Snow Drifts at Windward Roof Steps

Snow drifts are frequently observed at roof steps, that is, at the common wall between two nominally flat roof surfaces at different elevations. When wind blows from the lower roof towards the upper roof, some of the snow on the lower roof is transported towards and settles at the roof step, resulting in a windward roof step drift on the lower-level roof. When wind blows from the upper roof towards the lower roof, some of the snow on the upper roof settles on the lower roof adjacent to the roof step, resulting in a leeward roof step drift on the lower roof. Current building code provisions relate the size of these drifts to the size of the snow source area (i.e., upper-roof length for leeward drifts). In the 1998 version of the ASCE 7 Load Standard, provisions for windward roof step drifts were changed. These changes result in somewhat higher loadings compared to previous versions of the ASCE 7 Standard. Herein, we attempt to justify these new provisions for windward drifts by reviewing the research upon which the previous provisions were based and by analyzing a group of new windward roof step case histories.