Roof snow loads; drifting against a higher wall

Snow drifts at the change in elevation of a multilevel roof are considered. An analysis of actual field measurements indicates that the height of the triangular drift is related to the length of the upper level roof, suggesting that triangular drifts form at backward facing steps. Conversely quadrilateral drift heights appear to be related to the length of the lower level roof, suggesting that quadrilateral drifts form at forward facing steps. Among large drifts, the triangular shape is the most common. A previous empirical relationship for the drift height of triangular shapes is reviewed and a new empirical relationship for predicting quadrilateral drift height is presented. For the vast majority of quadrilateral shapes in the database, the measured ground load and drift were less than the design ground load and drift height specified in the 1982 American National Standard Institute provisions.