Drifts on Lower Roofs

Roof snow drifts account for roughly 75% of all snow related roof damage. Drifts accumulate differently when the higher-level roof is on the windward side from when the higher level roof is on the leeward side. Windward drifts start out as quadrilateral shapes owing to a downward curling vortex, which forms when wind impinges on the vertical wall directly beyond the drift. For most roof steps, a leeward drift or a windward drift or some combination of the two is possible. However, in design the windward and leeward drift heights are calculated separately, and the larger value is used to establish the design drift loading. Although the adjoining roof step case is more common, occasionally the lower level roof is simply adjacent to the upper level. The adjacent roof drift presumably would start with a rise-to-run of roughly 1:4 like the adjoining roof drift.