Rising temperatures in urban areas increase the importance of vegetated structures that use evapotranspiration (ET) to naturally provide cooling the outdoor microclimate and reduce the amount of heat transferred through the envelope to the interior. The article discusses the theoretical calculation of evapotranspiration from meteorological data according to the FAO PM method. The measured values were compared with the data obtained by a weather station with the determination of the daily ET on an experimental vegetated roof in Dubnica nad Váhom. The redistribution of solar energy falling on the roof structure and the influence of the main weather input parameters on the ET efficiency were analysed. The research concluded that approximately 50% of the incoming solar energy is able to be absorbed by the ET process during sunny days. In the theoretical-experimental analysis, it was demonstrated that efficient ET can significantly reduce cooling energy in summer, especially for large roof areas of industrial buildings, thus saving costs and improving the indoor climate. However, the long-term effect of ET requires a sufficient water supply, which emphasises the importance of designing irrigation systems in the preliminary design stages.