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Old 04-23-2012, 11:37 AM   #16
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Adding 16.5" to length of deck, sistering 2x6


Maybe I missed something earlier. Are you saying your existing 2x6 joists span 16 feet right now? By IRC, the maximum allowable joist span for a 2x8 (the smallest allowable joist per code) is 10'6" if you use Southern Pine and the joists are spaced 16 inches on center. So you are way outside standard code.

The problem here is that you are outside code, yet you want to use the code allowable cantilever of L/4, and you are looking for a method to sister an undersized joist to accomplish the outside of code cantilever.

I get it that you are not going to rebuild your deck just because the joists are too small. Presumably the deck has been performing adequately so far as you are concerned. And I assume you are not going to talk to the building inspector, who might have some problems with the undersized framing on your deck. So that leaves you in a bit of a conundrum.


The best I can tell you is that when sistering joists under a floor, common practice is to use 10d or 16d nails spaced approximately 6 inches apart, in a staggered pattern. Common practice when sistering floor joists is to use a full length sistered joist of the same dimensions as the existing joist. It is also possible to use structural screws to connect the joists (rare), some people use structural adhesive in addition to nails (unnecessary but harmless), and some people use through bolts, typically 1/2 inch diameter. I recommend nails, they are simple to install, they work well, and the code official understands them.

In your case, you may want to follow a similar strategy. The fact that your joists are undersized makes the entire project dubious at best, and certainly not code compliant, but I assume that is not an issue in your case. Certainly it would be possible for an engineer to compute the maximum moment on your joists to determine what the safety factor is against failure, but unfortunately the problem is surprisingly difficult given the variable moment of inertia along the joist, the cantilever, and the method of attachment of the two joists. Given the difficulty of the calculations, it would be expensive, so I assume not gonna happen. The best advice would come from someone who had undertaken a similar project, perhaps they can relate what they did and how it worked.

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Old 04-23-2012, 11:39 AM   #17
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Adding 16.5" to length of deck, sistering 2x6


be sure not to line all the bolt patterns up in a straight line....stagger them..... and maybe rethink the 16 inch spacing
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Last edited by hammerlane; 04-23-2012 at 11:45 AM.
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Old 04-23-2012, 11:48 AM   #18
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Adding 16.5" to length of deck, sistering 2x6


First, the method of attachment will definitely be staggered.

Secondly, my deck is built with 3 beams and 16" joist spacing. The joist span works out to just under 7'. Yes SYP wood species.

So costs aside, will it be better to go with 16d nails staggered at 6" or 3/8" bolts staggered at 16"? Nails would be easier so if they are sufficient, I will go that route.
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Old 04-23-2012, 05:03 PM   #19
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Adding 16.5" to length of deck, sistering 2x6


Please check with your local building permit office and inspector. Where I live, the max cantilever is 24", no matter what size of joist you use (2x8, 2x10, etc).

Not saying you're not ok, but every region has different codes and it is very important to stay within codes as they are there for your safety, and your family/friends who are also on the deck. Lawsuits are a no-win for either party.

Your permit office/inspector will also give you the required sistering tolerances, where to stagger the fasteners, what type of fasteners are required (galvanized or stainless steel depending on whether the joists are ACQ pressure treated or ACC -- the older type and below-grade quality treatement), etc and definitely use construction adhesive between the sistering boards.
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Old 04-23-2012, 11:37 PM   #20
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Adding 16.5" to length of deck, sistering 2x6


Construction adhesive between them will effectively trap water, leading to rot. I'd check with your local AHJ, 1/2" bolts anywhere on a deck are minimum; http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

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Old 04-24-2012, 06:47 AM   #21
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Adding 16.5" to length of deck, sistering 2x6


Quote:
Originally Posted by GBR in WA View Post
BTW: hammerlane, those diagrams are copyrighted
Not a problem if you have permission which I do.
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Old 04-24-2012, 09:53 AM   #22
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Adding 16.5" to length of deck, sistering 2x6


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Originally Posted by GBR in WA View Post
Construction adhesive between them will effectively trap water, leading to rot. I'd check with your local AHJ, 1/2" bolts anywhere on a deck are minimum; http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

Gary
BTW: hammerlane, those diagrams are copyrighted; #4: http://www.decks.com/terms

If the normal continuous "S" shape is used with the adhesive, yes, it can trap water. Local contractors showed me years ago to use it in straight 45 degree lines leaving a 2" gap at the bottom of the "V" to allow for water drainage. I had sistered deck joists done this way the year I bought my house and when I tore it down a couple years ago to build a new deck, I cut these joints up so they fit in the truck to haul away and the sistered joists had no rot in the middle after 14 years.
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Old 04-24-2012, 09:56 AM   #23
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Adding 16.5" to length of deck, sistering 2x6


Wow, great advice. I would have never thought to do that with the adhesive. Water should run down the top of the adhesive and out the 2" gap. Thanks for that!

