# Blocking Between Floor Joists - Important?



## hellohello (Oct 17, 2013)

When is blocking/bridging (or whatever you call it) necessary between floor joists? What is the purpose of this blocking? To keep the joists from moving back and forth? Wouldn't the plywood subfloor basically be achieving that same idea?


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## sixeightten (Feb 10, 2009)

Bridging is used to help stiffen a floor. This does not make it able to support more weight, but helps the loads to be transferred to several joists and lessen the movement of the floor. The plywood merely prevents the top of the joist from moving.


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## hellohello (Oct 17, 2013)

sixeightten said:


> Bridging is used to help stiffen a floor. This does not make it able to support more weight, but helps the loads to be transferred to several joists and lessen the movement of the floor. The plywood merely prevents the top of the joist from moving.


Ah, thanks, I see now. How often should the joists be bridged?


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## sixeightten (Feb 10, 2009)

I think the code says that any joist longer than 8' requires it. Anything over 16' requires two rows.


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## cortell (Nov 27, 2010)

hellohello said:


> Ah, thanks, I see now. How often should the joists be bridged?


IRC requires bridging on joists deeper than 12" (nominal), at least 8'OC. 

I've read one trade article (in a respected journal) where the author swears bridging does little more than cause squeaks. He swears it's much better to just go above code on the subfloor thickness, and nail-n-glue it down, and use 1x3 strapping under the joists.


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## cleveman (Dec 17, 2011)

I think solid wood blocking is the way to go.

Firstly, it is nice when you are framing, before you install the sheathing. The joists won't move while you are installing the sheathing. It also lines them up properly so you won't have to move them as you install sheathing.

I like to screw the blocking in. This way, if there is a block in the way of some hvac or whatever, I unscrew it and screw it back in flat towards the bottom of the joists.

I can't say enough good things for it.


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## GBrackins (Apr 26, 2012)

if you are located within a hurricane prone region floor bracing adds to the strength of gable end walls. it also provides bracing for the top of foundation wall parallel to the direction of the joists.

http://www.awc.org/pdf/WFCM_110-B-Guide.pdf


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## Arkitexas (Mar 10, 2011)

Bridging between joists (or beams) actually serves three purposes. 

First, bridging resists the tendency of a joist to roll under load. As a joist begins to roll, its load capacity is reduced and it will deflect more than if held vertical. Underside sheathing alone will not insure that a joist will not roll, it just insures that all the attached joists will roll in unison. I have opened up several old floors which lacked bridging and found the joists permanently rolled over as much as 20* - 25* out of plumb. Rolled joists create fracture problems at the walls and ceiling as well as severe deflection in the floor surface.

Second, where floor loads are concentrated on one joist, the bridging will distribute some of the load to the adjacent joists. Thus the floor structure supports loads in a "team" like manner and reduces the amount of floor deflection that would occur had only one joist been loaded. 

The blocking also keeps the joists evenly parallel for alignment and nailing of dimensional sheet goods such as decking and gypsum board.

If a floor displays excessive squeaking I would be asking the builder why he didn't use panel adhesive between the joists and the decking and screws instead of nails. Where blocking is called for in the building codes, there is no such thing as going "above code" on the decking that will substitute for the blocking. 

Rick


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## cortell (Nov 27, 2010)

Arkitexas said:


> Bridging between joists (or beams) actually serves three purposes. [...snip...]


I'll play devil's advocate here (it wasn't my article, and I'm not necessarily convinced on his point of view, but I also don't dismiss it as easily as you)...

Joist rolling: that can be prevented with under-joist strapping. The author did mention the need for strapping.

Joist alignment: there's much more cost-effective ways to lign-up joists pre-sheathing.

Load distribution: I suspect you're right on this one, but then again code does not require bridging except for very deep joists. But lets assume a bridging-less floor has more deflection. That's a serviceability issue. What is the author's motivation? Avoiding squeaks. That's another serviceability issue. This to me sounds like an informed judgement call on which is the lesser of two evils. Also, thicker sheathing will for sure increase load distribution, and make up some of the difference.

Finally, as for the cause of squeaks, author attributes his past issues to friction in the bridging, not to friction between the sheathing and joists. To quote: "We'd install the [...] bridging tighter than a guitar string only to return a year later and find it had loosened and was causing squeaks. Even though we use kiln-dried material, the seasonal changes in humidity cause the joists to shrink and swell enough to render any type of bridging worthless." [Arnold and Guertin, Fine Homebuilding]. (Sorry. Did not find the article online.)


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## tony.g (Apr 15, 2012)

cortell said:


> "We'd install the [...] bridging tighter than a guitar string only to return a year later and find it had loosened and was causing squeaks. Even though we use kiln-dried material, the seasonal changes in humidity cause the joists to shrink and swell enough to render any type of bridging worthless."


One way to maintain rigidity of the joists is to use the traditional herringbone bridging, usually cut from 2x2.
The theory is that as the joists shrink more in the vertical plane than the thickness, the 2x2s actually get compressed tightly against the joist sides.
A lot more time-consuming than solid bridging, though.


