# Adding 16.5" to length of deck, sistering 2x6



## hammerlane (Oct 6, 2011)

How far back are the sistered cantlivered joist going into the existing joists?


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## NickWa (Nov 17, 2011)

They are 8 foot boards right now, there are 8 joists with additions being added to them. I was undecided if I would cut them shorter or not, depending if the full 8' was needed or not. As I said they are just temporarily screwed in for now.

Trying to figure out what type of hardware I should use and how much of said hardware. Carriage bolts, lag bolts, some sort of brackets, etc.?


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## hammerlane (Oct 6, 2011)

NickWa said:


> They are 8 foot boards right now, there are 8 joists with additions being added to them. I was undecided if I would cut them shorter or not, depending if the full 8' was needed or not. As I said they are just temporarily screwed in for now.
> 
> Trying to figure out what type of hardware I should use and how much of said hardware. Carriage bolts, lag bolts, some sort of brackets, etc.?


So you have 8 foot 2x6's that you want to cantilever out 26 inches. That leaves you with 5 feet 10 inches of the sisterd 2x6 going back into the existing joist. You should verify that is enough lumber for that amount of overhang. Ive read some posts where it says 4 foot back for every 1 foot overhang. Maybe a 2x6x10 would be a safer choice.


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## NickWa (Nov 17, 2011)

That is true, although I am not sure if it makes a difference that they will be attached to existing joists which are cantilevered out 9.5" right now? These existing joists are full 2x6x16 boards. Thus the total unsupported cantilevered addition will be 16.5" with 9.5" of sistered joist cantilever.

If I were to use 2x10, would I just notch out for the beam? I can't have the sistered addition be higher than my existing joist!


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## hammerlane (Oct 6, 2011)

SO right now you have a 9.5" cantilever off of a 2x6x16 joist. And to this you want to add 16.5" more.


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## NickWa (Nov 17, 2011)

I will try and clarify :laughing:

Right now, 16' 2x6 are being used for my joists of which are cantilevered 9.5" past my final beam. Thus, I have 9.5" of unsupported board out of a total 16' 2x6.

I want to extend the deck by 3 deck boards (16.5") which would create a total cantilever of 26" of which 16.5" would be unsupported totally and 9.5" would be supported by the existing 2x6 joist cantilever and however long the rest of my board ends up, will be supported by the 2x6 joist over the joist span.


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## hammerlane (Oct 6, 2011)

OK I get the picture now. Trying to look up how far back the sistered joists need to be on the existing joist.


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## Daniel Holzman (Mar 10, 2009)

You may want to discuss this with your building inspector, assuming you have a building inspector (you have not told us where you live). Where I am, the minimum size for a joist is 2x8, so clearly you either work off a different code, or possibly your deck predates code, or maybe there is no code where you are. The L/4 maximum cantilever rule only applies to allowable size joists, so in my town (which uses IRC 2006), even if your deck predated existing code, you would not be allowed to extend the overhang since the building inspector would then require you to bring the deck up to minimum code, which among other things requires minimum 2x8 joists.


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## NickWa (Nov 17, 2011)

So if 2x6 were allowed, one could do 16/4 = 4 foot cantilever? I am only going 26". My deck is rock solid and has never bounced or flexed whatsoever, even with multi-family bbq's and such. 

I will tell you right now, I am not rebuilding deck with 2x8 joists just to add 16.5" to the length - that is just silly. :bangin:

If a person had a deck built with 2x8 joists and was extending it in the same manner as I would like to, how would they secure the sistered joists to the original/beam?


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## hammerlane (Oct 6, 2011)

NickWa said:


> If a person had a deck built with 2x8 joists and was extending it in the same manner as I would like to, how would they secure the sistered joists to the original/beam?


Do you mean how would they secure the sistered joists to the original joists?

Also how high is your existing deck from the ground?


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## NickWa (Nov 17, 2011)

Yes how would secure a sistered joists to original joists which are up to current code standards. Top of the deck surface above grade is 22".


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## hammerlane (Oct 6, 2011)

If you decide to go along with your plan of sistering 2x6's to the original joists, go as long as you can for the back-span part of the sisters. 

Few tips:
-use construction adhesive along with lag bolts
-use larger diameter washers to spread the force of the bolts out over a larger area.


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## NickWa (Nov 17, 2011)

Was indeed planning to use an adhesive between the two boards.

I am thinking to just leave the boards at the full 8', adhesive them together and bolt every 16" with 3/8x4" hex head bolts and multi-sized washers.

To me this seems like it would be adequate but I came here to look for advice so I will take any and all into consideration.


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## hammerlane (Oct 6, 2011)

something else found at: http://decks.com/


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## Daniel Holzman (Mar 10, 2009)

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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## hammerlane (Oct 6, 2011)

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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## NickWa (Nov 17, 2011)

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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## dpach (May 12, 2009)

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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## Gary in WA (Mar 11, 2009)

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


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## hammerlane (Oct 6, 2011)

GBR in WA said:


> BTW: hammerlane, those diagrams are copyrighted


Not a problem if you have permission which I do.


