The answer depends, but it is unlikely that the 2x6 PT lumber is capable of a 16' span. Here's the deal. You need to know your snow (live) load requirements for your specific area. This information is available from your local building department. We also need to know what roofing is proposed here (dead load). IF this is an open roof structure with lattice or something on top, snow loads will not accumulate and we are not talking about a conventional roof. If you are enclosing the roof with sheathing and roofing, you don't need PT lumber.
PT lumber is not structurally rated as high as select structural untreated lumber. For a 16 foot span, that is what will likely be needed. The capability to make a 16' span will depend on load requirements in your area. IF you have a snow load less than 10 psf and dead load less than 10 psf, AND use either select structural, No. 1 or No 2 structural doug fir rafters on 16 inch centers, the 16 foot span is OK.