Snow Drift Loading on Multi-Stepped Roof 1

[MNach]

I have a Building with multiple roof heights. The roof steps up twice in a relatively small distance. The middle roof height drift load can be easily calculated using ASCE-7. The middle height roof drift length comes to about 23' so 2' short of the entire length. So for the lower roof height do I use the length of the middle roof height or the total length of both higher roofs. To me using the total length of the upper roofs would be very conservative but is using just the middle roof length not conservative enough. Attached is a sketch for clarification. Thank you.

 

[ajh1]

The only code I've seen that says anything in this regard is the Massachusetts Building Code.

1608.4 Drifts on Multiple Level Roofs. For multiple stepped roofs similar to that shown in
Figure 1608.4.1, the sum of all the roof lengths upwind above the drift under consideration, lu*,
in Figure 1608.4.1, shall replace lu in Figure 7-8 of ASCE 7. For multiple level roofs similar to
that shown in Figure 1608.4.2, if the total calculated height of a drift and the underlying uniform
snow layer on the upwind side of a higher roof (hd + hb) is equal to or greater than 0.7(hb + hc),
then the length, lu*, as shown in Figure 1608.4.2, shall be used in place of lu in Figure 7-8 of
ASCE 7.
(See attached for the figures)

Our company has had a number of issues over the years where the snow drift was clearly driven by the overall length of the upper roofs, not just the most immediately adjacent one.