Spend time on a tar or asphalt parking lot on a hot sunny day. Then head to a nice open grassy spot. Notice the difference? It feels much cooler on the grass, even in open sun. Believe it or not, the same scenario plays out on more and more rooftops throughout the United States every year.

No matter what we devise to seal rooftops from the weather, the combination of plants and soil beat all. Tar, shingles, concrete, wood and other materials all break down over time from incessant heat, cold, rain, snow, ice and wind. Plants grow and soil just lies around, enjoying the weather.

Following tips from Mother Nature­—and ancient history—companies in this country backed by innovative work in Germany, offer ingenious methods to take the combination of soil, grass and other plants to new heights, to create roofing systems that offer significant savings and ecological benefits. Though up-front costs are greater for such “living roof” systems, long term cost- benefits are substantial.

“Green roofs are vegetated roof covers, with (light-weight) growing media and plants taking the place of bare membrane, gravel ballast, shingles or tiles,” according to information found at www.greenroofs.com, a resource site for information about the concepts, technology and products associated with the systems.

“Green roofs reap so many different benefits,” said Linda Velazquez, publisher of the website. “There’s stormwater reduction, prolonged life of the roof, energy savings, even habitat. It cleanses the rain water, which gathers toxins in the air,” she said.
Stormwater runoff reduction is a key factor in today’s land development process.

“Green roofs can absorb anywhere from 50 to 90% of stormwater that falls on site,” Velazquez claims.

The concept is far from new. Humanity’s use of the earth as a source of building material goes back millennia. Antiquity’s most famous examples are the Hanging Gardens of Babylon, with their lush roof gardens built between the 8th and 10th centuries BC and the sloping, tree-and-shrub-covered walls of the Ziggurat of Nanna, built around 2100 BC, according to the website.

Around 1000 AD, sod-covered roofs were used in Iceland and Scandinavia, a concept later brought to the United States and used by settlers to the Midwest and northern parts of the country. Frank Lloyd Wright and other modernist architects incorporated roof gardens into their designs.

Today’s renewed wave of interest began in Europe.

“It started in Germany as stormwater mitigation, where they needed to handle their stormwater problems on site,” Valazquez said. The first green roof systems were introduced there in the early 1970s.

By the 1980s, according to a report from Pennsylvania State University, as the technology developed, the refined systems gained considerable acceptance in that country.

“Industry figures suggest that 10% of German roofs are greened. Between 1989 and 1999, German roofing companies installed nearly 350 million square feet of green roofs and the rate is increasing,” the report states.

“Common theory holds that roof life can be at least doubled, and perhaps tripled or more, with a green roof. The main reason for this is that the multiple layers protect the waterproofing membrane and structural elements from damaging ultraviolet rays, wind, and temperature fluctuation extremes,” Velazquez said. “It creates a living sponge with living plants.”

Environmentally, green roofs can have an impact on the urban “heat island” effect that drives up energy costs.

“This phenomenon describes urban and suburban temperatures that are 2 to 10°F (1 to 6°C) hotter than nearby rural areas,” according to the Environmental Protection Agency (EPA). “Elevated temperatures can impact communities by increasing peak energy demand, air conditioning costs, air pollution levels, and heat-related illness and mortality.”

A system called “cool roofs” can be a much less expensive alternative to green roofs, but they don’t have nearly the longevity, according to EPA information. Most are a smooth, bright white surface that reflects, rather than absorbs solar radiation, which can extend the life of the roof and save on the air conditioning bill.

However, over time, the surface loses its reflective properties due to the accumulation air pollution, dew and dust. The accumulation accelerates with an increase in the slope of the roof, which will draw more particles to taint the surface.

“Studies done at the Florida State Energy Center show that during the first two years of a cool roof’s life span, solar reflectance can deteriorate up to 11% if regular maintenance is not provided,” the EPA website states.

Green roof systems incorporate at least one, but usually multiple layers, depending on the complexity of the system and design. The layers include a single to multi-ply waterproofing layer, drainage, growing media and plants. Such systems can range anywhere from 2 to 3 inches for the simplest plans, and up to 15 feet for actual roof gardens.

Soils have to be light, yet allow for the free flow of nutrients and hold water.

“Most U.S. companies have looked to the Germans to see what has worked for them,” Velazquez said. “Some U.S. companies have modular systems, where you can order it one day, and practically the next day you can have a green roof. It’s a portable-unit technique.”
The multiple layered systems are the root cause of the cost build-up, so there is potential sticker shock for those investigating the green roof concept.

EPA estimates listed on its website show the up-front cost for the least expensive green roof in the United States starts at about $8 per square foot, including materials, preparation, and installation. In comparison, the cost of a traditional built-up roof starts at about $1.25, while cool roof membranes start at approximately $1.50 per square foot.

Estimates for the higher-end green roof systems could range from $25 - $40/sq. ft and up. However, the larger the project, the greater the economies of scale.

“The Ford Motor Company River Rouge Plant green roof in Dearborn, MI, for example, came in around $4/sq. ft., but we’re also talking about an extensive green roof that’s almost 500,000 sq. ft in size,” greenroof.com states.

Velasquez likes to use the example of Italian lasagna to create the mental image of how a green roof system goes together.

“The noodles would be the different natural layers of protection or waterproofing,” she said. “Typically, you start from the deck up. The deck can be wood, it can be metal under certain circumstances. Concrete would be ideal. There should be insulation, above or below the roofing deck or above the water proofing. From there, you would have the water proofing, the drainage layer, a filler fabric, then the engineered soil or growing medium for your plants.”

There are two basic types of green roof systems—extensive and intensive—which can range from virtually maintenance-free to elaborate, garden-like layouts.

The extensive green roof system utilizes fewer and thinner layers, is lighter and less expensive and often appeals to the owner who is looking for an ecological roof cover with limited or no access. Plants are limited to grasses and hardy, drought resistant varieties.

The intensive green roof systems are more like a roof garden and can incorporate a much wider range of plants, including flowing shrubs, trees, and even vegetables. To accommodate more elaborate designs, there needs to be more and deeper layers, which would then require proper roof support structure.

The growing medium is a critical component of the green roof model. It is an “engineered soil”, not garden or top soil, which would be too heavy and contain pathogens, weeds, and insects. The special growing medium for green roofs must be extremely light weight, contain a high ratio of inorganic to organic materials and be able to hold water while also allowing for proper drainage and aeration. It also must not settle or lose volume or blow away, according to Velazquez.

“Inorganic components in the growth media could include materials such as expanded slate and shale, extruded clay, crushed recycled bricks and concrete rubble, lava, rock wool, and perlite. Much smaller percentages of organic matter may be added, such as well-rotted humus material …augmented with organic fibrous material and a small amount of clay particles,” stated Velazquez in a research paper on the subject.

“There are basic guidelines that need to be followed and there are certain professionals that need to be involved, especially a structural engineer, an architect or someone who can actually design a system,” Velazquez said.

Such systems can be created on existing structures as well as new construction.

“Almost any building can be retrofitted, but again, they have to be structurally sound,” Velazquez said. “There would be several factors involved. It has to be in water-tight condition and you have to go by local city codes.”

Right now, green roofs are most applicable to commercial buildings and urban areas have an advantage as well, because that is where most installers are.

“The way it started was in the larger cities, because of the suppliers,” according to Velazquez.

Residential prices are going to be higher because the projects are going to be smaller but the potential is there for residential applications and there is work being done on such systems. Green roofs can be put up rather easily, up to a 33-percent slope, she said.

“Green roofs offer so many benefits all in one package,” she said. “The sky’s the limit.”  SLDT