Penn State measured a green roof during a June 2002 heat spell; its membrane peaked near 30°C, while the uncovered roof surface climbed to nearly 70°C

A week in June 2002 produced a stark comparison for two test roofs at Penn State. Researchers at the Penn State Center for Green Roof Research monitored two identical roofs, one covered with a plain waterproofing membrane and the other covered with plants. One bare roof reached almost 70 degrees Celsius in the summer sun. At the same time, the membrane beneath the plants stayed near 30 degrees Celsius, a few degrees below the surrounding air temperature, according to the center's monitoring data.

A bare roof turns into a heat trap without shade or moisture

A typical roof absorbs a lot of heat when the sun comes out. Dark membranes and shingles absorb solar radiation, and the only way to escape the heat is to warm the building below and the air above. The Penn State team found the same pattern in another test on a hot day. With the air at about 32 degrees Celsius, an exposed black plastic surface reached about 63 degrees Celsius, while sedum plants on a green roof stayed near 28 degrees Celsius.


A green roof breaks that pattern through evapotranspiration. The US Environmental Protection Agency has observed the same effect nationwide. Their findings suggest that green roofs provide shade, cool the air, and lower temperatures in both the roof surface and the surrounding air. In contrast, conventional rooftop surfaces may exceed the air temperature.

The heat gap reaches the room below, not just the roof

Roof temperature does not stay on the roof. It moves into the ceiling and the rooms below. In the same June 2002 test, the average interior temperatures of buildings with bare roofs soared past 40 degrees Celsius, while the green-roofed buildings kept interior temperatures close to the outside air for most of the period.

Over an entire month in August, researchers found that small test buildings with green roofs at Penn State's Rock Springs facility used about 10 percent less energy than similar structures with flat black roofs, per the center's findings. Ten percent is not an incredible amount, but it can add up over the many hot days of a typical Midwest or East Coast summer.

The federal government is testing the same idea at scale

The U.S. General Services Administration now has more than 80 buildings with green roofs, covering 2.2 million square feet of roof space from coast to coast. One federal roof, after factoring in the costs of stormwater management, horticultural maintenance, and energy savings, had a payback period of six years on its initial investment. Over the roof's expected lifespan, GSA calculated a total return on investment of 224 percent. A government agency has reached the same conclusion as academic researchers about the energy-saving potential of planted roofs. The key point is not how much power a building with a green roof puts back into the grid, but how much energy it saves the owner.

What renters and homeowners can take from this

Most people will not install a rooftop garden, and a full roof retrofit is a major construction project. But the principles behind a green roof still apply on a smaller scale. Anything that shades a rooftop surface and keeps it from directly absorbing sunlight, from reflective coatings to potted plants on a level rooftop deck, can have similar physical effects to those documented by Penn State and the EPA. The EPA highlights cool roofs as a similar option, using reflective material rather than vegetation to provide some of the same benefits to surface temperatures while being cheaper to install and a viable choice for those who cannot afford a green roof. The research offers a practical lesson for anyone in an urban area facing rising summer electricity bills: a roof's color and covering directly affect how much heat enters the building below.

Green roofs are becoming a real cooling strategy for US buildings

As heat waves become more common in American summers, rooftop vegetation is increasingly being used for cooling as well as appearance. A city block with heat-absorbing rooftops will have different summer temperatures than one where some roofs have been converted into green patches. Penn State's experiment and the federal government's projects both show that roofs are an important tool for reducing summer cooling loads.