Abstract: Based on measured velocities in conical vortices by flow visualization, a simplified two-dimensional vortex model is established by adding transitional region between the vortex core and the potential flow region of the Rankine vortex. Through this flow model, a relationship linking the flow above the vortices, the curvature radius of flow streamlines in the vortices and the suctions beneath vortex cores is obtained. Two factors contributing to the intensity of vortices are identified as the curvature radius of flow streamline and the velocity in vortices. A larger curvature of flow streamlines and a faster vortex spins lead to a greater intensity of vortex. By flow visualization and pressure measurement, the ratio for parameters of conical vortices on a flat roof and two saddle roofs with different rise-span ratios are analyzed. Then the intensities of conical vortices on the two saddle roofs under different wind directions are given, based on the standard intensity of conical vortices on the flat roof. The quasi-steady theory is improved by including the effect of vortices. With this two-dimensional vortex model and the improved quasi-steady theory, the mean and peak suctions beneath cores of conical vortices on the two saddle roofs are predicted, which are verified by comparing calculated and measured pressures on larger-scale models of saddle roof.