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Captain.Sassy 08-06-2010 11:38 AM

Jacking and Sistering Floor Joist
Hey guys,

So I've got a crack in one of my floor joists in the basement. The joist looks like it's a pretty important one (it's a bit wider than the others). Right above the joist, on the first floor, there is also a column that seems to support the second floor above. So I'm gonna try and jack the joist up and sister some 2X10s on either side.

I don't want to remove the wiring that's going through the joist, so I'm planning to put some notches in the sister boards to fit over the wiring.

The crack has appeared where there is a notch in the joist for an air heating duct. The notch is actually fairly small- about a half inch deep by a foot across.

The reason I think the joist is cracking is because the previous owner of the house (or one of them- the place is over a century old) dealt with a slope in the first floor by pouring a concrete floor over the hardwood from the front of the house to the kitchen in the back. I think the concrete floor is about 2 inches at its thickest. Needless to say, I don't relish the thought of pulling up all that concrete (and honestly, I'm retiling the kitchen this fall and putting in new counters and cabinets, and the concrete does give a pretty flat surface to work on).


With all this in mind, my plan right now is to

0) Lay a strip of 1" thick plywood along the bottom of the joist, screws to fix it on one side of the crack. The strip will be about 3" wide- to fully cover the bottom of the two sister beams and the current joist

1) use a hydraulic jack and 6X6 post to jack up the joist and close the crack (the crack is very small right now- can't see to the other side, and only looks to have separated a few milimeters at the widest). I will use a short section of 6X6 runnign parallel to the joist, and on top of the 6X6 post, to try and distribute the force of the post pushing up (like a 6X6 T on top of the jack)

3) put a steel jack-post directly under the crack. On top of the jack post I intend to have two pieces of 1" thick plywood to help distribute the force; the piece of plywood directly on top of the jack post's top plate will be slightly wider than the top plate, and the piece of plywood on top of this will be a bit wider still. Kind of like a small, inverted pyramid.

- MY QUESTION HERE IS IF I NEED TO POUR A CONCRETE BASE FOR THE JACK POST- I'M Planning to leave it permanently in place. I read about pouring a concrete foundation with a re-bar X in the middle. If this is the most cautious thing to do, then I'll mix some concrete and pour a 2'X2' square down there with a couple of 33" rebar pieces crossed in the middle. I'm not sure how thick I should make the concrete base though. The floor of the basement looks like it's poured concrete.

4) After putting the jack post in place, I plan to then put a bunch of really strong wood glue on the two sister pieces, then clamp them togehter sandwiching the joist between them. Then I'll put some screws up through the plywood strip at the bottom into the joist and both sisters, and then I'll put some bolts through the sandwich. Two bolts at either end, then one kind of offset a little further down, and then maybe a couple more near the middle.

The sistering pieces I plan on making about 5' long.

SO, my plan is to leave the jack post in place permanently to give extra support, and also to have the joist sistered (while it's not cracking too bad, it is splitting a bit.)

I wanted to bring in photos but the camera died before I could copy them off of the camera and onto teh 'puter.

Cheers, and thanks for any suggestions


jklingel 08-06-2010 02:43 PM

You've got a tough one there, but it sounds overall like you will be better off when you are done. If it were mine, I'd cut those electrical wires in a heart beat; notches in beams, obviously, don't do anything to help w/ load carrying capacity. If I recall, a notch may reduce the beam to a beam even less high than what the depth of the notch reduces it to (make sense?). The post will make a huge difference in what the beam will carry. I think a uniformly loaded beam carries weight inversely proportional to the cube of the length of span; ENGINEER, correct me here. As for your 2' concrete pad, why not over kill it and use 5" or 6" thick, 3' x 3', and put in more rebar; two pairs about 4" apart, perpendicular to each other? I'm not an engineer, so I go extra heavy when it is cheap and easy to do so. The problem is, what is below this? I'd have to assume that the concrete that is in place will distribute the load "pretty well", but I'd go wide on the new "footer".

