There’s a reason these epicenters are referred to as "concrete jungles." Some of the most populated American cities are inundated with physical structures. New York City may at first glance appear to be an outlier because it boasts one of the most famous urban parks in the world, Central Park, but in reality, the difference in raised structures versus parks is striking: The Big Apple has an estimated 1 million buildings, and only 1,700 parks and other recreational facilities.
While New York City is currently undergoing a veritable parkland revolution, an entirely new concept is redefining how green spaces are utilized within otherwise flora-starved metropolises. Although largely unnoticeable from street level, public and private sector buildings are increasingly incorporating green roofs on conventional rooftops to reflect sunlight and absorb rainwater.
As governments are confronted more and more with the harsh realities of an ever-warming planet, green roofs are becoming all the rage. It’s not only municipalities interested in transforming existing roofs, either. Private enterprises, including real estate developers, are considering green roofs to broaden the appeal of office or living spaces and maximize their ancillary environmental and cost-saving benefits.
For those in the industry, the grass may only get greener from here. Green Roof for Healthy Cities, a nonprofit green roof trade association, reports the industry grew by 10 percent from 2015 to 2016. According to its survey, Toronto and Chicago led the continent in 2016 with the addition of more than 600,000-square feet of green roofing, followed by Washington, D.C., Seattle and Philadelphia.
Green roofs transform sun-drenched rooftops into ecological oases that help reduce the so-called “Heat Island Effect” plaguing congested areas, by cooling the surrounding air. These living, breathing roofs are also known to reduce energy costs, among myriad other benefits, in addition to their aesthetic appeal.
That’s exactly what’s happening across the country, albeit a little later than in several nations across the Atlantic.
The modern history of vegetated roofscapes dates back to the mid-20th century, as European countries foresaw the advantages of adding greenery to existing rooftops. In the 1970s, Germany realized the impact of green roofing on lessening the burden on storm drains. As technology evolved and agricultural architecture became fine-tuned, the many extraordinary and positive effects of installing a green roof become clearer and clearer by the day.
New York City, which has more than 1.6 billion-square feet of rooftop, is among a growing number of cities considering inventive ways to be more energy efficient and reduce carbon emissions. Although NYC lags behind Chicago and Canadian cities, it has identified green roofs and solar panels as critical keys to increasing sustainability. To that point, three-quarters of New York’s carbon emissions originate from “homes, schools, workplaces, stores, and public facilities,” according to the city.
The Big Apple was also one of the first cities in the United States that pledged to adopt the principles of the historic Paris climate accord after the Trump administration announced it would back out. One of its goals is for the city to become carbon neutral by 2050. Among the ways it intends to accomplish this is by investing in green roofs.
Design and construction varies, depending on the pre-existing roof and local climate. Still, a growing body of research appears to support an industry-wide belief that green roofs are reliable and smart alternatives to the way roofs have been built for decades.
Paul Mankiewicz, a biologist and plant scientist who is the executive director of the Bronx-based nonprofit Gaia Institute, said if New York City, in particular, transformed a small fraction of asphalt rooftops into green roofs, there’d be a noticeable “local dropping in temperature.”
Meanwhile, there are other benefits that often get overshadowed, such as increasing the lifespan of roof membranes—systems designed to move water off its surface—boosting the natural habitat, and leveraging green spaces for educational and agricultural purposes.
“When you see a rooftop, it’s kind of getting to the point where, why wouldn’t you have a green roof?” said Eric Dalski, founder and partner of Highview Creations, which designs and builds green roofs in the New York City region.
Dalski singled out the High Line, a converted railroad trestle and one of the most-trafficked public spaces in Manhattan, as the most famous example. Not only has the elevated park introduced visitors to plant species they'd otherwise never interact with across such a sprawling city, he credited the rise in real estate values to its appeal, too.
“It just shows to prove that people appreciate green space,” Dalski said.
