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By Robert C. Lichy
Green roofing is thought to be a relatively new idea, but in fact it has existed since ancient times. Records show that ancient Mesopotamians installed gardens on their stone-stepped towers, called ziggurats, as early as 600 B.C. A Benedictine abbey with rooftop gardens was built in France in the 13th century. In Europe, sod roofs have been used as protection against the weather for centuries. Rockefeller Center in New York had five rooftop gardens installed in 1936.
While there have been examples of green roofing throughout history, it emerged as a viable modern roofing concept about 45 years ago in Germany. It was developed to cope with the country’s rising energy costs and to reduce the storm water burden on its aging sewer system. Ultimately, green roofing spread across the continent before coming to North America. In the past 10 or 15 years, we’ve become more aware of the benefits and green roofing has started to gain popularity. Cities across the country are encouraging green roof development in the form of mandates or tax benefits.
The number of green roofs has steadily increased during the past several years. Several large U.S. corporations installed green roofs on their buildings. Ford Motor Co., H.J. Heinz Co., the Gap, Starbucks and Quaker Oats are only a few of the companies taking advantage of green roofs.
Thermal resistance is the key feature. Green roofs can provide increased thermal resistance all year long. They’re particularly helpful, however, during summer months or in warm climates, because they effectively reduce cooling loads. It’s not unusual for a typical black roof membrane in those conditions to reach surface temperatures of 175°F. The vegetation on a green roofing system generally doesn’t reach a temperature greater than 5° above ambient. This reduction in roofing system surface temperature translates to a considerable savings in cooling costs.
“The typical green roof both reduces the amount of water run-off and spreads it out over a longer period of time.”- Robert C. Lichy
Then, there’s the matter of storm water retention. Most traditional roofing systems do little to reduce or retain storm water during a heavy rain. A typical green roof assembly, on the other hand, can delay the peak in storm water flow by as much as four hours. The total run-off can be reduced by 50% to 90%, depending on system design. When the water running off the roof is combined with storm water from paved areas, it can present a real burden on the plant’s storm water system. The typical green roof both reduces the amount of water run-off and spreads it out over a longer period of time.
A green roof offers environmental benefits. Manufacturing plants located in urban or suburban areas frequently are guilty of contributing to what has been termed the Urban Heat Island effect because of large, black parking lots and dark roofs. While a white roof might reduce the negative effect, a green roof might actually mitigate the heat island. Green roofs not only cool and humidify the ambient air, they filter out dust and smog particles. Plants absorb aerosol contaminants in the ambient air, leaving the air safer for all of us.
Don’t forget the increase in the roof’s expected usable life. There’s currently little hard data upon which to predict the expected life of a green roof over that of more traditional roofing. However, it’s reasonable to expect the green roof to last longer. The waterproof membrane in a green roof assembly is protected from ultraviolet degradation and weathering such as rapid temperature changes, strong winds, hail and ice.
Like other types of protected membrane systems, the membrane temperature in a green roof remains nearer that of the interior space and doesn’t fluctuate much. Additionally, the waterproof membrane is protected from damage that can occur from maintenance traffic. Some preliminary studies indicate that the life expectancy of single-ply membranes might be increased by as much as 25 years and the life expectancy of some other types of roofs might be doubled.
Finally, many local, state and federal governmental entities provide incentives to encourage projects that save energy and improve the environment.
There are two classifications of green rooftop: vegetative systems and green roofs. The two classifications are known as extensive and intensive, respectively.
The extensive, sometimes called low-profile, is designed for good thermal and storm water retentiveness while having minimum weight load. These generally have only a few plant types. The thickness of the growing medium is as shallow as 2 in. to 4 in. The approximate load of an extensive roof when wet is 11 lb./sq.ft. to 22 lb./sq.ft.
The intensive, or high-profile, has many more plant types, sometimes including large plants and trees. The thickness of the growing medium can be 5 in. or greater. The intensive roof is sometimes referred to as a roof-top garden.
Traditionally, green roofs have been built in place; each component being assembled on-site. More recently, preassembled and planted modules containing the various components are available. The preassembled modular roofs are usually extensive in that they contain limited depth and number of species.
The variations of green roof design include several components. Sometimes one element carries out multiple functions. Start with the waterproof membrane. A number of different membrane types have been used successfully in green roof assemblies. Perhaps the most popular type is a single-ply membrane because of its low cost and application simplicity.
Polymer-modified asphalt membrane systems also are frequently used in green roof designs and have a demonstrated history of good service. Hot-fluid-applied systems also have proved successful. These hot-fluid systems include both polymer-modified asphalt and coal-tar. Polymethyl methacrylate (PMMA) membranes also demonstrate considerable promise of providing a quality, long lasting, waterproof membrane for green roofing. These membranes are installed directly over a concrete or other solid structural deck. If the building has a fluted metal roof, cement or gypsum board might have to be installed to support the waterproof membrane.