This work presents the concept and structural design of a 1000-m2 glass fiber-reinforced polymer (GFRP) sandwich roof replacing a timber roof over an indoor swimming pool. The advantages offered by the lightweight, multifunctional GFRP-polyurethane (PUR) sandwich construction for the roof replacement are described. The roof structure was designed for two potential construction scenarios (with and without including a green rooftop), whose pertinent dead loads on the roof significantly differed. The resulting designs for both cases, considering long-term effects and applying two design recommendations (Eurocomp and BÜV) are compared and discussed.