Section 7.9/Sliding Snow
ASCE 7-22 Section 7.9 requires design for snow sliding off a sloped upper roof onto a lower roof, distributed over a 15-foot width on the lower roof.
Loads caused by snow sliding must be considered for sloped upper roofs that discharge snow onto lower roofs. The total sliding load per unit length of the lower roof eave is calculated as SL = 0.4 times pf times W, where pf is the flat roof snow load on the upper roof and W is the horizontal distance from the eave to the ridge of the upper roof. This sliding snow load is distributed uniformly on the lower roof over a distance of 15 feet from the eave of the upper roof. The sliding load is superimposed on the balanced snow load already present on the lower roof. Sliding snow must be considered for any roof surface that is slippery enough or steep enough for snow to slide, including metal standing seam roofs, membrane roofs, and glass surfaces. Snow guards or retention devices on the upper roof can prevent sliding, but they must be designed for the retained snow load.
Why this section exists
When snow slides off a steep upper roof, it lands on the lower roof as a concentrated load near the wall. Combined with the existing balanced snow and possible drift loads from wind, the sliding snow can create total loads that are several times the balanced roof snow load in the first 15 feet from the wall. This load concentration has caused structural failures in canopies, covered walkways, and lower roof sections adjacent to taller building masses. Metal roofs are particularly susceptible because their smooth surface sheds snow in large sheets.
What plan reviewers look for
Plan reviewers identify upper roof surfaces that slope toward lower roofs and check for sliding snow calculations. They verify the upper roof slope is steep enough for sliding (typically any slope greater than about 1/4 on 12 for smooth metal surfaces, higher slopes for rough surfaces). They check that the sliding load is distributed over 15 feet on the lower roof and superimposed on the balanced snow load. They verify the lower roof framing within 15 feet of the wall is designed for the combined load. They check whether snow guards are specified on the upper roof and whether the guards are designed for the retained snow load.