Green Roofs Can Help Cool Cities In The Summer Heat, But Building Them Is Only The First Step – Many Don’t Survive

The public Transbay Joint Powers Authority built an extensive park and walking path covering the roof of a four-block-long bus terminal in San Francisco known as Salesforce Park. When it opened in 2019, developers estimated it would save $6,500 annually in energy costs. Photo courtesy Fullmetal2887 via Wikimedia Commons, CC BY

As summer temperatures soar across the United States and Europe, cities are being forced to confront the difficult reality that they’re getting dangerously hot.Record-breaking heat waves strain power grids, send people to cooling shelters and can make the top floors of some apartment buildings unlivable.

One way to help cool buildings is to install green roofs – living rooftops covered by plants that can transform heat-absorbing rooftops into cooling solutions.

Green roofs have long been promoted as a method to help cities stay cool. Often cities offer developers incentives to install them. But just building a green roof isn’t enough.

My research with colleagues shows that what happens after a green roof is built is as important as the decision to install a green roof in the first place. We found that many green roofs fall into neglect and don’t survive beyond a few years, likely due to the cost of maintenance. When buildings’ operational budgets are cut, landscape elements – including green roofs – are often altered or maintenance is scaled back to save money.

The view on the Chicago City Hall roof. Conservation Design Forum, CC BY-SA

What Is A Green Roof?

Unlike a traditional roof made of asphalt, metal or tiles, a green roof is made up of layers, including insulation, water proofing and drainage, that allow plants to grow atop the building.

Depending on their design, these roofs can support everything from low-growing sedums to shrubs and trees. Rockefeller Center’s rooftop terrace gardens in New York City were designed in the 1930s to hold trees, grass, shrubs and seating areas to double as parklike respites far above the traffic. The roof of the California Academy of Sciences in San Francisco is fully covered by 2.5 acres of wildflowers, succulents and other native coastal flowers.

The California Academy of Sciences in San Francisco features a green roof with hatches that open to an atrium below. Anna Fox via Flickr, CC BY

These roofs are more than just decorative landscaping.

Plants absorb sunlight that would otherwise heat up the building below. They also cool the surrounding air through a process called evapotranspiration, in which they release water vapor. Additionally, they can reduce the amount of energy needed for heating and cooling buildings by acting as insulation. Green roofs also capture rainfall and prevent it from reaching storm drains, reducing flood risks and improving water quality in the community.

Rockefeller Centers’s rooftop gardens are designed like parks rather than wildlands. David Shankbone via Wikimedia Commons, CC BY-SA

Because of these benefits, organizations like the European Commission and the U.S. Environmental Protection Agency have promoted green roofs in recent years as a way to reduce urban heat islands, make cities more resilient to extreme weather and improve biodiversity by supporting birds, butterflies and other insects.

Some cities even help pay for them by offering incentives, such as tax credits, stormwater fee reductions or sped-up permitting to encourage developers to install green roofs. Philadelphia, for example, offers businesses a tax credit that can cover up to 50% of green roof construction costs.

These programs recognize the public benefits that green roofs can give, but they tend to focus on getting the roofs built rather than on keeping them functioning long term.

Without maintenance, all the work and money invested by builders and the communities can quickly be lost.

What Happens To Green Roofs In The Long Term?

Most green roof conversations focus on their design: How deep should the soil be? What plants should be used? How much can they cool a building? Less attention is given to what happens to the roof five, 10, or 20 years down the road.

To better understand the long-term fate of green roofs, we studied 46 green roofs across the southeastern United States. Rather than asking how well a newly installed roof performed, we asked whether it was still functioning and being kept up years after construction. The results surprised us.

Nearly half of the roofs were no longer functioning as intended. Nine had been abandoned, with vegetation loss and no maintenance. Twelve roofs had been removed completely.

Even more interesting was what predicted a green roof’s success, or lack of it.

It didn’t matter whether the green roof had received LEED certification, formally recognizing it as sustainable design, or whether it was owned by a private corporation or a nonprofit. What mattered was whether building ownership had changed.

Every time buildings changed hands, green roofs either ended up abandoned or removed altogether, possibly due to a loss of knowledge about the roof. There is also the issue of cost; people who buy buildings with green roofs may have different budgets or priorities and do not necessarily want to spend the time or money to pay for them.

Larger-scale implication

Green infrastructure requires ongoing monitoring and care.

Plants need to be watered, fertilized and sometimes replaced. Irrigation systems need repairs. Invasive species need to be managed. Drainage layers need inspecting to prevent disastrous leaks that damage the building below. Maintenance on green roofs can cost anywhere from a few cents to over $3 a square foot per year and can be time-consuming depending on whether a roof needs to be weeded and watered by hand.

Without that care, many of the benefits used to justify the original investment can disappear.

Imagine planting thousands of street trees, then never watering them again. That would be a poor investment. Yet people often assume green roofs will continue working indefinitely after installation without stopping to consider the work involved in keeping them alive.

Our research suggests that this assumption deserves a closer look.

Looking Toward The future

Research consistently shows that green infrastructure plays an important role in helping cities adapt to a warming climate.

It can reduce urban temperatures, improve stormwater management, create habitat and make neighborhoods more comfortable.

Walter Reed Community Center in Arlington, Va., outside Washington, D.C., features a green roof that helps insulate the building. Arlington County, Va., CC BY-SA

But for green roofs to keep delivering environmental benefits decades after installation, communities may need to think beyond incentives for construction.

Long-term maintenance plans, clear ownership responsibilities and funding for stewardship may prove as important as the installation. Climate adaptation isn’t just about building new infrastructure, but also about making sure that infrastructure can continue to thrive years into the future.

Taylor Wishart is a Ph.D. student in Climate Change, Earth System Science, Environmental Science.

This article is from The Conversation, an independent nonprofit news organization dedicated to sharing the knowledge of academic experts. Find out more about us or subscribe to our weekly highlights.

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