This house basically has no subflooring, built on pier and beam in the 50's. Throughout has tongue and groove, probably 3/4 thick hardwood, which must be the subflooring and is original. The areas with out carpet over hardwood (two bedrooms and hall) are finished wood surfaces. So I guess, they intended for the hardwood to be the subfloor as well as the top layer? How common and why is this done, what are the risks of having this? The quality is superb even after 50 years. There are two areas which one of the planks have split, still in tact but give and return under weight. How would replacement of these single planks be done? Any advice is appreciated, Thank You in advance.
"So I guess, they intended for the hardwood to be the sub-floor as well as the top layer? How common and why is this done,"
Cause that' the way they used to do it.
I've seen single hardwood floors in both balloon framing and in masonry built structures, all more than fifty years old but how long that method continued by builders in some areas I have no idea.
If the "bad" boards are not unsightly and you don't plan on refinishing the floors, you could reinforce the boards from below by cutting a 1x8xthe distance between your joists getting some construction adhesive and screws and gluing and screwing it to offer support for the bad board.
If you decide to replace the bad boards there are several websites that give detailed descriptions and some with drawings though most refer to flooring over sub-floor the principals are the same except location of the joists that the board rest on will be needed to give the replacement support along with the support I mentioned above for just reinforcing the bad boards.