The parallel and perpendicular orientation of lamellae in free-surface thin films of symmetric diblock copolymers is studied in the strong segregation approximation. In contrast to films, which are confined between two hard walls, the period of the parallel lamellar structure in the free-surface films is determined from the equilibrium conditions. As a result, the period depends on the surface tension coefficients (spreading parameters). Both wetting and nonwetting conditions are considered. We have shown that the perpendicular orientation of the lamellae can be stable even in thick films if the different blocks interact with the substrate and air similarly, i.e., when the values of the spreading parameters of both polymers are close to each other. Also stability of the perpendicular lamellae is predicted for thin enough films with dissimilar spreading parameters under wetting conditions for both species (attractive interactions between film and substrate). Otherwise, the lamellae have the parallel orientation.