Using 6x6 beams for roof joists
Probably not.

Without any particular input from you on actual required loads in your area, I tossed a 16 foot span and 48" spacing at the Wooden Beam Calculator here:

https://courses.cit.cornell.edu/arch264/calculators/example8.1/

And it spat out 4x16 and 6x16 as options (for defaults of 50 PSF live (snow, etc) and 20 PSF dead (structure). Thus, a 6x6 is going to be FAR less beam than needed, probably.

Beam strength goes as the CUBE of beam depth, so that's in the range of 12.5-19 times less beam that you'd need. Going to a 30 lb live load (which is the ICBO minimum if memory serves) only shrinks it to a 4x14, (a beam 8X stronger than a 6x6, roughly) and at a 1/2:12 slope you will need to be very concerned with roof deflection in rain events, however rare they are - once the roof deflects enough that it does not drain, the roof load can become quite extreme on the day it does rain, if it's a gully-washer.

You also add the problem of what you are spanning the 48 inches with - plywood alone won't be happy, so you'd need additional framing (purlins) to get the deck span in range, or you'd need to use a material that can span that distance at load, such as stressed skin panels.