He may have a point. Currently, more buildings—both public and private—are being celebrated for their green roof installations, including Seattle City Hall and the famed Getty Center in Los Angeles.
It doesn’t end there for these gardens in the sky. From brand new high rises and municipal buildings to apartments and public schools, green roofs are now a top priority for urban planners. With scientists blaming three consecutive years of record heat on climate change, and more and more cities confronting the many threats posed by an ever-warming planet, green roofs have been taking on a much more urgent role.
What follows is a comprehensive explainer that defines what exactly a green roof is, breaks down their many key benefits, outlines how they’re used to combat carbon emissions and other pollutants, and examines other reasons why the public and private sector are increasingly incorporating such roofs into urban areas where they live and work.Back To Top
The process of installing plants, shrubs and grass atop a building is well known to Europeans, but it has only recently entered the mainstream in North America. Green roofs act as natural force fields against the sun’s blistering rays. And the concept is spreading.
As aforementioned, Germany is considered the pioneer of green roofing, as it sought to lessen the burden on storm drains in the 1970s. Fast-forward four decades, and Toronto reportedly became the first North American city to require green roofs on certain buildings.
In the United States, New York has taken strides to make green roofs more ubiquitous, and will allocate $1.5 billion to green infrastructure through 2030. Chicago, the third-largest U.S. city, has been at the forefront of the green roof trend, now boasting an estimated 5.6 million-square feet of vegetated roofing, according to the city.
While design and implementation of a green roof may require help from a licensed landscape architect, the concept is not too difficult to comprehend.
Most people are familiar with solar panels being affixed to roofs to absorb heat and convert it into energy. With green roofs, architects literally plant vegetation atop roofs as a way to decrease temperatures thousands of feet above ground level, mitigate stormwater pollution, drive down energy costs, and/or various other purposes, several outlined below.
Naturally, you may ask: “So what? Why go through all the trouble of creating a garden where virtually no one can see them?”
If implemented correctly, green roofs can bring the rooftop temperature down to ambient, which is the air temperature on that particular day. So instead of the mercury reading on a roof pushing 150 degrees on a 92-degree day, for instance, the ambient reading will be closer to that day’s temperature. What that means is the existing roof is taking less of a beating from the sun, the building’s temperature itself decreases, and structures become more insulated. Naturally, with the surrounding air at a more manageable temperature, HVAC units can work more efficiently, thus minimizing carbon emissions generated by these machines.
Dalski, the founder and partner at Highview Creations, explains:
“Basically, if you’re on a green roof...you’re looking at ambient temperatures, whereas, if you’re on a rooftop in New York City that doesn’t have any sort of temp reflective or green technology to it, temperatures could be as high as 130 to 140 degrees.”
Mark Morrison, president and CEO of Mark K. Morrison Landscape Architecture, MKM, a New York City-based landscape architecture firm, said he also takes into account the solar orientation of the space (how much light shines on the roof), wind strength, and whether all legal requirements are satisfied. The biggest factor is how much a load the roof can hold, he said.
After analyzing the composition of the roof, it’s imperative to decide what type of green roof to install: either extensive or intensive.
According to the Environmental Protection Agency (EPA), extensive green roofs typically include sedum and require less maintenance, whereas intensive roofs are more comparable to an actual garden, and allow for a diversity of plants. Both can absorb a considerable amount of rainfall, which can mitigate the impact of stormwater runoff and alleviate pressure on sewer systems. Extensive green roofs are made for structures that can’t support a heavy load of soil.
The key to all this are plants, of course. Architects traditionally try to incorporate native plants, because they work well in the local climate. Green roof vegetation undergo a process called evapotranspiration. That’s when plants emit absorbed water back through their leaves, resulting in a cooler air temperature, according to the EPA.
“Plants are natural air conditioners,” said Morrison.
J.R. Kramer, a landscape architect from Charleston, S.C., who along with his wife founded their own firm in 2007 called Remark, agrees. He said conventional roofs almost fully absorb the sun’s rays, providing little relief.
