[john70t]

Can you educate me on plywood?

The framing I assume is ballon-type.
The floor joists are true 2x6 and true 2x5, one foot on-center.

After about 140 years, the existing tongue-in groove floorboards are about the same weight as balsa.
They are held in place with a few #6 nails and are hacked up over the years.
There has been some repeat cracking in the ceiling above.

I want to sheet with 5/8" or 3/4" plywood, using liquid nails and deck screws to lock it all in place.

Weight is of great concern...

There is CDX and RTD(Resistance Temperature Detectors) type plywood but I'm not sure which to use and whether the 5/8" would conform to code.