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We built our first pergola (20'x 18' x 10' tall). We used cedar with 6 - 6x6 posts with 30-inch concrete footings attached with 1/2" J bolts to OWT/OZCO post bases. We have double 2x8 beams that span ~10ft with 2x6 rafters that span ~18ft and are connected with Simpson Hurricane Ties. Post to beam connections were made with 8-inch OWT timer bolts. I also put ~2ft braces up everywhere that I could. There were some issues with the patio not being level so I used some wood and stainless steel shims to ensure that the posts make good contact with the post bases and reduce any extra play.

On paper, I felt like everything seemed okay, and in a static situation or with light force the structure seems to be stable. The issue I noticed is that if I give it a hard push or if I try to rock it back and forth in the direction parallel with the rafters, the structure still wobbles a bit. It is pretty solid when you try to move it in the direction of the beams (this is the direction with 3 posts instead of 2) so I am not really worried about that.

We really like how it turned out but wanted to get other opinions on whether or not this would be considered safe. I don't want anyone to get hurt for the sake of my pride if we were to leave it alone.

Are there any suggestions on how to make it more sturdy? Would larger braces on the side with only 2 posts help? I also saw Simpson has some knee brace stabilizers and was thinking maybe those would work better than just the timber screws I used for the braces?

Before braces After adding braces Another angle

Edit: Adding photos of diagonal brace connections Corner Center Post

Video of wobbling when forcefully shaking one of the posts: https://drive.google.com/open?id=15B0GaEV4My2gBfh_8d0WrulkR2FXMe2_

  • Have you seen diy.stackexchange.com/questions/13636/… ? – RvdK May 20 at 9:16
  • "I don't want anyone to get hurt for the sake of my pride if we were to leave it alone." earns my vote! – FreeMan May 20 at 11:41
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    Can you provide close up shots of how the rafters are attached to the diagonals and to the beams? – FreeMan May 20 at 11:50
  • When you shake it parallel with the joists/rafters, do the joints at the knee brace open up? – Aloysius Defenestrate May 20 at 14:10
  • @RvdK yes I checked that one out and that's where I got the idea for the 45 degree braces. I followed as much as I could from that one but my structure is slightly different as it is freestanding versus being connected to the house via a ledger. – pergola-newb May 20 at 16:47
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I think you've got several issues:

  1. I don't think that the single screw holding each end of the bracing in place is sufficient.
    1. You might consider adding a 2nd screw at each end of each diagonal brace.
    2. I would put the 2nd screw at a different angle than the original (i.e. if the original lower screw is horizontal, add a 2nd screw at a 45° angle up).
    3. This may help provide some additional stiffness. You may want to try one pair to see if that helps before doing all of them. (Or wait for someone to give a more authoritative answer instead of a guess/suggestion.)
  2. Those hurricane clips look nice, but they provide minimal stiffness along the rafter direction.
    1. A rafter tie like this would probably work better. enter image description here
      Image courtesy of Lowes.com. No endorsement implied or expressed
    2. Yes, this one is ugly, but they make nice ones for visible work.
    3. Adding a 2nd hurricane clip between the rafter and other joist may help improve the stiffness.
  3. Unfortunately, I think the biggest issue is actually the most difficult to solve. I don't think that surface mounting the posts with those connectors is sufficient.
    1. I took a look at the web-site for the brand you mentioned and they seem reasonably beefy, but they seem to be targeted more at railing posts than pergolas.
    2. I mounted 2 short posts to my concrete steps with similar (though admittedly not as beefy) surface mounts and the posts wobbled horribly. I ended up removing them and half-lapping the post off the edge of the step and bolting it horizontally. Now it's rock solid.
    3. I'd think you would have been better served by burying 1/3 of the post in concrete rather than trying to mount them on top. Unfortunately, this is a major rebuild at this point - hopefully someone will have a simpler solution.

Finally, it does appear from the video that there is a fair bit of wobble. How hard do you have to work to produce that wobble? Is it obviously evident from just casually leaning against a post, or do you have to put both hands on it and give it the ol' college try?

If you have to work hard to get that wiggle going, it may be sufficient to just leave it as is. If it wobbles like that from a stiff breeze blowing, then you'll probably want to do some strengthening.

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  • Thanks for the thorough response! The wobble isn't immediately evident and you have to use both hands to get it to wobble like the video. Unfortunately, I agree with what you said about the post base being the big issue. If I could do it again I would bury the post (I was worried about water & rot) or get a monolithic post base instead of a 5-piece. Unless someone else has a better solution, I'll try what you said and maybe find rafter saddles that fit my beams or clip the rafter to the other side. I'll try to strengthen the brace connection and maybe get thicker lags for the post bases. – pergola-newb May 20 at 18:14
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    I would seriously consider "leave it alone" as a viable option, @pergola-newb. If you were supporting a 2nd floor deck with this structure, I'd be quite concerned, but the most you'll have up there is sunshine and maybe some vines. – FreeMan May 20 at 18:17
  • Heck, even if you were to decide to cover it with fabric for shade, and climbed up there and walked across the rafters dragging the fabric behind you, that would be a 1-time, careful endeavor and I wouldn't worry about the wobble other than it possibly throwing you off balance. Moving slowly probably wouldn't cause enough wobble to worry about. – FreeMan May 20 at 18:22
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    I generally agree, though many a pergola (including some I've built) use posts that aren't buried. I think improving the connections of the braces will take 90% of the movement out, here. Consider threaded rod running completely through. Then you can snug it up as the wood shrinks and not worry about wood threads working loose. – isherwood May 20 at 19:03

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