In the thesis, we analyse and design a beam which is part of a roof structural system. For the chosen element, we analyse two variations, first one for reinforced concrete and the second one for prestressed concrete element. The beam is 24.7 m in length and its height varies from 1.25 meter at the bearings to 1.5 m in the middle. The calculations were made accordingly to the requirements of Eurocode DIN EN 1992-1-1:2010. In the first part of the thesis load, load combinations and structural analysis was made. We used the results to dimension and prove all of the requirements for serviceability and ultimate limit state. Reinforcement was designed for both beams for their shear and bending resistance. Control for torsion carrying capacity was made due to their unsymmetrical cross section. For the prestressed beam the rebar was designed for the states of transport and mounting. Necessary tension forces in the cables were calculated for the prestressed beam. Loses of its tension were determined. Vertical displacements of the beams were checked for uncracked and cracked cross sections. Positive effects of prestressing were also included. Reinforcement drawings are shown at the end of the thesis.