How Are Boise Roofs Designed for Snow Loads?

The ground snow load is the starting point, not the answer. ASCE 7 and the local jurisdiction publish ground snow values derived from decades of weather data, and the engineer converts that to a roof load with a set of adjustment factors. The slope factor reduces load on steep, slippery roofs that shed snow and increases the effective demand on low-slope roofs that hold it. The exposure factor accounts for whether wind can scour the roof - an open, windswept site carries less than a sheltered one. The thermal factor reflects heat loss through the roof: a well-insulated cold roof holds snow longer than a warm roof that melts it from below. The importance factor then scales the load to the building's risk category - a hospital or school gets a higher factor than a storage shed, because the consequences of failure differ. Minimum roof snow loads apply regardless of the calculation, so very low ground-snow cases still produce a designable load. The engineer documents each factor and its source in the calculations, because the plan reviewer checks the chain from ground snow to member design. A missing or misapplied factor is one of the most common corrections in structural plan review.

Drift is the silent overload. Wherever wind-blown snow piles against an obstruction - a parapet, a rooftop unit, the step where a low roof meets a taller wall - the engineer calculates a drift surcharge and applies it as a triangular load on the structure below. The drift height grows with the upwind fetch and the ground snow load, and on large commercial roofs the surcharge can govern the design of entire bays. Unbalanced loads cover the windward-scour, leeward-deposit pattern on pitched roofs, plus the sliding snow case where snow sheds off an upper roof onto a lower one. Each of these is a separate load case in the analysis, not an afterthought. Re-roofs deserve the same rigor as new designs. Swapping roofing, adding insulation, or installing solar changes the dead load, the thermal factor, or both - and the original structure may predate current snow provisions entirely. The engineer surveys the existing framing, confirms member sizes and spans, and runs the current loads through the actual structure. Where capacity falls short, the fix might be sistering joists, adding beams, or limiting the new assembly's weight. It is far cheaper to find the shortfall on paper than under a February snowpack.