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I recently purchased a house in New Jersey and the 2nd floor was filled with old carpet that we wanted to replace with hardwood flooring. I ripped up the carpet only to find particle board under all of it. I proceeded to remove all the particle board to find the 5/8" (19/32" really) CDX Subfloor over 16" OC Joists. We are looking at a rather expensive hardwood so I want the installation to be ideal so I'm going to add a 2nd layer of plywood underlayment over the existing subfloor. Here are my questions:

  1. What thickness underlayment should I use. The top step of the stairs is currently 1" lower than the rest so 1/4" plywood is not a problem but should I put 1/4" or go to 1/2" to make sure that the floor is stable with over 1" of subflooring under the hardwood.

  2. What type of plywood should I use CDX? BC Sanded? etc.

  3. The house is 'L' shaped due to a renovation done in the 90's. The front of the house (short leg of the 'L') has the joists running E-W and the back of the house (long leg of the 'L') has the joists running N-S with the subfloor running perpendicular to the joists in each leg. At the line of the renovation the subfloor is cut and turns 90 degrees. Because of the look we are going for we would like to run the hardwood flooring perpendicular to joists on the long leg of the house which would then run parallel to the joists in the short leg. Should I run the underlayment all in the same direction or match the subfloor and cut it at the renovation line? Is there going to be any issue at the transition point or something I can do to stiffen that line up (it's only an opening at the top of the stairs for the hallway approximately 4' long, everything else is under a wall)?

  4. How should I attach the plywood underlayment to subfloor. I was told by one of the hardwood installers to glue and screw (or ring shank nail would be allowable) to the joists. Or I've read that I should not attach the underlayment to the joists and avoid them and attach underlayment to the subfloor using staples spaced tightly (Every 6") around the perimeter of each underlayment sheet and scattered around field?

Any help is appreciated as I want to get this right the first time. Please let me know if you need any other information.

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    This is a lot of questions all put together, our preference is to break them out when possible into separate questions. – BMitch Feb 16 '16 at 20:33
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  1. Your current plywood is suitable for hardwood installation. I do not ever like to use 1/4 subfloor as it is brittle and when screwed sometimes doesn't lay flat. If I added to your subfloor I would at least put in 1/2 inch but like I said you don't need it with 16" OC. Note that a thicker sub might have benefits but at the same time you are adding weight to the joists.

  2. Depends on area of country and moisture levels. CDX is fine in most places.

  3. I would not try to match the current subfloor. I would do the opposite.

  4. I do not ever attach additional subfloor layers to joists. When the joists swells - and they will in 80% of the world - your sub will be pulled or pushed and then you will have a screw or nail that is working its way out and possibly squeaky floors. The whole reason for the second layer is extra inflection levels. By attaching to the same thing you get the same thing just thicker.

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    I agree with @dmoore. If you do use 1/4" to match the height stagger the subflooring so the seams are not in the same place. This makes a much more stable / quiet floor. – Ed Beal Feb 16 '16 at 21:58
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In agreement with the above and further suggest: 1) Walk the existing floor in silence, use 1-1/4" screws to eliminate real or perceived squeaks. 2) Pay attention to the butt joints, if they are swelled knock them down with a belt sander. 3) At the top of the stairs be sure the entire area is screwed down well, it takes brutal traffic. 4) Go opposite the existing plywood pattern if you install the 1/4-1/2"and be sure to glue & screw it, cut a small hole on the tube to prevent any lumpy, hardened dried globs. Good luck, hope it looks nice & is personally satisfying.

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