A Grassroots Movement Benchmark

Published in Today’s Facility Manager (September 1999)

A Grassroots Movement

Green roofs created of native grasses and plants are growing around the world

By Michael Newland

Imagine yourself walking through a lush, green field. Wind ripples through the grass and clouds float overhead. It may sound like a daydream, but it’s actually a roof inspection for the increasing numbers of people embracing the idea of a literal green roof, where vegetation is the roofing medium. More than just pretty faces, green roofs may offer quite tangible benefits regarding water runoff and energy efficiency.

According to Canton, MA-based Sarnafil Roofing and Waterproofing Systems, the definition of a green roof is an area of planting on a waterproof substrate at any level that is separated from natural ground by a man-made substrate.

While they’re certainly not the norm (and not appropriate for every situation), green roofs are increasingly being planted around the world. When Gap Inc.’s award-winning 195,000 square foot office building in San Bruno, CA was built, for instance, it was determined a green roof was a viable and cost effective alternative after an extensive study performed by various parties involved in the project, including Charlottesville, VA-based design architects William McDonough + Partners.

What did the study find? A building becomes less susceptible to exterior temperatures when it’s covered by a green roof. The insulating value of soils lowers the cost of heating and cooling a building.

For instance, the thermal mass of the 69,000 square foot green roof at Gap Inc. modifies the rate of change of temperature and results in an energy savings three times greater than that of a conventional roof membrane. Originally, the payback period for the roof was estimated at eight years, but today Gap Inc. estimated payback will more likely be five to six years, according to a source affiliated with the project.

Gap Inc. representatives comment, The roof of 901 Cherry is part of the landscape…and it puts an actual ecosystem where tar and gravel might have been. The self-sustaining, low maintenance grass roof provides extraordinary thermal and acoustic insulation and will contribute to increased energy savings year ’round.


So how is a green roof created? The two principle types of green roofs are defined in terms of their purpose and root depth, according to Sarnafil representatives. An extensive green cover features thin soil, harsh growing conditions, little or no irrigation with a moss, herb, and grass species at 0% to 30% slope. On the other hand, an intensive garden roof features deep soil, benign growing conditions, irrigation, with a wide range of garden species at a 0% to 2% slope.

According to Erisco-Bauder, a United Kingdom-based supplier of green roofs, a typical system is made up of 11 layers: primer; vapor barrier; a two layer waterproofing system; a separation film capped by a protection mat; a drainage or water retention layer; a filter fleece; and finally the growing medium and vegetation.


In addition to the aforementioned thermal benefits, green roofs offer other advantages. Some touted benefits include:

Aesthetics. A green roof provides an engaging landscape for employees and the surrounding area. The selection of native grasses and plants can reflect the natural environment in color, texture, and structure, and may attract birds and butterflies.

Improved acoustic insulation. Buildings with green roofs are quieter. Due to their mass, green roofs can attenuate sound transmission by 50 decibels.

Extended durability of membrane. The insulating membrane will be protected from ultraviolet degradation, mechanical puncture, and temperature extremes because it is completely covered by a green roof. The life expectancy of a membrane can be well beyond that of a conventional roof.

Environmental impact. Green roofs modify the local microclimate, affecting temperature, humidity, and rainfall runoff. The plant materials of a green roof provide increased oxygen to the air and counteract some of the carbon dioxide produced by a building.

Water retention. At the Gap Inc. facility, the average annual rainfall in San Bruno is approximately 30 inches, which equals 520,550 gallons of water on the 69,000 square foot area of the green roof. Most of this average rainfall would normally be absorbed by the green roof, creating little run-off.

Municipalities Taking Notice

The combination of benefits green roofs offer may have particular advantages to the concerns of municipal governments. And some cities are beginning to take notice. In Stuttgart, Germany, for instance, new building construction can only take up 40% of the original site — but with an exception. If a corporation creates a green roof, the requirement expands 60% and the number of allowed parking spaces increases as well, says Matt Carr, product manager of Garden Roof TM with Chicago, IL-based American Hydrotech, one of the most well known green roof companies in the U.S.

Stuttgart’s requirements are largely due to the fact that garden roofs can be much less taxing to municipal stormwater systems, he says. Seventy-five percent of a typical rainfall is usually absorbed by a green roof, according to Carr.

