So here is my dilemma
. Recently moved in, removed the carpet from a floor in the den, removed the parquetry underneath to be able to put in hardwood. Managed to level parts of the floor from beneath due to missing joist hangers. Also bought some self levelling underlayment from some tricky areas that not possible to fix from beneath. However found out that towards the wall there is a little dip at least 1/4". Found out that below the wall there is a 15' x 9" joist sitting on a major beam and sill plate, however there is only one joist beneath this wall! And there is another wall on the second floor in the same place.
The joist seems to have sagged slightly but with no damage to sill plate or beam.
1. Lift the joist and install a sister joist (not so easy as I have to remove part of the ceiling beneath and some electric wires (hopefully no pipes)
2. Suck it up and just put some self levelling underlayment (problem with this though is it is almost the entire wall length (14 ft) therefore the additional plywood have to put cannot be screwed down close to that part (unless I block off pre-designated areas with some parquety to avoid underlayment going there and then screw the ply there and glue ofcourse).
How much of a sag, well the pictures will tell.
Thanks for all your feedback!