Footings for Jack Posts in Basement?

molsonbcJanuary 9, 2008

I'm planning on putting in 2 jack posts in my basement to slowly raise up a few joists to level in preparation to install a new support beam, but I was unsure about what sort of footing I should have for them. It's a concrete floor on a two story house, so I was thinking a couple of 4x4s on the floor would work.

And then when it comes time to install new permanent columns for the beam, I was unsure about footings for those, too. My thought was to cut a hole for each column, dig down 2 feet and use a little rebar and concrete to build up a footing. But I guess I wasn't sure if that was deep enough to dig. Any ideas about how much of a footing I should plan on?

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Ask your city/county what the code is and if you need a permit. If a permit is required (probably is) you don't get it, it will come back to haunt you when you try to sell down the road.

    Bookmark   January 9, 2008 at 3:53PM
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It's not only the depth of the footing, but also the size of the pad. A typical size footing would be two foot square and one foot thick. I would plan on supporting more than a few of the joists using two or three adjustable columns. As long as the concrete floor is at least a few inches thick, it should be ok for temporary.

    Bookmark   January 9, 2008 at 6:30PM
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If the posts are temporary then nothing of them will remain after the new beam is placed.

I usually use old sections of railroad tie about 5 feet long or a couple of layers of 2x12 to spread the load.
It does not have to be as good as a permanent footer for temporary use.

Permanent footers will need to meet the code for the load they will be carrying and the type of soil they rest on.

    Bookmark   January 9, 2008 at 8:04PM
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For the new footings, if your soil is undisturbed sand and gravel, and this is typical residential construction (nothing requiring engineering), than a 2 foot by 2 foot by 1 foot thick concrete (with steel rebar reinforcing) footer is generally sufficient. The top of the new footers should be about 6 inches below the top of the concrete floor in the basement. Again, this is for well drained, well compacted, sand and gravel, and typical non-engineered residential housing loads. If your specifics vary, it would be wise to ask a local soils engineer for specifics for your area.

If a permit is required, the AHJ will tell you how to do it.

    Bookmark   January 9, 2008 at 8:42PM
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It all depends on why you are jacking the floor joists. If it's just to take out a sag, and there's no load from a wall etc above, then a couple of 2 x 12's on flat are fine. Same holds for the permanent columns, if they are only carrying the weight of the floor above plus the live load and the new beam, there is no way they would ever punch through the existing slab, unless it is unrealistically thin. A cool way to take sag out of floor joists is to jack them straight and screw steel strips along the bottom edge, which is in tension when the beam (joist) is loaded. The load would then have to stretch the steel or shear the fasteners in order to deflect the joist. You could do every other joist.

    Bookmark   January 18, 2008 at 3:58PM
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