By Kelly DiNardo

October 9, 2019

When David Michaels moved to Chicago this year, he chose the Emme apartment building in part because of the third-floor green roof, which has a lawn, an area for grilling, fire pits and a 3,000-square-foot vegetable garden.

“The green space was a huge factor in choosing this apartment,” Mr. Michaels said. “My wife and I are out there every other night, grilling or relaxing. And we like that they host classes out there.”

The Emme actually has two rooftop gardens — the one visible to residents on a deck on the third floor and a 5,000-square-foot garden on the roof of the 14-story building. Both are run by the Roof Crop, an urban farm that grows food for restaurants on a handful of roofs in Chicago. Residents at the Emme can also subscribe to regular bundles of rooftop-grown fruits and vegetables.

As concerns about climate change and dwindling natural resources grow, green roofs have become increasingly popular. The Toronto-based organization Green Roofs for Healthy Cities estimates an increase of about 15 percent in the number of green roofs in North America since 2013.

Replacing black asphalt and shingles with plants can lower the surrounding air temperature, filter dirty stormwater and reduce a building’s energy use.

While it is difficult to calculate the savings, as utility costs vary from city to city, the National Research Council of Canada estimates a green roof can reduce air-conditioning use in a building by as much as 75 percent.

Beyond the energy savings, by lowering air-conditioning demand, green roofs also help to reduce air pollution and greenhouse gas emissions, according to the United States Environmental Protection Agency.

As understanding of the benefits grows, more cities around the world are passing green roof legislation. In 2010 Copenhagen began requiring green roofs on all new commercial buildings with a roof slope of less than 30 degrees. In 2016, the city of Córdoba in Argentina issued a bylaw that directed all rooftops — new or existing — of more than 1,300 square feet to be turned into green roofs. The same year, San Francisco began requiring that 15 to 30 percent of roof space on new buildings incorporate solar panels, green roofs or both. More recently, the New York City Council passed a suite of measures to reduce greenhouse gases, including a requirement for green roofs, solar panels or a combination of both on newly constructed buildings. Other cities support green roofs through non-legislative measures; Washington, D.C.’s stormwater regulations and Philadelphia’s tax credit both encourage green roofs.

Toronto was the first city in North America to pass a green roof law, in 2009, requiring new buildings or additions that are greater than 21,000 square feet to cover between 20 and 60 percent of their buildings with vegetation. Developers can opt-out for a fee, but fewer than 10 percent choose to do so, according to Jane Welsh, the project manager for environmental planning at Toronto’s City Hall. Many that do seek exemptions are simply looking to install smaller green roofs than are required by the Green Roof Bylaw.

Since the law was enacted, roughly 640 green roofs, covering more than five million square feet collectively, have been constructed, effectively changing Toronto’s architectural DNA and making the city a leader in the green roof movement.

Simply put, a green roof is one that allows for the growth of vegetation, but the process is more involved than plopping down a few potted plants. Typically, a green or living roof is constructed of several layers including a waterproof membrane, a root barrier, a drainage layer, a growing medium — soil is too heavy — and plants.

As green roofs have become amenities for residents and employees, they often also include picnic tables, benches, fire pits, and other extras.

Before Toronto’s green roof bylaw took effect, the Hugh Garner Housing Co-operative in the city decided to build one when its roof membrane needed replacement. Residents were presented with three designs — one that evoked a circus, one modeled after an English garden and one that brought to mind a city park. Residents voted for the park in the early 2000s, and today, the 22,000-square-foot roof features flower beds, trees, an herb garden, gazebos and specially-made picnic tables that are wheelchair-accessible.

“In the summer, every picnic table will have folks eating dinner out there,” said Beata Domanska, a resident and former board member who advocated the green roof. “People are up there playing cards, reading, sun tanning. We have member events there. We’ve even had weddings up there. It’s become part of the culture here.”

Of course, green roofs are not entirely new.

“We’ve been using soil and plants as a roofing material for thousands of years,” said Steven Peck, the founder and president of Green Roofs for Healthy Cities. “The Vikings would flip their boats over and cover them in sod because it’s a great insulator. What’s new is the research the Germans have done. They essentially invented the lightweight extensive system.”

