Plants have the ability to reduce the overall heat absorption of the building which then reduces energy consumption. The primary cause of heat build-up is insolation, the absorption of solar radiation by roads and buildings and the storage of this heat in the building material and its subsequent re-radiation. Plant surfaces however, as a result of transpiration, do not rise more than 4–5 °C above the ambient and are sometimes cooler. This then translates into a cooling of the environment between 3.6 and 11.3 degrees Celsius (6.5 and 20.3 °F), depending on the area on earth (in hotter areas, the environmental temperature will cool more). The study was performed by the University of Cardiff.

A study at the National Research Council of Canada showed the differences between roofs with gardens and roofs without gardens against temperature. The study shows temperature effects on different layers of each roof at different times of the day. Roof gardens are obviously very beneficial in reducing the effects of temperature against roofs without gardens. If widely adopted, rooftop gardens could reduce the urban heat island, which would decrease smog episodes, problems associated with heat stress and further lower energy consumption.

Aside for rooftop gardens providing resistance to thermal radiation, rooftop gardens are also beneficial in reducing rain runoff. A roof garden can delay runoff, reduce the rate and volume of runoff. Permeable substrates are replaced by impervious structures such as buildings and paved roads. Storm water runoff and combined sewage overflow events are now major problems in North America. A key solution is to reduce peak flow by delaying (control flow drain on roofs) or retaining runoff (rain detention basins). Rooftop gardens can delay peak flow and retain the runoff for later use by the plants.