Maximum span for 2x8x16' beam for pergola

Old 05-01-12, 11:48 PM
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Maximum span for 2x8x16' beam for pergola

I am planning for a pergola with overall dimensions of 16' wide by 12' deep. The material will be pressure treated pine. I want to limit the support posts to 4 and need to know the maximum span between the posts to avoid sagging. I am planning on using 6" x 6" x 12' posts set in concrete. I can inset the posts about 1' from the corners to lessen the span to give me about 13' on the front and back beams which will be 2x8x16'. These beams will be supporting 2x6x12' rafters on 16" centers. I can double the 2x8x16' beams and rest them on top of the posts. Will this be prone to sagging? If so, would a 3rd 2x8x16' sandwiched and bolted or 45 degree corner bracing help? If not, would you have any other suggestions other than adding another support post? Thank you for your time and consideration.
Old 05-02-12, 03:42 AM
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Welcome to the forums! Here's a link to a calculator you may find helpful in determining your spans: Maximum Span Calculator for Joists & Rafters You'll need to input your parameters, but it will help.
Burying post in the ground will shorten the life of the pergola. You should, instead, dig 12" footings to the frost line (in your case probably a foot or so) and pour footings in sonotubes. Then apply Simpson Strongtie post bases, either prior to the concrete setting, or after it sets, depending on which base you choose. Then your posts will rest off moisture laden ground and will be fastened to the post bases.

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