“If we’re able to incorporate a green roof, you have the evapotranspiration effect, where it’s going to be absorbing the sun’s energy,” said Kramer. “It’s going to be creating energy, it’s going to have moisture release, and so it would have more of a cooling effect then just having an asphalt roof.”
Besides providing critical relief from sweltering temperatures, these skyscraping canopies also absorb rainwater. According to a 2017 EPA report titled Reducing Urban Heat Islands: Compendium of Strategies: “The reduced energy demand from green roofs also reduces air pollution and greenhouse gas emissions associated with energy production. Further, because ground-level ozone forms more readily with the rise in air temperatures, green roofs help slow the formation of ground-level ozone by lowering air temperatures.”
Green roofs also act as natural sponges, soaking up a significant amount of rainfall and releasing it back into the air.
“Studies have shown that extensive roofs will typically capture between 50 and nearly 100 percent of incoming rain, depending on the amount of growing medium used, the density of vegetation, the intensity of an individual rainstorm, and the frequency of local rain events,” the EPA report found.
Less rainfall flowing down onto the street can also reduce stormwater runoff, which fuels erosion, pollutes the environment by collecting and transporting hazardous chemicals into local waterways, and eventually the ocean, and substantially strains water treatment facilities.
Municipalities are increasingly prioritizing stormwater retention, landscape architects said.
“If you have a green roof that has eight inches of soil and has this native meadow where there’s more water actually attaching to the plants, it’s a way to capture that water, clean the water and capture some of it, and let it slowly release versus just running it into a gutter...and sending it to your storm drain system,” said Kramer, the Charleston-based landscape architect. “It just slows the water down.”
When we put the question of which plants thrive best in an urban landscape to the aforementioned trio of landscape architects, they unanimously agreed that the best options are those already existing within the location’s natural habitat. Not only can these flourish in that city’s particular climate, but native plants also attract other creatures, such as bees and migrating birds.
Morrison said pollinator plants are also useful, because bees pollinate about 80 percent of what we eat.
“If we don’t take care of the bees, we got a big problem,” he said.
Incorporating native plants into a green roof is a no-brainer, according to Kramer, because “they do well in this climate” and they attract pollinators, butterflies included. He added that the soil is just as crucial to the effectiveness of a green roof.
Most green roofs are built upon an engineered growing medium, which typically includes soil, explained the 2017 EPA analysis.
“The planting media used in green roof systems are usually engineered to provide the best support for plants with the lightest weight and can be tailored to maximize water retention without water-logging the plants,” it stated.
Green roofs are not conducive to heavy soil, especially with many architects specifying an 8-inch standard depth. A thicker layer enables landscapers to plant a variety of vegetative species, such as native grasses and perennials. A 4-inch depth does not have enough soil mass for such plants, Kramer said.
Highview Creations’ Dalski said his firm uses specialized soil blends typically engineered to withstand harsh elements. The blend he uses has good drainage, is wind shear-resistant, and lightweight.
Most green roof growing mediums consist primarily of inorganic material (about 70 to 80 percent), such as Utelite shale, which expands when heated up to 2,000 degrees, and the remainder is organic matter. Another option is Gaia soil, developed by Mankiewicz, the Gaia Institute founder, that is ultra-lightweight and uses a recycled Styrofoam that’s top-dressed with compost or mulch, Dalski said.Back To Top
While each landscape architect may develop green roofs differently, there are certain industry standards. Of course, there’s more to green roofs than just shoveling dirt onto a roof and planting seeds.
Typically, architects begin by installing a protective membrane—or 'root barrier'—that separates the drainage system and actual roof membrane, said Dalski. The next layer includes a drainage mat that catches excess water. Because protecting the soil is so crucial, architects will buttress the bottom layer by adding a filter fabric that retains soil and does not impede the flow of water during periods of heavy saturation. Finally, green roof medium and plants are added, to complete the process.
