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Old 03-06-2010, 05:38 PM   #1
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This is whats happening on my sagging floor.


I'm wondering what the proper methodology is for raising the sagging floor joist. As you can tell in the diagram the outside wall is 8" concrete blocks on top of a footer with a poored concrete basement floor. Center beam is a 6x8" old timber beam and the support poles are 3-4" steel posts. The floor joists are true 2x4x12 that measure those exact dimensions. The joists are hard as hell and diffcult to even pound nails into.

Not all the joists are sagged. In fact, in the garage section of my basement I have 8 joists and the 3 nearest the center of the house are sagged as much as .75" to 1.0".

I want to sister modern 2x8x12's in place along the sagged boards after raising them back to the original height. I'm wondering what the best way is to do this? My plan was to put a new beam 90 degrees to the 8 floor joists directly at the sag location and use new steel supports on each end? After getting the new beam at the right height I'd like to then sister each of the floor joists. My question is how should I raise the new support beam to get the joists the right height? I have access to heavy duty bottle jacks. I've heard that raising the joists too fast can cause them to fail or cause cracks in my plaster walls upstairs. I've also heard that the plaster will crack no matter how slow I go if I'm moving them joists an inch.

What should be my plan of action?

EDIT: I just noticed my diagram shows the block wall on top of the concrete pad. The bl0cks are actually laid on top of a footer and then the concrete pad was poured up to the wall.
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Last edited by speedster1; 03-06-2010 at 05:40 PM. Reason: Noticed a mistake in diagram
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Old 03-06-2010, 07:15 PM   #2
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This is whats happening on my sagging floor.


if those are old growth timber they may not straighten out without moisture. I myself would consider notching/ saw kerf in the center jack them straight and sister at that point

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Old 03-06-2010, 07:49 PM   #3
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This is whats happening on my sagging floor.


Your diagram shows the joists are true 2x8 inches, but your post says they are true 2x4 inches. Please clarify.

Assuming they are true 2x8, you say you want to sister modern 2x8 boards onto the old ones, after jacking up the old ones. I checked the strength of the old joists, and they check out OK as far as strength and deflection, assuming they are 2x8 not 2x4. Again assuming they are 2x8, and assuming you have 50 psf dead plus live load, and the joists are 16 inches OC, they have apparently deflected more than the theoretical deflection of about 1/4 inch.

Modern 2x8 boards would be expected to deflect approximately 0.4 inches under the loading I described. So unless you get the two boards to work together, you are not going to improve deflection.

Anyway, once you clarify the actual size of the joists, we can discuss options further. Check out the spacing too, I did not see it in the post.
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Old 03-06-2010, 08:07 PM   #4
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This is whats happening on my sagging floor.


Daniels question need answering and tpolks point is very true-----

I have a suggestion for jacking---Do you know what a jack wall is? I've used them to life amazing weight--- I'll write you out a description if you aren't familiar with the jack wall.

You bottle neck jacks will be dangerous and infective.--Mike--


Here is a link to an old post with a description or a jack wall.
How To Replace A Rotted Sill In An Older Home - DIY Chatroom - DIY Home Improvement Forum

Last edited by oh'mike; 03-06-2010 at 08:15 PM. Reason: Found old post
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Old 03-06-2010, 08:12 PM   #5
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This is whats happening on my sagging floor.


could you add another header down the middle for the entire span? Jack it up till your happy with the floor strength and straightness. It takes away a little from the basement but overall might be easier and stronger. Dig out a few footings and youre in
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Last edited by II Weeks; 03-06-2010 at 08:23 PM.
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Old 03-06-2010, 10:36 PM   #6
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This is whats happening on my sagging floor.


Thanks. To clarify, the floor joists are 2x8x12 and are 16" IOC.

My initial thought was to do something like what II Weeks suggested. But my thought was that the second header would be temporary and would allow me to then sister all the joists really well and then remove the header in order to safe my space. The problem is my garage is about 12 feet wide. I'd be okay with adding a permanant 12 foot header on two support poles that set near each wall. But if a support pole was placed at the 8 ft location it would be in the middle of my garage and not allow me to pull my truck into the garage.

I have 5 jacks total. 3 of them are really nice and were used in coal mines to support roofs. They are tower style with a huge rod on the top that you rotate in circles. They work well because they are very accurate. The other 2 jacks are rachet style. They can lift a lot of weight but aren't cuite as precise. About 1/4" per crank. Since there are really 3 main joists that are sagging theroetically I could place a jack beneith each floor joist and crank each one seperately until I get to my desired floor height. At that time I could sister then joists using a framing nailer. I wasn't aware about the lack of bending in the old growth wood. I may have to kerf cut them in order to get them to bend up.
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Old 03-07-2010, 01:14 AM   #7
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This is whats happening on my sagging floor.


I would not attempt to jack an old growth 12' d/f joist in the middle for 1" gain. The idea of sistering is to correct any sag with the new joist. Start reading on page 134- 136: leveling a floor, and the case studies....... http://books.google.com/books?id=1uY...lywood&f=false

Be safe, Gary
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Old 03-07-2010, 08:57 AM   #8
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This is whats happening on my sagging floor.


