Sliding Snow Loads (ASCE 7 §7.9) Explained

Metal roofs, glazed surfaces and some steep asphalt roofs shed snow rapidly in a warming event. When that snow slides off the upper roof, it lands as a concentrated surcharge on whatever is below: a lower roof, a covered walkway, a sunroom addition, or parked vehicles. ASCE 7 §7.9 requires a check for this load case.

When §7.9 applies

A sliding snow check is required when a slippery roof (smooth metal, membrane, glass) is adjacent to a lower-level roof, a porch roof, or any structure in the path of sliding snow. The upper roof must have enough slope to actually shed snow, generally steeper than 2:12 on a slippery surface.

Calculating the sliding surcharge

The sliding load is the weight of snow that could slide off the upper roof onto the lower structure. ASCE 7 §7.9 sizes it as: sliding surcharge = Pf (upper roof) × lu × (width of lower roof slope receiving the snow). The surcharge is distributed over a distance of 15 ft measured from the eave of the upper roof. If the lower roof is narrower than 15 ft, all the snow lands on the actual width.

Combined load on the lower roof

The lower roof must be designed for its own balanced snow load PLUS the sliding surcharge. The two loads do not cancel each other: the lower roof supports the worst case of its own accumulated snow plus what slides off the upper roof in a melt event.

Design implications

For new construction, place covered entrances, walkways and secondary roofs outside the slide path of upper slippery roofs. For existing buildings, post warning signs in the slide path and consider snow guards on the upper roof to control or prevent sliding. Snow guards redistribute rather than prevent shedding; they reduce the sliding surcharge but do not eliminate it in a major melt event.