Max span for a pressure treated 2x12 on a pergola
I am building a simple pergola on my back patio. I have four 6x6 posts in the ground (33" deep) that are set with concrete. It is 12' x 18.5' rectangle. I will have fourteen 2x8 slats on the top (spaced about every 18"). for the 18.5' span I have two 2x12's bolted to the 6x6's (Kind of a 2x12 6x6 2x12 sandwich) by four 1/2 inch galvanized carriage bolts.
Ideally, I really only want 4 posts supporting this thing. Can I do this project with the 18.5' span on the 2x12's or do I need to put a post in the middle and make it a more managable 9' span.