# Sistering joists



## greg24k

You can also use steel wall-bracing straps, wrap the joist with it and nail the straps every so many inches with tico nails using the tico palm nailer,they sell them now for $50 and that will work very significantly.


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## Winchester

bridging and strapping.

I've also always wanted to try glueing and grk'ing a 2x2,2x3, or 2x4 piece of wood along the bottom, (on the flat)

the bottom of the joists are in a lot of tension and this would really help with that if it was bonded effectively. Similar theory as an I joist. All the 'meat' where the largest forces are applied.

as far as I know, the more depth, the better, but you may not want to lower the height of a ceiling...

drew a sketch (not to scale ). What do you guys think? I've seen the second one used but not the first one.


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## Bastien1337

Winchester said:


> bridging and strapping.
> 
> I've also always wanted to try glueing and grk'ing a 2x2,2x3, or 2x4 piece of wood along the bottom, (on the flat)
> 
> the bottom of the joists are in a lot of tension and this would really help with that if it was bonded effectively. Similar theory as an I joist. All the 'meat' where the largest forces are applied.
> 
> as far as I know, the more depth, the better, but you may not want to lower the height of a ceiling...
> 
> drew a sketch (not to scale ). What do you guys think? I've seen the second one used but not the first one.


I feel like hitting both side of the joist with the 2x2 or 2x4 would be more benefical then a 2x4 underneath. 

For me I see bonding on the sides giving more strength in the tension portion of the joist, while strapping the bottom would not. It doesn't take much for 2x4 on the flat to deflect, it would just follow the crown without giving any support. On the side however, if the joist wanted to continue to deflect it would have to force the 2x4s on edge to go into tension and I think they would put up more of fight to bend thus strengthen the floor Or at least adding more rigidity.


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## aptpupil

sistering will help a lot.
this was talked about a lot on this thread:
http://www.contractortalk.com/f14/fun-wood-x-bridging-68111/
also, check out this which has some actual data behind it:
http://blog.luxorcorp.com/2008/12/08/the-simplest-way-to-fix-bouncy-floors/
ultimately, it's going to be up to you about which method is going to be best for your situation/budget.


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## KennMacMoragh

greg24k said:


> What are you better at...being dumb or being a plumber?
> 
> Read what you wrote again...you already answered your own question :thumbsup:


Then maybe I'm dumb too, I'm not so sure how much that would stiffen it since he can't span the sistered joists the entire length. 

So you have a crawlspace underneath? Or a basement? A method I've used for a crawlspace is to build strongbacks underneath. Take two 2x8's nailed together, two nails every six inches, and nail it perpendicular to the joists. Cut some braces so you can shove them up tight to the joists. I'd put in three of them spaced a few feet apart, right in the middle. Then to support the ends you can put treated 4x4's at a 45 degree angle up against the footing. I repaired several over spanned joists this way, the repair was all designed by an engineer. 

If you have easy access to the joists and lots of room to work, then sistering might be a better option. But in a crawl space, using strongbacks is much faster and more effective.


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## jimmyb21

Wow, thanks for all of the replies! :thumbsup: There's some very good info. there. There is some sagging but there is tile on the floor of the kitchen so I'm just gonna leave good enough alone. I like the idea of the plywood but I'm gonna need to see how much I'll need and go from there. It may be cheaper just to use dimensional lumber. I think anything at this point would help. I would put a beam in but I don't want to loose any head room so that's out of the question. Thanks to everyone again :notworthy


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## Kanding

Winchester said:


> bridging and strapping.
> 
> I've also always wanted to try glueing and grk'ing a 2x2,2x3, or 2x4 piece of wood along the bottom, (on the flat)
> 
> the bottom of the joists are in a lot of tension and this would really help with that if it was bonded effectively. Similar theory as an I joist. All the 'meat' where the largest forces are applied.
> 
> as far as I know, the more depth, the better, but you may not want to lower the height of a ceiling...
> 
> drew a sketch (not to scale ). What do you guys think? I've seen the second one used but not the first one.


This is a good question. You are adding more material in one scheme but placing the material at a lower depth in the other scheme. Barring math errors, a quick calculation gives me about a 60% increase in moment of inertia (I) from a base 2x8 (about 48 in^4) for the 2nd scheme (2-2x4s) and a 250% increase for the first scheme (1-2x4). Adding the ½ the material at a lower depth clearly wins. Thus bending stiffness would be expected to be increased by these proportions (assumes wood species/grade and thus wood modulus of elasticity is the same throughout, and that sufficient fasteners are provided to prevent any horizontal slip between surfaces).

These % increases will be less, and the difference between them will decrease, as the base joist size increases. Because the floor also adds some additional stiffness that I neglected, you wouldn’t see increases exactly this large in reality.

