Experimental Investigation of Rain-on-Snow Load Processes in Roof Snowpacks under Varying Rainfall and Snowpack Characteristics

Abstract

Rain-on-snow (RoS) events pose a threat to structural safety because they can significantly increase roof snow load under adverse roof snow, geometry, and drainage conditions. Most snow load codes consider RoS load as a uniform load throughout the roof, potentially underestimating large differences in load concentration at different roof sections. Although previous RoS experiments show increasing load until the outflow equals the inflow, the potential of nonuniform water retention leading to localized load concentration in roof structures has not been experimentally explored. Given concerns about hydraulic retention and redistribution in roof snowpacks, a series of artificial RoS load experiments was conducted in a temperature-controlled wind tunnel to investigate the impact of factors such as roof geometry (slope), roof snow depth, and rainfall intensity on the evolution and localization of RoS load. The results show that greater snowpack depth significantly increases water retention when roof slopes have low inclination due to delayed drainage. Although the steeper roof slope significantly reduces the RoS load, it causes load concentration with water retention localizing toward the roof eaves, consistent with coupled effects of rainfall intensity, gravity-driven flow, and unsaturated (capillary-controlled) retention. The observed shift in water retention from a trapezoidal to a lower triangular profile with increasing slope requires special attention at cantilevered roof edges. Furthermore, significant compaction with observable sliding was observed, which may have contributed to load localization near the roof edges. Hence, these findings suggest reinforcing roof edges in lightweight structures with overhangs to mitigate risks of structural damage or collapse.