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-   -   Blocking Between Floor Joists - Important? (http://www.diychatroom.com/f19/blocking-between-floor-joists-important-188712/)

hellohello 10-16-2013 09:10 PM

Blocking Between Floor Joists - Important?
 
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?

sixeightten 10-16-2013 09:14 PM

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.

hellohello 10-16-2013 09:15 PM

Quote:

Originally Posted by sixeightten (Post 1254397)
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?

sixeightten 10-16-2013 09:35 PM

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

cortell 10-16-2013 09:47 PM

Quote:

Originally Posted by hellohello (Post 1254398)
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.

cleveman 10-16-2013 11:22 PM

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.

GBrackins 10-16-2013 11:23 PM

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

Arkitexas 10-16-2013 11:54 PM

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

cortell 10-17-2013 08:24 AM

Quote:

Originally Posted by Arkitexas (Post 1254458)
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.)

tony.g 10-17-2013 11:30 AM

Quote:

Originally Posted by cortell (Post 1254528)

"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.

Chris Sweeney 10-17-2013 12:27 PM

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?

cortell 10-17-2013 12:41 PM

Quote:

Originally Posted by Chris Sweeney (Post 1254623)
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.

hand drive 10-18-2013 08:16 AM

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

cortell 10-18-2013 09:52 AM

Quote:

Originally Posted by hand drive (Post 1254934)
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).

ddawg16 10-18-2013 11:28 AM

Quote:

Originally Posted by Arkitexas (Post 1254458)
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

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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