# what size beam for jacking up floor joists



## ponch37300 (Nov 27, 2007)

I posted my sagging floor joist issue a couple weeks ago and have decided on jacking up the existing floor joists using a temp beam supported by 4 columns at the middle of the floor joists. Then when all is level I will sister 2x10s to the existing 2x8s. My question is what size beam should I use for the temp beam? The beam will be 23 feet long and I'm thinking 4 support columns, one at each end and 2 in the middle so it would be right around 7 ft in between each column. This temp beam only needs to support the middle of the floor joist and what's on them. There is no supporting walls on top of it so it doesn't need to hold the whole house up. I'm thinking that double 2x6 or 2x8 should do for this? Here is a drawing that helps to explain the situation.


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## tpolk (Nov 7, 2009)

how you getting the 2x10 over the beam


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## Big Bob (Jul 27, 2007)

Can you post a link to your previous postings on your floor joist issues?

It might help us or at least me understand why you are taking this approach. Is the sag uniform at the middle for 23 ft? What does the string tell you?

what is your plan for sistering the 2x10 to the 2x8 at the temp beam area. notch your temp beam or block up directly under the 2x8 from the temp beam. OR Notch the 2x10 around your temp beam?

Think about renting some post shores (screw jacks).

Let us know.


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## ponch37300 (Nov 27, 2007)

Here is the link to my other thread http://www.diychatroom.com/f19/need-add-floor-support-62366/. The floor is sagging becuase it has 2x8s spaced at 16" that span about 15'. 

I bought 4 "screw" jacks since I'm not sure how long this project is going to take so renting might get a little expensive.

I plan on using a spacer in between the beam and existing 2x8s so I can slid the 2x10s in. I'll have to do a test to see if I can slide them in with the temp beam in place.


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## jlhaslip (Dec 31, 2009)

Don't forget that you need to be careful with the bottom of the jacking system. 
It will be placing a whole bunch of weight onto the floor system it is raised on. 
Check to see if you need to support the floor underneath with other jacks/posts before they get loaded up.

Best to carry the temporary loads all the way to the basement/ground.


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## ponch37300 (Nov 27, 2007)

jlhaslip said:


> Don't forget that you need to be careful with the bottom of the jacking system.
> It will be placing a whole bunch of weight onto the floor system it is raised on.
> Check to see if you need to support the floor underneath with other jacks/posts before they get loaded up.
> 
> Best to carry the temporary loads all the way to the basement/ground.


 
This will be in the basement to jack up the first floor so as long as the cement which is in pretty good shape can hold the weight I'll be alright.


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## jlhaslip (Dec 31, 2009)

ponch37300 said:


> This will be in the basement to jack up the first floor so as long as the cement which is in pretty good shape can hold the weight I'll be alright.


I would still place some 2 by's and plywood under the system to spread the weight around. 
Do you know how thick the concrete is or are there footings near there? Better to jack at the footings.


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## ponch37300 (Nov 27, 2007)

I was thinking about laying a 2x12 about 2 feet long down flat to help spread the weight a little. I doubt there is any footers in the middle of the slab. The post on each end of the beam should be close to footers since they will be by the foundation wall, but the two posts in the middle probably will just be on the slab. 

Thanks


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## scott j (May 4, 2009)

*Cracks*

I have a brick fireplace being held up from the basement that is on 2x8 joists with screw jacks. I still see concrete cracks even though the previous owner used 1 2x8 wood under the jacks.


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## ponch37300 (Nov 27, 2007)

I think I'm going to cut 2 foot long peices of 2x12s and a peice of 3/4 plywood 23"x24" and make a pad that will have 2 2x12s under a peice of 3/4" ply and hope for no cracks. This should spread the weight out over a 2x2' area. This is just going to be a temp beam and posts, just using it to raise the joists and level things out before I add the sister joist.


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## Big Bob (Jul 27, 2007)

tpolk and I cross posts... but I think the same thing has got us scratching our heads.

your in a basement.. assume these are solid walls and you don't have a row of window to stick the boards out...

I don't think you will have enough angel to get the full length sister board in place once the beam is in place. Maybe with a good fire cut end...?

1. I guess you could tack the sisters in place then set your temp beam.

2. If it were my project I would forget the temp beam... run a lot of string and line levels... keep them tight... once every hour... and work the sisters in one at a time. I think that beam is going to be in your way alot.

