I see where you are getting messed up David. For the time being forget about what what something is made of and instead try to understand this by using construction "terms".

In the era your house was built the normal construction method was to build the floor joist system and nail down a subfloor and build the walls on top of the subfloor. After the house was dried in they would have come back and added a a square edge underlayment to make an additional layer. The first layer that runs underneath walls is termed the Subfloor, the second layer is termed an underlayment, does not matter what what was used or what is was made of it's still considered an underlayment.

Construction methods changed after that era to start using a single layer subfloor that was thicker. tongue and groove, has better glue, and better layers of intermediate plies with no voids. This way of building serves the purpose of a subfloor "and" underlayment. That practice still stands today, so your lumber yard man is absolutely right. They normally don't use underlayments on new construction anymore because the better subfloor material nowadays serves both functions for a hardwood installation.

Think of it in terms of layers instead of what something is made of.

So now you need an underlayment replaced. Well, you could use several different things for that depending on how thick you need it and what will be the finished floor going over top. In your case it will be hardwood so you should use a plywood that is whatever thickness you need and with square edges because you will be using this as an underlayment by definition. A CDX grade plywood is suitable for this. It has square edges, exterior glue and the voids in the intermediate plies won't really matter because you have a "subfloor" underneath it.

Got it?

Thanks. The problem with my situation is that the frame of my house is set on top of the particle board underlayment as well. As I removed it I noticed that it extends under the 2x4 framing plate. So I removed the particle board and had to gnaw out the particle board under the drywall back to the framing plate.

My confusion was that I was hearing that I could NEVER put hardwood over OSB--only over plywood--but then found that NOFMA says, Yes, "OSB is a comparable substrate." That is what the Lumber Co. guy said though I resisted it. So I was hearing conflicting information. Whether it's called "subfloor" or "underlayment" I didn't really care. What I cared about is what I should put down once the particle board is removed: OSB or Plywood. Knowledgeable people gave me conflicting answers.

I am very happy to put down the plywood (which seemed to be either the only thing I should use OR the better of the two to use). It costs only $1 more than the OSB at the Lumber Co AND has the virtue of being delivered free (Home Depot charges a bit less per sheet but tacks on a $55 delivery fee).

Because of the kind of confusion I encountered, I suggested that the Sticky be a bit more explict about what we novices really encounter--what kind of Underlayment can be used. In particular, is OSB really In or Out as the material to be used under the hardwood flooring.

Here's the email from the technical director of NOFMA (I have the joist system):

MY REPLY-----

Follow manufacturers directions for proper subfloor/underlayment. From NOFMA instructions check www.nofma.org Publications, How to Manuals, Installing Hardwood Flooring, and the subfloor/underlayment materials are described as a proper subfloor. Typically ¾” plywood on slab construction and either ¾” plywood or OSB on joist systems.

Mickey Moore
Technical Director
901-526-5016
901-526-7022 Fax