Before any project gets off the ground, err, roof, Kramer said his company inspects its composition, identifies what maintenance may be required, and perhaps most importantly, learns the goals and objectives of the client—which vary, depending on whether the desired roof is intended for stormwater management, reducing emissions, or aesthetics.
While not every green roof may have an irrigation system, Dalski said he recommends clients have one installed if plants are “growing in the dead of summer,” or, for intensive systems, incorporating more perennials.
Perhaps one of the best aspects of green roofs is they require minimal maintenance once the plants have stabilized.
MKM’s Morrison said his firm maintains the Visionaire building in Battery Park City, but it only requires monthly visits in the summer.
“If you layer planting [and] if you plant ground covers, shrubs and trees, and give them adequate space the first couple of years...they grow together and they work well and they look good together,” he said.
Maintaining a green roof is also simpler if you don’t have bulky materials, said Morrison, who recommended being judicious with mulch and making sure you stick to best practices, such as keeping up on weeding and pruning. On top of that, using an irrigation system that saves on water is always helpful, he said.
“You really don’t have too much maintenance,” Morrison said.Back To Top
If you’re considering installing a green roof to a new or existing building, there are a few factors consider.
If you were to properly construct a green roof, you’d require a landscape architect, structural engineers who can assess the existing roof and the building itself, and a landscape irrigation consultant, according to MKM’s Morrison.
A soil expert may also be consulted to better clarify the ideal weight and mix that may work best for a particular project.Back To Top
Architects typically first point to their ability to reduce heat and associated energy costs. Second, stormwater retention is a huge plus, especially for municipalities experiencing a rise in sea level. Third, green roofs can potentially stretch a roof’s usefulness far beyond its original lifespan—which averages about 20 years—extending it several decades longer. And there’s so much more.
Green roofs are great to look at. You don’t need a green thumb or an appreciation for horticulture to understand the beauty of a green roof. Not only are green roofs eye-catching, but they’re far more likely to inspire relaxation among owners and visitors to enjoy a drink, read a book, or marvel at the panoramic views.
Among the projects Morrison’s firm MKM designed is the Visionaire Sky Island in Battery Park City, overlooking New York Harbor. The roof includes 160 different species of plants, an eight-tree food orchard, six garden plots, an Asian garden, and “secret” alpine garden, Morrison said, adding that the eye-popping views are also an appeal for visitors.
“We have a pergola with a dining area,” Morrison said of the award-winning green roof. “And when people go there, the views are unbelievable. They get a drink. They walk out to the southern end of the terrace and they look out at New York Harbor and they ooh and aah. They come back for dinner [and] they never go back to look at anything else that night.”
Reduce Temperature & Urban 'Heat Island Effect'
When people refer to the so-called “Heat Island Effect,” they're typically describing how one area—traditionally, a city—is much hotter than a surrounding rural area. According to the EPA, the “annual mean air temperature of a city with one million people or more can be 1.8 to 5.4 degrees Fahrenheit (1 to 3 degrees Celsius) warmer than its surroundings. In the evening, the difference can be as high as 22 degrees Fahrenheit.”
Increased temperatures contributes to greater energy usage. People like to pump their air conditioner units on steamy summer days, which has the collateral consequence of producing more greenhouse gas emissions.
The reason asphalt rooftops radiate so much heat is that they’re almost completely unable to reflect the sun’s rays. According to a recent report by Yale School of Forestry & Environmental Studies: “Fresh asphalt reflects only 4 percent of sunlight compared to as much as 25 percent for natural grassland and up to 90 percent for a white surface such as fresh snow.”
Green roofs can play significant roles in bringing down temperatures at heat islands, acting as natural coolers.
“During the summer months when the sun is beating down on rooftops, you basically have full exposure and a lot of the membranes that are typically used in roofing kind of absorb that heat,” said Dalski, the Highview Creations founder. “So not only does it raise temperatures directly from the sun, but it actually releases...the roof’s temperatures slowly, [and] much slower than any other service.”
