Drifts on Lower Roofs - Snow Loads - page 56

Chapter 7

Drifts on
Lower Roofs

In the past, roof step drifting (such as that as shown in Figures G7-1 and
G7-2 ) accounted for roughly 75% of all snow-related roof damage (O'Rourke
et al. 1982). With the advent of modern code provisions that relate the size
of the drift to the upwind snow source area, the amount of snow-related roof
damage caused by roof step drifting has decreased. Based upon the samples
presented in Table G1-2 , roof step drifts now account for approximately 25%
of all snow-related losses with other drifts (parapet wall, gable roof, and com-
bined) accounting for another half.
Drifts accumulate differently when the higher-level roof is on the wind-
ward side as opposed to when the higher level roof is on the leeward side.
As shown in Figure G7-3, leeward drifts are nominally triangular in shape.
Windward drifts are more complex. Windward drifts often start out as quad-
rilateral shapes because a vortex forms when wind impinges on the vertical
wall directly beyond the drift. However, as the windward roof step drift
grows in height, the vertical distance between the top of the drift and the top
of the wall diminishes. If the wall is short enough the wind is redirected over
the top of the wall. At this point the windward drift begins to morph into a
nominally triangular shape. As shown in Figure G7-4, when the wind stream-
lines no longer hit the wall, the shaded area fi lls with snow, resulting in a
triangular shape. If the wall is tall enough, the wind continues to impinge on
it (that is, the vortex remains), and the windward drift will remain at some
distance from the wall. In one case history, the windward drift was located
about 25 ft upwind of a 9-ft parapet wall.
The ASCE 7-10 provisions relate the size of a roof step drift to the
amount of snow in the source area upwind of the step. The amount of snow
is related to the ground snow load, p g , and the upwind fetch area or length.
For the windward drift on Roof A in Figure G7-3, the fetch is the lower roof
length upwind of the windward step, C C . For the leeward step at Roof C, the
fetch is something between the upper roof length, C u and the sum of the upper
and lower level roof lengths upwind of the step C C + C u . In other words, for

wind from left to right, some of the snow originally on Roof A will end up in