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I am looking to extend my 4x4 corner posts on a deck to build a Pergola over top of the deck. I also plan on sleeving those 4x4's with a composite sleeve for the full length from the 4x4. So the extensions need to be flush, so plates and other things protuding cannot be sticking out of the sides.

So what is my best and strongest option to join these 4x4s?

Prelimimary Pergola design

This is my preliminary Pergola design. The deck where I'm adding the pergola is not huge. It is basically a 10x12 Pergola design.

Existing deck 1 Existing deck 2

Please note the 2 posts on the left side away from the large square deck section. That is the old deck section that I sleeved the old posts. As alignment wasn't perfect, the post in the center is actually a "floppy post" to make the steps more aesthetic on each side of the steps. But with the composite sleeves make them look like 6x6's and adds structure to the 4x4 posts.

22" 4-way angle supports?

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  • Yes, They are anchored into 12" diameter concrete footings 5' deep. I put the deck in just last year and had a friend with a skidloader auger that put the holes in for me and we poured them with reinforcement. Now my wife wants a Pergola over it. I don't relish the idea of digging those monstrosities out to redo every corner of the deck. That is why I'm looking at extending the existing posts.
    – Brendan Sanders
    Jul 14, 2017 at 14:12
  • Photos and added preliminary pergola design. I can easily place 6x6 angle braces up from the vertical posts and slide them between the horizontals. I would want to limit the angles on 3 sides since I don't want them to restrict the walking areas as 3 of the 4 legs are either near stairs off the deck or the narrow part of the deck that lead off the remainder of the narrow wrap-around deck to my house. Would 12" braces added each direction be enough? I could add 2' braces each direction on the one corner, which is where the predominant winds would come from. What do you think?
    – Brendan Sanders
    Jul 14, 2017 at 15:00
  • A 12" brace is tiny. Typically you should get close to 1/3 the post height. It looks like your pergola will be freestanding, which makes them even more critical.
    – isherwood
    Jul 14, 2017 at 15:07
  • Well the legs extend below the deck and are put in 12" diameter, 5' deep reinforced concrete footings.
    – Brendan Sanders
    Jul 14, 2017 at 15:09
  • I just drafted it in rough in my design. The long side of the brace would be 22" long and I could brace it in all 4 directions in each direction and lag bolt the brace to each of the 2 horizontals at each post. The areas where I don't have places that people would be smacking their heads could be longer. Then I'd have 4 locations that would have beefier cross-braces. Would that be more helpful to my design? See the link at the bottom of my original post.
    – Brendan Sanders
    Jul 14, 2017 at 16:22

2 Answers 2

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To accomplish the join I'd acquire some 1/4" by 3-1/2" by 24" steel plates and let them into opposing sides of the posts to act as gussets. I'd use countersunk flat-head sleeve bolts, piloted accurately and minimally, to sandwich the whole works.

Be sure to consider whether the existing posts are adequately anchored, as there will be substantial torque applied to them at their bases. It doesn't matter how well you attach the extensions to floppy posts.

That said, I think your plan seems flawed. A pergola creates some fairly unique structural concerns, namely the lack of diagonal bracing that a conventional roof would provide, as well as increased wind drag.

I'd be replacing your 4x4 posts with 6x6 posts that run all the way through. They'll do the job much better than spliced 4x4s, and they'll probably be more appropriate from an aesthetic standpoint, depending on your design. You don't want chicken legs under your nice new pergola.

If you elect not to do that, consider substantial diagonal bracing below the ceiling line connecting the posts to the primary horizontal members of your design in appropriate directions (where connection to a fixed structure doesn't provide support).

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  • That is why I plan on placing sleeves over them. They make them the same diameter as a 6x6 and the composite material does add some structure, not just a metal sleeve. I sleeved the posts on another older section of the deck and it looks quite nice. I could see the advantage to diagonal bracing each direction, and it would add to the look quite nicely. Thanks for the suggestion. I'll try and post some pictures of the existing deck. I will reset the rail between the legs which should add to the stability of the existing legs somewhat too.
    – Brendan Sanders
    Jul 14, 2017 at 14:21
  • Would I be better off cutting the footing posts off a the deck level and then attaching new 6" posts to the deck be a better option? I'd think that would reduce the stability over using the 4x4's that are anchored in the ground. If I would have just known that she wanted to then put a pergola on later, I would have upgraded them to 6x6 and placed full length needed at the time of building the deck last year. But what I've got now is what I've got. I just need solutions if it is at all possible to be done.
    – Brendan Sanders
    Jul 14, 2017 at 15:26
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What I have done is to put 1/2" copper tubing into the center of the existing 4X4 and extend it into the new 4X4 above. It requires drilling holes about 6" deep in the centers of each 4X4 and about one foot lengths of copper tubing ( or steel pipe). You now have normal looking 4X4 that will take any vertical load. They are not very resistant to side forces, to get that stability I rely on the overhead structure/roof- that is any lateral force is resisted by all 6 posts together. I would not hang a swing from the top but it will support all the hanging baskets you want.

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  • Any diagonals you can add would be great.
    – blacksmith37
    Oct 18, 2017 at 14:41

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