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Old 06-11-2009, 01:05 PM   #1
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Jacking Up Floor Joists in Tight Crawl Space


Hey all, I've been doing a good amount of reading on this topic and seems like there are some experts on this forum. There are some specific things I haven't found an answer to just from reading, so I'm here to post a couple of questions.

First, the scenario: I have an older house (1946) with a standard concrete block foundation under most of it. THere is one 9'x15' addition which just has a crawl space under it. It runs long ways from the front of the house to the back and is positioned on the side of the house (3 outside walls). The house also sits on a hill, so the back of the house foundation is buried, and the front is walk-out. Thus, the crawl space under the addition is very tight in the back (maybe 8-12 inches of space, not enough to crawl under) and open in the front (2.5-3 feet).

The floor in the addition has started to sag, of course, at the tight end of the crawl space. We would like to install new floors anyway, so I started chopping down through to see what I could find. 1 carpet and two hardwood floors later, it seems the joists are in decent shape (no rot).

What I thought I might do is open up the floor the width of the addition, and maybe 4-5 feet long (so 9' by 4' or so), get in there with a shovel and start getting some room under the joists, pour a concrete form, jack up the joists and shim under them with some blocks or metal posts.

How deep should I pour my form? Some say past the frost line, but its a very rocky area and I'm not even sure if that's possible. Since the ground is sloped, how to I ensure that my form/repairs will not shift over time with rain water and ice?

Thanks in advance.

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Old 06-11-2009, 03:17 PM   #2
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Jacking Up Floor Joists in Tight Crawl Space


Your situation is not that uncommon. Before going to a lot of trouble to jack up floor joists, which is never a simple job, you should determine a few things.

1. Are the joists adequately sized for the spacing? If not, you can jack them up, but they will sag again over time.

2. If the joists are adequately sized, they may already have achieved a stable configuration, and it may be unnecessary to jack them up. Perhaps all you need to do is to level the surface by shimming or use of levelling compound.

3. I don't understand what you mean by "pouring a form". A form is used to allow controlled placement of concrete. You don't pour a form, you place concrete in a form. But in this case, why are you doing it at all? Is it to support the jacks? Or to support the joists after they are jacked into position? The use of jacks is generally a temporary measure to get your joists back to level, as long as the jack is adequately supported during the jacking operation, the long term stability of the jack support is not important, since you will be removing the jack anyway. Of course, if you are planning to install a permanent support (i.e. a lally column) after jacking the joist, that is a different matter, then you would need an adequate, permanent column support.

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Old 06-11-2009, 03:23 PM   #3
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Jacking Up Floor Joists in Tight Crawl Space


Daniel, thanks for the reply.

I was probably unclear, but you seem to have figured out what I meant. Yes, I meant I would jack the house into position and then add a permanent support. The front of the house has a concrete block wall just like the foundation of the rest of the house supporting it. The back end, however, just has a couple of blocks in each corner and, as far as I can tell, really nothing supporting it in the middle.

So I figured I would dig down, pour a flat slab of concrete and use that as a foundation for a perminant support that I would place under each joist after it is in position. I assumed I could use concrete blocks for this or a steel post. My concern is that my slab or the blocks will move out of position over time...
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