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I have a 20'x20' pergola on a concrete patio. Construction is 8"x8" posts sandwiched with 2"x8" rafters.

I notice some swaying during some thunderstorms with heavy winds and I can push on a post with some effort, that moves the whole pergola.

What size knee bracing should I install (width and length)?

enter image description here

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    8"x8" will fit/bolt between the rafters and match the posts.
    – Alaska Man
    Commented Jul 7, 2020 at 16:52
  • Thanks Alaska Man. How long should they be from the longest end of a 45 degree cut?
    – JohnnyT
    Commented Jul 7, 2020 at 17:00
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    I am not an engineer. i know longer is better but You do not want to completely block off the ability to walk through and you must consider (not) hitting your head. I do not know the height of you pergola. Take 2x4 and hold it up to get an idea. i would see what it looks/feels like if starts about 5 or 6 feet away from the top if the post. Too short will not provide enough resistance to racking but i do not know what too short is, Others will.
    – Alaska Man
    Commented Jul 7, 2020 at 17:14
  • Thanks again. I should’ve mentioned the posts are 10’ long. 6” rafters means the higher rafters are 9.5’ from the ground, while the lower rafters are 9’ from the ground.
    – JohnnyT
    Commented Jul 7, 2020 at 18:22

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That's a beautiful pergola, and to preserve its style I would use the same timbers as you used for your posts. I'd make the vertical and horizontal legs of the triangle 1/3 of the post height, unless that causes serious headroom interference, then reduce a bit to no less than 1/4 of post height. Much less than that and you won't see much benefit.

I'd make all the knee braces the same length, so that they interface with the posts at different heights due to the rafter height.

The connection between the rafters is fairly straight-forward. The connection at the post is less so. Large lag screws, countersunk and level, or carriage bolts all the way through would be good. That connection will have the most tendency to slip, reducing the effectiveness of your effort.

Here's a similar example courtesy of StrongTie. In this case the knee braces connect to the post at the same height, but I pictured yours being vertically offset (to match the height of the rafters). I would also deviate from this photo by running the bolts in horizontally. You'll need shorter bolts and I think you'll end up with a better connection.

enter image description here

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  • Thank you isherwood. Would it be a bad idea to go with a 4x4 sandwiched with 1’ long 2x6’s in the rafters? Cutting and lifting 8x8 pieces seems awfully hard to do. I would bolt through the rafters and put in lag screws horizontally, as you suggested.
    – JohnnyT
    Commented Jul 7, 2020 at 20:51
  • That would work, but I'd tack one up first to be sure you're happy with the appearance. It might be worth the extra effort. If you look over some examples on the web you won't see many with smaller braces.
    – isherwood
    Commented Jul 7, 2020 at 21:07

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