Snow Loads at Parapets

According to ASCE 7-22 [1], drift loads at parapet walls shall be considered in the design of both the main frames and the purlins. In addition to increasing the demand on the rafters, these drift loads can significantly increase the demand on the purlins located in these areas. Therefore, in such areas, either larger purlin sections or a greater number of purlins may be required.



Drifts on Lower Roof & Sliding Snow on Lower Roof

Snow drift on the lower roof and sliding snow can become governing actions and must be included in the snow-load calculation. The step in roof height promotes drift accumulation along the interface, and sliding snow from the upper roof can deposit on the lean-to roof, increasing local demand on purlins, rafters, and edge details.



Structural designers often do not take this drift load into account. This can significantly underestimate the demands on both the main frames and the purlins, leading to substantially lighter designs that are neither safe nor code-compliant.


References:

[1] American Society of Civil Engineers. "Minimum design loads and associated criteria for buildings and other structures." American Society of Civil Engineers, 2022.