Captain.Sassy 08-06-2010 02:57 PM

I hear you on the 'If it's cheap, go big'. For sure. The only thing with making the base wider than 2'X2' is that I may eventually want to finish the basement, which will mean framing and drywalling the post to make it look like it was always supposed to be there. :P A 3X3 pillar in the middle of the basement takes up a lot more room than the 2'X2' would... If I knew where to get a sheet of steel and get some holes drilled in it I'd probably use a 2X2 steel sheet instead and tapcon it into the concrete floor, cause you can just put nice floor around and over that and frame a few inches around the jackpost.

Also, what you say about the notches kinda does make sense to me if you think about the wood grain being attached to other wood grains, I can imagine that the split wood grain at the notch would like 'pull' the un-split wood grains apart. But maybe I'm just imaging stuff. Anyways, one reason why I want the 1" plywood strip on the bottom is to help keep the strength of the beams where they're going to be notched, to kind of pull teh two edges together.

Daniel Holzman 08-06-2010 03:47 PM

Couple of comments. First, make sure the post you are planning to use is rated by the manufacturer for permanent use, not just temporary use. Second, you are going to need a solid footing, therefore you need to break or cut through the concrete floor, dig out any weak (read organic, loose, or soft clay) soil, and install a footing suitable for the post. Do not install the footing above the floor, that is not suitable for a permanent post.

Not knowing the soil strength, and without details about how much load the post will support, I cannot comment on the required footing size, check with your local building inspector (I assume you are getting a permit).

Captain.Sassy 08-06-2010 06:46 PM

Thanks for the suggestions, Daniel. The concrete floor is pretty solid, and I'm hoping most of the weight will be held by the new joists anyways.

I was planning on leaving a telescoping support column in there as extra support. After your comment I think I will try and find a single piece column that is certified for permanent use.

I've found a metalworking place that could do me two 2'X2' sheets of 1/4" thick steel with holes punched so I can fix the post to the sheets and the sheets to the concrete floor, so I'm planning on using these (one on top of the other) as the base. Would that really not work?

jklingel 08-07-2010 12:34 AM

Captain: On the way home, I was thinking exactly what Dan said about your new footer. (Seriously and no kidding.) As for the 2 sheets of 1/4" steel, that will help, but that is not the greatest use of a plate. Try a piece of paper that way, vs ripping it apart when lying on a table. Usually, supporting columns have gussets welded to them, and to the post, to help spread the load. Sure the concrete looks solid, but you don't weigh much. I'd go w/ Dan's idea, personally. j

Captain.Sassy 08-07-2010 09:38 AM

I think I'll try and just pull all those wires out and sister two new joists along the existing joist from end to end then, and forget about the post for now. Worse comes to worst, I'll cut a square out of my basement floor and pour a footer and put in a post if the sistered joists don't do the trick i.e. if there's any new cracking after the fix. (I might need to do that or something even sturdier eventually anyways if I ever put a roof deck on the top of the house)

There's a telescoping steel post down there which looks like it used to be holding up the joist right where the crack is now. Whoever was last in here moved it over a couple of feet to be right close to the wall , and I think that's when the joist started cracking (good job buddy! :thumbsup:). The post looks pretty old too!

Thanks for the help, any further recommendations always welcome!

Here are some photos of the joist in question.

epson 08-07-2010 10:19 AM

Ok when sistering your existing joists you should span the entire length. Use liquid nail adhesive and either nail or bolt right through with nut and washer. One of the reasons your joist is cracking is because the previous owner decided to add a 2’’ concrete floor and the existing joists could not handle the load and now you want to add tile and kitchen cabinets which will add more load. Also the old spacing of your joists might not be adequate.
1) Go to your local city building department and tell them your situation and ask what the required joist and spacing should be for your requirement.
2) As per your footing/pad it should be 3’ x 3’ square and at least 3’ deep for the frost line, then you need to add 10mm re-bars one row bottom and one row top in the opposite direction.
3) As per your column, when you have your pad in place and your joists temporarily jacked and straight you can measure from the pad to the underside of your joist and have a fixed steel 3 ½’ round or adjustable column made for your application. The column should have a base plate 6’’ x6’’ x3/8’’ thick minimum and a top plate in a “U” shape wide enough to accommodate your sistered joist with 4 holes 5/8’’ diameter to accept ½’’ bolt and washer and when put in place drill right through “U” plate and joist and bolt.
4) Now your existing basement slab should be approximately 3-4’’ thick and your new pad will be lower by approximately 4’’ so when your column sits on the pad you can you fill in with concrete and level it off to your existing floor and it will be permanently fix.
Hope this helps...

