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Old 12-29-2012, 01:40 AM   #1
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Bracing sistered joists


I am currently trying to strengthen and stiffen a floor. The existing floor uses 2x10 joists (16 in oc) spanning ~14 ft without any cross bracing. I want to sister (most of) the 2x10s and use solid blocking for bracing at approx. the midpoint of the span. I'm also going to use a thicker subfloor material (currently 5/8 in., looking at 3/4 or 7/8 Advantech, or maybe just CDX plywood). The problem is how to fasten the blocking - since the joists are sistered, simply nailing through them, as recommended at http://www.renovation-headquarters.c...20bridging.htm, figures 5a and 5b, isn't easily doable and/or would require extra long fasteners (due to the increased thickness). Can I just use really long screws (I'm thinking the longer Timberlok variety) instead of nails? Can I stagger the blocking to simplify the fastening and then "double" them (with angled screws - from one side - through the joists into the ends of the second, doubling block, in addition to a couple of nails between the blocking pieces themselves)? Or am I limited to metal or wood bridging (x-style)? Is there any way to do this (solid blocking and sistered joists)?
Thanks in advance for your thoughts...

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Old 12-29-2012, 03:00 AM   #2
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Bracing sistered joists


I toenailed with screws (driven at a 45 deg angle), by first predrilling. Predrilling helped ensure the blocking stayed at the proper height. I'm not sure if it's the correct way, but that's what I did and I feel pretty confident it will stand the test of time.

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Old 12-30-2012, 12:57 PM   #3
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Bracing sistered joists


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Originally Posted by Seattle2k View Post
I toenailed with screws (driven at a 45 deg angle), by first predrilling. Predrilling helped ensure the blocking stayed at the proper height. I'm not sure if it's the correct way, but that's what I did and I feel pretty confident it will stand the test of time.
If you want, offset the bracing, and end screw, just like when building stud wall. I'd favor that over toenailing. I'm not a big toenailing fan...I toenail only when there isn't any other way to do it. And, I understand that toenailing is perfectly acceptable and widely used...just, by choice, not my cup of tea.

Just my two cents...
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Old 01-01-2013, 05:56 PM   #4
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Bracing sistered joists


Thanks for the replies.
So I gather I'm not crazy, or, at least, I'm not alone in my craziness!

Thanks for the reminder to predrill - that'll help avoid splitting the 2x blocking (since the span will be rather short, relative to the depth).

My understanding is that offset and end nail/screw is generally not recommended (and many inspectors will note it as a deficiency) because the bracing / load distribution is most effective when the bracing is in line. However, I agree and appreciate that end screwing is generally easier and often more secure than fastening at an angle - that's why I'm thinking of a "compromise" of offset and end screw that provides a well secured bracing and then "doubling" those to effectively eliminate the offset - the end effect would be doubled bracing that's all in line. (And it does seem reasonable, since I'm doubling the joists, too.) Only the second, doubling block would be fastened at an angle (minimally - probably only 2 screws at each end, from one side vs. the normal 4 - 2 from each side). The blocks would be directly fastened also, through the first into the second. I *think* that would still satisfy most concerns about load transfer, but I also think I'll have to be careful about fastener placement to avoid splitting the blocks.

It may be overkill, but since I'm doing the work (i.e., labor is "free") and the additional materials are relatively a small increment in cost, I'd rather overdo it than have an insufficient undergirding. IMO, it's cheap insurance to avoid sagging/squeaking floors that would require tearout/rework to remedy.
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Old 01-02-2013, 09:51 PM   #5
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Bracing sistered joists


Can you lose 2" of headroom to sister with 2x12's, pp. 92: http://books.google.com/books?id=zjJ...ilever&f=false

Good read; http://www.ewpa.com/Archive/2004/jun/Paper_278.pdf

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Old 01-07-2013, 12:15 AM   #6
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Bracing sistered joists


Thanks for the additional thoughts!

> Can you lose 2" of headroom to sister with 2x12's

Not easily - HVAC plenum and ductwork, plumbing and gas pipes, etc.
Even if that weren't the case, I'd still have to notch the ends so they sit on the existing beam and sill (or not sister the entire joist length). As the provided link confirmed, the strength is directly proportional to the square of the depth, and the stiffness to the cube of the depth - in my case, going to a 2x12 rather than a 2x10 would result in a sistering joist that's 44% stronger and 72.8% stiffer. That would be nice, but I think it's too much work to make it worthwhile - I am hoping that the steps I've already outlined will be sufficient. (Of course, if I'm wrong, I'll be cursing myself later...)

The second link was indeed an interesting read, but it left me wondering about other, untested scenarios. I was surprised that cross-bridging with a 1x4 strapped across the bottom of the joists actually performed better than solid blocking; I wonder if strapping under solid blocking also has benefit. I think it might, since the strapping presumably spans across several joists (I didn't see this explicitly mentioned in the article). I remember seeing something in a Fine Homebuilding article about metal strapping along the joists helped because it spread load along a joist; 1x strapping probably does something similar, except it is across multiple joists. I wonder, too, if the results would be similar for standard dimensional (non-LVL) joists. (of course, I could always use LVLs as the sistering joists! Hmm...) Also, the author was only measuring vertical (vs. lateral) deflection; I still think solid blocking may still be better to prevent lateral deflection / roll - that may be an important consideration in earthquake zones (not a real requirement or concern for me, though). Reinforcing braces with metal (aluminum or steel) brackets seems interesting, but my gut says it might be overkill and unnecessarily expensive. I wonder how the results would have changed had the author used two or more bolts (rather than a single bolt) to attach the bracket to the joist. Also, it would be more "real world" if there were some test cases that used both traditional nailing with the bracket (as would result from a "retrofit" where the metal bracket is added after the fact to "shore up" a weaker floor system).

I confess that I may have a bias against cross-bridging - not necessarily in the method itself, but because I have seen a few installations where it was not fastened properly (not attached at either the top or bottom). I suppose improper installation happens for solid blocking, too, but I haven't seen it. (Admittedly, I've personally seen *far* fewer solid blocking installations in general - in fact, only one that I can recall.)

I apologize if I sound overly critical or cynical of any proposed ideas - I do appreciate the feedback/discussion. If any of my thoughts don't hold water, feel free to shoot 'em down!
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Old 01-07-2013, 12:30 AM   #7
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Bracing sistered joists


Just food for thought but you may want to nail your blocking instead of screwing.

Nails have a higher sheer value than screws but screws have a better holding power. In the case of the blocking, you are trying to prevent up and down movement and the nails will hold better.
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Old 01-08-2013, 03:14 PM   #8
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Bracing sistered joists


That's good to know - thanks!

My problem is that since the joists are sistered, I have to nail through 3" of dimensional lumber before I even get to the blocking. So those would be some really big nails, and driving them would be painful. I see that there are 30D, 40D, and 60D nails that are long enough, but they're really quite thick, and I'm not sure the blocking wouldn't split. (Of course, I could go with 4x4 blocking, too! ) Does anybody have any experience with really large/long nails and how to drive them? I certainly don't! (I'm assuming it's a bit more complex than simply whacking at them.) Unless I have a solution to that issue, I'm going with screws.

One thought is that I could drill a hole through one 2x so that I'd only have to drive the nails through a "normal" thickness. Of course, driving them in a hole like that presents its own challenges - and I'm again limited by my experience and tool availability. So unless I have an epiphany (or a forum reply kicks me in a better direction ) and a more doable solution presents itself, I'm sticking with the screws.

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