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Discussion Starter · #1 ·
tl;dr: Trying to add support in a crawl space to lift a sagging floor. Will a 24" x 24" x 8" footing with rebar do?

I have been reading a lot about jacking up a floor and would be delighted if the fine people here could critique my Googling.

The floor in question is framed with 2x6 joists, is not well-supported, and is - predictably - sagging. I would imagine the span is about 18 feet with sporadic supports here and there. The house has a single story and does not have any remarkably heavy objects over the problem area. It does have an exterior wall parallel to the joists.

It would be nice to sister the 2x6 joists with 2x8, although the crawl space is extremely small and it may not be possible to get new joists in there.

General consensus seems to be I should install a beam of two 2x10s, possibly two beams if they'll fit, jack the beams up with bottle jacks 1/8" per day, and support them with screw jacks.

There seems to be some disagreement about what to place the screw jacks on. I have read:

  • Pavers are fine.
  • Pavers will not support anywhere near that much weight.
  • Concrete footings are fine.
  • Poured concrete is better than concrete footings.
  • Poured concrete should be 8" deep and should be placed at the point where the earth becomes hard.
  • Poured concrete should be 12" below the frost line (that would be 9 feet deep here)

While I can see the value in going below the frost line I'm extremely skeptical that will be possible given the very limited space to work in. Also I'd be concerned about attempting to dig a hole 3 feet deeper than I am tall, while under an already unstable house. If I get out as much dirt as I can, pour a 24"x24"x8" footing with rebar at 4" in a # pattern, can anyone here speculate on what results I would see?

Even if I could get the floor close to level and adjust the screw jacks with the seasons, that would be an improvement.

Thanks to all.
 

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  1. Can you give us a rough idea of where you’re located so that we can judge the temperature range. “Southern prairie provinces” is close enough, as an example.
  2. Do you know what the coldest wintertime temperature in the crawlspace is?
  3. How far from the external foundations will the permanent screw jacks and their support platforms be?
  4. Have you contemplated insulating (and possibly heating) the crawlspace to maintain an above zero temperature there and obviate the need to consider frost heaving of the earth under the support platform?
Chris
 

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Discussion Starter · #3 ·
Hey, thanks a lot for replying.

1) Central Alberta.
2) There's a furnace nearby so I would guess no cooler than 15ºC / 60ºF.
3) The closest ones would be pretty close I would think. 12" or so.
4) That would solve a ton of problems. I did not realize it would be as easy as that.
 

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Is the whole house on peers or is there a foundation all the way around the house if there is a foundation all the way around the house you probably don’t have to worry much about the frost line


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It doesn't take much to cut 2x2' hole in the block foundation to work through. Poured is different. 24" cut will not require you to install a header and that is up to 30", most bsmt windows are 30" with band joist. Not if there is a load bearing beam/post there in the floors/roof above. Footing inside does not require at frost depth but you must dig until you find "hard" and undisturbed earth. That depends on how the house was built and if the crawl space ground was filled or not dug up at all. If dug up and filled later, not sure how you can put a footing there since indoors will see not much settling without even the help of rain.
Easier maybe cut the floor and work from above and sister/replace the joists. Probably faster since lifting a sag and bowed joist takes time. You may not be able to lift the joist overnight, example. How much sag? Sag means the lowered part is deepest at middle so you could leave as much as a foot along the perimeter and still see fairly flat floor when finished. My floor was about 1.5" sag at deepest though I handled it with some supporting wall underneath (full bsmt) and shimming the floor. Small house but I shimmed just half the floor and the rest was at less than 1/8 and becoming flush and just 3/8" engineered floor. 2.5" strips. I had to use 3/8 not to change the stairs first riser much.
 

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On the subject of frost heaving of a support pad for the jack(s) - if the house has a below frost-line concrete foundation and the crawlspace temperature is 15C, then it is unlikely that this will happen. Here is an interesting article that discusses the subject. The other thing to consider is the moisture content of the ground under the house, which in Alberta could be very dry. Without moisture, frost heaving is also not an issue.

If you were to upload some photos to give an idea of the ground composition, the specs on the screw jacks that you intend to use (maximum force and baseplate size), there are some folks on the forum who might suggest the appropriate dimensions for a poured concrete support pad.

Chris
 
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