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I live in Florida and currently have a deck support by ten 9" round, 10' high pilings. The outer ledge boards are double 2x10x10 bolted to the piling which are notched to accommodate the double 2x10s.

The current deck is only 5.5' wide. The pilings are spaced 9 feet apart with one piling on each side of the deck every 9 feet to support it. I only want to extend the deck 30".

Instead of tearing the whole deck off and redoing the supports to cantilever it which can only be 24", I thought I could use 2x10s for a knee brace on each side of the piling and run 14" galvanized bolts through it. Then I would run a 2x10 parallel to the house on top of the knee brace on the outside and use lag screws to attach it to the knee brace. Then use 2x8x30" long with galvanized joist hangers to attach to the existing double 2x10 which is resting in a slotted space on the piling and bolted to it And the 2x10 bolted on top the knee brace. 

____________________________
        | <-- hanger      |/|
    old | new             |/|
  joist | joist           |/|
________|_________________|/|
|   |/|/|             /     /          
|   |/|/|            /     /                
|   |/|/| <-- beam  /     / <-- knee brace      
|   |/|/|          /     /                 
|   |_|_|         /     /
|       |
|       | <-- piling
|       |
|       |

Do you think this would work or do you have any other ideas?

To answer some of your questions. The pilings are pressure treated wood telephone poles. They are 9" round. I have attached a side view of the knee brace I'm considering using,

enter image description here

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    The brace would negate the cantilever in my opinion so I would think it would be acceptable. With the deck that high the rail around the deck would be important this is where I have had inspectors verify that the rail could handle side loading of 200 lbs and that a 4" ball could not pass through. – Ed Beal Feb 27 '18 at 18:57
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That seems reasonable, but your local inspection office is the ultimate authority. You should sketch it up and meet with an inspector.

I'd be a bit concerned about drilling completely through the pilings, though (which I assume are concrete). That's a lot of rough treatment for a relatively thin structure. Instead, I'd use expanding anchors and lag screws.

You'd also have to connect the top of the knee brace to either the pilings or the existing joists to keep it from rotating outward.

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It all depends on what reason the code restricts cantilevers to 24".

  • If it's the bending and shear in the joist, then yes: a knee brace will allow to go beyond 24".

  • If it's the bending in the piling, then no: the bending moment in the piling will be the same

  • If it's some safety thing to do with overhang if you fall off or drop something, then: no: propping the cantilever is unlikely to solve this

Even given the above it all comes down to the Building Inspector: you would need him to know why the 24" limit exists, and be able to convince him that your knee bracing fixes that issue for a cantilever over 24".

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  • Section R507.5 of the IRC varies the overhang distance just based on the joist characteristics; so that suggests that the effects on the pilings are not a (primary) concern. That said, an added effect to consider in your answer is the weight of the overhung portion and any seesaw (or more mild bouncy / vibration effect). (codes.iccsafe.org/content/IRCSigChanges2015/…) – DaveInCaz May 30 at 10:10
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In certain cases, I've had to add on to existing floor joists, because the homeowner decided they wanted a bay window. A general rule is to add two times the length of the cantilevered section to a joist. Check with your local building inspector on that. That means that if you extend 30" past the front of the deck, you would extend 60" back underneath the deck and against the existing joists. That would, of course, eliminate the knee brace.

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  • The OP states that 24" is the limit, presumably due to local code. – isherwood Feb 27 '18 at 21:00
  • @ Isherwood. He also said: "Then use 2x8x30"long with galvanized joist hangars to attach to the existing double 2x10 which is resting in a slotted space on the piling and bolted to it And the 2x10 bolted on top the knee brace." The point is trivial. The answer I gave is not how much you can cantilever; it's about the building technique. The "24" he mentions could be based on the method of construction he want's to use. – user81998 Feb 27 '18 at 21:31
  • This doesn't address the bending or shear capacity of the joist, or the bending capacity of the piling. – AndyT Mar 7 '18 at 15:34

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