This paper presents the methodologies for numerical modelling of snow deposits and snow transport on long-span roofs for steady and unsteady flow. The calculation of snow loads on long-span roofs is a complex problem, solving which often involves deviating from the building code recommendations. Experiments in wind tunnels, although widely used, do not allow reproducing the full-scale effects of all snow accumulation processes. At the same time, the continuous improvement of mathematical models, numerical methods, software and computer technologies makes the development and implementation of numerical modelling technologies in real construction practice and regulatory documents inevitable. In this paper it is shown that the use of the well-known erosion-deposition model, supported by field observations and experimental data, allows reproducing reasonably accurate snow distributions on long-span roofs. The importance of the “synthesis” between physical and mathematical modelling and the application of the building codes is emphasized, as only the joint use of approaches can comprehensively describe modelling of snow accumulation and snow transport and provide better solutions to a wider range of related problems.