I am looking to extend my 4×4 corner posts on a deck to build a Pergola over top of the deck. I also plan on sleeving those 4×4's with a composite sleeve for the full length from the 4×4. 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?
This is my preliminary Pergola design. The deck where I'm adding the pergola is not huge. It is basically a 10×12 Pergola design.
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 6×6's and adds structure to the 4×4 posts.
Best Answer
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).