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## Chris Sweeney (Apr 20, 2012)

When bridging, do you guys stagger? For example, do you put your bridges right next to each other in the adjacent joist bays, or do you offset them from one another?


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## cortell (Nov 27, 2010)

Chris Sweeney said:


> When bridging, do you guys stagger? For example, do you put your bridges right next to each other in the adjacent joist bays, or do you offset them from one another?


I've only found one person that insists in-line is important for maximum performance. I'm skeptical.

Many framers stagger solid blocking so they can end-nail both ends. I don't find it any harder to toe-nail one end, so I go in-line.


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## hand drive (Apr 21, 2012)

if the floor system is reaching its span/size limits for the application then I can see where the blocks could squeak after time and everything loosens up some but I've found that joist blocking when used correctly in an adequately built floor system (low bounce factor) provide lots of support and make a strong floor even stronger. if the crawlspace or basement has moisture issues and the wood moves a lot because of it then squeaking could be a factor, then it goes back to fixing the moisture issue and not so much the blocks themselves


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## cortell (Nov 27, 2010)

hand drive said:


> if the floor system is reaching its span/size limits for the application then I can see where the blocks could squeak after time and everything loosens up some but I've found that joist blocking when used correctly in an adequately built floor system (low bounce factor) provide lots of support and make a strong floor even stronger. if the crawlspace or basement has moisture issues and the wood moves a lot because of it then squeaking could be a factor, then it goes back to fixing the moisture issue and not so much the blocks themselves


That sounds very reasonable to me, and I for sure would not abandon bridging based on one person's theories and experiences. There's empirical evidence showing bridging does in fact substantially reduce deflection. I just wanted to share an informed dissent on the matter, particularly since it's in an area where base code does not dictate (again, other than 2x12s and deeper, IRC does not require bridging).


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## ddawg16 (Aug 15, 2011)

Arkitexas said:


> Bridging between joists (or beams) actually serves three purposes.
> 
> First, bridging resists the tendency of a joist to roll under load. As a joist begins to roll, its load capacity is reduced and it will deflect more than if held vertical. Underside sheathing alone will not insure that a joist will not roll, it just insures that all the attached joists will roll in unison. I have opened up several old floors which lacked bridging and found the joists permanently rolled over as much as 20* - 25* out of plumb. Rolled joists create fracture problems at the walls and ceiling as well as severe deflection in the floor surface.
> 
> ...


Hopefully Daniel H will chime in...but I'm willing to bet that he would agree with the above.

I'm finishing up a 2-story addition now...and per my dwg's...any span over 8' requires blocking....

I did mine in a line....when using a nailer...you just put the head at the edge of the existing block and run it in at an angle...about as close to butt nailing as you can get.

I can't stress how nice it is to have those blocks in while your trying to put down the sheathing...it's not fun walking on joists that are flopping around....and with the blocking....you know the joists are straight...nailing goes a lot quicker...

As for sheathing....3/4" seems to be the common standard...my architect had me use 1 1/8" T&G. That sh!t is heavy.....and my floor does NOT squeak....or bounce....walking on my second floor is almost like walking on concrete. And those sheets were only about $10 more than 3/4"....but worth so much more.


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## cortell (Nov 27, 2010)

ddawg16 said:


> Hopefully Daniel H will chime in...


What would we do without Daniel and the Garys to keep us amateurs in check?! :thumbsup:


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## Arkitexas (Mar 10, 2011)

Dear advocate of the Devil,

Alignment: This is not a code requirement but rather a side benefit, I apologize if I made it sound otherwise. 

Rolling-Bracing: Bottom chord bracing (strapping) is another method to brace the joists and is permitted by the R502.7.1 of the IRC. In my personal opinion (and not required in the IRC) bottom cord bracing should have structural terminations (triangulation, panelizing, X bracing, bridging, etc.) at each end of the run to be as effective as continuous blocking. There come those pesky squeaks again. In commercial steel construction such terminations are standard. 

Squeak Sources: I don't contest that blocking was the source of the writer's squeaks and it appears that he has found a better floor assembly that is quiet. That said, it has been my experience floor squeaks have nearly always been attributable to poor deck attachment to the joists and/or marginal joist sizing. In any case, thicker floor decking and adhesive is a good upgrade which, in addition to improved load distribution, also adds mass bettering the sound deadening qualities. 

Dismissive: I disagreed with the first paraphrase of the article "bridging does little more than cause squeaks" and the second quote that "......seasonal changes in humidity cause the joists to shrink and swell enough to render any type of bridging worthless". Wood framing does in fact shrink and swell but that doesn't make the bridging worthless. I think I would have been more agreeable had he described bridging as unacceptably annoying.

Other Alternatives: a) Use galvanized steel bridging which does not vary dimensionally with humidity levels. b) Switch from wood framing to steel. 

Rick


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