In a 2013 study, researchers found a healthy green roof cooled air by 1.58°C at night, while drought damage cut that measured benefit to 0.78°C

Rooftop gardens have been part of the contemporary American skyline for quite some time, often touted as being an easy solution to the problem of the urban heat island effect, which subsequently causes downtown pavement to radiate heat long after sunset. Cities from New York to Chicago have paid hefty sums to have green roofs installed, on just that premise. However, a 2013 peer-reviewed study titled “Reduction of the urban cooling effects of an intensive green roof due to vegetation damage,” published in the journal Urban Climate, adds a caveat to that assertion that US cities that invest in rooftop greenery might want to take note of.

The four researchers behind it, namely Andrew Speak, James Rothwell, Sarah Lindley, and C.L. Smith, found that a well-maintained and healthy green roof reduced the air temperature on top of it by up to 1.58°C (2.8°F) at night. But during a drought, one of those same roofs suffered damage and had a night cooling ability of only 0.78°C, which is less than half of the night cooling power it gave before. It's not just a bit of trivia. It has clear implications for the way that American cities should think about the summers that they are now facing, especially when they see the installation of green infrastructure as the end of the project, and not the start of a new one.

The study that should worry every city planner


The team followed the performance of an extensive green roof project in Manchester in the UK and compared it to a neighbouring conventional concrete roof. They found that a month-on-month median temperature measurement taken 300mm above the surface of the green roof shows a difference of 1.06°C from the background temperature; this difference was most conspicuous at night when the concrete and asphalt store up the heat accumulated during the day.

Next came the turn. Some of the roof was damaged due to mismanagement during a drought and now has areas with bare soil. In the study's summer measurements, this damaged patch ran as much as 0.63°C warmer in the late morning than the healthy green roof nearby: a separate, daytime effect from the nighttime cooling numbers. At night, the damaged patch still cooled the air, just far less effectively than the healthy roof: 0.78°C of cooling compared to the healthy roof's 1.58°C. So an unmaintained green roof doesn't just lose its benefits over time. It can, in patches, run measurably warmer than a well-kept green roof close by during the day, though even in that damaged state, it was still cooling the air at night, not adding heat to it.

Back in the US, the stakes are higher

Here, extreme heat is not just an inconvenience; it's one of the deadliest weather hazards. As per a 2023 NOAA feature on urban heat mapping, there are some alarming facts: urban heat islands can be up to 20°F hotter than surrounding greener areas, and for 30 years, heat has been the top weather-related health hazard in the US. Cities turn to green roofs as one of the weapons with which they can fight it. The EPA's overview of green roofs as a heat island strategy puts the cooling potential even higher, noting that green roof surfaces can be 30-40°F cooler than conventional roofing and can help provide a reduction in the ambient temperature in the city.

Those numbers are true, but represent roofs that are in fact being cared for. The Manchester results should remind everyone that there is a difference between "green roof installed" and "green roof functioning" and that many cities in the USA are making the leap from installed to functioning without being aware.

Green is not a permanent setting, and the real takeaway

This is where the not-so-comfortable part comes in. It is costly to install a green roof, and, as it can be shown, it requires regular maintenance and watering if it is to continue to provide what it was designed for. When there is a dead sedum in a drought-stressed patch, it's not an error but a measurable reduction in the exact benefit that taxpayers were promised.

None of this is an excuse not to pursue green roofs or to argue against them. The cooling effect is well known, and it pays to pursue. But there's an argument against considering them a one-and-done solution, particularly one that has become more popular in recent years. Rooftop greenery is not just a climate infrastructure investment; it is a maintenance expense, too. If not, that seemingly photogenic rooftop garden may not be doing all the good it's supposed to for your block.