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Recently I had some waterproofing done on my basement. This included removing some concrete that anchored/lifted the stringer's supports so that old concrete could be removed and new stuff poured (see image 1).

I don't have a problem fixing this once the new concrete has finished setting (I was fully aware that this was on me after the job was done), but unfortunately I have no idea how to approach it.

Image 1. The darker area on the floor in the image was originally a mound of concrete, maybe 1/2" to 3/4" high stretching between both supports (you can see some remnants under the support on the left, all was removed near the wall).
Currently there's just a scrap board holding up the stairs near the bottom end (top left of image 1 in the background), and I'm just trying to go downstairs as carefully and infrequently as possible for now.

Image 2. You can see there is a slight height difference between the bottom of each stringer and the floor. There was a difference before, as there is a floor drain not too far from the lower end, but I'm sure the new concrete slightly increased the difference. A scrap piece of plywood temporarily lifts one end. It looks like there may have been a small patch of concrete lifting this part as well, but I'm not 100% sure.

Eventually I'll need to rebuild the landing that was at the bottom of the stairs which I had to tear up for this (it hides some sort of old cast iron capped off sewer pipe I have no intention of touching, just off the left of image 2).

My current plan is just to keep a scrap block under the stringer, just clean it up a little/hide it. Of course the landing I build will need to be slightly raised on that side as well, but that's no big deal. But if there's a more "legitimate" approach I'm more than happy to attempt that instead.

How sould I best approach this?

stringer supports stair base

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Consider installing a pressure-treated 2x8 base plate onto the floor slab, underneath the stringers. That's roughly how you'd build a new staircase by today's standards.

  1. Notch the bottom of the stringers so the new base plate will fit. This will be the trickiest part since you can't disassemble the staircase.
  2. Slide the base plate into place. It should be a tight fit requiring a little hammering or lifting of the stringers.
  3. Fasten the base plate to the slab using your preferred concrete fasteners.
  4. Fasten the stringers to the base plate using galvanized or stainless steel framing nails, or if not those, at least deck screws compatible with the PT lumber.

If you search for connect stair stringers to slab you can find a variety of drawings and photos that should be helpful. The connections I suggest are more like the inner-stringer of a 3-or-more-stringer staircase, than then outsides. That's due to using the base plate as a permanent shim.

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