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Snow Drift

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I have two roofs sloping perpendicular to one another with a parapet wall in
between.
The upper roof is 5:12, the lower roof ( with an average height of 20 feet
lower & Pg=25psf for both) is 3:12.
In calculating snow drift, does it matter that the roofs are sloping
perpendicular?
Or is the key parameter just the difference in height?  

UBC 1641.3.2 covers nonparallel ridge lines where the roofs are "connected"
(looking at figure A-16-12) almost treating it as sliding, but what if there
is no connection to these roofs except from a parapet.

  _________
  |____^____|   <Lower Roof
  |       |       |
  |       |       |            
  |   <  |  >   |    <Upper Roof 
  |       |       |
  |____|____|     
  
(Excuse my crude sketch)

Any help would be appreciated.

Todd Hill
TK Architects, Inc.