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Max span for a pressure treated 2x12 on a pergola

Posted by bakes5 (My Page) on
Thu, May 3, 07 at 8:42


I am building a simple pergola on my back patio. I have four 6x6 posts in the ground (33" deep) that are set with concrete. It is 12' x 18.5' rectangle. I will have fourteen 2x8 slats on the top (spaced about every 18"). for the 18.5' span I have two 2x12's bolted to the 6x6's (Kind of a 2x12 6x6 2x12 sandwich) by four 1/2 inch galvanized carriage bolts.

Ideally, I really only want 4 posts supporting this thing. Can I do this project with the 18.5' span on the 2x12's or do I need to put a post in the middle and make it a more managable 9' span.



Follow-Up Postings:

RE: Max span for a pressure treated 2x12 on a pergola

I went 14' between posts with this "sandwich". In 7 years I didn't have any sag to it. It was holding up an oversized roof too.

I'd build it without the middle post. If you ever noticed sag in the future you could alway jack it back straight and add the middle post at that time.

This is not a professional opinion by any means. :)

RE: Max span for a pressure treated 2x12 on a pergola

You should be fine with using 2x12's spanning that distance. I was doing some research on designing a deck and I came across a handy guide at:
Deck Construction Guide.pdf
It lists maximum spans and spacing for various sized lumber.
It says a 2x12 can span up to 18'10" if used as a joist and I think your purpose is as good as if using one joist.

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