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?

  • 5
    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
    Commented Jun 3, 2012 at 19:11

5 Answers 5


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.

  • Also make sure to paint "end cut solution" which is typically Copper Sulfate (III), all over any cut/exposed grain, assuming you're using pressure treated. Ignore this if you're using cedar.
    – William S.
    Commented Mar 20, 2015 at 0:38
  • I think you can spring for the extra to put galvanized everywhere that isn't a coast... and for a coast, you need very thick galvanizing, or stainless. Don't screw the next guy with a bunch of bright steel outdoors. Commented Apr 17, 2023 at 21:50

Here's an excellent pictorial breakdown and quite detailed explanations of most types of joinery: US DOT

According to Chicago's exceedingly stringent Deck Code (pages shown are labeled 34 and 35) you can do either a half lap or a butt joint. Both require hot dipped galvanized (min. 1/2" or 5/8", depending on splice type) through bolts, washers and that they be 6x6's in the first place.

Having to transfer this to a 4x4, I'd pay special attention to the minimum edge distance of 1.5" and probably do a lap splice for its staggered pattern, cheating the distance of 2.5" between the fasteners and maybe a 1/4" off the MED.

  • This code probably ignores hurricanes and earthquakes.

  • DO NOT OVERCUT. This would provide a shear plane much like a glass cutter does.

  • The splice should avoid the location of the hand rail and begin a minimum of 6" off the deck.

Half Lap Post Splice:

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Post Splice Using Plates:

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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:

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Put the two 4x4.s together and drill two 1/2 inch hole,s 8" from the top and bottom and install 1/2 inch threaded rod,s and use correct washer,s install nuts -last use metal mending plate,s with 2" screw,s front and back of post. roof support should require full length post.! . Robert Cooney Construction

  • This could be a good solution depending on possible torsion/shear forces in play. Have you seen this application in use? Commented Mar 20, 2015 at 14:57
  • Even hot dipped threaded rod can fail very quickly, especially with the ends cut off; use the appropriate length bolts. Those 2" screws should be 1/4" diameter stainless steel lags.
    – Mazura
    Commented Apr 17, 2015 at 5:34

I would get 4x4s the full height of the desired roof and bolt it to the outside edge of the current 4x4 the whole length using lag bolts as suggest above. Bolting a 4x4 on top of a 4x4 is never going to be that strong without a lot of cost for the mend plates etc.

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