The topic of this final paper is „theory, calculation and application of planar and threedimensional truss girder“. The theoretical part summarizes the basics of girders, their division, determination of static definiteness and differences between spatial and planar truss girders. The part where the calculation is provided shows the determination of the reactions of the supports, the calculation of the forces in the rods by analytical, graphical and grapho-analytical methods, on a simple school example. This section contains explanations of the procedures and conditions under which the calculation was performed, that is, idealization. The task was also solved in MD Solids with the aim of comparing the results and verifying the calculated. Finally, two examples from practice were calculated: the roof girder of the industrial hall and the space girder loaded with the roof panel. In each example, respecting the constraints, ie. taking into account only the static stresses, the forces were calculated and based on them, sizing was performed. By adopting the dimensions and selected standard rods, modeling is performed in the Dlubal RFEM software, which uses the finite element method. In this way, the real reactions of the rods made of structural steel S235 are shown. And the aim of the above was to show the differences (shortcomings) of idealization in relation to realistic solutions.