Most species used in residential construction are Pine, Spruce, Fir.
Floor Joist spacing is usually 16", 19.2" and 24" on center.
Stud spacing is usually 16" and at times 24" on center.
Rafter spacing is typically 16" for stick frame and 24" for trusses.
The information that you did not provide us is the span of the enclosed porch.
Your question should be…
I have a porch with a span of ___ feet with 2x8 rafters at 24" on center using
___________ (species of wood), what would be the minimum for a slope of less than
3/12 with a moderate to heavy snow load?
At any rate, 2x8 rafters will typically span almost 14 ft. at 24" on center.
Let me also say that you did not provide sufficient pictures to accurately assess you question… the span of the deck may be 12 feet…yet the rafters may be going back another 4 foot up the roof to get the 3/12 (probably 2/12) pitch, unless the framer busted the load down with bracing then your true span in the above scenario would be 16 ft.
If you want your question answered then you need to provide more info…