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So I've got a scenario where I'd like to re-enforce my joists, and I don't currently have the ability or time to sister the entire length of the 12' joists. Now I could remove blocking, insulation, and pipes and sister about 6' of them on the sill side, but that got me thinking:

Would adding "sisters" between the existing blocking actually work well? To me it seems like the small "sister" board would carry any bending/shear forces of the original beam in that area to the blocking. This would then result in either compression forces on the blocking, or rotational forces, both of which seem much more tolerable. It would also result in sharing the load between the joists. Obviously the sister will be glued, clamped, and nailed to the existing joist.


I'm trying to determine the pros/cons of each approach:

A) Sister between blocking, and block the "center" of the sister

B) Sister between blocking, and block the ends of the sister

C) Sister between blocking, and double up the existing blocking

enter image description here (Blue = existing joists/blocking/sill) (Red = new "sister" boards) (Orange = potential new blocking) (Joist are 16" OC with blocking ~4ft staggered)

[Edit] Although some comments/answers have been provided I think it would be best to provide some supporting information as to why one way or another might be best.

[Edit 2] I think people are missing the actual question. I do know that the more of a joist you can sister, the better in most situations. I understand that to do this blocking would need to be removed. I also understand people want to suggest things targeted towards a specific problem/solution.

The question however: why the suggested re-enforcement methods will or will not provide any increase in structural integrity. Logic dictates that it will indeed increase the load sharing capabilities from one joist to another and outward to the foundation, and although not quite as strong as an actual continuous sister, it should mimic one as there are no gaps to allow the smaller "sisters" to move in any way. They would simply transfer the forces to the blocking, and then onto the next smaller "sister" on the other side of the blocking.

  • How much space between the blocking? – mmathis Oct 25 '16 at 14:50
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    What is the problem you are trying to fix by reinforcing your joists (bouncy floors, termite damage, etc.)? Without that information, no one can tell you whether it is sufficient for what you are trying to accomplish. – statueuphemism Oct 25 '16 at 14:54
  • The spacing between the blocking now is roughly 4 feet. There is no issue as of now, I recently replaced part of the sill beam due to insect damage, and this floor will now hold a bathroom and therefore a tub full of water. So while I'm in this section I'd like to do what I can to increase the structural integrity of the flooring system. – While-E Oct 25 '16 at 15:05
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None of the three options you've shown are likely to add substantial strength or stiffness to your floor. To add either, you'll need a single, continuous additional member to span the majority of the joist length.

The blocking is probably not critical, especially if you're doubling the joists for most of their length. It's typically installed to prevent deflection, and that's not a significant concern where you have doubled joists.

My suggestion would be to remove blocking from every second joist space and double for 80% of the joist's length in those spaces. The remaining blocking will serve to stabilize all joists.

If the original joists are sound (not showing rot), compression is probably not a concern. Even a tub of water doesn't add up to enough weight to compress healthy lumber.


Regarding your comments on the original ideas, you will see some gains if 1) you fit your additional blocking extremely well, and 2) you fasten it all together with screws and construction adhesive. The result will essentially be a load-bridging honeycomb that will improve rigidity and strength. However, the flooring already provides substantial load-sharing, so gains will be minimal.

The primary reason that it's not widely hailed is that it's a very putzy, time-intensive plan that's ultimately inferior to a proper joist sistering. You'll spend more time cutting and fitting all those blocks than you would tearing out half the blocking and slapping new lumber alongside the joists.

  • Thanks for the input. I figured as much. I just had the thought and was trying to justify why it wouldn't add any strength. Mostly because I figured it will increase the ability to share the load at any given point in the floor, and even though it's not a "continuous member" it should act like one given there's no gaps, only spots where it "rests" on the blocking. – While-E Oct 25 '16 at 15:31
  • Can't edit... meant rests/attaches to blocking, not rest "ON" the blocking. – While-E Oct 25 '16 at 16:04
  • I don't know about tossing blocking to the wind but if there wasn't any in the first place, it's not like I'd add some. +1 – Mazura Oct 25 '16 at 22:46
  • We installed plenty of 1x3 X-bridging back in the day when we were still using solid lumber joists. In my suggestion I'm leaving half the blocking. That and the doubling of the joists should be plenty adequate. – isherwood Oct 26 '16 at 13:26
  • I truly appreciate the suggested fixes, thank you, and I will likely do something very similar to what is suggested, but I came here more for insight on the solutions provided as outlined in my final edit. – While-E Oct 26 '16 at 15:56

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