My family has been in the roofing and siding business for over 70 years and for a roof the general rule is replace it once it starts showing signs of moisture or leaks on the underside of the roof deck. 

Plus paint the underside.

I've had 2 separate plumbers out to look at it, and they cut a hole in the downstairs ceiling to access the underside of the tub. 

.• No storage is allowed under the stair unless the underside of the stair is fire rated• All doors that open into the stair must be fire rated except any door that opens from the stair to the exterior• At least one window in every bed room must, when open, have a clear opening of 5.7 square feet at the second floor and 5.0 square feet at the first floor.

I am looking for a solution to mitigate as much of this airborne sound as possible. the core problem is that of the sound transferring through the stairwell, ceiling insulations seems to be getting the job done.Additional Information:- I have access to the underside of the stairwell, but am unsure what material would provide, if any, sound reduction from the underside.- The stairwell will not be used by the upstairs tenant, apart from entering onto the first landing.Does anyone have a viable solution for preventing this?

What I thought could be an easy fix, turns out to be the entire roof that sweats on the underside of the entire roof when it’s cold out.

Also something else to consider is having open attic space and basement space if you are over under. 

I have heard comments that homes in TX or GA need a major rehab after 10 years, builders use MDF for trim, pressboard under siding instead of OSB sheeting etc As an investor we buy 1960s homes, they have good bones and are worth an update.

Box sills, windows, doors, outlets, switches, lack of house wrap under siding etc.. 

@Amiris Brown Yes chalk that one up to learning, there are always deals out there and the more numbers you crunch the better you'll get at your estimates.Regarding the mold, I've never heard of anyone removing and sanding the undersides of the hardwood, but that's not to say its not done.