I need an advice. We just bought 100 yrs old house, everything in the basement seems to be straight, however second and third floors are sagging in the middle (about 2-3 inches), after talking to several engineers/architects nobody seems to be able to pinpoint the problem and all they say that the house is old and it's natural. I still want to level the floor, I removed thin hardwood and it looks like I have 3/4 planks as a sub floor, on the third floor somebody actually laid hardwood right on the joists without a sub floor, and in the bathroom I have about 5 inch of concrete.

So I am planning:

1. install 1/2 inch plywood on top of the sub floor on the 2 nd floor leveling with planks and shingles, does it make sense? would it be better to remove sub floor and replace it with plywood?

2. remove hardwood on 3 rd floor, install plywood sub floor and then put hardwood, does it make sense? would installing sub floor add much weight?

3. remove concrete floor in the bathroom on the 3rd floor and install plywood with cement boards, existing concrete is cracking on joists and I can probably fix it but I think if I remove it I'll reduce the weight it puts on the house.

Any suggestions, opinions welcome.

Thank you.