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Building pergola with two rows of 3 posts. I'd have 2x10 beams mounted on the posts supporting the 2x8 rafters, but I'm curious how far apart I can push my rows of beams. Ideally I would love to have them 13' apart with the rafters cantilevered 18" on either end but I'm worried I will have sag.

I should note that I am planning on having 2x2 slats on top of rafters spaced every 12". Also, will be using PT lumber. Thanks

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    Is there going to be a roof/covering that may hold snow ( or weight of any kind ) – Alaska Man Jul 1 '19 at 20:30
  • Got a picture/drawing to make sure we're all talking (thinking) about the same thing? – Greg Nickoloff Jul 2 '19 at 16:15
  • @GregNickoloff I had something similar to this in mind. Hoping to have the two rows as far apart as I can while avoiding sagging. No solid roof as mentioned in other comment. – Kelly Jul 2 '19 at 23:14
  • @AlaskaMan No, nothing solid. Just the slats for shade on top of 2x8s. Very similar to image linked below. – Kelly Jul 2 '19 at 23:15
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I would think you'd be safe at 13'. Although it's not an exact comparison, 13' seems to be "in the ballpark" considering there won't be much load on them.

Here's what I'm basing my opinion on:

Max. Live Load 20 lbs/ft2 (956 N/m2)

Max. Live Load 20 lbs/ft2 (956 N/m2) https://www.engineeringtoolbox.com/horizontal-roof-rafter-span-d_1484.html

And also using this calculator to try various wood species and spans, etc:

https://www.awc.org/codes-standards/calculators-software/spancalc

I think how the "rafters" are attached might have some significance for you too.

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