fabric 5 Membrane (adhered) 6 Dens Deck 7 Approved insulation 8 Substrate     Image Carlisle Syntec

 

Deep Assembly Diagram 1 Growth media 2 Protection fabric 3 Drainage gravel 4 Protection fabric 5 Root barrier 6 Extruded polystyrene 7 Membrane (adhered) 8 DensDeck 9 Approved insulation 10 Substrate     Image Carlisle Syntec

 

 

Intensive roofs have a deeper growing medium (over six inches), produce relatively higher structural loads, can accommodate a wider variety of plants, shrubs, even small trees, and require a level of maintenance comparable to that of conventional ground-level gardens. They can be furnished with walkways, arbors, benches, and other amenities. They are usually feasible only on new construction engineered to carry the extra load of the deeper soil and the water it can absorb. Some roofs combine extensive and intensive to capture the advantages of each. Structural analysis may reveal portions of an existing roof that can support the heavier loads of intensive vegetation, while leaving other areas to be scarcely planted.

The roofing industry has stepped up to the plate to offer protective materials and educate architects in the application. In general, a green roof of little or no slope has over a half-dozen requisite layers. Over the roof deck is thermal insulation, an all-important waterproof membrane, a drainage layer, a filter mat, and the growing medium. For intensive roofs with larger plants, a root barrier is also needed to protect the layers below. The growing medium should strike a balance between water retention and drainage, while meeting the requirements of the intended plantings. Between engineering the structure, specifying the built-up layers, and laying out the gardens, green roof design is often a multidisciplinary effort, involving botanists as well as landscape architects, engineers, and architects.