I'm planning a cedar pergola over my deck. Posts will be 6"X6" and will be notched to accept a "sandwich" beam of two 2"X?"s. Across the beams, the rafters will be 2"X6"s on ~8" centers, which should be sufficient given their 4~6' spans. If I need a beam to span 13~14', will a pair of 2"X8"s sandwiched around the posts be sufficient? I assume that 2"X6"s would be too small. I live in Colorado so wind and snow loadings are in effect. The pergola may someday support vines. I've found some online span charts, but they don't really apply to pergolas, which carry so much less load than floor/ceiling framing. Any help on the beam size is appreciated.

because the only load they carry is the dead load of their own weight or a slight dab of snow, the 2x8s will probably handle it, but proportioqantely they might look small. Since a pergola is decorative as much as it is functional, I would use 2x10s

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Off hand I'd agree with the 2x10 but 14'... plus it's cedar, that's got to be close, plus you need to apply the math on the elasticity when wet due to the exterior nature of the project. If you added a post no problem... 7' w/2-2x10 ... 14'... I don't know... even if the rafters are only 6' I wouldn't want to call it engineered to code.

But also I'd add that here in this county in CA they treat a pergola just like a roof so we still have to apply a 40# /sq ft live load I believe it's due to the fact someone is going to get up there, be it a painter, or the next owner stringing xmas lights or even someone might apply roofing, you now that corragated crappolla... so I'd check with you local buidling dept if you have any doubts.

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