The greatest potential of greenroofs lies in their capacity to cover impervious roof surfaces with living, breathing, permeable plant material. Greenroofs are healthy, sustainable, and regenerative roof landscapes that can help protect our environment by diminishing developmental impacts on our communities. They are one sustainable design element in the palette of today’s ecological designer.
Integrated Living Roofs
Integrated design is essential for delivering a cost-effective green building. Successful practitioners have come up with ways to get the whole team collaborating effectively and thinking outside the box. Not only are they delivering green projects within conventional building budgets, but many are doing it for a conventional fee.
As designers and community and business leaders, we can choose to mitigate the many negative effects of a building’s footprint by incorporating various green design principles. As an alternative to imposing our built structures onto the land without considering the function of under-used roof surfaces (beyond waterproofing), we can incorporate organic greenroof architecture as a way of designing with nature to evoke displaced landscapes and restore a measure of greenspace. Imagine looking down from an airplane with a bird’s eye perspective. Instead of seeing huge expanses of concrete or black tar roofs imposing themselves on the natural environment, you see moving stands of flowering, multi-colored plants.
The roof now blends into the landscape as a naturalistic meadow scene. Or designed gardens and parks create a new “fifth” façade for human recreation with flowering shrubs, trees, and vegetated spaces. These scenarios are technologically possible, and greenroofs do not require particularly high-tech design. It is important to understand, however, that these are engineered systems consisting of various material layers which must work in tandem to perform correctly.
A Quick History of Greenroofs: From Ancient Mesopotamia to the 21st
Century
Combining plants with architecture is not a new idea, and neither are greenroofs. Since early recorded times, natural and created landscapes have been integrated into the urban fabric.
Designed elevated greenspaces have existed as long as humanity has been concerned with architecture. Manipulating our living spaces logically also includes using natural areas and garden designs as artistic expressions and a way to connect back to nature.
The sloping walls of the Ziggurat of Nanna, built around 2100 B.C., were covered with trees and shrubs. The fabled Hanging Gardens of Babylon, which included lush roof gardens and terrace greening, represent the earliest known interpretations of roof greening, built between the 8th and 10th centuries B.C. Earth-sheltered huts dating from the Viking era have been found in Ireland and Scotland. In addition, around 1000 A.D., sod-covered roofs were used in Iceland and Scandinavia. Later on, early 19th century settlers in Canada and the northern United States introduced grass roofs.
"Garden cities" have been developed from Persia to Renaissance-era Paris, and later from Russia to Berlin, London, and New York. Modernist architects such as Le Corbusier, Frank Lloyd Wright, and Roberto Burle Max promoted the benefits of roof gardens, and incorporated them into the fabric of their designs. Still-successful modern greenroofs from the 1930s include the five famous Rockefeller Roof Gardens in New York, and the Derry and Tom's Garden in London (the modern Kensington Roof Gardens).
Greenroofs today can be found throughout Europe and around the world. But the development of greenroofs from an expression of vernacular architecture to a viable sustainable construction roofing alternative took place in modern Germany. There, greenroofs have evolved through trial and error, the repeated testing of materials, and ultimately the development of industry standards and codes. It is estimated that Germany now has over 800 greenroof projects.6
Modern Greenroof Pioneers in Germany and North America
True modern greenroofs were introduced in Germany in the early 1970s by manufacturers, landscape architects, and university researchers. In 1971 Gerda Gollwitzer and Werner Wirsing outlined the principles of modern greenroofs in their book entitled Roof Areas Inhabited, Viable, and Covered by Vegetation. Hans-Joachim Liesecke outlined the basis for intensive greenroofs in his 1972 report entitled Dach und Terrassengärten [Roof and Terrace Gardens]. Others followed, notably Kolb, Hans Luz, Hans Kienle, and Bernd Krupka.
Acceptance of greenroofs in the European marketplace came in the 1980s, when systems were enhanced through use of reliable root barriers and sophisticated forms of buildup that guaranteed safety and a long lifespan. Credit for many of these developments goes to the German greenroof companies ZinCo,optima (now split into two companies, optima and Optigrün), and Bauder. They were the pioneers of modern greenroof technology, especially with regard to root resistant bituminous waterproofing.
Pioneers and proponents of greenroofs in North America from the early 1990s include Tom Liptan, ASLA, of the City of Portland, Oregon; Charlie Miller, P.E., of Roofscapes, Inc.; Katrin Scholz-Barth, a civil and environmental engineer; and especially two prominent veteran landscape architects from the sphere of traditional roof garden design -- Cornelia Hahn Oberlander, FCSLA, FASLA, in Canada, and Theodore Osmundson, FASLA, in the United States.
Two Greenroof Types: Extensive and Intensive
Greenroofs are vegetated roofs with engineered soil (also known as the substrate or growth media) and plants layered above a concrete, wood, or metal roof deck. They can substitute for gravel, shingle or tiles. Imagine a roof lasagna-like assembly with a meadow on top.
The bottom line is that the plants are planted directly onto the roof, not just in containers. The layers vary from system to system, and certain elements vary in their placement above the roof deck. At the very least, however, all greenroofs include waterproofing (single or multi-ply), drainage, soil, and plants.
Over the past 35 years, sound German engineering, technology developments, and testing standards have led to greenroof systems that range from virtually maintenance free to quite elaborate.
There are two main types of greenroofs -- extensive and intensive (also referred to as low-profile and high-profile); the names indicate maintenance requirements. The two designs can also be combined.
The type of greenroof that is appropriate for a given application must be determined by the site owner and designer, with a view to how the roof is to function. Greenroofs can be used successfully in both new and retrofit construction. They are limited only by the slope or pitch of the roof, existing load requirements, and budget factors.
See Exhibit 1 on the following page for a chart describing the differences between extensive and intensive greenroofs.
Extensive vs. Intensive Greenroofs
Low-Profile/ Ecoroofs High-Profile/ Roof Gardens
• Low growth media: 1” – 6”
• Lightweight: 12 – 50 lbs/sf
• Low growing plants: 1”– 24” H
• Less variety of plants: Alpine
types, succulents, herbs, some
grasses and mosses
• Usually non-accessible
• Slopes up to 30° & higher
• Less expensive: $12-$25/sf
• Low water requirements
• Low maintenance
• > 6”-15’ and deeper
• Heavier weights: 50 lbs/sf +
• Trees, shrubs and more
• Huge variety of plant selection,
depending on loads, design &
budget
• Designed for human recreation
• Relatively flat
• More expensive: $25-$40/sf +
• Irrigation usually necessary
• Higher maintenance
Extensive Greenroofs
Extensive greenroofs employ fewer and thinner build-up layers, and thus are lighter and less expensive systems. They are used when the owner primarily desires an ecological roof cover with limited or no access for recreation. Less growth media is used, and the appropriate plants are low-growing, hardy Alpine types.
Plants for extensive greenroofs must be tolerant of high heat, drought, wind, and frost. They must also be self-regenerative in nature, and have low maintenance requirements overall.
Media depths range from one inch up to about six inches. A popular misconception is that a flat roof is ideal, but in fact flat roofs present drainage issues. Ideally the roof should have a gentle slope of at least 1.5 - 2% to allow for natural drainage properties. Generally, extensive greenroofs can be installed on slopes of up to 30°, although there are greenroofs with 40° slopes.
Reinforcement will be necessary at steeper pitches using cross battens or underlying grid structures to hold the plants and engineered soil in place, as well as to deal with wind shear.
Roofs with strong wind uplift or with slopes 15° and higher should be protected during establishment with an erosion control net in the form of jute or other natural biodegradable fiber.
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