Hi im looking for help on planning a covered patio to a 18x16 Deck I plan on adding to my house. The deck will be 18' wide and extend out 16' from the house. I want to build a patio extending out 12' from the house and have atleast a 1' overhand on each side making the patio cover 12x20. I plan on using Polygal as roofing so it does not need to support much weight. Building in a location that does not experience snow. What size post and beam do I need to use to span the 18' between support post? attached is a image of the concept design i made.  

I'm looking for help on planning a roof over a deck I plan on adding to my house. The roof will be 18' wide and extend out 12' from the house. I want to have at least a 1' overhang on each side making the roof 12x20. 

I plan on using polycarbonate sheets as roofing so it does not need to support much weight. Building in a location that does not experience snow. What size post and beam do I need to use to span the 18' between support posts?

Here is the concept design.