ABSTRACT: In this paper results of the structural calculations of typical warehouse purlinless steel roof structure were presented. The objective of this research was to compare, currently most popular, approach to designing trapezoidal sheet based on 2D beam model on rigid supports with 3D modelling one, which takes into account stiffness of whole structural system. It was concluded that stiffness relations between roofing sheet and main structure may significantly influence the distribution of internal forces. This effect is usually omitted by structural designers.