How Is Snow Load Engineering Done for Alpine Townhomes?

The engineer works the ASCE 7 snow provisions as a set of cases, not a single number. The balanced load covers the uniform snow the roof carries in calm conditions; the unbalanced load covers the wind-redistributed case on sloped roofs, with the windward slope stripped and the leeward slope surcharged; and the drift load covers the triangular piles against parapets, rooftop units, and roof steps. For townhomes, the roof-step drift is usually the governing case: the engineer computes the drift height from the upwind fetch of the higher roof and applies the surcharge to the lower roof's framing, decking, and connections. Parapet drifts get the same treatment along every party wall. Sliding snow from an upper roof onto a lower one is a separate impact the engineer does not ignore — the code's sliding load adds a surcharge to the lower roof, and the engineer sizes the lower framing for the combined drift-plus-slide case where the geometry creates it. Snow retention systems, where used, are engineered for the full sliding force with attachment details into the structure, not just the decking. The engineer documents every snow case on the structural plans so the plan reviewer and the framer see the same loading the design assumed.

Use this checklist before the structural set is issued: • Adopted ground snow load confirmed in writing with the authority having jurisdiction for the exact parcel • Site-specific snow study engaged where the elevation exceeds the code's mapped limits • Balanced, unbalanced, and drift loads computed for every roof plane, step, valley, and parapet • Sliding-snow surcharge applied where upper roofs shed onto lower roofs • Cold-roof detailing: full insulation, airtight ceiling plane, eave-to-ridge ventilation, ice barrier at eaves • Roof drains on low-slope areas detailed against freezing; overflow scuppers provided • Snow retention or managed shedding designed above entries, walkways, and neighboring units • Footings below the local frost line on the soils report's bearing values; differential settlement checked at party walls • Jurisdiction confirmed: town-adopted snow load amendments vs county adoption, and the plan-check submittal path for each