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Up On the (Living) Roof

May 15, 2009

“Right smack dab in the middle of town, I’ve found a paradise that’s trouble proof.” It’s right where the Drifters said it would be: Up on the roof!

In town park from the Girls Gone Green blog

In-town rooftop park; photo from Jubie's "This Girl's Gone Green" blog

This week’s post explores green roofs – planted roofs that offer huge benefits by stemming storm water runoff, cutting heating and air conditioning bills, and reducing air and noise pollution. Green roofs are fine places for birds, butterflies and bipedal buddies to visit. Chefs too; at Trent University in Ontario, Canada, the roof of the Environmental Sciences building grows vegetables that are served in a university cafe called the Seasoned Spoon.

I fondly recall sitting on a roof outside my rented room above 13th Street in Boulder to study while I was an undergrad. Roofs were a popular student perch then, possibly because when I went to college in the 1970’s, neither dorms nor rental houses in Colorado were equipped with air conditioning. In the breeze under a shady, overhanging tree was the best place to sit and read when the mercury started to edge up.

Despite the recent eco vogue that has made green roofs au courant – topping buildings from Singapore to Dearborn, Michigan, where Henry Ford’s original River Rouge Truck plant now sports a 10-acre vegetated roof – grassy roofs are really old hat. More than a century ago, high plains settlers were nicknamed “sodbusters” both because of their work and their built-into-the-prairie housing. And grass-covered roofs were venerable even then.

Norwegians and Icelanders were building green roofs three and four centuries ago! The vegetation that kept their houses warm during the long Scandinavian winters could also feed the goat during the summer. Some of those Nordic houses are still intact. An Icelandic house, standing in its original spot, can be seen at the end of this blog. A number of grass-roofed historic houses have been saved and moved to cultural museums such as the Norsk Folkemuseum in Oslo. Interestingly enough, the Norsk Folkemuseum’s website notes that when tile began to be produced locally, sod roofs became unfashionable. As villagers replaced their roofs, they discovered that “tile roofs do not insulate as well as sod roofs and many people put in paneled ceilings for warmth.”

Why Green Roofs are Cooler – Literally

EPA infrared photo shows heat island effect in Atlanta, Georgia

EPA infrared photo shows heat island effect in Atlanta, Georgia

When I stepped out of my window onto the roof to study during my college days, I usually threw a bath towel out first. In bare feet, that roof was often too hot to touch.

A black tar and asphalt roof can push the mercury up as high as 180 degrees Fahrenheit, and accumulated acres of black roofs and pavement lead to a phenomenon called the “Urban Heat Island” effect. Because dark surfaces concentrate and reflect heat, cities are commonly 6 to 10 degrees warmer than green areas around their perimeters. No wonder that by July, we urban dwellers are, to paraphrase the Lovin’ Spoonful, “people lookin’ half dead, walkin’ on a sidewalk hotter than a matchhead.”

Because it gets so darn hot in the summer in the city, one-sixth of all electricity consumed in the US goes to cool buildings. That causes air conditioners to spew out exhaust, and ironically, more heat. It’s a vicious circle that, according to the EPA, leads to:

  • Increased energy consumption – Higher temperatures in summer increase energy demand for cooling and add pressure to the electricity grid during peak periods.
  • Elevated emissions of air pollutants and greenhouse gases – Increasing energy demand generally results in greater emissions of air pollutants and greenhouse gas emissions from power plants. Higher air temperatures also promote the formation of ground-level ozone.
  • Compromised human health and comfort – Warmer days and nights, along with higher air pollution levels, can contribute to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality.
  • Impaired water quality - Hot pavement and rooftop surfaces transfer their excess heat to storm water, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Rapid temperature changes can be stressful to aquatic ecosystem.

Green Roofs in the Birthplace of the Skyscraper

In 2000, a demonstration rooftop garden was planted atop Chicago’s 11-story City Hall. Since then, the birthplace of the skyscraper has been sprouting green roofs right and left. Partly because of $5,000 grants that the City awarded to dozens of residential and small commercial projects, Chicago is home to more green roofs than any other US city.

Green roof on Chicago City Hall

Green roof on Chicago City Hall

Chi-Town’s City Hall garden mitigates heat island effect by replacing a black tar roof with greenery. The garden absorbs less solar heat and manages to keep City Hall cooler, using less air conditioning and energy. Temperature differences between the green roof and the black roof on the nearby County Building have proved impressive. For example, on August 9, 2001, at 1:45 pm, when the temperature was in the 90’s, the thermometers read:

  • City Hall Roof (paved area) 126 – 130 F, (planted area) 91 – 119 F
  • County Roof (black tar) 169 F

City Hall’s roof garden holds more than 100 species of plants that have been selected for the sunny rooftop environment, plants that can handle the windy, arid conditions common in Chicago. In addition to shrubs and vines, two trees also live on the roof. Most of the greenery there consists of prairie plants native to the region. Thanks to all the vegetation, the roof garden can soak up 75% of a one-inch rainfall before storm water runs over into the sewers. In addition, the plants filter the air, improving air quality by using excess carbon dioxide to produce oxygen.

