How Are ASCE 7 Snow Loads Designed for Commercial Buildings?

ASCE 7 snow design takes the ground snow load from the code maps or the local jurisdiction, applies exposure, thermal, and importance factors plus a roof slope factor, and produces the balanced roof snow load. Then the engineer works through the additional cases the chapter requires: windward and leeward drifts at parapets and projections, drifts at roof steps between high and low roofs, sliding snow from sloped upper surfaces, unbalanced loading on gable and curved roofs, and rain-on-snow surcharge where the climate warrants it. Each case produces its own loading diagram on the roof framing, and the members are designed for the worst combination. The calculations document the ground snow load, every factor used, and each drift geometry — drift height and width are computed from the upwind fetch distance, so the roof layout itself feeds the load. Reviewers in snow jurisdictions check drift cases closely because that's where under-design hides.

Unbalanced loads deserve attention because they surprise owners: wind strips snow from the windward side of a gable roof and piles it on the leeward side, so one slope can carry multiples of the balanced load while the other carries almost nothing. The framing has to handle that asymmetry, not just the average. On curved roofs the pattern shifts with the geometry, and open structures like canopies get their own unbalanced provisions. Then there's the serviceability side. Snow is a sustained load — it sits for weeks — so deflection under snow matters more than deflection under a brief wind gust. A roof that deflects excessively ponds meltwater, and ponding plus snow is a failure mode with a long history. We check deflection limits and ponding stability as part of the snow design, not as an afterthought, and we pay special attention to long-span roofs and roofs with interior drains where ponding risk concentrates.

Make sure the structural documents address every one of these before they go to plan check in a snow region.

• Document the ground snow load source — code map value or the jurisdiction's amended minimum
• Show exposure, thermal, and importance factors with the reasoning for each selection
• Compute drift at every parapet, roof step, and rooftop projection with the fetch distances shown
• Check sliding snow from upper roofs and sloped surfaces onto everything below
• Evaluate unbalanced loading on sloped, gabled, and curved roof geometry
• Verify deflection and ponding stability under sustained snow plus rain-on-snow where applicable