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I'm starting my backyard renovation project. I've put in a large concrete slab (18'x26'). Now I'm building a Pergola over the slab.

My pergola construction is as follows:

6 - 6x6x10 posts (Grade 1) 12 - 2x12x20 beams (Grade 1)

Posts are held to slab using Simpson Bases anchored into slab Beams are held to posts using notches and double through-bolted on each post

Rafters are 2x8x16 that overlap each other 24" and are through-bolted diagonally at the seams.

The posts,beams, and rafters are built level (no slope)

Above the rafters I have some 2x4x8 runners (sloped)

Above the 2x4x8 I plan to add 2x2 runners

This is my basic construction. I'm experiencing a little flex/sway in the structure but I'm not finished tying it all together yet. I'm using Simpson H2.5 Hurricane straps placed diagonally on every intersecting rafter/beam/runner.

Will adding all of the hurricane straps help tie it all together, or, should I consider some cable tensioning "X" braces on my structure? I was thinking of adding (2) "X" braces if it doesn't stabilize after I'm done adding all my Hurricane ties.

Anyways, any advice (good or bad) you can offer is highly appreciated. This is my first pergola project and I'm not an engineer/carpenter. I'm trying to do it as right as I can realistically do within my budget and skillset, so, be gentle. :-)

Notes:

The 2x12 Beams Span 13' Between the posts

The 2x8 Rafters Span 11' Between the posts

The 2x8 Rafters are spaced 18 inches on center

The 2x4 Runners are spaced 16 inches on center

The 2x2 Runners will be spaced 12 inches on center

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    Looks great, but why is there diagonal bracing in one direction and not the other? Which direction is unstable? I would not add any more metal hardware. That won't solve your problem and tarnishes the appearance.
    – isherwood
    Commented Jul 14, 2020 at 13:32
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    Also, are the diagonals really only fastened at the bottom with small screws? They're normally attached with large lags or carriage bolts.
    – isherwood
    Commented Jul 14, 2020 at 13:35
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    Which way is it swaying? Referencing your first picture, is it moving in and out of the page (in the same direction as the diagonal bracing), or side to side?
    – SteveSh
    Commented Jul 14, 2020 at 13:51
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    It's not as difficult as you might think to notch the braces. Run a circular saw set to depth from the sides, full depth from the ends, and finish from the ends with a hand or reciprocating saw. Full bracing, properly fastened, should solve your problem almost entirely.
    – isherwood
    Commented Jul 14, 2020 at 14:03
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    Nice work for a first big project! I agree that finishing up the knee bracing in both directions should remove the sway. To reinforce the knee brace joints, you might consider a flat plate with holes that will attach to the post and the bottom of the knee brace with lag bolts from each side or carriage bolts all the way through. The make 'em in fancy black finish so they look nice.
    – FreeMan
    Commented Jul 14, 2020 at 14:13

2 Answers 2

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Triangles are stable, squares are not.

Finish the knee braces.

You might be able to make them more simply by making them from narrower material that will fit between the beam members as they are (ie, they don't need to be the same width as the post, so they can be the same width as the cut-down section of the post the beam is wrapped around.)

Depending on height and why you have double posts on one side, you could also X brace on that side between the double posts.

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  • Thank you, the double posts on one side are either going to be a swing (picture a beam above those two posts, or, may be the bbq area under open air. I havent decided yet. All of the area to the left of the slab will get pavers and we'll clean up the palm trees to have some foliage as well.
    – Sunny Side Up
    Commented Jul 14, 2020 at 13:59
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With the addition of the knee braces, the structure is considerably more stable now. Thanks to everyone for your input/advice. She's coming along nicely. Although, I'll never do this again. This is way more work than I'd ever imagined! LMAO....

I made a table and a bar and had the slab tiled. I think it came out pretty good all things considered. The bar top is a 12' long butcher block I stained to match the barrels and poured 2 coats of epoxy resin over. I added a back wall and two tv's for entertaining as well.

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  • It actually is. I'm saying the additional knee braces stopped the swaying. Move along.
    – Sunny Side Up
    Commented Oct 8, 2020 at 12:42

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