Rain-on-snow roof surcharge load: A review of recent collapses, design standards, current research, and challenges

Rain-on-snow (ROS) events can significantly increase roof snow loads in buildings, particularly as retained rainwater adds to the snowpack's weight. Recent roof damages from ROS load have been observed in both old and new structures, as climate changes exacerbate such events. Evaluating ROS load in existing snow load codes is crucial to determine whether they adequately address climate-driven impacts, including changing precipitation patterns and extreme weather events. This paper presents a systematic review of the ROS events, damage cases, current methods, and changing climate which shows that snowpack, its properties, and flow type are the key areas of focus. The review of ROS load standards exposed a reliance on historical data and snowpack characteristics, with limited consideration of climate change impacts on ROS surcharge loads. The bibliometric analysis of keywords revealed that environmental factors, snowpack properties, and climate change effects need to be considered for modeling ROS load. Among the available models, snowmelt and snowpack models are effective for long-term ROS simulations, particularly regarding snow depth, density, and pore distribution. In contrast, flow models can provide practical solutions for estimating ROS load while accounting for both uniform and non-uniform wetting front flow conditions in older snowpacks. The distinction between uniform wetting front flow and matrix-preferential flow is pivotal in determining the complexity of ROS load models, as these patterns significantly influence water retention in snow. As global warming can intensify ROS events, reassessment of ROS load standards with climate models is imperative to understand the future effects of climate non-stationarity.