San Francisco and Toronto mandate them, most of European countries promote them and even the Vikings knew about them: green roofs.
Now Colorado’s capitol, Denver, made up the leeway with a controversial mandate that every new building with a gross floor area of 25,000 square feet or bigger must be built with a roof garden or a combination of a green roof and solar energy collections. Proponents praise the manifold benefits for a better overall quality of life, but opponents fear rocketing housing prices.
Denver’s mandate goes even further then similar approaches, as Brandon Rietheimer, head of the Denver Green Roof Initiative, explained: “We are doing something different here. We are trying to incorporate existing buildings.” Rietheimer admitted, this might be a challenge, “but nobody else does existing buildings. So, that would really increase the amount of acreage that we see.”
And it will make Denver, known for bad air quality, a pioneer in demanding green roofs in the United States.
“I am sure, you have driven around and seen signs that are like ‘Ozone warning, don’t drive today’ and things like that. We are 11th worst in the nation for our ozone,” explained Rietheimer.
In fact, Denver is ranked 8th amongst the most ozone-polluted American cities according to the American Lung Association’s “State of the Air 2016”.
Ozone occurs naturally in the upper atmosphere (ozone layer), where it shields the earth from the sun’s ultraviolet radiation. Whereas ground level ozone, that is formed in a chemical reaction of nitrogen dioxide and UV light (see graphic below), is harmful to our health.
Nitrogen dioxide (NO2), produced by burning fuels and emissions from vehicles and power plants, splits into nitrogen (NO) and atomic oxygen (O) under the influence of sunlight. The released oxygen atom combines with atmospheric oxygen (O2) to ozone (O3).
Under the Clean Air Act of 1970, average ozone levels in the U.S. have declined with a notable drop after 2002, according to the EPA. But it is still a problem for cities like Denver, where an average of 300 sunny days per year boosts the chemical reaction that create ground ozone.
But more vegetation and particularly atop roofs, can reduce the ozone pollution. Trees and plants does this by taking in ozone molecules through little pores in their leaves, called stomata.
In Chicago, researchers measured the impact of green roofs on air pollution. They found out that in one year, 50 acres of rooftop gardens removed 3,700 pounds of air pollutants out of the atmosphere. Ozone comprised for 52 percent of those pollutants.
Furthermore, plants indirectly reduce air pollutants by lowering surface temperatures. They provide shade and absorb light, which both decrease the photochemical reactions that form ozone (see graphic above).
Green roofs use the sunlight for photosynthesis and do not store the heat, like black roofs do. Traditional black roofs incorporate the sunlight and heat the building, even after sunset. “You have a warmer evening period, now you have to cool your building down even more, when it is dark out,” explained Rietheimer.
Plants instead release humidity by evapotranspiration, a photosynthesis process, and therefore act like swamp coolers, which creates more balanced temperatures throughout the day. Rietheimer said, the difference is huge. He refers to the green roof of Denver’s EPA building, where an ambient temperature of 80 degrees was measured in mid-July. “And across the street, at the Alliance Center, their roof was 170 degrees,” added Rietheimer.
Green roofs not only manage the sun, they also manage the water.
Washington D.C., for example, has a strict stormwater management mandate “that pretty much forces green roofs,” said Rietheimer. Here’s why: “When it rains, instead of immediately running off, it [the water] gets absorbed into the substrate and then it slowly filters through and then it is slowly released,” explained Ritheimer. By slowing down the water and releasing it drop by drop, the stress on the stormwater management systems can be reduced enormously and overflows can be prevented.
Studies show that the effect can be significant. According to DeNardo et al. (2005) green roofs can delay the start of water runoff in average by 5.7 hours, compared to a direct runoff on black roofs with no water retention.
Besides this, Samuel Cotture from the Swiss team that won the 2017 Solar Decathlon in Denver experienced another benefit: “As soon as we received our green roof on the competition site, we noticed a lot of butterflies in the middle of the plants.”
Research shows that Cotture’s observation was not a fluke. Butterflies, birds, invertebrates like snails and pollinators like bees use the urban habitats – which are probably one of the last refuges in cities for species that have alarmingly declined in number.
Scientists recently quantified the loss: In average the biomass of flying insects in German nature protection areas declined by 76 percent in the last 27 years.
Rooftop gardens could be a safe harbor for these animals, as Kadas (2006) showed. He found out that at least 10 percent of species that are rare o scarce in Great Britain, reside on London’s green roofs.
Not only the smallest animals but also mammals are attracted by roof gardens.
“Facebook has a huge green roof on their building in California,” Rietheimer said. “It is a very extensive green roof – and there’s actually foxes and baby foxes that live on it. They made their way up the stairs somehow.”
There are other benefits too. Residents and workers can enjoy the green spaces provided by the roofs, vegetables can be harvested atop of buildings and a growing industry promises jobs.
But in order to be able to provide these benefits, plants have to stay alive and the system has to keep working.
“Plants native to Colorado, which inhabit areas with shallow, rocky, well-drained soils, are good candidates,” according to EPA’s quality assurance plan for green roofs in Colorado.
But green roofs can be difficult to implement and maintain. For example, even drought tolerant species can struggle in the dry and hot climate.
In fact, some green companies may recommend against green roofs, according to Katie Kruger, CEO of the Denver metro Commercial Association of Realtors. “Most of them would say: Don’t do the green roof. […] The green roof is expensive. And in Colorado it’s really, really hard to keep the plants alive,” she said.
Kruger is also member of Citizens for a Responsible Denver, a coalition of Colorado businesses that campaigned against the mandate.
“Watering and maintaining soil moisture in trays on the green roof has been a challenge,” said Richard Mylott, Public Affairs at the EPA in Denver. “The system requires regular maintenance and calibration of the irrigation system, as well as re-soiling and replanting of the trays.”
“There is more maintenance with the green roof,” added Kevin Seiler, Facilities Director at the Denver Community College. The college’s main building, the Confluence Center, was designed with a green roof in 2011 and planted with a special type of grass.
“This roof does absorb a lot of heat,” explained Seiler, which means the soil dries out quicker than on ground level. “We do need to irrigate it. When the irrigation system wasn’t working, you could see where plants were stressed. But these type of grasses can go dormant, when it is a drought type condition. So, most of them have sprung back, once we got the irrigation working.”
Drought resistant plants typically follow one of three strategies. They either avoid dry conditions, which would mean they literally die when forced onto a Colorado green roof. Or they escape, saying they would keep their life cycles and so their growing times short, which makes green roofs unattractive.
Third, they root deeper to access groundwater. It is obvious that deep rooting plants are not a good solution for green roofs. The soil depth and with it the load the roof has to carry would be enormous.
Even with a much thinner soil depth, existing buildings would struggle.
Some of the older buildings, said Kruger, probably had to be structurally bolstered to hold the higher load of a green roof.
With all these additional costs, critics fear that affordable housing could be unaffordable with the mandate.
“Affordable housing is a huge problem here,” said Kruger. “The costs have to be passed on to somebody and it will be the tenants.” She worries that the house-owners are not prepared for the additional costs or new investors will be backed up by the upfront investments.
The area of green roofs in Denver can add up to 57.5 million square feet by 2033, a cost –benefit study shows. This would be almost two times the area of New York’s Central Park on Denver’s roofs.
With an even more growing population in the future and the impacts of climate change, cities will meet several challenges, speaking of transportation, housing, air quality or food production. Green roofs can for sure help to mitigate the impacts and make the cities of the future a vital place.