You should contact a civil or structural engineer to verify if the existing members can handle the loads for a deck.
Your roof’s live & dead loads are less than the live & dead loads of a deck system, so it is very important that the roof members be verified. If needed, additional supports should be added to handle new live & dead loads being brought upon by the conversion to deck system.
Roof Loads are typically
Dead Load = 15psf
Live Load (Slope greater or equal to 4:12 = 15psf
And
Deck loads are typically
Dead Load = 10psf
Live Load = 40psf
I also recommend that you verify the information will your local building codes.
Good Luck.