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In BIPV design with c-Si cells, ventilation is important in order to keep cells as cool as possible. To allow a good ventilation it is therefore generally preferred to mount the modules separated from the existing roof. In the case of sloped roofs the modules are superimposed to the existing roof and for flat roofs separated tilted mounting structures with wind loads are used instead, but both are no real building integrations.In this paper we analyse the behaviour and the energy yield of a 15.36kWp PV system based on flexible triplejunction thin-film amorphous silicon modules laminated together with a single ply roofing system based on a flexible polyolefine (FPO) membrane which acts as a waterproofing system.Compared to c-Si modules, a thermal isolated BIPV design matches the best the thermal behaviour of a-Si technologies. BIPV DESIGNBuilding Integrated PV systems (BIPV) combine in an harmonious way the architectural functions of building materials (protective, good-looking, …) and the electrical production. The PV material assume a new role with a double function: architectural and productive.For PV systems is required an high architectural quality, particularly regarding the 7 criteria of integration define by the IEA PVPS task 7 [1]. These criteria, that help the incorporating of the PV during the building design and assess the architectural quality of BIPV projects, are important for architects in determining why a BIPV project is or is not aesthetically pleasing.Joining together more functions in one element, will introduce often compromises that limit one of the two functions.For example in BIPV design with c-Si cells, the ventilation is important in order to keep cells as cool as possible and with high efficiency. This constraint complicates the integration of crystalline silicon PV modules into a building and leads to higher installation costs. To allow a good ventilation it is generally preferable to mount the modules separated from the existing roof.In case of sloped roofs the modules are superimposed to the existing roof and for flat roofs are mounted on a separated tilted structures with wind loads, but both are not really in the building integrations concept.Thus, is necessary to make an effort for finding some PV building elements adapted to different kind of integration. FLAT ROOFFlat roof is a particular type of roof not adapted for good-quality BIPV design with double function elements. However it is possible to use a special cover: single ply roofing system based on a flexible polyolefine (FPO) membrane which acts as a waterproofing system.In this field single ply roofing systems today accounts for approximately 20 -25% of the flat roofing market in Europe.FPO's (also known as TPO, thermoplastic polyolefines) has a good ecological profile and good longterm properties. Under normal conditions they will have a service life in excess of 40 years. These membranes are therefore compatible with PV modules with a life time of at least 25 years ore more.The roofing membranes can be joined by means of...