Green roofs offer environmentally friendly alternative

A green roof — also called an eco-roof, nature roof, or living roof — is simply a roof that's covered with some sort of plant material (green being just one of the possible colors). The name is a reference to the environmentally friendly "green" approach, which is definitely the direction the architectural, development, and construction industries are moving.

By Michael D. Perry, President, Building Logics, Virginia Beach, VA August 6, 2003
Key Concepts
  • There are two types of green roofs: intensive and extensive.

  • Advantages include lower total life cycle cost, reduced stormwater runoff, and lower energy costs.

  • Traditional roofing technology is still the foundation for green roofs.

    Basic types
    Environmental concerns
    Special considerations
    Ten acres of sedum

    A green roof — also called an eco-roof, nature roof, or living roof — is simply a roof that’s covered with some sort of plant material (green being just one of the possible colors). The name is a reference to the environmentally friendly “green” approach, which is definitely the direction the architectural, development, and construction industries are moving. The most common plant types are sedum or stone crop. Sedums are the most draught-resistant and freeze-resistant plants available; they can actually grow in places where no soil is present.

    Historically, green roofs are really nothing new. The ancient world had the Hanging Gardens of Babylon and many other spectacular examples. More recently, the movement has its roots in places like Iceland and Scandinavia, where limited resources resulted in sod-covered homes and other early green roof forebears.

    Green roof technology as we know it today began in Germany in the 1960s. Because of its positive and dramatic impact on the environment, the concept slowly but surely gained in popularity throughout Europe as green roof techniques and technologies steadily improved. Currently, more than 10 million sq ft of green roofs are installed every year in Germany alone.

    Basic types

    There are two types of green roofs, intensive and extensive. Intensive involves landscaping that can include manicured lawns, shrubs, trees, and flowers. These spaces are intended for humans to enjoy. They’re also expensive and impractical for most applications. Extensive, which is what’s driving the current green roof surge, is much less involved and practically maintenance free. This type of system uses a very thin layer of soil and a groundcover-type plant. For most applications — including industrial — an extensive green roof system can be extremely practical, cost-effective, attractive, and beneficial in many ways.

    There are a number of extensive green roof systems available, but they all have certain things in common. First (from the bottom up) is a waterproofing membrane. Next comes a root barrier that prevents the plants from penetrating the waterproofing layer. A drainage layer draws excessive moisture away from the roofing membrane. A filter fabric layer prevents soil from clogging the drainage system while enabling water to penetrate and nurture the plant life. And finally, a soil substrate layer and a surface layer of plant life, customized to meet the functional and aesthetic requirements of the building, make up the exposed layer. On steeper slopes of up to 45 deg, a raised grid structure is installed to stabilize the soil medium.

    Environmental concerns

    Environmental problems associated with traditional roofing are many. The tools of the trade include tar, asphalt, solvents, asbestos and many other ecologically unfriendly chemicals and materials, all of which wind up in landfills at the end of their 10-yr-or-so lifespans.

    Roofs also contribute to the problem of stormwater runoff. Practically 100% of the rain that falls on a typical roof washes, in a great rush, straight into sewers and drainage systems. In urban areas, the result can be flooding, pollution of water supplies and waterways, sewage backup, and millions of dollars spent trying to alleviate these problems.

    Last but not least is the urban phenomenon known as the “heat island effect.” Roof surfaces (as well as roads and parking lots) absorb solar radiation and re-radiate it for hours. On a hot summer day, a typical roof can reach temperatures exceeding 140 F. The cumulative effect is that, during the summer months, cities are typically 10 deg hotter than surrounding areas. That’s enough to change weather patterns, cause smog and other pollution problems, and make things pretty uncomfortable for people who live and work there.

    How do green roofs help? For one thing, they typically double (or even triple) the lifespan of a roof, keeping roofing materials out of landfills. In a life-cycle cost analysis, this fact alone usually negates the higher initial-cost differential.

    Green roofs retain 70% to 90% of the annual rainfall that lands on them, which stabilizes water flow and relieves overburdened drainage systems. That water, retained in the soil and the plants themselves, is released slowly as humidity, which cools surrounding areas. In addition, instead of absorbing and re-radiating heat from the sun, plants use that energy for sustenance. This cycle eliminates excessive heating on rooftops and goes a long way towards eliminating the heat island effect. A recent study by the Lawrence Berkeley Laboratory showed that a mere 5% increase in green space in a large metropolitan area would reduce the average summertime temperature by 4 deg, and would reduce smog by 10%. These kinds of statistics are inducing cities and the federal government to begin mandating or providing incentives for the use of green roofs.

    Additional energy savings are realized from the added insulation qualities and transpiration associated with a green roof system. The lower ambient temperatures associated with these roofs reduces air conditioning requirements. This reduction equates to energy savings and greater life expectancy of air conditioning equipment. In certain circumstances a green roof could eliminate the need for air conditioning in a building.

    There are other societal benefits from green roofs. Pollution is reduced as the plants’ leaves collect dust and other pollutants from the air. The roots filter water. The plants generate considerable amounts of oxygen. And finally, green roofs help turn an ecological dead zone into a habitat for birds and insects.

    Special considerations

    When considering installing a green roof system, whether it is intensive or extensive, there are several fundamental issues that must be considered. The key word in green roof is roof . Because of the environment in which the roof membrane must function, the best-performing, time-proven green roof membrane available should be used. The membrane and root barrier must meet the rigorous FLL Standards for root resistance. This is the only internationally accepted standard for green roof waterproofing membranes.

    Second, adequate drainage is paramount for a successful green roof application. Any water not absorbed by the green roof assembly must be removed from the roof.

    Third, all green roof assemblies should include a water retention system. Proper water retention will help ensure that plants have enough nutrients and moisture to thrive during conditions of limited rainfall and can eliminate the need for additional irrigation.

    Fourth, the soil substrate should be designed to accommodate the desired vegetation. Soil mixes should not exceed 20% organic material, should be lightweight, and should absorb and store as much water as possible.

    Because the green roof concept is new to this country, anyone considering a green roof should consult a trained specialist. A successful green roof project requires a team approach that includes the building owner, architect, manufacturer, landscape architect, and roofing contractor.

    More Info: Mike Perry is available to answer questions regarding this article. Contact him by phone at 757-340-4201 or by e-mail at . Article edited by Richard L. Dunn, Editor, 630-288-8779; e-mail .

    Ten acres of sedum

    If there are doubts about the benefits of green roofs, Ford Motor Co. doesn’t want to hear them. The company is investing in what is believed to be the largest (almost half a million sq ft) green roof ever installed. It is part of the new $2 billion Rouge Center scheduled to start up in late 2003.

    Business justification behind the roofing system is to reduce storm water runoff and eliminate the engineering and construction costs of elaborate storm drain systems. The roof, in combination with other environmentally friendly features such as wetlands, a porous surface on parking lots, and ivy covered walls, are estimated to save some $35 million compared with conventional systems, according to Tim O’Brien, director, Ford Environmental Quality Office.

    The sedum is being grown on special mats developed by a German company specifically for green roof applications. When the time comes for installation, the mats and sedum plants will be rolled up like sod and moved to the rooftop. The original plan to grow the sedum in place in 4 in. of soil was dropped because of the weight of the soil and the potential for wind erosion.

    The roof is expected to last 20 to 40 yr.