Cantilever deck allowable span question

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Old 07-12-19, 09:29 AM
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Cantilever deck allowable span question

Hi all, I'm working on replacing a second story deck at the moment. I've read the DCA 6 decking guide cover to cover and I've found a few parts unclear. I'm getting ready to pour footings this weekend and I'm looking for a sanity check first.

The section I'm confused about is Table 2 (joist spans and overhangs) and Figure 1A (joist span). The guide refers to L, Lj, and Lo, and I almost feel like L and Lj are backwards. With that said, I'll get to my question.

I see a max allowable span of 14' for a 2x10 joist, and a max allowable overhang of 3' 5". I'm confused as to whether 14' is the max length of the entire joist (including overhang), or if this is the max length for the main span (from ledger to beam for example).

The DCA 6 guide specifically says that Lj is the full length (including overhang) and that the Lj max is 14'. This seems to contradict what I've read elsewhere, which makes me think 14' is the max main span and you can extend it the length of the overhang past the beam. If that's not the case why cantilever in the first place.

FWIW my deck plan is 20'x16', with the beam 13' out. Also, there is next to no code in my area and no permit required so I can't really consult with my local inspector.

Thanks!
MSal
 
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Old 07-12-19, 05:30 PM
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I don't really know why you are confused... its pretty clear in the DCA6. But you do have a number of things confused. Let me walk you through it.

L is your span. As shown in Figure 1A, span is the distance from your ledger to your beam. You say it's 13', So L is 13'. L must be =/< Lj. The table says Lj is 14' 0" if you are using Southern Pine 2x10's 16" on center. Lj is the maximum span allowable. Your span is 13' (assuming you measured the distance from the inner edge of the ledger to the closest part of the beam, just like its shown on Figure 1A.) You are good so far. You aren't overspanned.

The table also says that your overhang is the lesser of 2 numbers. Either Lo, which the table says is a maximum of 3' 5" or L/4. Since 13/4 = 3.25 or 3' 3", you must use the lesser number as your maximum overhang. That means your overhang can be a maximum of 3' 3" which is L/4.

So as shown in Figure 1A, your total joist length is L + beam width (3"?) + L/4 (3' 3"). And this does not include your ledger (1 1/2") and rim joist thickness (1 1/2").

Add it all up and the MAXIMUM length your deck could be is 13' + 3" + 3' 3" + 1 1/2" + 1 1/2" = 16' 9" Plus any overhang on your deck boards. Since you say you want the deck to be 16' long... you have it made.

If you want your decking to be exactly 16' long and flush with your rim joist, you would make your cantilever 9" shorter than 3' 3". If you want your decking to hang 1" past the rim joist, you would make the cantilever 10" shorter than 3' 3".
 
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Old 07-14-19, 06:33 AM
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Thanks for the sanity check! Table 2 specifically says Allowable Span (Lj) in the header, and a max 14' for 2x10. IMO this is confusing and made me think L was the full joist length, especially since figure 1a indicates L<=Lj. Oh well, It sounds like I'm set.

Edit: It just clicked. Lj is your max while L is your actual. D'oh! Thanks again!
 
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Old 07-14-19, 06:53 AM
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Allowable span is not full joist length because it is only the maximum distance between 2 points... the ledger and the beam, not including the cantilever.

I think you've finally got it.
 
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