Scientists reveal how green roofs cut rooftop temperatures by 40°F

Cities often become warmer than surrounding rural areas due to the heat absorbed by buildings, roads, and rooftops. Rooftops, in particular, can become extremely hot during summer, placing additional stress on buildings and those who inhabit them. To combat this urban heat problem, an innovative solution has emerged - green roofs. These roofs are covered in vegetation, soil, and carefully designed layers that help buildings endure rising temperatures.

Green roofs function through several mechanisms, including shading and evaporation. The presence of plants and water on the roof surface reduces direct heating caused by solar radiation. Additionally, water stored in the soil slowly evaporates, further cooling the roof.

Studies have shown that surface temperatures of planted roofs can be up to 40°F lower than those of traditional roofs during hot days, specifically in sunny weather. This cooling effect is primarily attributed to the shade cast by vegetation and the evaporation of water contained in the soil. In southern Italy, a study published in Energy and Buildings found that a traditional roof reached temperatures of 74.3°C in June, while a vegetated roof maintained surface temperatures 57-63% lower, effectively reducing heat fluctuations throughout the year.

Beyond heat reduction, green roofs offer other benefits such as improved insulation, rainwater absorption, and habitat creation for wildlife. By allowing rainfall to be absorbed gradually by the soil and vegetation, green roofs can alleviate pressure on urban water networks during heavy storms. Furthermore, the presence of plants and flowers can support various forms of urban wildlife, including pollinators and birds.

Green roofs differ from intensive green roofs based on factors such as building conditions, budget, and intended use. Extensive green roofs are lighter and require minimal maintenance, making them suitable for a wider range of buildings. Intensive green roofs, on the other hand, support deeper soil, larger plants, and sometimes trees, but require stronger structures, more maintenance, and greater investment.

While both green roofs and cool roofs aim to reduce rooftop temperatures, they employ different methods. Cool roofs primarily change how sunlight affects a building, whereas green roofs introduce living elements that can manage heat, retain water, and create ecological spaces. In some projects, these two approaches can be combined to achieve optimal results.

However, installing a green roof typically comes with a higher cost than fitting a standard roof. The final price depends on the type of system, the condition of the building, and the level of maintenance expected. Nonetheless, the benefits of green roofs, such as heat reduction, improved insulation, and habitat creation, make them a worthy investment for cities seeking to alleviate the urban heat island effect.