Abstract: The friction velocity of building roof is an important factor to determine the drifted snow load. In this paper, CFD (computational fluid dynamics) method is used to simulate the friction velocity of the flat roof. In addition, combined with its distribution characteristics, the variation trends of snow transport rate, snow erosion flux, and exposure factor with span are analyzed. To verify the effect of the turbulence model simulated by CFD on the friction velocity of the flat roof, the numerical simulation results are compared with the Irwin probe wind tunnel test. The results show that the simulation results of the Realizable k-ε turbulence model are most close to the friction velocity results obtained from the wind tunnel test. According to the CFD simulation results of four roofs with different spans, when the ratio of roof span to height is 4, the mean friction velocity of the roof is the smallest. And with the increase of roof span, the position of the minimum roof friction velocity and the area of snow deposition are both gradually closer to the windward eaves. Meanwhile, it is found that the mean snow erosion flux of the roof decreases with the increase of the roof span, while the snow transport rate and snow load exposure factor increase with the increase of the roof span.