Roof loads resulting from rain on snow: results of a physical model

Abstract

A physical model is used to calculate roof loads due to rain on a snow covered roof. A snow depth of 0.5 m and the twenty-five year rainstorm in Hanover, New Hampshire, are used in the examples. For a flat roof with 10 m parallel flow to gutters, the total liquid weight can increase the roof load by about 50%. The weight of the transient liquid is greatly increased if the mode of flow is radial to central drains and is decreased if the roof is slightly inclined or if significant melt channels form in the basal layer. However, the wetting of the snow over its entire depth will still cause a significant weight of transient liquid. Snow drifting can cause very large, local loads but the effects of snow temperature and antecedent moisture are not too important.Depending on the circumstances, the largest load can occur for either a long duration, low intensity rainstorm or a short duration, high intensity rainstorm. The former occurs if the saturated layer makes a significant contribution to the total live load whereas the latter occurs when the liquid weight is due mainly to the unsaturated layer. Further study is needed to establish the joint probabilities of combined snow and rain loads, especially when rain and snowmelt occur simultaneously.