Hardwood flooring rignt over floor joists ?
 

I have some soft spots in my hallway I am trying to repair . This is all plywood, but it will butt right up against the original hardwood flooring that the rest of the house has .
It appears as if this is how the original flooring is installed. I do not have it lifted it yet, but judging by the thickness, it looks like they put it straight down on the joists .
This would be great if I can do this, as I can blend it in very nicely with only one little seam . If I have to use plywood subflooring , I am going to have a step up .
A quick internet search says yes, my bud who is a carpenter says no way .
This is about 4 feet above a crawl space with dirt floors underneath
As always, thanks !

If you want to keep the same height you can put plywood subfloor between the joists. Screw and glue some 2x4 or 2x3 to the side of the joist low of the joist top at required height to make the top of the new floor line up and lay in strips of ply, then floor over top. I did this years ago and it worked very well, it also stiffened the joists.

How thick is the hardwood ? And what is the joist spacing ?

That will work. Old houses around here had the cut the joists down. The thickness of the ply to be used would be enough. It would help to have a power planer, but some kind of sideways track saw setup would do fine. You'll need to cut near the walls with a Sawzall.

Our 1920s home has ~7/8" douglas fir tongue and groove right on the floor joists and the floor is pretty solid. There spacing of the most is roughly 24".