You might want to try a dry run and see how your plan is going to work.:thumbsup:


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## Daniel Holzman (Mar 10, 2009)

I'm with Bob, I cannot visualize the advantage of your approach versus jacking up each joist individually, of course very slowly, until it is level, then sistering the new joist to it. One jack, one pad, no interference. The only advantage I see about the beam is you get to jack all the joists simultaneously, but I don't see how this will save you any time, considering the difficulty of setting up the beam and jacks, advancing each jack individually, checking each joist for level, etc.

By the way, the joists you jack up to level are going to be under considerable stress because you are changing their shape. You are going to need a lot of nails or structural screws to adequately connect the new joist to the old joist. Also, after you release the jack, the paired joist is going to move downward due to the release of energy in the old joist, so if you are looking for a level floor, you are going to need to overcompensate during the initial jacking. Exactly how much will depend on the species of wood, dimensions, amount of deformation of the joist, and the location of loads above the joist.


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## ponch37300 (Nov 27, 2007)

I was thinking that putting a temp beam in would allow me to raise the whole floor at once, avoiding raising one joist and not the one next to it. Just thought that it would be a lot of stress on the sub floor if I raised one joist and not the one next to it. I was planning on giving this a test run to see if I can slide the new joists in over the beam. It would probably be faster to do it without a beam but I was worried about the stress on the other joists. 

I'm not necessarily trying to get a perfectly level floor, more trying to stiffen up the floor because it has a "bounce" to it. 

Thanks again for all the advice and ideas!


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## HooKooDooKu (Jan 7, 2008)

My thoughts are along the same lines as Bob.

You could jack the floor up on joist at a time if you first block all the joists together. That way as you jack one joist up, the blocking will pull the neigboring joists up with it. Presumably it won't put much strain on the sub-flooor. Conceptually as you jack one joist up 1", the next joist will raise 3/4", the next on 1/2", etc. You could even use a short beam to jack the neiboring joists up in this type of pattern. At one end of the beam, jack it up to level the floor. The other end just barely jack the joist up. As you get one joist sistered, move the system down to the next joist. To avoid raising and lowering the floor, use two beams. Rather than take down the 1st beam, use the 2nd beam to jack the next set of joists up.


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## ponch37300 (Nov 27, 2007)

HooKooDooKu said:


> My thoughts are along the same lines as Bob.
> 
> You could jack the floor up on joist at a time if you first block all the joists together. That way as you jack one joist up, the blocking will pull the neigboring joists up with it. Presumably it won't put much strain on the sub-flooor. Conceptually as you jack one joist up 1", the next joist will raise 3/4", the next on 1/2", etc. You could even use a short beam to jack the neiboring joists up in this type of pattern. At one end of the beam, jack it up to level the floor. The other end just barely jack the joist up. As you get one joist sistered, move the system down to the next joist. To avoid raising and lowering the floor, use two beams. Rather than take down the 1st beam, use the 2nd beam to jack the next set of joists up.


This sounds like a good method. I have 4 jack posts right now so I can get one more and then will have enough to jack one truss up and also have 2 beams so I can just keep going. This idea eases my questions about raising one beam at a time. Thank you all for your suggestions.


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## HooKooDooKu (Jan 7, 2008)

ponch37300 said:


> This sounds like a good method. I have 4 jack posts right now so I can get one more and then will have enough to jack one truss up and also have 2 beams so I can just keep going. This idea eases my questions about raising one beam at a time. Thank you all for your suggestions.


Why do you need a fifth?

And I'm even going to modify the suggestion just a little bit...

First of all I said something about blocking the joists together... well you can't do that until AFTER you've sistered new joists to them.

So here's a modified suggestion:

1. Use the beam and two jacks to jack the 2nd joist (and several after it) on the slope previously discussed. Make sure the 2nd joist is jacked to the final height desired.
2. Use a 3rd jack to jack up the joist to be sistered.
3. Sister the joist.
4. Add blocking from the 1st to the 2nd joist.
5. Remove the 3rd jack.
6. Use the 3rd and 4th jack to jack up the 3rd joist (and several after it) on the slope previously discussed. Make sure the 3rd joist is jacked to the final height (watch out for the 1st beam to make sure it doesn't fall).
7. Remove the 1st beam.
8. Return to step 2 (moving down a joist each loop).


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## ponch37300 (Nov 27, 2007)

Well the little picture in my head for the five posts was 1 to jack up the joist I am working on, 2 for a beam on the next couple of joists, and 2 more for another beam which will be put in to move the first beam. This resulting in not having to move the first beam and let the joists down without supporting the next couple joists. This way I can keep alternating the beams everyother joist all the way down and the next couple beams will always be supported. I just have a fear of letting down the beam to move it, raising and lowering and raising the joists over and over. I would think it would be better to raise once and keep them supported till they are all supported.


