What do I do if I have hardwood floors with no subflooring?

I've purchased a small home and it appears by viewing the flooring from the basement that the previous owner had put down hardwood without any subflooring (the wood planks are notched underneath). How unusual is this practice and what would I need to do to "correct" it? Was there never any subflooring?

Answers:
In many older homes the subfloor was also tongue and groove. Now if you have oak floor and your sure thats its the only floor there I cant see any real problems unless the joists are further than 16" on center. As far as unsual I would have to say yes it is but oak is very strong and should hold up.As far as correcting it, short of pulling up the oak you might as well just go over it with another layer of flooring.

Other answers:
When you say notched underneath, are they tounge and groove. If so you don't need a subfloor. By the way, what is the thicknesss of the floor?
When you say notched underneath, are they tounge and groove. If so you don't need a subfloor. By the way, what is the thicknesss of the floor?
older homes used tongue and groove sub flooring before plywood came about. chances are that is what your looking at
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