How to install “subfloor” over top of existing hardwood

Question

I recently removed a few layers of different flooring/underlay to expose original hardwood flooring. I’ve been told by two different hardwood refinishers/installers that the hardwood isn’t salvageable unfortunately. So now I am going the route of 1/2” plywood over the top of the existing flooring to level it out. Finish floor will likely be a vinyl. I know there are rules for the true subfloors and orientation to the joists and whatnot but didn’t know if same would apply for what I’m planning.

Plan would be to use 1/2” plywood, let it acclimate for a couple days, then screw to flooring every 6” on perimeter of sheet and every 12” otherwise.

For reference, previous build up of flooring was: Original hardwood-1/8” underlay screwed to hardwood-1/2” plywood glued and screwed-1/8” LVT

Answer 1

Orient the short plywood edges parallel to the joists to maximize the extra stiffness that you'll get. If there are any low spots, now is a good time to fill them. You should probably decide upon your finish floor now so that you can get a specification for how flat you need to make the floor.

Answer 2

In some builds, flooring is made of 1" wood planks which appears as hardwood floor. Perhaps this is why it is not salvageable. Or it is hardwood on top of a subfloor.

In either case, the assembly you have (provided it's done to spec) does not need further strengthening or stiffening, and your additional 1/2-plywood is a suitable way to even it and level it out. The orientation does not matter, and you do not have to use T&G.

You can perform a walk test to see if you prefer extra stiffening, and then use 3/4-in plywood and place it perpendicular to the joists. A 12in fastening schedule is sufficient if you are laying vinyl flooring. I also glue my second subfloor where I anticipate the screws, to prevent squeaking.