## 1 Introduction  
Climate change is having increasingly detrimental and noticeable effects on our global environmental systems and on the way we live [1]. The intended aim of the \"Paris Agreement\" was to limit the global warming caused by greenhouse gas emissions to a temperature difference of well below 2 \({}^{\circ}\)C compared to preindustrial times [2]. However, especially in densely populated areas, the summer months are getting warmer, and more and more people suffer due to the urban heat island (UHI) effect. The UHI effect describes the phenomenon where temperatures in urban areas exceed those in rural areas, as the city offers other environmental conditions that do not occur as dominantly in lesser-populated areas. In their \"Urban Heat Island Strategy Plan\", the city of Vienna stated that increasing vegetation and de-sealing surfaces are suitable methods for enhancing positive microclimatic effects and environmental conditions for the inhabitants of cities. Roof greening, as one of the suggested actions [3], does provide multiple benefits, such as storm water retention, CO\({}_{2}\) sequestration, noise and dust-particle reduction and habitat functioning [4; 5]. Biodiversity through added green spaces provides numerous ecological benefits, while the urban microclimate can be significantly improved, as the effects of the urban thermal mass are balanced by heat-absorbing greenery. Additionally, green infrastructure, such as intensive roof gardens, offer onsite recreational zones and adequate places for urban gardening, which can help inhabitants to stay healthy [6]. Using roofs to implement plants has thus increasingly become a viable measure for enhancing the urban environment. At the same time, the integration of renewable energy, in particular photovoltaic (PV) systems, into the building fabric is important in gradually reducing the share of electricity produced from fossil fuels. With both applications competing for the same (urban) roof areas, research must focus on suitable applications for building integrated systems that provide these much-needed synergies. Especially for existing buildings, solutions for retrofitting existing roofs must be sought in order to transform the current building stock into energy-generating green habitats. As the effects of climate change are already noticeable, not only are actions against climate change needed, but measures for adaption and mitigation are also needed to provide a healthy and livable environment for the future, especially in densely populated cities [7]. The project \"PV Rooftop Gardens: Innovative Systems for the Future\" focused on how photovoltaic systems and green roofs can be integrated within a single roof area. The objective of the project was to find prototypical designs that combined recreational green areas for building occupants with renewable energy generation and storm water retention within the same roof surface area, thus providing multiple benefits per square meter of roof. Within a strong interdisciplinary team, various designs were developed for prototypical applications that were suitable for a wide variety of building layouts. Architectural and constructional aspects, the ease of assembly, plant species and growth, water permeability and retention, as well as photovoltaic system performance all had to be considered within one application. Specific attention was also given to the retrofitting of existing flat roofs so that the proposed system could be used to update the current building infrastructure. The prototypes were analyzed and tested based on structural design aspects, suitable plant choices, and energy output. The key results of the study are summarized in this paper. In the next section, Section 2, the relevant background and the state-of-the-art research related to intensive and extensive green roofs and photovoltaic system applications (and the combination of both) are outlined. Section 3 describes the key aspects of the methodology in relation to the design development of the construction and the green roof.