Rain-on-Snow Surcharge Loads

A rain-on-snow surcharge is occasionally one of the contributing factors in a roof collapse, although the author is unaware of case histories wherein rain-on-snow was the sole source of an overload that led to a collapse. The rain-on-snow surcharge associated with rain percolating through a roof snowpack is composed of two portions. The first, typically small portion is the load caused by vertical percolation through the upper unsaturated layer. The second, larger portion is the load caused by rain flowing downslope in the saturated layer immediately above the roof. As expected, the surcharge caused by flow in the saturated layer increases with increases in rainfall intensity, rainstorm duration, and eave-to-ridge distance, and it decreases as roof slope increases. All other things being equal, a low sloped roof with a large eave-to-ridge distance will have the greatest rain-on-snow surcharge.