Quote:
Originally Posted by GBR in WA View Post
I would not attempt to jack an old growth 12' d/f joist in the middle for 1" gain. The idea of sistering is to correct any sag with the new joist. Start reading on page 134- 136: leveling a floor, and the case studies....... http://books.google.com/books?id=1uY...lywood&f=false

Be safe, Gary
Thanks. I read that section and I'm still a little unclear as to what I should do. What if I would do what I shown in the below diagram?
  • Uniformly use the mine jacks to raise the new header into it's final level position.
  • kirf cutting the old timber if necessary in order to get it to bend.
  • At the time the floor becomes level I can sister new 2x8's alongside the existing joists with a 3.5" framing nailer.
I could indeed leave the header in place if I need to as long as I only use two supports. One at each end. I'd hope that the header along with sisters joists would create a pretty strong flat floor.

What do you think?
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Old 03-07-2010, 04:23 PM   #9
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This is whats happening on my sagging floor.


the problem with sistering is existing wiring and plumbing usually. If thats not an issue, than go with it and no need for support columns near existing foundation walls. They'll work with either notching a little or steel angle iron bolted through.

PS didnt see this was for a garage
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Old 03-07-2010, 06:41 PM   #10
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This is whats happening on my sagging floor.


Quote:
Originally Posted by II Weeks View Post
the problem with sistering is existing wiring and plumbing usually. If thats not an issue, than go with it and no need for support columns near existing foundation walls. They'll work with either notching a little or steel angle iron bolted through.

PS didnt see this was for a garage
No serious issues with plumbing or wiring.

So you think sistering the joists alone will be enough and there is no need for a new beam? Interesting.
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Old 03-07-2010, 08:34 PM   #11
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This is whats happening on my sagging floor.


Your old growth, true size 2x8 is almost certainly stronger than any new dressed 2x8 you can buy at the big box store. After you kerf cut the old joists to get them to bend without breaking, they have effectively no strength, so you are in essence replacing a full size 2x8 with a dressed 2x8.

If your goal is to flatten the floor, you may want to consider shimming the existing joists (insert shims between the joists and the flooring) by using a string line to define level. Those beams have almost certainly taken a permanent set, and are not likely to deflect much more if at all over the next few years. Sistering a joist by kerfing the existing beam adds no strength, but sounds like a real killer project given the lack of room.
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Old 03-08-2010, 06:19 AM   #12
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This is whats happening on my sagging floor.


Quote:
Originally Posted by Daniel Holzman View Post
Your old growth, true size 2x8 is almost certainly stronger than any new dressed 2x8 you can buy at the big box store. After you kerf cut the old joists to get them to bend without breaking, they have effectively no strength, so you are in essence replacing a full size 2x8 with a dressed 2x8.

If your goal is to flatten the floor, you may want to consider shimming the existing joists (insert shims between the joists and the flooring) by using a string line to define level. Those beams have almost certainly taken a permanent set, and are not likely to deflect much more if at all over the next few years. Sistering a joist by kerfing the existing beam adds no strength, but sounds like a real killer project given the lack of room.
What is the process for shimming between the floor and the joist? Not sure how I can see that even being possible considering the floor is nailed to the joists with god knows how many nails?

Yesterday as a test I took one of the mine jacks and placed it directly under a sagging joist in the most deflected part of the floor. No load bearing walls above it, just open floor space. The mine jack easily pushed the sagging joist up. I placed another jack on the joist beside the other one and it also pushed up quite easily. I didn't hear anything cracking. I realize this will probably be a differenct story if I try to use them to raise another area in my basement that is below a load bearing wall. So considering that the jack did indeed push the sagging joist up without breaking or cracking it I believe I probably could sister a 2x8 beside the old joist. I think I may have to actually push the joist up a little more than what it will require to make it level because once I sister the new board and remove the jack I suppose it will deflect back down to some degree. Although how much I'm not sure.

Another quick qestion: If I'm spanning 12 feet what would make a stronger beam? Three standard 2x8x12's or Two 2x8x12 LVL's?
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Old 03-08-2010, 08:11 AM   #13
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This is whats happening on my sagging floor.


I have found over the years in working with historic homes that it really depends on the species of the wood as to wether the sage would come out. If they seen to be jacking up easily then I would go ahead and jacke them up. Make sure that you are lifting a larger section of the floor at once or you will be putting too much strain on the one joist. If there seems to be significant pressure ( which it sounds like there isn't) then you could always go with and engineered lumber (LVL) instead of demensional lumber to sister on with. As far as anchoring the lumber I have found that gluing and screwing or bolting works best into the old lumber. Use clamps to make sure that you have a tight fit before anchoring it. Also Jack the floor a little higher than you ultimately want it. Only go about 1/8 to 1/4 higher. This way when you let the jacks down they will then relax into the desired position. Good luck.
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Old 03-09-2010, 12:39 PM   #14
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This is whats happening on my sagging floor.


Just jack the house up an 1/8 inch a day. not really that hard. There is also no need to put a 2x3x3 footer down below. Why would they be sagging under normal load? Unless you got ALOT of **** up there. New 2x8 are rated for more than 12ft at 16"o.c. If your that worried about cracking plaster than just leave it alone. It isn't going nowhere.
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Old 03-09-2010, 03:18 PM   #15
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This is whats happening on my sagging floor.


I've decided I'm going to build a 12' beam from two 2x8x12 sandwiching 1/2" plywood and construction adhesive. I will run the beam 90 degrees to the joists at the halfway point of the span.

I will raise the beam uniformly but may need to indivually shim certain joists based on whats happening with the upstairs floor. If I do indeed need shims are there a specific type rated for these types of loads? Or will standard wood shims (type used to set door jambs) work?

Thanks everyone for your help and advice.

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