Section 7.6/Unbalanced Roof Snow Loads
ASCE 7-22 Section 7.6 requires unbalanced snow load analysis on gable, hip, curved, sawtooth, and multi-level roofs where wind causes asymmetric snow accumulation.
Unbalanced snow loads must be considered on roofs where wind action causes snow to be deposited unevenly. For gable roofs with a slope between 2.39 degrees (1/2 on 12) and 30.26 degrees (7 on 12) and a horizontal eave-to-ridge distance W greater than 20 feet, the windward side is loaded at 0.3 times the balanced flat roof snow load (pf) while the leeward side carries the balanced sloped roof load plus a rectangular surcharge. The surcharge intensity (hd x gamma) is calculated from the drift height formula. Hip roofs, curved roofs, sawtooth roofs, and multi-level roofs each have specific unbalanced load patterns defined in Figures 7.6-1 through 7.6-4. For gable roofs steeper than 30.26 degrees, no snow is assumed on the windward side (it slides off) and the leeward side carries the full balanced load.
Why this section exists
Wind blowing across a ridgeline scours snow from the windward slope and deposits it on the leeward slope. This creates an asymmetric load that produces the maximum bending moment in ridge beams and connections, net uplift on windward rafters and trusses, and lateral forces on interior bearing walls. Many structural failures during heavy snow events result from unbalanced loading rather than the balanced snow load. The unbalanced condition often produces higher member forces than the balanced case, particularly for ridge beams, hip rafters, and frames with long spans. The 20-foot minimum W threshold excludes small structures where the unbalanced effect is negligible.
What plan reviewers look for
Plan reviewers check the roof geometry (slope, span, eave-to-ridge distance) to determine if unbalanced analysis is required. They verify the structural calculations include both balanced and unbalanced load cases. For trusses, they check that the truss design includes the unbalanced loading pattern. For ridge beams, they verify the beam is designed for the eccentric loading from the unbalanced case. They confirm the unbalanced load is included in the load combinations and check for uplift on windward connections.