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## HooKooDooKu (Jan 7, 2008)

ponch37300 said:


> Well the little picture in my head for the five posts was 1 to jack up the joist I am working on, 2 for a beam on the next couple of joists, and 2 more for another beam which will be put in to move the first beam. This resulting in not having to move the first beam and let the joists down without supporting the next couple joists. This way I can keep alternating the beams everyother joist all the way down and the next couple beams will always be supported. I just have a fear of letting down the beam to move it, raising and lowering and raising the joists over and over. I would think it would be better to raise once and keep them supported till they are all supported.


Your basically doing just that with the plan I've tried to outline. Basically, you use one of the jacks that will be used for the second beam on the 1st joist. When you bring that jack down off the 1st joist, the joist will already be in position never to be moved again. Then you've got both jacks available to setup the second beam while the 1st is in place. This should really be the case if one side of the beam already has the next joist upto it's final height. When you take the 1st beam down after the 2nd beam is in position, the "next" josit to work on should pretty much be in position.

Now you are correct that when you take the 1st beam down, the joist you are going to work on next will drop ever so slightly. Hence the need to use a jack on it. But I don't think the amount is going to be much of anything at all.

And like I said, you could modify it such that the beam is holding the joist you are sistering and the next set. That way, when you put the 2nd beam into position, you'll be moving the 2nd joist into postion (but again, watch out for the 1st beam trying to fall).


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## ponch37300 (Nov 27, 2007)

hooKooDooKu, I think we are talking about the same thing, just in different ways. I think I understand what your saying and maybe I"m worrying to much about the joists around the one I'm raising. I think with the advice you guys have givin me I have a pretty solid plan of attack and should be good to go. The real challange is going to be doing the joist for the second floor sometime down the road, not looking forward to opening up the ceilings to do them.

Thanks again to everyone for all your suggestions and advice.


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## HooKooDooKu (Jan 7, 2008)

ponch37300 said:


> ...The real challange is going to be doing the joist for the second floor sometime down the road, not looking forward to opening up the ceilings to do them...


Ouch! I had enough problems and made enough of a mess just working in a basement with a HVAC ducts in the way... but that was about it. But I must say that you don't back down from a challenge. Good Luck and do it safe.


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## Thin & Crispy (10 mo ago)

I know you're here seeking more answers and less questions, but I do have a couple of questions. How much upward force do you think it will take to push the existing sagging joists up into a level plane? Won't the new sistered-on floor joists be under much more stress if they have to overcome the downward force of the sagging joists which want to resume their former profile in addition to supporting the general weight of the house above?


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## Nealtw (Jun 22, 2017)

Thin & Crispy said:


> I know you're here seeking more answers and less questions, but I do have a couple of questions. How much upward force do you think it will take to push the existing sagging joists up into a level plane? Won't the new sistered-on floor joists be under much more stress if they have to overcome the downward force of the sagging joists which want to resume their former profile in addition to supporting the general weight of the house above?


This thread is 12 years old.


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## Thin & Crispy (10 mo ago)

Nealtw said:


> This thread is 12 years old.


I don't think the age of the thread is important. I was searching for some information on dealing with sagging floor joists and found some insight from what other contributors had shared. Please tell me why you were reading a 12 year old thread.


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## Nealtw (Jun 22, 2017)

Thin & Crispy said:


> I don't think the age of the thread is important. I was searching for some information on dealing with sagging floor joists and found some insight from what other contributors had shared. Please tell me why you were reading a 12 year old thread.


When you post to an old thread it shows up on the list of new posts where all the regulars here are hanging out. 
Some people add a helpful message to an old thread and there is nothing wrong with that but the problem has likely been solved and members have moved on. 
Some people post to old threads hoping for help with a similar problem, and the help may never come because the regulars who could help tend to ignore threads with real old dates. 
There is nothing wrong with posting to an old thread but there is nothing wrong with warning people as some have not noticed just how old it is.


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## 1865wingate (12 mo ago)

ponch37300 said:


> I was thinking about laying a 2x12 about 2 feet long down flat to help spread the weight a little. I doubt there is any footers in the middle of the slab. The post on each end of the beam should be close to footers since they will be by the foundation wall, but the two posts in the middle probably will just be on the slab.
> 
> Thanks


Are you sure the ends need to be jacked up? they should be set in the foundation. then use extras to space out the center.You will then distribute the load at more points and it will be easier. Could you then put the new beam beside the old and catch any bypassed joist ends?And footings allow that?


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