Photovoltaic (PV) can supply all or a significant part of the electricity consumption of a corresponding building without depletion of finite fossil fuel resources. They can be mounted on buildings' roofs or facades. Energy production in buildings by renewable energy sources has recently gained importance and has been referred in laws and regulations regarding energy efficiency policies. Although Turkey has one of the most solar electricity generation potential in Europe, there are only few PV applications in buildings. These examples include mostly PV systems mounted only on low-slope roofs as alternative constructional designs are still unknown in Turkey. Moreover, there is not a guide, regulation or standard regarding the construction of PV roof systems. In order to assists architects, constructers and roof covering material producers to develop correct design alternatives, a model has been developed for the constructional design of PV roof systems. The aim of this paper is to present the model proposed for the design of PV mounted steep roof systems and based on the model, a case study in which a PV mounted steep roof system is designed for Istanbul, Turkey.