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I have a deck I am going to put a roof over. It has 4x4s for vertical supports that I need to extend upward to attach my trusses to. What is the best way to connect two vertical 4x4s?

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I'm guessing the current 4x4s are the support posts for a typical railing, thus about 3 feet tall? And you're looking to extend them to 8 feet? How far below the surface of the deck to they extend? How secure are they? The "right" way to do this would likely be replacing the existing supports with longer ones, thus avoiding a massive shear weakness. But more details may help open more options. –  Scivitri Jun 3 '12 at 19:11
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Current accepted practice that meets code in NC and is hurricane wind rated - cut a rabbet 24 inches in each piece. Sandwich the rabbet joint between 2 - 18 inch steel plates (galv for coastline) thru bolted with 4 equally spaced bolts. Old method - 12 inch rabbet with 2 - 24 inch steel plates. 2 bolts thru the rabbet and 1 bolt above and below the joint. I think the old method is stronger esp. for sheer but code is code.

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While it may not be the "right" thing to do, you can definitely connect them without creating too much of a weak point.

Replacing them completely will look more streamlined and will be stronger. However, if it would be indefeasible to do so (it'd practically be like re-building the deck) you can minimize the shear weakness by doweling the connection. You'll probably want the dowel to go about 4 inches deep in each side, and you'll want a good sized dowel. Make sure to use a template when drilling so that the 4x4's line up well. Chances are there will still be a lip where the two meet though, and you might want put some trim on that. You could consider a metal dowel too, just check out the galvanized rod in your local hardware store's metal department.

Alternatively, you could make the supports sorta architectural looking by cutting 2 4x4's so that they meet in a V into your existing post:

enter image description here

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