Introduction During the last century, huge movements of people have been observed between rural and urban habitats. Currently, about 50% of the world population lives in urban areas [4]. With the increase of people in cities, the so-called urban heat island (UHI) effect has become a bigger and bigger problem for health and comfort. Indeed, urban materials like asphalt and concrete trap heat fluxes inside the urban centres. [2] This effect increase locally the temperature, which may lead to several hazards, particularly during summer [1]. A huge amount of studies has tried to describe the role of green roofs as a solution against the heat effect (e.g. Takebayashi & Moriyama, 2007[5], Kleererkorper et al, 2012[3], Zinzi & Agnoli, 2012[7], Susca et al, 2011[6]). Among others, Takebayashi & Moriyama proved by measuring experimental data that the heat flux on green (vegetalized) surfaces would be significantly smaller than on a grey one [5]. Vegetation may then have a reducing effect on heat increase. Susca et al (2011) studied the effect of vegetation and green roofs against UHI in the city of New-York, USA, and used some monitoring stations spread in the city to obtain measures during a complete year. They demonstrated the advantages of having green roofs against UHI, but also for biodiversity and air quality improvement [6]. In this paper, we will focus on the city of Geneva, Switzerland. We will try to measure a reduction of heat measurement in the locations where a green surface is present, and confirm the propositions made previously.