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## dpach (May 12, 2009)

GBR in WA said:


> 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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## NickWa (Nov 17, 2011)

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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## hammerlane (Oct 6, 2011)

dpach said:


> 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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## NickWa (Nov 17, 2011)

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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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## woodworkbykirk (Sep 25, 2011)

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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## dpach (May 12, 2009)

hammerlane said:


> 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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## hammerlane (Oct 6, 2011)

like this....say the red v's are the glue?


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## Daniel Holzman (Mar 10, 2009)

"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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## NickWa (Nov 17, 2011)

Sure :thumbsup:

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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## dpach (May 12, 2009)

hammerlane said:


> like this....say the red v's are the glue?


 
Yeah, that's the jist of it. You can put them closer together; just a 2-3 inch gap between bottom tips is plenty for water to run out.


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## hammerlane (Oct 6, 2011)

Damn the torpedoes.....Stop talking and start sistering. Go as long as you can for the back-span part of the sisters, use the tip on the adhesive, mind the staggered bolt or nail pattern and measure twice cut once.

Post a photo or two when completed.


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## NickWa (Nov 17, 2011)

Yes I apologize for being short but I didn't initially come here to ask if I should rebuild my deck with 2x8 or if I can cantilever an additional 16.5" but rather the correct way to sister joists when a cantilever is involved. I know adhering to the code is important and I do want to do that.

So from now on, I will speak theoretically, as if I was going to do this but not actually doing it (thereby not breaking any codes).

On a serious note, if my deck did (theoretically) have 2x6 joists, will I be forced to tear it apart if I had a building inspector come over? If it is old construction, would it be "grandfathered" past the current building standards today? Furthermore, could monetary fines be imposed if it was discovered that it is using 2x6 joists?


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## goosebarry (Mar 28, 2012)

NickWa said:


> On a serious note, if my deck did (theoretically) have 2x6 joists, will I be forced to tear it apart if I had a building inspector come over? If it is old construction, would it be "grandfathered" past the current building standards today? Furthermore, could monetary fines be imposed if it was discovered that it is using 2x6 joists?


Assuming,

 it was built to code at the time
a building permit was issued and the work inspected
you are not making any modifications
Most state level enabling laws limit inspections to "new work". There are exceptions to that rule depending on your location and the type of building (single family, multi-family, mixed use, etc). If you let us know where you live so one may be more familiar could help.

In some cases parts of the building codes can be adopted in other parts of the law. For example, MGL requires all decks higher than ____ inches to be surrounded by an approved railing. If you had previously (to the law taking effect) built a deck without a railing, you would be required to add a railing or tear it down. 

My town past a general H&S by-law requiring *all* in-ground and above ground pools must be surrounded by an approved fence and gate. 

Some actions are triggered by an event, such as, when a house is sold or permitted electrical work performed, hardwired fire detecters must be installed and inspected.

The "codes" define current best safety practices. It is up to your State, Province or local authority (LA) to adopt, interpret and enforce them.


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## NickWa (Nov 17, 2011)

I'm in Ontario Canada and there would have been no need for a permit when the deck was built (although one may have been pulled, I don't know), because the deck is under 24" from grade and not attached to the house.


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## woodworkbykirk (Sep 25, 2011)

so its a floating deck.... even worse.. i can see this going bad.. .


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## NickWa (Nov 17, 2011)

Why? Do you think my joists are floating on the beams? Or the beams are not attached to the posts? Or the posts are not attached to the footings. 

Joists are 16' long secured across 3 beams which are secured to 9 posts which are secured to 9 footings. I don't see why it would go bad if it is floating as compared to secured with ledger? It isn't going to "tip" over with excess weight on one side...


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## no1hustler (Aug 11, 2010)

delete


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## NickWa (Nov 17, 2011)

Yes I know, I was asking why the poster above me thought that made a difference as opposed to being attached to the house. The deck is essentially 1 piece, everything is attached, screwed, nailed, bolted together and whether it is floating or attached to the house, it is solid.


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## hammerlane (Oct 6, 2011)

Nick...could you post a photo of the deck as it exists now. Show an overall view and maybe a close up of the cantilevered section?


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## NickWa (Nov 17, 2011)

Sure, will have to wait until I get back to the house but can get some after that.


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## hammerlane (Oct 6, 2011)

Caught an episode of Holmes on Homes the other night called "A Pane in the Glass". Topic was about an improperly installed bay window that had no support under it. The crew removed the window and sistered new joists to the existing joists to make a cantilever for the window.

Couldnt find the episode on line anywhere.


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## EdQuinn (May 1, 2012)

Couple of issues I see are first the 2x6 joists are a little light for the span is the 16 foot with the 9.5" overhang only supported by one beam and the ledgerboard on the house? If so here is what I would recommend first double check the ledger make sure it is bolted or lagged to the house Look in the basement to make sure they are installed properly. Next I would use 2x8x8 and notch them around the outer beam glue and carriage bolt every 16" with an additional backer block on the opposite side. I do have one question and concern that may change this recommendation why do you need the additional 16.5" ? if you are installing a bench that will ad weight to that edge I may rethink the issues and consider adding an additional beam at the outer edge and eliminate the canter-lever all together at that point I would go beck to 2x6x8 and glue and bolt so the deck does not have any walking issues with the added piece. Please keep in mind that a on a deck when people gather they have a tendency to gather at the railings this adds a concentration of weight in this area greater then on the rest of the deck which is why I would really want to check the other end of the Joists to see how well it is anchored.


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