Re: Huge Hump in subfloor - HELP!
Have you observed the hump over a period of time?
If you have time, go rent a good laser level (one that projects to all walls from the center of the room). Put it on the center of the floor and measure from the line to the floor in multiple places around the wall. Do this every three months for a year in the exact same locations and see if the measurements vary. A contractor will also probably do this for you, but remember that their diagnosis will be influenced by their specialty...
If it's stable, then it could be the crown of the floor joists, in which case it's permanent and needs to be worked around. I'd probably shim the low points and use self-leveling compound. Or it could be that the center beam has been lifted too high for some unknown reason, which brings us to the shims -
The shims may have been put in because the pillar didn't reach the beam at installation. Or maybe the pillars sank and someone put in the shims fearing that the beam wasn't supported, and then they raised again (a seasonal thing) and that lifted the floor. If you try to remove them, do so carefully and slowly. Jack the beam to relieve the pressure - no more than 1/8" above the current height. Remove the shims. Now lower the jack 1/8" a day until the beam rests on the pillar (IF it will reach it - remember that they could just be too short). Any more than 1/8" movement too quickly and you'll be patching and dry-walling for the next few weeks...
If it changes over time, it's probably movement in the pillars holding up the center beam. That can be solved by supporting the beam, removing the old pillar, breaking up the concrete under the old pillar, and then sinking a pier down to the bedrock which will support a new pillar.
If there is a stud wall running along under the beam, then the studs may also need some sort of slip joint so that they don't start taking the weight of the beam once the pillars have been made stable, since the rest of the basement floor could still rise and fall taking the wall with it. Or you could remove the wall and replace it if desired with a shorter wall that won't touch the beam even when elevated.
Anyway, that's my 2c on the matter. For the record, I am not a contractor, just a homeowner that has a similar, though less pronounced, problem. I feel like I've read every piece of material on the topic out there...
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