Before you can begin to determine the size of your framing members you need to know the total of the loads to be supported. The following are typical summaries (often called "Loading Lists"). These are examples only - your materials will probably be different than those listed.
For A Roof...
Total Dead Loads = 3.0 + 1.05 + 1.05 + 1.1 = 6.2 psf
Live Load = 20 psf; may be less depending on roof slope.
For A Roof Which Also Supports A Ceiling...
Total Dead Loads = 6.2 + 2.8 + 1.0 = 10.0 psf
Live Load = 20 psf; may be less depending on roof slope.
For The Bottom Floor Of A Single Story Residence, No Basement...
Total Dead Loads = 4.0 + 2.3 + 2.2 + 1.8 = 10.3 psf
Live Load = 40 psf for residential occupancies.
For A Floor Which Also Supports A Ceiling Below...
Total Dead Loads = 10.3 - 1.8 + 2.8 + 1.0 = 12.3 psf
Live Load = 40 psf for residential occupancies.