Over its first five years, the roof saved the City about $25,000 in energy costs, a saving due to shading, insulation and evapo-transpiration effects. (Evapo-transpiration occurs when plants secrete or “transpire” water through pores in their leaves.) The key features that affect the roof’s energy use include an increased layer of insulation under the main roof, plants and walkways that cover 20,300 square feet, and an irrigation system that provides adequate water to the plants.

The Academy Comes Alive in San Francisco

The checkerspot butterfly, a threatened species, has found a friendly habitat on the green roof of the Academy of Sciences in Golden Gate Park in San Francisco

The checkerspot butterfly is one of two threatened species that have lost habitat and are slated to find a home on the green roof of the Academy of Sciences in Golden Gate Park in San Francisco.

The California Academy of Sciences in San Francisco – which calls itself the “greenest museum in the world – is a new LEED platinum-rated building that is topped by a 2.5 acre green roof, the largest green roof of any natural history museum in the world. It covers 197,000 square feet, is 6-7 inches thick, and cost $17 per square foot. The roof retains 2 million gallons of rainwater, preventing 70% of it from running off and flooding city drains. The water that does run off the roof is collected in cisterns in the basement. It’s used to irrigate the roof’s plants, nine indigenous species and the most concentrated area of native wildflowers in the city.

The Academy’s roof is visually arresting. Designed by Renzo Piano, it rolls in imitation of the Bay Area’s coastal hills. That sloping shape presented some technical challenges that were solved by the development of something called the BioTray®, a biodegradable, reinforced, modular plant propagation tray made from rapidly renewable coconut coir fibers. The tray holds the growing medium in place while the plants put down roots and later helps the plants to hang onto water.

According to the museum’s website, this unusual roof also provides thermal comfort inside the museum:

The steep slopes of the roof act as a natural ventilation and cooling system. Fresh air, cooled by the vegetated surface, is funneled into the entry plaza, whose retractable skylights peel back to allow cool air to sink into the building to offset mechanical cooling demand with natural ventilation. Additionally, the thermal mass, surface moisture, and insulation in the roof assembly are expected to maintain the building’s interior an average of 10 degrees cooler than a standard roof would.

The Academy’s new home earned the United States Green Building Council’s platinum rating, the highest possible LEED sustainable building rating. The museum’s commitment to sustainability is evident at every turn, from the bike racks and rechargeable vehicle stations out front to the solar cells on top. Down below, radiant heating provides warmth from beneath the floors. The museum’s designers even thought about what’s inside the walls; the insulation was made from recycled denim!

Details at the Root of the Matter

Green roof on university in Singapore

Green roof on School of Art, Design and Media at Nanyang Technological University in Singapore

Whether a vegetated roof sits atop a house, a library, a factory, a school or even a church, and whether it’s one story in height or hundreds of feet above the ground, the details of construction and maintenance differ very little.

Green roofs come in two general varieties: shallower “extensive” roofs and thick “intensive” roofs that can support deep-rooted plants and even trees. Extensive roofs can cost as little as $7 a square foot but more commonly run $10-15 per square foot. An intensive roof runs around $15-$25 per square foot.

An extensive green roof may weigh no more than a slate roof (which is still pretty darn heavy), but an intensive roof is a hugely weighty matter. A special structural design is needed to support the weight of the growing matrix plus plants and water.

From the structural framing up, a green roof is an open-faced Dagwood sandwich: there’s a thermal insulation layer, a waterproof membrane, a drainage layer, a filter layer, and then the growing medium, which is not just soil, because plain old dirt would weigh too much. The plants are the garni on top. (Note: The durability of the waterproof membrane is important. I see from the online faculty meeting notes at Trent University that after 10 years, the membrane on the green roof on the Environmental Sciences Building “is shot” and needs to be replaced.) A living roof may be constructed layer by layer from scratch, or it may be constructed using a modular green roof system. Several of those can be located using the links below.

Like standard gardens, vegetated roofs require maintenance. How much and how often depends on what kind of plants you choose and how hardy they are in the local climate. A zeroscaped roof, which requires water only when the young plants are establishing their roots, may need tending only once a year. A roof planted with vegetables may need daily attention, which means that the roof will be need to be planned not only with the garden in mind, but also with the expectation that the roof must provide access and support for frequent foot traffic. After all, people are going to want to eat those strawberries when they ripen!

To Learn More

Icelandic

Traditional green roofed house in Iceland

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Up on the Roof

When this old world starts getting me down
And people are just too much for me to face
I climb way up to the top of the stairs
And all my cares just drift right into space
On the roof, it’s peaceful as can be
And there the world below can’t bother me
Let me tell you now

When I come home feelin’ tired and beat
I go up where the air is fresh and sweet (up on the roof)
I get away from the hustling crowd
And all that rat-race noise down in the street (up on the roof)
On the roof, the only place I know
Where you just have to wish to make it so
Let’s go up on the roof (up on the roof).

At night the stars put on a show for free
And, darling, you can share it all with me

Right smack dab in the middle of town
I’ve found a paradise that’s trouble proof
And if this world starts getting you down
There’s room enough for two
Up on the roof…!

- Gerry Goffin and Carole King

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One comment

  1. Yeah, really the green roofs are cooler as it absorbs less heat.



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