Replies

1. 
   charlie_sullivan | Dec 15, 2014 10:50pm | #1

   What are the goals of the plywood or OSB between the old and new floor boards? Providing an air barrier? Providing a flatter surface for the new floor? Preventing the floor from flexing or squeaking? What's wrong with the old floor--is it structurally sound?

   Without knowing the answers to those questions, my generic recommendation would be 3/8 or 11/32" plywood would be fine. But this isn't really my expertise, so let's see if others agree.

2. GBA Editor
   Martin Holladay | Dec 16, 2014 04:38am | #2

   N. Madrid,
   This is a judgment call, and a decision would be based in part on the condition of the existing pine floor, as well as any need to match the height of the existing flooring in adjacent rooms. If the pine floor is relatively flat and in good shape, then 1/2" plywood underlayment may be all you need. A thicker subfloor -- either 3/4" plywood or 3/4" Advantech -- will certainly work just fine.

   One of the main virtues of Advantech is its ability to resist warping and swelling due to rain during construction -- a factor that doesn't apply in your case.

3. 
   Richard Beyer | Dec 16, 2014 10:46am | #3

   Martin gave good advice. In my own house we have 6-8" wide fir boards fastened to the floor joists. Before installing the wood floors over them we screwed each board down with a lot of screws and then installed the wood floors directly over the fir floor in the opposite direction. Unless your floor is abnormally uneven there's no reason to waste money on ply or OSB. That is unless your building up the floor to match an adjoining floor as Martin pointed out.

4. 
   madrd | Dec 16, 2014 10:05pm | #4

   Thank you so much for the answers.

   The goals of the plywood is air barrier, and some added strength. The majority of floor is strong enough, minimal flexing in most areas, mostly flat, but a few weak areas ( at exterior doors, front of kitchen sink, partial bathroom floor). I am updating the home so the entire floor should be sound. I hated the idea of a new wood floor, but thought it was my only alternative, and thought the ply underneath would be needed; I really don't know.

   I removed the peel and stick tile, pulled up 1/2 plywood to expose the original 1945 floors. I used a drum sander to remove the black felt and some cupping, but didn't complete the sanding process, because I think I did it too soon -should have given the floors more time to adjust to the room temperature. So next, I began filling the numerous (all the way through) nail holes and weak areas, put a temporary seal to protect the floors since sheet rock, etc, was still needed.. I live in the south, we don't have many cold days. But, we had 4 days of an artic blast, and it was an experience. The intake air seemed to create a vacuum pulling the cool air through every teeny-tiny space. I decided that the nostalgia of an original floor was second to a solid floor. My research suggested what I posted above.

   Everything fits perfect with the original floor height. The 1/2" plywood that I removed was made to fit at the two doorways by adding a 1/2 piece over the door framing. It worked alright. The front door is very small already, so it will need adjustments.

   Unfortunately, this original part of the house also has only about 12" underneath, so adding a subfloor under the existing floor isn't feasible.I believe the rest of the house has a tongue and groove subfloor, then a tongue and groove flooring, and the floors are great.

   Thank you for your input. If I skip the plywood, do you suggest a membrane in between the two floors? If I do skip the plywood, I'll sand the floors again before adding a new hardwood.

   Thank you all again.

   NM.