Second plywood subfloor over uneven subfloor for floating laminate

Discussion in 'Laminate Flooring Q&A' started by FoolsErrand, Dec 4, 2018.

  1. FoolsErrand

    FoolsErrand New Member

    Hello - I am looking to install about 1000 sq ft, of floating laminate flooring on the first floor (over a basement) of a 30 year old house. The subfloor is 3/4" tongue and groove OSB that was previously carpeted and appears to be in good in shape but is full of significant dips and peaks - upto 1/8+" in spots.

    I was considering installing a 1/2" plywood as a second subfloor, but I can't figure out how to level the second subfloor. I was planning to run the second floor in the same direction as the first, offset the 4x8 panels, and screw them down with 1 1/4" screws; however, the more I think about it the more I am convinced that I am going to end up with a second floor with the same peaks and valleys as the first subfloor. Is what I want to do possible or advisable? Is there a way to actually make this work / level? Your help is genuinely appreciated.
     
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  2. Tom Potter

    Tom Potter I Support TFP

    As long as your first layer is solid with no deflection it's good to take laminate. No need for more plywood.

    Sand your highs & fill your lows.
     
  3. FoolsErrand

    FoolsErrand New Member

    Thanks for the input. There isn't any deflection; however, there are some pretty significant peeks and valleys. For example, there is one spot where there was previously a ~40" peninsula wall that has left a substantial valley in the floor. Sanding and filling seems virtually impossible over this much space. Is there an alternative option?
     
  4. Jim McClain

    Jim McClain Owner/Founder Administrator

    I'm pro-underlayment, in situations like this. You can use a grinder to take down some of the high spots, then use some 15 or 30# felt paper to fill the low spots. I'm guessing most of those are between the joists. You can layer it, but not too thick, or you'll create a soft spot. Then go with your underlayment. Half-inch will provide more bridging power, but it might be okay to use 3/8". Staple (or screw) into the subfloor, but not into the joists. A fastener every 4" in the field and 2" along seams.
     
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  5. FoolsErrand

    FoolsErrand New Member

    Thanks, Jim. I appreciate the response and advice.
     
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