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Snow drift loads on adjacent sloped roofs

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Title: Snow drift loads on adjacent sloped roofs

Okay, I got another one for you guys, this question relates to my previous question "Question on wood Roof Trusses".
I'm planning on modifying my existing gable truss, by adding a piggy back truss to one side, essentially making a big triangle (single slope) the high point of the triangle will meet the high point of my new structure which has a 68'-0" wide flat roof. My question is what should I do about snow loads? 

With a wd of 68' (using ASCE 7-95) I end up with a addition drift of 53 psf, this of course would be added to the 21 psf roof snow load.  (region is phila, pa).  (roof slope is 30 degrees)

Since the "lower roof" is not flat, and not "really lower", what I am planning on doing is treating the new load as a "unbalanced load" and multiplying my 21 psf by 1.3.

I'm obviously not totally convinced this is the correct approach, since compared to the "drift" approach it is not conservative.

Any advice is appreciated.
Keith Hyndman
Exton, PA