Abstract: It was analyzed the stress and deformation of a simple purlin with two sag rods under wind suction，whose roof sheets were connected to the upper flange of the purlin by self-drilled screws; the analytic results were compared with those of FEA. It was found by the analyses that the theory of twisting about upper flange could predict more accurately the lateral deformation of the lower flange，the reaction of the sag rod，and the reaction of the roof sheets close to the non-sag rod zone. There was a maximum compressive stress at the lower flange lip on the section of the sag rod，whose result calculated by EC3 method agreed well with that of ANSYS; there was a maximum tensile stress at the upper flange lip on the section of the sag rod, hence the effect of the distortional deformation must be taken into account. In such case the theory of bending and twisting must be used to calculate the reaction of the sag rod，only then can the positive stress due to the distortional deformation be introduced.