Not sure that this is the right forum for this but.....

I need to jack up a cement bathroom floor in order to sister new joists onto rotten ones. I could really use some advice on the best way to go about this!

The cement slab floor is 5 1/2' x 7 1/2' and is 3 1/2 to 4" thick with ceramic tile on top. The cement was poured into a shallow box which extends a couple of inches below the tops of the floor joists. The area of basement just below this floor is dirt. At some time in the past, probably when the old subfloor was replaced with cement, the dirt under the bathroom was dug out to make it possible to stand below it so the distance between the ground and the bottom of the slab is approx. 5 to 5 1/2'. I need to raise the cement about 3/8 to 1/2" on one end in order to get the new lumber in place.

My first concern is what type of jack to use to lift the weight of the slab. Is there a formula for calculating the weight of something like that? I already own a couple of 5 and 10 ton hydraulic jacks, would they work if I used a 4x4 post between them and the bottom of the slab or should I check into renting or buying some other type of jack? Since the width of the slab where I'll be lifting it is 5 1/2', should I use more than one jack? Any thoughts on how best to go about this would be really appreciated! I'm completely in the dark! lol

I figure that the cement slab with probably crack during the jacking process so I plan on breaking it apart after the joists are repaired and replacing it with new subflooring.

My description leaves a lot to be desired so if you'd like I can post photos to make it clearer. Just let me know..........
Thanks,
Pegdot

 

Sounds like you have about 13 cubic feet of concrete in the bathroom floor to jack up. If I remember correctly it's about 50 - 60 lbs per cf, so a car jack is plenty. If I understand correctly, you have a 4" cement slab poured over floor joists?? Wonder why it sagged....
If you're sure you want to tear out the slab, I'd so that before jacking anything. If you want to leave it in place, I'd reinforce the joists and use a cross beam on a 4x4 post. Make two, use one for jacking and one to leave in place, maybe a foot from the low end. Hope that helps...
Matt

 

Thanks for the info.!
The shower has apparently been leaking slowly for a long time and had turned two joists directly underneath it to nothing more than rotten mush. Since this is at one end of the slab it has tilted down slightly.
I'd love to be able to break up the cement and then repair the joists but I'm afraid that my trusty sledge hammer & I would both likely end up in the basement along with the cement! lol