Going to use some heavy duty adhesive (suitable for exterior and pressure treat) and 1/2 bolts to secure the sistered extensions.
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Old 04-24-2012, 09:59 AM   #24
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Adding 16.5" to length of deck, sistering 2x6


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Local contractors showed me years ago to use it in straight 45 degree lines leaving a 2" gap at the bottom of the "V" to allow for water drainage. .
Can't picture what you mean.
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Old 04-24-2012, 10:01 AM   #25
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Adding 16.5" to length of deck, sistering 2x6


\
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If all the lines of adhesive are at 45 degrees with a space at the bottom for water to "run out" any water that gets in between the sistered joists should find its way "down" the line of adhesive and out of the bottom where the 2" gap is left adhesive free.
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Old 04-24-2012, 10:34 AM   #26
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Adding 16.5" to length of deck, sistering 2x6


before you do anything check with your local building officials. locally the codes state that the maximum cantilever for deck joists is no more than the width of the joist stock.. meaning a 2x8 joist cant be cantilevered more than 7 -1/4" past the beam

and for a regular floor system its a 6 :1 ratio.. for every one foot of cantilever the joist must be 6' back the opposite way and thats if a engineer puts a stamp on it
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Old 04-24-2012, 10:55 AM   #27
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Adding 16.5" to length of deck, sistering 2x6


Quote:
Originally Posted by hammerlane View Post
Can't picture what you mean.
This is how mine were done years ago:

/\ /\ /\ /\

Upside down V's with a 2" gap between bottoms for drainage. On the bottom of the sistered joists, I could see the trail where water had ran out, and there was no rot between the boards when I cut them up to haul them away.

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Old 04-24-2012, 11:08 AM   #28
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Adding 16.5" to length of deck, sistering 2x6


like this....say the red v's are the glue?
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Old 04-24-2012, 11:08 AM   #29
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Adding 16.5" to length of deck, sistering 2x6


"The total cantilever will only be 26" and I have read that maximum cantilever for 2x6 joists is 30" so I should be ok in that regard."

So I understand that the span between beams is 7 feet (84 inches). The L/4 allowable ratio referenced in the IRC would allow you a maximum of 84/4 = 21 inches, but you want to go 26 inches. This fails the IRC test.

However, you have already indicated that you are using 2x6 joists, which are also sub minimum. So I believe it is fair to say that you are not too concerned about code, unless you operate under a local code which is more permissive than IRC, but by now on this long thread you probably would have mentioned that.

Since you are well outside code, the interesting question is how your deck will perform, which of course we can all speculate on indefinitely. Unless someone has actually built a deck like yours, or wants to do the very difficult math problem of computing load, deflection, and moment on a cantilevered deck with variable moment of inertia, we are just guessing.

I will note that code is sometimes conservative, but generally based on some sort of reason. There may actually be a good reason why the cantilever is limited to L/4, and this reason may in fact be based either in experience, or based on mathematical analysis of cantilevered decks. I would be interested in a report back from you as to how the deck works when you are done.
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Old 04-24-2012, 11:23 AM   #30
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Adding 16.5" to length of deck, sistering 2x6


Sure

At this time, I can report that a 14' walkway at our cottage which extends from land to just out over the water to the dock, has a 38" cantilever with no beam underneath but instead 4x4 legs which sit in the water. It has been there for >15 years and aside from replacing the legs once, has had no problems at all. We sit here, kids run over it, dogs jump onto it, heck, I run on it once and a while.

If I were not sistering joists, I would have no problem with a 26" cantilever using 2x6 joists, however, I want to be sure that the cantilever will be sturdy as I have never done any sort of thing before.

If at some point, an inspector throws the book at me, I will address it then but if what you say is true and 2x6 joists are against code anyways, I am not losing anything by adding this 16.5" of 2x6 cantilever on a less than 2' high residential deck. I don't think the deck is anymore than 5 years old so perhaps it was against code when it was built then too.

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