# Sistering Joists



## shadetree (Aug 31, 2011)

What is the minimum overlap needed between new and old joists when sistering?


I'm in the processes of repairing tons of rot and there are two 2”x12”x12' joists that the last few inches have rotted off the ends and they are now only resting on about an inch of joist material. I'd like to repair these so that they are bearing on the full top plate, but would rather not put in entire new joists if at all possible. Does anyone know if it's acceptable to use say a 3 foot long repair if the first connector is within 1 foot of the end of the old joist?


----------



## lewisthepilgrim (Dec 8, 2011)

Its not a good idea to join a joist in mid span. This puts all the tension onto your hardware, instead of the wood itsself. Unless you are using some NICE hardware, it will NOT be rated for it. You need to join boards over a support colum.

Whats the situation? Deck ?


----------



## shadetree (Aug 31, 2011)

Nope, 2nd floor joists. Although they are on the interior side of a deck. Previous owner didn't install flashing that was even remotely close to functional and it rotted everything near it.


----------



## hand drive (Apr 21, 2012)

if the rot is at the very end of the board you could splice two 4' sister splices per joist. one splice on each side of joist member. nail it like crazy or use heavy duty through bolts to attach the sisters to the joist. in general you want about 4 or 5' of splice past where the rotted area of the joist is, if the joist is rotted back 1' from the end then 5' splices will be needed, etc...


----------



## lewisthepilgrim (Dec 8, 2011)

What size are the joists? Is the floor sagging at ALL ?? If it its, it might be an excuse to beef up the floor anyway...


----------



## cortell (Nov 27, 2010)

hand drive said:


> if the rot is at the very end of the board you could splice two 4' sister splices per joist. one splice on each side of joist member. nail it like crazy or use heavy duty through bolts to attach the sisters to the joist. in general you want about 4 or 5' of splice past where the rotted area of the joist is, if the joist is rotted back 1' from the end then 5' splices will be needed, etc...


The problem with this is that it's not prescriptive. Such a solution could be structurally sound, but it would require making complex calculations to determine that. I.e., it would require the expertise of a structural engineer to tell you what bolts to use, how many to use, and where to put them. Short of that, you're going on gut feeling, and that's never a good idea. That's why we have prescriptive code--so you can build something without requiring an engineer. All the calculations have already been made and you're mostly assured to have a sound design if you follow code. Deviate without an engineer, and you're rolling the dice. Not to mention, if will almost certainly fail inspection if you've pulled permits.


----------



## robertcdf (Nov 12, 2005)

Cost of joists are cheap compared to fixing it again when the "cheap fix" fails later. Joists are designed to go from bearing point to bearing point, do that and your golden.


----------



## lewisthepilgrim (Dec 8, 2011)

Cut out rot, sister the joists. And you'll have the strongest floor on the block !


----------



## shadetree (Aug 31, 2011)

Joists are 2"x12"x12' spanning a 10 foot opening, 24" OC. None of them are sagging including the rotted ones, but that is mostly becasue there is no load on them other than the sub-floor (this is an almost empty storage building behind my house)

Riddle me this: Would it be more acceptable to sister with a full length 12ft 2x8 on either side of each the rotted joist? (or 2x10, but I'm pretty that a single 2x8 #2 SYP can span the 10', so two should be fine? http://www.americanpoleandtimber.com/pdf/Spans_joists.pdf) There are obstructions in the way that would make it difficult to maneuver a full length 2x12 into position


----------



## bill01 (Apr 12, 2012)

Depends why they used a 2x12 and how close it was to the load limit. If you look at the maximum loads a 2x12 can take and compare it to other combinations you will find that you would have to go to 2 2x10 to have similar number in all categories. This shows why depth is more important than width in a beam. Those table are the best way to go without a engineer.... or you can do the math. If the load you actually anticipate does not actually require a 2x12 then you could in effect sister 2x8 to the 2x12 and be strong enough. Of course this assume the 2x12 is stable enough to be joined to 2x8 since you are still using the top of the 2x12 to transfer the load from the deck to the 2x8.