In the 1970s, German horticulturists, construction companies and others began developing waterproofing technologies and researching blends of growing mediums that would be lighter than soil. In the 1980s, Germany passed a mix of local and federal laws encouraging green roof development and today the country features approximately 925,000,000 square feet of living roof. The movement began to take root in the United States and Canada in the early 2000s, with developers, architects, and designers looking to imitate Germany’s achievement.

Today, the University of Toronto’s Green Roof Innovation Testing Laboratory has grown into a leader in green roof research. The state-of-the-art facility uses 33 testbeds, a weather station and more than 250 sensors gathering data on soil moisture, flow rates, temperature, rainfall, humidity and amounts of sun and wind to help researchers assess and improve green roof performance.

Research like this helped overcome some early hurdles when ambitious designs, plant choice, lack of irrigation or a combination of factors left plants dying and green roofs turning brown.

“The focus has shifted from pretty to performance,” explained Vanessa Keitges, the chief executive of Columbia Green Technologies, the firm behind more than 1,500 green roofs in North America, including Amazon’s headquarters in Seattle and the Zella Apartments there, which feature a deck with an herb garden, outdoor kitchen, dog run and plenty of seating. “We’re getting better at fine-tuning the plant palette so you don’t end up with a brown roof. We’ve moved to drip irrigation instead of spray. We’re designing systems that are much easier to maintain. We want them to be goof proof.”

City planners are looking to green roofs to mitigate the urban “heat island” effect, which describes a metropolitan area that’s significantly warmer than surrounding rural areas because of human activity. Air temperatures in cities, particularly after sunset, can be as much as 22 degrees Fahrenheit (around 12 degrees Celsius) warmer than less developed regions. The elevated temperatures raise energy consumption, increase air pollutants and compromise human health, with extreme heat now causing more deaths in the United States than all other weather events combined, according to the Centers for Disease Control and Prevention.

The vegetation on a living roof cools the surrounding air through evapotranspiration, the process in which moisture in plant leaves evaporates into the air, essentially working like an outdoor air-conditioning system. In addition, the increased insulation and decreased need for air-conditioning means less overheated air is put back into the surrounding environment.

The plants on green roofs also work like a giant sponge, soaking up water and therefore reducing the amount of rainwater that reaches street level, lowering the risk of flooding, minimizing sewer system overflow and filtering dirty runoff.

“Cities are built to be impervious,” Mr. Peck said. “The water runs off buildings and streets, and when it hits streams and estuaries, it can cause erosion. It’s also incredibly contaminated. If you want to have drinkable, fishable, swimmable water in and around our cities, we need to capture stormwater and use it as a resource, instead of just getting it away from the buildings as soon as possible.”

Beyond the long list of financial, health and environmental benefits, green roofs have become valued amenities in residential buildings.

“We were really keen on finding a place with outdoor space,” said Carolyn Kushner, who chose the apartment building 525W52 in New York in part because of the roof, which features lounge chairs overlooking the Hudson River, a large lawn, grilling stations, bocce ball and Ping-Pong and shuffleboard tables, all surrounded by shrubs and greenery. Dr. Kushner and her partner — who proposed on the roof — go up there several times a week. “We have a glass of wine and watch the sunset. The city can be really overwhelming if you don’t get that kind of greenery. It’s nice to have someplace to escape to.”

One early cause of hesitation in adopting living roofs was the upfront cost. Green roofs cost two to three times as much as a traditional roof.

“With so many environmental initiatives, it often boils down to money,” Ms. Domanska said. “One of the ways we’re able to sell this is that a green roof significantly extends the life span of a roof. Over time it saves money. That was very attractive to the people who live here. Now people see all the benefits, but initially, it was about the savings.”

Studies bear this out. A 2006 study from the University of Michigan compared the expected costs of conventional roofs with the cost of a 21,000-square-foot green roof and found that over its lifetime, the green roof would save about $200,000. Almost two-thirds of that would be in reduced energy needs for the building below.

“We’ve established best practices for creating green roofs,” Mr. Peck said. “We have political leaders looking for ways to improve the health and well-being of the people in their cities and also address climate change. We have research that shows the benefits of green roofs. It seems clear that the cities that invest in green infrastructure will be the cities that thrive.”

A version of this article appears in print on Oct. 12, 2019, in The New York Times International Edition.