Rafter spans from the AWC Span Calculator are calculated as maximum horizontal (projected) spans. Snow loads used to calculate the maximum horizontal spans are assumed to be uniform and independent of the snow loading condition (i.e. flat roof versus sloped roof, warm roof versus cold roof, balanced versus unbalanced, etc). The designer first determines the appropriate snow load condition and then he/she can use the AWC Span Calculator to calculate the maximum horizontal span for that loading condition.
ASCE 7 Minimum Design Loads for Buildings and Other Structures contains provisions to determine the loads for specific loading conditions based on adjustments to the ground snow load. The AWC Span Calculator does not have the means to calculate these adjustments, so rather than providing drop-down options for snow loads (e.g. 20 psf, 30 psf, etc.), the AWC Span Calculator requires the roof snow load to be calculated and entered manually.
For roof rafters where the maximum horizontal projected rafter span from eave support to ridge is less than 20 feet, ASCE 7 provides a simplification that sets the unbalanced snow load equal to the ground snow load (see Snow Provisions in ASCE 7-05 for more information) which simplifies the determination of the snow load condition. This simplification also applies in ASCE 7-10.