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Adding second layer of plywood to existing subfloor

Adding second layer of plywood to existing subfloor

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Old 07-09-05, 04:55 PM
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Question Adding second layer of plywood to existing subfloor


Homeowner (me) Putting down about 850 sq ft 5/8" bamboo flooring to replace carpet. (I have questions in the flooring board on that subject)

House Specs
  • Built in 1979
  • In Maryland, USA, in a humid (June-September) area
  • Existing subfloor is 5/8" plywood
  • 2x8 Joists 16" OC, unsupported span about 11' 6"
  • Joists are a hard lumber, yellowish/orangish color. Likely fir
  • Unknown if current subfloor is glued to joists.

  • 5/8" subfloor is the bare minimum for this new floor, and I don't like that.
  • Couple bouncy spots on the floor, the main one being in the dining room where the previous owner had a very heavy piece of furniture for a long time. Oddly enough, the adjacent kitchen has 12" tile that has never shown any mortar cracks or other problems. Not sure if this is a joist or subfloor issue.
  • The existing subflooring appears to be butt jointed with no expansion gaps.
  • Existing subfloor has seam height issues. Height difference is 1/8" or more in some places at the plywood seams, but doesn't look quite like I'd expect buckling to look (may be fixed by nailing down - TBD)

Plan of Attack

I plan to put another layer of 5/8" plywood over the existing subfloor, and screw that down using coated deck screws, and likely glue with liquid nails. I will offset the joints to overlap the current subfloor joints.


1. What grade of plywood should I get to put over the current subfloor? I assume a B/C exterior grade, good side up?

2. I've read conflicting information in the archives here about whether or not I should screw that plywood to the joists, or go out of my way to miss the joists and only screw to the existing subfloor. Which is correct for this situation? I assume screwing into the joists is correct.

3. Which is a better idea: glue with liquid nails (and screw), or put down some paper in between the two floors and screw only? I'm inclined to go glue & screw, but don't want to make a noisy floor.

4. Do I need to remove all those existing staples from the floor (from the carpet pad), or will the second layer of plywood smush them down well enough? From my experience in laying pergo in another room, I fully expect to find thousands of staples from a couple carpet changes. It took us hours to remove all the staples from that 12x10 room floor.

5. 2x8 joists are not all that big. In fact, they are consistently 1/4" smaller than our current 2x8 standards (7 1/4" vs. 7 1/2"). Sistering the joists is possible, but would be a huge pain given the drop ceiling, utilities etc. in the finished basement. Should I put cross-blocking (not sure of the term) between the joists to shore them up? If so, any recommendations on size, attachment mechanism (hangers or through-nail) and spacing?

6. Do I need to cut a saw kerf along the subfloor joints to open up an expansion gap before I put down the second layer?


Old 07-10-05, 08:03 AM
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New Sub Floor

Pete: That is quite a list of questions and hopefully I will not miss one of them. I would put down 5/8" T&G Subflooring. That is what is is made for. No expansion joints, glue and screw. Screws - 2 1/2" use the star head, 12" on center, I use 6" myself. I also screw the whole field. You can just hammer down the staples flat. No need to pull them. 2x8's are the correct joists, and they seem to be in fine shape, so I would just let the sleeping dog lie. Hope this helps. Good Luck
Old 07-10-05, 09:08 AM
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I'm agree with what Jack said. Have a few extra points to add too..

- Nail/screw the crap out of the original 5/8 flooring. I've found a few builders that think skipping whole joists is ok. Or a nail every 16-20" is ok. If you have access to a framing nailer, use 8d ringshanks.

- Screw the new plywood down. Star or square drive are easier to use and dont strip out like phillips.

- Make sure there isnt any electric lines or water lines in those joists you can hit with the nails or screws you use.

- Do yourself a favor and fasten the new plywood 6" on center. The better its fastened, the less chance of squeaks.

- Run the plywood the opposite way the original was layed if possible.
Old 07-10-05, 05:07 PM
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Thumbs up

Thanks a bunch, guys. That really helps, especially the bit about running the ply in the opposite direction. That was my gut instinct, but the information regarding that on the net was contradictory. I think it was mainly for initial subfloors, though.

I'm actually looking forward to this project. A new floor to get rid of this ugly old carpet will make a huge difference.

Any comments on the glue vs. paper between the two layers of plywood?

Old 07-11-05, 04:18 AM
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You do not want to put paper between your floors. As far as glue is concerned, I think you want to use PL 400. But when you go to get it, read to make sure PL 400 is the right one. I am sure it is not PL 200, but am going from memory. There are 6 different glues in the PL line. Maybe Hellrazor knows off the top of his head. I have too many things to remember as it is. If I were going to buy it I would read the lable myself. Good Luck
Old 07-11-05, 07:32 AM
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Thanks Jack, will do.

Old 01-03-13, 03:55 PM
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Hey Psychlist1972, on the second layer of plywood did you provide 1/8" spacing around each sheet of plywood?

I'm working through something very similar now and am debating the need of the 1/8" spacing for the second layer of 1/2" plywood im putting on. Right now, i'm looking at Subfloor Liquid Nails and screwing into both Joists and Subfloor and then a 5/8" bamboo finish (with underlayment of course) - each sheet of ply is tight to the next.


Thanks in advance.
Old 01-03-13, 05:53 PM
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Thread is 7-1/2 years old. The OP may no longer be with us.
Old 01-03-13, 07:01 PM
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Yeah, thread closed. See architectonik's other posts on this subject.

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