Pergola design help needed
Hey everyone. This is my first post here and I hope someone can help me out with my little project.
I am building a paver patio in the back yard of my Southern California home and want to incorporate a freestanding pergola. However, due to the design layout of the back of the house, particularly the placement of the kitchen window and a sliding glass door, I was thinking about doing a rather large rectangular pergola using four 6 x 6 posts with faux Tuscan columns slipped over them. Two of the posts would be 20' apart (the long side of the rectangle along the back of the house) and the other two would extend out 15 feet from there. Accordingly, I was looking at essentially spanning 20 feet using either 2x10s or 2x12s bolted to either side of the 6x6 posts. 2x6's would form the rafters spanning the beams and then 2x2s on top of those.
From what I've read here and on other websites, it sounds like the 20 ft span is too large even for the 2x12s and I run a serious risk of sagging and I'd have to look at a 6 post design at a minimum. As stated above, the design of the house makes the six post style problematic because it would likely place a post right in front of a window or door. What's the largest recommended size for a rectangular four post pergola? If I do go with a 6 post design, I would end up with a narrow structure projecting away from the house as opposed to a wide structure along the width of the house (if that makes sense). The latter would be more usable given the layout of the yard. I'd like my large, 4 post design to work but I don't know if it is possible from an engineering standpoint short of going with steel beams. If I ran the beams across the 15ft posts, I would then have to span the 20 feet with the rafters which would likely sag given that they would be less substantial than the 4 2x10s (or 2x12s).
Any help would be appreciated.