Does the homeowner's patio deck and substructure become the responsibility of the HOA
Or the individual homeowner regarding the repair of dryrot and the installation of a
Water barrier?

Thank you in advance for any advice.

It is probably the Association's responsibility but you will have to read the master deed and by-laws to be positive.

It depends on the deed. If this is a private deck used only by the homeowner, its probably their resonsibility.

Scott has the crux of the matter. Does the homeowner own the deck, or is it (exclusive-use?) common property? In the first case, the homeowner pays. In the second, the HOA. If it's something that the homeowner added with HOA approval, then it's most likely the homeowner's responsibility.