Section 7.7/Snow Drifts on Lower Roofs
ASCE 7-22 Section 7.7 covers snow drift loads that form on lower roofs adjacent to higher roof surfaces, walls, or parapet projections.
Where a lower roof is adjacent to a higher roof, wall, or parapet, snow drifts form against the vertical surface on the lower roof. Both leeward drifts (snow blown from the upper roof) and windward drifts (snow blown along the lower roof surface) must be evaluated. The drift height hd is calculated from Equation 7.7-1 using the ground snow load pg and the upwind fetch distance lu (the length of the upper or lower roof contributing snow to the drift). The drift surcharge is a triangular load superimposed on the balanced flat roof snow load on the lower roof. The drift width extends horizontally from the vertical surface for a distance of 4 x hd, with intensity equal to hd x gamma (where gamma is the snow density, 0.13 x pg + 14 in pcf, not to exceed 30 pcf). The greater of the leeward and windward drift loads governs the design. Drifts are not required where the difference in roof heights is less than the drift height hd divided by hb.
Why this section exists
Snow drifts at roof steps are one of the most common causes of structural overload and roof collapse in snow regions. Wind carries snow from the upper roof and deposits it against the wall on the lower roof, creating drift loads that can be two to four times the balanced snow load. Many roof collapses attributed to snow loading occur at these step locations rather than at mid-span. The drift load is concentrated near the wall, producing high localized forces on the lower roof structure. The unbalanced snow load on gable roofs and the drift load on lower roofs often produce the governing design forces for roof framing members and their connections.
What plan reviewers look for
Plan reviewers identify all locations where a lower roof meets a higher surface, including setbacks, mechanical penthouses, parapets above 3 feet, and adjacent buildings. They verify that drift loading is calculated for each location. They check the upwind fetch distances (lu for both leeward and windward drifts) and the resulting drift height and surcharge intensity. They verify that the structural members at the drift zone (typically the first 15 to 30 feet from the wall) are designed for the combined balanced plus drift load in the load combinations.