Rain-on-Snow Surcharge for Roof Design

Abstract

A study of structural loading on gable roofs due to rain falling on an existing roof snowpack is presented. Rain-on-snow provisions in various existing standards are reviewed, and a summary of structural damage in the United States arguably related to rain-on-snow is presented. Three separate approaches are then used to quantify the intensity, duration, and return period for a rain-on-snow event. The first approach is case history information from a 1996/1997 rain-on-snow event (unknown return period). The second is a statistical approach in which the ground snow load plus daily rain with a 50-year return period is determined (unknown intensity and duration of the rain). The third is a two-year return period rain (unknown snow condition). Based upon the resulting roof surcharge loads, revised provisions for the ASCE 7 load standard are suggested. One possible revision is to only require a rain-on-snow surcharge for certain roofs with low slopes and short eave-to-ridge distances. An alternate revision is to simply eliminate the surcharge.