He comments, I think over the next five years you’re going to see more municipalities encouraging these projects — as much for the rainwater runoff benefits as anything else.

Chicago Goes Green

Indeed, her in the U.S. the City of Chicago is expected to plant a rooftop garden on 20,300 square feet of the 38,880 City Hall roof this month. It wasn’t water retention benefits that attracted the City, though; it was the role green roofs may play in helping to reduce the effect of urban heat islands and smog.

(Urban heat island are a phenomenon caused by the increased use of dark, heat-absorbing building and construction materials and the corresponding reduction of shade trees in urban areas. As a result, urban areas can have higher temperatures — often two to eight degrees warmer — than the surrounding countryside. And higher temperatures correlate to higher levels of ozone pollution. For more on this, see TFM’s article Energy Efficient Roofs in the March 1999 issue.)

Chicago is one of five cities participating in an Environmental Protection Agency (EPA) program studying the effects of cool roofing — but the only one incorporating rooftop gardens.

Jessica Rio, a spokesperson for the Chicago Department of Environment says Chicago decided to include rooftop gardens in its EPA study to correspond with a citywide initiative toward greening that includes median plantings and recent ordinances encouraging greening of parking decks. The benefits are twofold: the City looks nicer, and the plants help combat pollution.

Exactly what impact the rooftop gardens will have on temperatures and smog remains to be seen, though. Researchers are eager to see Chicago’s results, says Virginia Gorsevski, program analyst with the EPA’s office of air and radiation, in an April 5, 1999 article from The Associated Press. (Green Roofs Cool City Rooftop Gardens In Chicago To Fight Smog, Heat.)

Rio says the City will be putting together a package in the near future on what it has learned so far relating to cool roofing and heat islands. Though it’s currently in draft form, the report will eventually be available from Brendan Daly at the Chicago Department of Environment. (Call (312) 742-0150 for a copy.)

In the meantime, regardless of scientific findings, Chicago residents will get to enjoy a bit more nature than usual. And as to the City Hall roof, Rio comments, I’m personally excited because my office window looks out onto it.

Taking Root?

Carr, the aforementioned product manager of American Hydrotech’s Garden Roof, says he sees a growing interest in green roofs and mentions recent high-profile projects like the $183 million Mashantucket Pequot Museum in Connecticut. We can’t keep up with the growth right now; it’s unreal the level of interest, it’s phenomenal.

According to Carr, William McDonough, the famed architect of the Gap Inc. building, is currently looking at green roofs for more than just new construction. He wants to start using it in reroofing, says Carr.

Carr isn’t surprised by the interest. From a design standpoint, the aesthetics are pretty strong, he comments and says he sees a market in hospitals where garden roofs could be beneficial to patients.

But Carr equally recognizes that green roof advocates still have a long way to go in convincing governments and the building industry of the practical advantages. The challenge will be quantifying the benefits, he says — especially when green roofs tend to cost 25% to 30% more than traditional systems. Generally, says Carr, green roofs cost $8 to $20 a square foot installed.

Carr’s product, Garden Roof, combines American Hydrotech’s roofing/waterproofing membrane with a patented system of drainage/water retention components from Germany-based ZinCo to offer a single source system. Hydrotech also offers a planning guide to landscape architects to guide them in the selection of appropriate lightweight soils and vegetation for the roof.

Another company, Soprema, based in Quebec City, PQ, is counting on the growth of green roofs. After years of research and experimentation and an investment of half a million dollars, it developed a specially formulated vegetation complex for growing plants on a thin, lightweight growth substrate. Sopranature, as the concept is called, is purported to transform roofs and terraces into garden oases. According to the company, With so may natural advantages, this avant-garde concept is sure to take roof as a way of doing things.

— Christine Menapace

This article contains excerpts from Client Profile: The Gap’s Green Roof by Michael Newland, a consultant with Cedar Rapids, IA-based Benchmark Roof Consultants. For more on Benchmark, call (319) 393-9100.

For more information on Sarnafil, call (800) 576-2358.

To contact Erisco-Bauder, visit www.erisco-bauder.co.uk .

American Hydrotech can be reached by calling (312) 337-4998.

For Soprema, call (418) 681-1224.

Special thanks to Alex Wilson of Environmental Building News.

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