I'm planning on building a pergola for my folks this summer. I'm going to use the plans from http://www.rd.com/17835/article17835.html. The only difference is my parents deck is 16' x 12' 2-1/2". I am trying to rework the design (using only half the design) to fit their deck.
My question is:
Can I use 4 4x4's as my posts connected by lag bolts onto the sides of the deck? Then span two 16' 2x8 across the top? Will that be too much load for that distance?
I think that 2x8's will be insufficient for the 16' ceiling span over time. Granted, they're not holding any load except their own wieight, but I think that they'll probably show a lot of sag after a couple years.
As for bolting the 4x4 posts to the rim joist of the existing deck...Absolutely not. The structure needs to bear directly on the deck's bearing points. If it is being inspected, I doubt any inspector would allow it either. There will be an incredible amount of weight on those 4 posts, not to mention that it probably won't be as solid as you think.
I have similar situation, I am trying to build 10X10 pergola(Post to Post length) using 12X12 (2X8)joists .
I am planning to install 3 posts on the side of internal joists spaced 5 ft apart and 2 on rim joist, although I plan to put 6 posts in total as even I think just 2 posts cant hold the entire weight over long time.
If you cant install Posts on the side of rim joists. whats the alternative, most of the books recommend the same.