This master thesis is a comparison between green roofs (vegetation placed on roof tops) and roof mounted photovoltaic (PV) systems, which is done with the method life cycle assessment (LCA). The aim is to compare the different installations from a sustainability perspective and to present how each installation contribute to a more sustainable community. For this purpose three different green roofs are investigated (one extensive, one semi-intensive and one intensive) and also three different PV systems (5,2 kW with a 43° slope, 8 kW with a 20° slope and 13 kW with zero slope). Beyond these six scenarios a seventh installation is designed and this is a PV green roof, which is an extensive green roof placed under an 8 kW PV system. The LCAs are done through an inventory of production data for each installation and by developing performance indicators for a number of parameters in the use phase of each roof, such as electricity production, storm water delay and carbon accumulation. In the result these numbers are presented and compared to each other, which shows that the production of PV systems have a greater climate impact than the production of green roofs. On the other hand the PV systems can have a greater climate impact compared with green roofs. With the PV green roofs the benefits of both installations are combined which is shown to be a preferred alternative. One of the main conclusions of the project is that each installation would have a greater positive impact on the environment if the materials were produced with renewable energy instead of fossil fuels. Another conclusion is that if considering installing a PV system the cost of adding a green roof underneath is small but the benefits increase, which again indicates that a combined roof is a preferable alternative.