----------



## cortell (Nov 27, 2010)

shadetree said:


> Joists are 2"x12"x12' spanning a 10 foot opening, 24" OC. None of them are sagging including the rotted ones, but that is mostly becasue there is no load on them other than the sub-floor (this is an almost empty storage building behind my house)
> 
> Riddle me this: Would it be more acceptable to sister with a full length 12ft 2x8 on either side of each the rotted joist? (or 2x10, but I'm pretty that a single 2x8 #2 SYP can span the 10', so two should be fine? http://www.americanpoleandtimber.com/pdf/Spans_joists.pdf) There are obstructions in the way that would make it difficult to maneuver a full length 2x12 into position


If you use a smaller joist, the joist will not _directly_ be supporting the floor load. If for whatever reason, the end of the rotted 2x12 gives way, because you have less than the code-required minimum 1.5" bearing, the load will be transferred from the 2x12 to the fasteners to the 2x10. You end up in the same boat; it's all about how strong those fasteners are. It continues to be a design whose strength is unknown and outside prescriptive code.

Personally, I would hire an engineer to examine the situation and design a solution. Sure, it's going to require money, but you'll sleep better knowing those joists are going to stay put now and in the future. And, again, if you've pulled permits, you may not really have a choice anyway.


----------



## shadetree (Aug 31, 2011)

bill01 said:


> Depends why they used a 2x12 and how close it was to the load limit. If you look at the maximum loads a 2x12 can take and compare it to other combinations you will find that you would have to go to 2 2x10 to have similar number in all categories. This shows why depth is more important than width in a beam. Those table are the best way to go without a engineer.... or you can do the math. If the load you actually anticipate does not actually require a 2x12 then you could in effect sister 2x8 to the 2x12 and be strong enough. Of course this assume the 2x12 is stable enough to be joined to 2x8 since you are still using the top of the 2x12 to transfer the load from the deck to the 2x8.


I couldn't imagine this room would ever be loaded up more heavily than what a standard room would be. I'd be putting a 2x8 on either side of the rotted joist (two 2x8s per joist) instead of having just one. I was also planning on building up the top plate under the 2x8s so that the top of the 2x8s were also supporting the bottom of the sub-floor. Seems stronger than attaching them even with the bottom of the rotted joist. Or would that not be acceptable?


----------



## shadetree (Aug 31, 2011)

cortell said:


> If you use a smaller joist, the joist will not _directly_ be supporting the floor load. If for whatever reason, the end of the rotted 2x12 gives way, because you have less than the code-required minimum 1.5" bearing, the load will be transferred from the 2x12 to the fasteners to the 2x10. You end up in the same boat; it's all about how strong those fasteners are. It continues to be a design whose strength is unknown and outside prescriptive code.
> 
> Personally, I would hire an engineer to examine the situation and design a solution. Sure, it's going to require money, but you'll sleep better knowing those joists are going to stay put now and in the future. And, again, if you've pulled permits, you may not really have a choice anyway.


The ends of the 2x12 have already effectively given way. One of them had a several inch gap between the rotted top plate and the rotted end of the joist. I agree that attaching the 2x8 to the bottom of the 2x12 seems a little suspect. I was planning on building up the top plate enough under the new the new 2x8s so that the floor above rests directly on them, relieving the old 2x12 from any load at all.


----------



## cortell (Nov 27, 2010)

shadetree said:


> The ends of the 2x12 have already given way. One of them had a several inch gap between the rotted top plate and the rotted end of the joist. I agree that attaching the 2x8 to the bottom of the 2x12 seems a little suspect. I was planning on building up the top plate enough under the new the new 2x8s so that the floor above rests directly on them, reliving the old 2x12 from any load at all. Does anyone know if this would be acceptable?


Ah, nice. Building up the top plate should be no problem. It then becomes a matter of whether the new joists are sufficiently strong for that span and load. You can use a span table to figure that out. With a little luck, the answer will be yes. If it's not, you're not necessarily screwed. Span is affected by spacing. The smallest spacing in span tables is 12"OC. If you're lucky, your joists are currently 16" OC or greater, which means you have some room to play with. You could install the new smaller joists 12"OC and that might make up the difference.

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp


----------