Captain.Sassy 08-07-2010 04:31 PM

If just sistering the joists doesn't work, I'll stick a permanent post there. Thanks for teh detailed instructions.

So if I'm reading this right, I dig out a 3X3X3'4" hole in the basement (i.e. 3X3X3 space with the top of the space just below the bottom of the poured concrete floor) and then line the bottom of the hole with rebar going in one direction, then line it with rebar going the other direction to make like a rebar grid. Then pour my concrete in there, let it set so I get a nice concrete cube for the base, then put my post (w. base plate as described) on the poured concrete cube, adjust the height to the height of the jacked up joist, then affix the U shaped metal piece at the top of my post to the joist sandwich?

jklingel 08-07-2010 10:58 PM

I'd modify epson's suggestion, myself, though doing "as is" would not hurt. Frost will not be an issue in the middle of the house. Here is something I copied from another site; a guy is quoting John Straube. "A slab on grade insulated to R-32 in Finland had an average heating seasons soil temperature of 12.5°C (55°F)". This is consistent with isotherms I've seen, too. However, if you have to dig down 3' to get solid soil, then dig, and compact your way back out. Hopefully, the earth was compacted below the slab before construction. Hence, going down 8" should be plenty. I'd then pour that whole hole full of concrete, right to the top of the slab. I'd put two rebar grids in, 2" from the bottom and top, maybe 6"-8" OC. Wire the lower rebar grid together and set it on a few small concrete blocks (broken block pieces) or rocks, and pour concrete. Once you pour up to 6" thick, stick the other grid in and complete pouring. Smooth that puppy off to the original floor and let it sit a few weeks. Then install the post. If there is any concern about the rebar being too close to the surface, dig a tad deeper and put the rebar grids at 3" from the bottom and top.

epson 08-09-2010 12:30 AM

Yes that’s it and the re-bar should be 2’-6’’ long. Also you should wire them together like jklingel suggested (I forgot about that) and put at least (5) bricks of equal height under the re-bar one in each corner flush with the re-bar and one in the middle so the re-bar will stay raised when you pour your pad.
As for the Steel column the “U” shaped piece should be at least Ό’’ thick and whatever the two joists measure in width (inside dimension) and the length and height should be a minimum of 6’’ and welded to the steel column before it is put in place. When this is done, drill through the plate which already has the 5/8” holes with a ½’’ drill bit long enough to come out the other side and line up with the other hole so you can slide your bolt in and tighten it with a nut and washer.
When this is done all you have left to do is pour the remaining concrete on the pad to cover the column base and make it flush with your existing floor and your done. Oh one last thing when you pour your pad you have to wait at least one week for the concrete to cure before you place a load on it.

COLDIRON 08-09-2010 07:11 AM

Maybe this would work but it is more involved.
Move all electrical out of the way.

Remove ductwork and install new joist from plate to plate.. jack up old joist and secure to new joist with through bolts.. resize ductwork to fit make it wider and cut the height down to give you the same airflow with a transition fitting.. now it's permanent and no need for support post to be installed.

boman47k 08-09-2010 08:24 AM

Could it not just be sandwiched between sheet steel?

How is jacking going to affect the leveler that is on the floor? None, if you just close the crack?

Not suggesting, just pondering the situation. I am not qualified to suggest.

epson 08-09-2010 11:53 AM

Just putting sheet steel on either side is not going to do anything with the sagging. The floor will continue to sage unless you put a support under the just and with the load that he has he needs a support. As per “jacking” the column is jacked under the joist until the floor is level remember the existing joist is cracking, that means the above floor is out of level and in order to put it back you have to jack it back up and put a support under it.

boman47k 08-09-2010 03:00 PM

Maybe sheet steel were the wrong words. I mean maybe from a 1/4 to 3/8" thick.

I agree, if there was a support there to start with supporting an upstairs load, it needs a support put back.

I also believe the floor leveler is subject to crack when the floor is raised back up, but that is no big deal considering.

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