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I recently bought an old home- 102 years old and have some flooring issues. The long story is we bought it knowing it had foundation problems in the rear right corner. The home had a poured concrete foundation with the exception of the rear back of the house that had 3 ft added on and a brick and mortar foundation that needed to be repaired. Basically with the brick was crumbling and had sank about a foot- pulling the house down on that side by about 10-12 inches.

We had the brick removed and replaced with rebar and a poured foundation in that area but the damage to the floor inside remains. When they replaced the foundation they did jack it up maybe an inch to replace everything but considering the age and how much it sagged we did not want the house to be jacked up back to its original level state.

It’s a two story home with original hardwood flooring that we have had restored. The subflooring in that rear room is damaged and basically is very spongy in places. I was told one way to fix it was to jack the existing subfloor up between the joist with and add a “subfloor” (plywood) and secure it in place by nailing 2x4 to the existing joist on either side to hold my new subfloor in place. I don’t want to compromise those existing joists though and this seems like a really good way to do that.

Is there another option out there or is this a legitimate way of fixing the problem without ripping up these beautiful 100 year old floors? There is a conventional foundation so I can crawl around there without issue.

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    My understanding is that you can jack up the structure to be level, but you can't do it all at once. You can do like a turn or two on a floor jack each month. I've seen other people advise as much as an 1/8" per day, but that is more aggressive than I would be comfortable with. My priority would be to fix that underlying structure before worrying about the flex of the floor; jacking the floor might resolve that issue anyway. – UnhandledExcepSean Nov 19 '19 at 16:15
  • I'm not as much worried about making it level, just not spongy. Only one section of the floor has a bit of an angle or slant. The other area's are either secure or spongy. – Trae H. Nov 19 '19 at 16:49
  • I'm unclear on what exactly is spongy. Is something rotten? Is something flexing due to lack of support? – isherwood Nov 19 '19 at 18:17
  • Agree that "spongy" is an indicator of failed support, or rot, or something. That is different from the slant due to support sag. – Carl Witthoft Nov 19 '19 at 19:54
  • Sorry, yes the subfloor between the joist has received significant damage- the joist have not. So when you walk on it, the floor kind of dips then is firm then dips as you walk from joist to joist. – Trae H. Nov 20 '19 at 14:11
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The given parameters are that the floor joists and the flooring are good, but the sub-floor has "received significant damage." The solution then, is to replace the sub-floor. Don't assume that removing the existing hardwood floor will destroy it. Re-using old hardwood flooring is common. It reads like the hardwood and the sub-floor have already separated. With a 100 year old home, there probably isn't any glue (construction adhesive) to deal with, just nails that have lost their hold. An added benifit is that after removing the bad sub-floor, you could shim up under the replacement to correct the leveling issue. A few floor boards could be damaged during the removal. And the removed hardwood needs to be prepped before re-installation. But those issues are part of a good floor restoration project. With a bad sub-floor, it is hard to imagine a practical or quality alternative.

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Go at it from beneath, in the crawlspace. If the old sub-floor is rotted or easily crumbles away, then crush and scrape it away until it's out of that area. If not, then just leave it alone.

Then, you can very simply toe-nail (or screw) 2x4 bridges into the joists between the joists to support the floor. The 2x4's would just be held against the sub-floor (or finish flooring, if the sub-floor has been removed) while toe-nailing (screwing).

You may also be able to use a length of 2x12 against the sub-floor or finish flooring as a Bridge Backer to run a length within a or many joist bays. The 2x12 would just float or be free and not be screwed to anything.

This method can also be used to jack up that area of flooring without ever affecting the floor nor its finish.

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