Dalski explained that the evapotranspiration process means when plants “uptake water, they give off ambient temperatures when they release oxygen into the atmosphere. And so it’s basically reducing the radiation exposure from UV [ultraviolet] as well as the heat from the sun.”
Insulate Buildings & Bring Down Energy Costs
Green roofs also function as natural insulators. What does that mean? Well, in the summer, structures outfitted with vegetated roofing stay cooler, which means air conditioner units won’t have to expend as much energy.
“When dry, green roof layers act as an insulator, decreasing the flow of heat through the roof, thereby reducing the cooling energy needed to reduce building interior temperatures. In the winter, this insulating effect means that less heat from inside the building is lost through the roof, which reduces heating needs.”
Overall, green roofs are known to decrease energy demand, according to landscape architects and various studies. How much energy is saved depends on a variety of factors, including the type of roof and the structure itself. MKM’s Morrison said savings can average about 35 percent, while the international nonprofit Natural Resources Defense Council (NRDC) concluded in a recent report that green roofs can cut the amount of energy required to cool the floor directly below a roof by a whopping 50 percent.
Such savings are significant considering how widespread air conditioner use is around the planet—and the demand for such units is only expected to increase in the future.
The International Energy Agency (IEA), a clean energy advocacy organization consisting of 30 member countries, including the United States, expects energy demand from air conditioners to triple by 2050.
The startling findings appeared in an IEA report titled “The Future of Cooling,” which also noted that air conditioner use currently accounts for 10 percent of the globe’s daily energy consumption.
How do green roofs come into play? As Dalski explained, a building with a conventional roof would have to convert 130-degree air to the ideal indoor temperature. So instead of an air conditioner expending considerable energy to turn 130-degree air into 70 degrees, a green roof could reduce the outside air temperature by 40 to 50 degrees on a hot summer day.
“There’s less energy that’s needed to convert that,” he said.
Reduce Air Pollution
There’s a couple of ways in which green roofs can drive down air pollution. On one hand, the plants themselves can naturally remove air pollutants, while reduced energy use means less greenhouse gas emissions in the atmosphere, according to the EPA.
The aforementioned EPA report cited one study that looked out how much Particulate Matter (PM) could be reduced from the atmosphere as a result of a single green roof. The study found that a single, 1,000-square-foot green roof could eliminate 40 pounds of PM from the air—the equivalent to the yearly emissions of 15 passenger cars.
Mitigate Stormwater Runoff
Green roofs can also clean polluted waterways by preventing significant amounts of stormwater runoff from entering local water bodies in the first place.
The aforementioned NRDC study analyzed the impact green and so-called “cool roofs” would have on Southern California, finding they could reduce stormwater runoff by “tens of billions of gallons each year.”
This same analysis examined what would happen by the year 2035 if 50 percent of new or redeveloped projects were outfitted with green roofs, and estimated a 20 billion-gallon reduction in runoff, annually.
If not falling onto rooftop plants—where water is absorbed and evaporated, and drench impervious surfaces—runoff could lead to “increasingly severe flooding and erosion and can greatly amplify levels of pollution in surface water bodies,” noted the report.
“When the increased volume of runoff flows over paved surfaces, it picks up higher levels of automotive fluids and debris, metals, pesticides, pet wastes, trash, bacteria and pathogens, and other contaminants and carries them to nearby rivers, lakes, and beaches,” it continued.
Dalski said his firm has worked with the New York City Department of Environmental Protection (DEP), whose “primary objective,” he said, is to capture stormwater and prevent it from going into the combined sewers.
A detailed report released by New York City in 2017 concerning its plan to improve waterways mentioned green infrastructure projects as a deterrent to pollution.
“GI (Green Infrastructure) also has many co-benefits such as neighborhood beautification, air quality improvements, and cooler temperatures in hot summer months,” it stated.
