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Snow Drift Loads on Roofs with Slope Cha

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This desert rat is looking for some advice from some of you in snow country 
or experienced in designing for snow.

I'm designing an A-Frame mountain cabin which has a roof pitch of 12 
(vertical) in 6 (horizontal).  A bump-out with a roof pitch of 5(V):12(H) 
intersects with the A-Frame roof.  Intuition tells me that this bump-out roof 
will be subjected to drift and sliding snow loads from the main A-frame 

With regards to 1994 UBC Appendix Chapter 16, Section 1641:

What do you take for W(b) in equation 41-1?  The width of the A-frame at the 
base or the width of the A-frame at the level that the bump-out roof meets 
the A-frame, or ???  ?

How do you calculate the width of the drift load?  4 * h(d) is easy enough to 
calculate [providing the appropriate W(b) is used in equation 41-1].  
However, 4[h(r)-h(b)] would come out as a negative number as h(r) is (as I 
understand it) the height of the vertical wall between the low roof and the 
high roof and the drift width is to be the smaller of the two.


A. Roger Turk, P.E.(Structural)
Tucson, Arizona