This paper presents recent research, concerning some preliminary design guidelines to be applied in GFRP composite sandwich panels, for civil engineering applications. For more than a decade, the foundations of preliminary design of concrete, wood and steel structures, have been fully established in structural design codes and technical recommendations. Since FRP structures at the moment possess no standard design code, its preliminary design guidelines are nowhere to be found. This work pretends to fill this gap in knowledge, by providing simple and clear design guidelines to be applied in classical known GFRP composite sandwich panels, more specifically in home buildings. Just like in concrete structures, for GFRP structures one of the most unfavourable design state is the serviceability, mainly due to the long term deflection. Therefore, this long term deflection will be used to estimate the minimum span/height ratio necessary to “indirectly” verify the serviceability state. This is performed using an extensive parametric analysis: of the span beam model; type of materials; type of support; load levels; and geometrical properties. In this work in order to correctly estimate the flexural and shear deflection, the Carrera Unified Formulation is used, therefore no simplified shear correction factor is needed.