Increase Lifespan of Existing Roof
This is potentially one of the most overlooked benefits of green roofing. When landscape architects build green roofs, they do so on top of the existing structure. Generally speaking, building owners do not have to replace the current roof before adding vegetation.
In most cases, roofs need to be replaced about once every 20 years, but just by adding plants and other vegetation to the roof, its life can be extended by several decades.
“While a typical roof membrane is exposed to the UV lights, all the elements, and it breaks down over time, a green roof protects the roof membrane,” said Dalski. “So what is typically a 20-year replacement can go on for 50 to 60 to 70 years, well beyond any of our lifetimes.”
That sentiment is shared by others within the industry.
“It’s been proven that...the same roof membrane is going to last 50 years or longer,” added MKM’s Morrison.
EducationThat’s right. In some cases, public schools and other agencies are using green roofs as educational tools.
Of the projects Highview Creations has spearheaded in the decade the firm has been existence, its “pride and joy” is the Lenox Hill Neighborhood House, a century-old nonprofit that helps people in need, according to Dalski.
Among its features: an intensive playscape for children that introduces them to native ecologies, a sensory garden that exposes kids to an array of smells, and two extensive green roofs on the sixth floor. Along with the educational aspect, one of these green roofs is home to a vegetable garden, and its provisions are used in the nonprofit’s kitchen.
“They [the kids] don’t know much about plants, they don’t understand how gardens work. And so by introducing it on the rooftop, we kind of integrated it into curriculum so the kids are understanding pragmatically how green roofs work,” Dalski said. “They can take a look and they can pick their own fruits. They can see and smell and kind of see what happens and [understand] the essentials that are needed to make gardens happen.”
Morrison’s firm also has experience working with educators. He said P.S. 6 on the Upper East Side of Manhattan was having a classroom shortage, so MKM was hired to create green roof equipped with a greenhouse, which kids could use for growing plants, science lessons, and as a non-traditional classroom. MKM also helped turn P.S. 62 in Staten Island into the city’s first and only “net-zero” school, which means it’s completely unreliant on the energy grid. The roof is entirely covered with photovoltaic cells, and includes an urban farm replete with raised planters, a greenhouse and a small outdoor classroom, Morrison said.
Simply getting kids out of the house and into the elements can have a positive effect on classroom performance, added Morrison, who said there’s been a noticeable rise in test scores at schools with green roofs.
“We do a lot of schools,” he added. “You’re finding test scores are going up... Forget about a green roof. If you can get kids outside, test scores are going up, behavior is much better, you have more attention being paid because you’re getting kids out in the environment and away from these god-awful cell phones.”
But the emerging consensus appears to point to green roofs as a net positive for cities, especially for those aggressively trying to reduce carbon emissions, relieve overburdened sewage systems or create a more inviting environment thousands of feet in the sky.
Kramer sums it up this way: “It’s really a great thing to do for the environment. It’s a way to save energy costs and you have aesthetics that typically you would never have, especially if you can get a green roof where your windows are looking out on it. Especially in a work environment — it’s just so much nicer to look out on a green roof than a [conventional] roof. And I think in the end you probably end up having better work performance out of your employees. That may be another benefit.”
These systems can also reduce pathogens and waste volumes and minimize vector attraction and unpleasant odors.
As explained earlier, microbes feed on the sewage sludge, human or animal refuse, or food scraps and other organics in the absence of oxygen. Next, the associated chemical reactions occur: hydrolysis, fermentation or acidogenesis, acetogenesis, and finally, methanogenesis. In a nutshell, complex organic matter (carbohydrates, proteins and fats) becomes soluble organic molecules (sugars, amino acids and fatty acids), which then becomes methane and carbon dioxide, or as aforementioned, biogas. Such technologies profoundly impact our local, regional, national and global environment in several significant ways.
The award-winning green roof at the Visionaire building in Battery Park City. Credit: Pelli Clarke | Pelli Architects
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