We've got a 1921 colonial. I was removing the kitchen ceiling to sister a 2x8 joist. An overzealous plumber had taken a 4" diameter chunk out of the joist to run a 2" pipe through. The cast iron tub didn’t mind but the floor outside the bathroom did. It has a slight sag, noticeable with a bounce when you walk over and by creaking.

After taking down the ceiling, I noticed that the 1” thick x 2” wide tongue and groove oak flooring is nailed directly to the joists with no subfloor. The joists are 16” OC, 2x8. Any suggestions? I was thinking to put up a ¾” plywood subfloor from underneath, in between the joists, wedge between 2x4s running along the length of the joists and the flooring itself. Thanks!