The use of composite materials and sandwich panels in civil engineering applications can be an efficient and sustainable alternative to traditional materials. A composite sandwich panel, composed by glass fiber reinforced polymer (GFRP) skins and a PET foam core has been designed to potentially substitute conventional roofs of canopy structures in train stations. Advantages are high strength-to-weight ratio, durability, lightness, limited interference on railway traffic during installation, factory production which reduces hazardous activities carried out in construction sites and the sustainability provided by the possibility of using recycled materials. However, in spite of the profound knowledge of these materials gained in the last decades in automotive, aerospace and aeronautic fields, their use in the construction industry is not straightforward due to different and various design requirements, ranging from adequate load bearing capacity at the ultimate limit states, to limited deformations in the serviceability limit state, and to a service life of 50 years. Moreover, the lack of consolidated National and European design Standards must be compensated by extensive experimental testing. The design approach, numerical modelling and experimental activity carried out on specimens and full-scale elements to characterize the mechanical properties and the durability of the materials are critically presented and discussed. From the results, it emerges that the geometrical characteristics, i.e. the thickness of the panel, are mainly defined by the fulfilment of the limitations imposed for the operational conditions, while the selection of the materials and, therefore, their mechanical and physical properties, are mainly determined by durability and fire reaction requirements.