Bastien raised the issue of the bending stiffness of the 2x4. Certainly, placing a 2x4 on edge has more stiffness than flatwise. However, the base I value of the 2x4 isn’t the issue, and contributes little to the built-up section. You are getting by far the most benefit from placing as much material as far away from the neutral axis (which is close to the middle) of the section as possible. Increasing depth is always your best option from a structural point of view—note just replacing 2x8 w/ a 2x10 more than *doubles * stiffness. Strength is a little more tricky because different adjustment factors are used for different member sizes, to account for knots and other imperfections.


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## jimmyb21

Ken, it's a basement. There's just a lot of utilities in some areas so trying to sister the entire lengths would be really difficult. I'm finishing the basement hopefully within a couple years so the walls will help but the area under my kitchen is the laundry & won't have any walls. That's the area I'm worried about the most since it has porcelian tile. I think that sistering even part of the joist would help in theory but wanted to see what other professionals had to say. I do think it would be better if I could sister the entire lengths but I'm just not sure if it's feasable. I think I like the idea of using the 3/4 plywood on both sides. I'm not worried about adding strength per say, more worried about the deflection. Thanks again for all the responses!


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## Kanding

You don’t need to span the entire length for there to be a significant benefit. Stiffening the very ends of the beams would help greatly for shear resistance but would provide little additional flexural stiffness.


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## Winchester

Kanding said:


> This is a good question. You are adding more material in one scheme but placing the material at a lower depth in the other scheme. Barring math errors, a quick calculation gives me about a 60% increase in moment of inertia (I) from a base 2x8 (about 48 in^4) for the 2nd scheme (2-2x4s) and a 250% increase for the first scheme (1-2x4). Adding the ½ the material at a lower depth clearly wins. Thus bending stiffness would be expected to be increased by these proportions (assumes wood species/grade and thus wood modulus of elasticity is the same throughout, and that sufficient fasteners are provided to prevent any horizontal slip between surfaces).
> 
> These % increases will be less, and the difference between them will decrease, as the base joist size increases. Because the floor also adds some additional stiffness that I neglected, you wouldn’t see increases exactly this large in reality.
> 
> Bastien raised the issue of the bending stiffness of the 2x4. Certainly, placing a 2x4 on edge has more stiffness than flatwise. However, the base I value of the 2x4 isn’t the issue, and contributes little to the built-up section. You are getting by far the most benefit from placing as much material as far away from the neutral axis (which is close to the middle) of the section as possible. Increasing depth is always your best option from a structural point of view—note just replacing 2x8 w/ a 2x10 more than *doubles * stiffness. Strength is a little more tricky because different adjustment factors are used for different member sizes, to account for knots and other imperfections.


That's what I was sort of thinking intuitively, but I'm glad someone can actually put numbers to it and verify. Adding material to the bottom is much better.

The only problem is how you would jack up a sagging floor _and_ add material to the bottom.


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## Winchester

Kanding said:


> You don’t need to span the entire length for there to be a significant benefit. Stiffening the very ends of the beams would help greatly for shear resistance but would provide little additional flexural stiffness.


I've heard smarter people than I say the most benefit is from stiffening the center 1/3rd


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## jimmyb21

Kanding, that makes total sense to me & thanks for the other information:thumbsup:,
Jimmy


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## Bastien1337

Kanding said:


> This is a good question. You are adding more material in one scheme but placing the material at a lower depth in the other scheme. Barring math errors, a quick calculation gives me about a 60% increase in moment of inertia (I) from a base 2x8 (about 48 in^4) for the 2nd scheme (2-2x4s) and a 250% increase for the first scheme (1-2x4). Adding the ½ the material at a lower depth clearly wins. Thus bending stiffness would be expected to be increased by these proportions (assumes wood species/grade and thus wood modulus of elasticity is the same throughout, and that sufficient fasteners are provided to prevent any horizontal slip between surfaces).
> 
> These % increases will be less, and the difference between them will decrease, as the base joist size increases. Because the floor also adds some additional stiffness that I neglected, you wouldn’t see increases exactly this large in reality.
> 
> Bastien raised the issue of the bending stiffness of the 2x4. Certainly, placing a 2x4 on edge has more stiffness than flatwise. However, the base I value of the 2x4 isn’t the issue, and contributes little to the built-up section. You are getting by far the most benefit from placing as much material as far away from the neutral axis (which is close to the middle) of the section as possible. Increasing depth is always your best option from a structural point of view—note just replacing 2x8 w/ a 2x10 more than *doubles * stiffness. Strength is a little more tricky because different adjustment factors are used for different member sizes, to account for knots and other imperfections.



It felt right, im glad to have some numbers myself.


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