I am building a pergola and I have 6x6 pressure treat upright posts. I am wanting to Span a 20 foot distance. What type of of beam would I need to do this. If necessary a can place an upright midway (at 10 ft) but I would prefer not to. The only load on these beams would be 2x6 that lay accross the top of course. These would be about 16ft long. This is my first post so I am hoping this works!
There are no span charts or requirements for pergolas since they don't have conventional roofing materials on top of them. Sagging is the real issue. I'm thinking something in the range of a 6x12 will be necessary to look good with the posts and still make the 20' span without too much deflection over time. If there's a lot of dead weight from other materials resting on the beam, I think a center support is in order.