<p> Flat-plate roofs are expected to oscillate up and down during strong winds. When a structure is oscillating, unsteady wind forces acting on it are affected by interactions between displacement and the flow. Aerodynamic instabilities such as vortex-induced vibrations can arise depending on the shape and dynamic characteristics of the structure. To date, these unsteady wind forces acting on large cantilevered roofs and the characteristics of wind-induced oscillation have been investigated using forced vibration tests and aerodynamic vibration tests in two dimensions.</p><p> Recently, as computer performance has improved, it has become possible to perform transient simulations of the flow around three-dimensional structures using computational fluid dynamics (CFD). CFD is capable of simultaneous and detailed investigation of three-dimensional wind pressure and velocity fields, which are difficult to investigate in conventional wind tunnel experiments. It is particularly suited for application to the unsteady wind forces caused by the interactions between an oscillating structure and the flow. However, the computational accuracy of surface pressures on oscillating structures simulated by three-dimensional CFD has rarely been examined.</p><p> The objective of the present study is to examine the relationship between unsteady wind forces and flow field around an oscillating flat-plate roof in detail using CFD. First, three-dimensional forced vibration test is carried out in a wind tunnel. The results confirm that there is a negative damping force on the roof when the non-dimensional frequency of the roof is relatively low. Moreover, the unsteady wind force on the roof varies depending on the cross section.</p><p> Next, the effectiveness of CFD with an overset mesh to simulate the flow around the oscillating roof is validated by comparing the numerical results with those of the wind tunnel tests. CFD is shown to be capable of approximating the wind tunnel results if the mesh range of the dependent region and the mesh resolution at the boundary between the master region and dependent region are set appropriately.</p><p> Finally, by applying complex proper orthogonal decomposition (CPOD) to the simultaneous wind pressure and velocity fields obtained in fine detail by CFD, the coherent structure of the fluctuating wind pressure field and the three-dimensional wind velocity field around an oscillating roof is extracted. CPOD is an effective technique for extracting any coherent structures in multivariate data and discussing the characteristics of various related phenomena. This analysis shows that the harmonic components of the unsteady wind force are caused by two-dimensional vortex shedding from windward edge of the roof when the non-dimensional frequency of the roof is relatively high. While, a negative damping force arises by the harmony of the roof oscillation and the three-dimensional vortex shedding when the non-dimensional frequency of the roof is relatively low.</p>