Pergola design help

Hi I'm new here any help would be appreciated. I am planning on building a 16 wide x20 ft long pergola. I would be using 4 6x6 or 8x8 post on each side. I want to keep the spacing short so I can put pull down shades for added privacy. The front 16 ft span I would like to keep open.
Question #1 what would be the best for the front 16ft span 2x8 or 2x10 and sandwich the post or put 3 together on top of the post I would be using diagonal braces to help support.
Question #2 the 20ft span would be the rafters what would be best 2x8or 2x10 and will I have to worry about sagging I also plan on using cedar for the whole job thanks

 

Welcome to the forums! I am sure there is more than one way to do this. Northern cedar in 2x10 will only span 14'4", so you will need some intermediate support. Not the end of the world with that width. I prefer to let in the band to the posts as depicted in this picture. We doubled it to the outside and bolted them to the post. You can't just bolt to the posts as the band must rest on the vertical part of the grain of the post.

 

I would recommend you go with rough cedar posts that are full dimension. Meaning the 6x6 is actually 6", not 5 1/2. You could use Simpson 1212HTPC's and sit the beams fully on top. 4 2x10's all sandwiched and siting on top would be 6", so you could bolt them together with galv. bolts. You will need diagonal gussets on each side of each post to keep the posts plumb, to prevent them from swaying.

You will have trouble finding 20' long cedar. You can get it special order through Menards.... 86.50 each. But that is much too long of a span for it to stretch without sagging. So you will need to plan for at least one 4 2x10 cross beam at the midspan.

You will likely also need blocking between the "rafters" to keep them straight... this blocking would best go directly above the beams that the rafters will be sitting on. And then some 2x2 perlins on top of the rafters, perpendicular to the rafter layout would not hurt... they will help tie everything together on your layout, keeping your rafters evenly spaced and straight.

 

Would there be any difference in using Doug fir instead of cedar? Has anyone used Doug fir and are there any pro's or cons to Doug fir vs cedar

 

Doug fir is stronger, but cedar will have more resistance to decay.