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justcallmetarzan t1_jac4164 wrote

I second /u/Spinaccio's approach for putting a post in. Or posts - I'm having some trouble envisioning how exactly the fireplace is supported by that (where is the ash pit?)

Another option to explore with an engineer... In our house (1931), I've got full access to the exterior basement wall "top plate" (concrete) and the interior wall top plate. If I had this problem under my fireplace, I would just raise the floor a bit temporarily with a jack and slide in at least two brand new joists. I don't know that you would want to sister these to the old joists, though.

But as for your dip... really depends. I've noticed a lot of uneven spots in my floors, but no damage to the joists underneath. What I suspect is happening is 100 years of uneven wear and settling and compression in the softer wood subfloor. If it moves, you could also have the classic subfloor board that ends just shy of a joist.


Spinaccio t1_jacco9l wrote

I’m guessing that it’s the hearth in front of the flue that’s the problem. The chimney itself would never sit on a wooden floor.


justcallmetarzan t1_jadv1r9 wrote

Yeah, I was thinking maybe it was a large hearth that had wings to the side. I don't know that the hearthstone on the floor would cause that sag though.