Subfloor Prep for Laminate Flooring

Q: I am ready to purchase 10mm laminate flooring for my family room. I pulled the old carpet and noticed some of the subfloor joints were uneven. I made the repairs by screwing the joints down, however I still notice minor cupping and swells on the subfloor which I’m not sure how to level it out. Will the laminate floor be able to have some flexibility to the contour of the floor or do I need to replace the floor or add leveling compound? I was hoping to just install the laminate floor which comes with an attached underlayment and also add a 6mm cork underlayment as well for better sound and moisture barrier. Do you think this is overkill or can this be done? Your advise is much appreciated.
Thanks,
Anthony

A: There are a few major reasons why a wooden subfloor will cup or bow. First off, hopefully your subfloor is staggered at a 90 degree angle to the boards next to it. If your subfloor is rising up at the seams, then the problem here is most likely that there was not a relief left between each board to allow for some minimal movement. Typically this relief only needs to be about as wide as a paper match (or so) and if you are not seeing this relief, you can create one by taking a skill saw and cutting a relief. This will allow your floor to move and should relax the floor a bit. From here you will want to sand the floor down a little (assuming this is one you can sand such as plywood, but not OSB).
Now, if you are experiencing a cup between the joists and not just rising at the seams, then this is usually because of one of two reasons. Either your floor joists are spaced improperly (too wide) or the material used for your subfloor is not up to spec (cheap material not meant for being a 3/4″ subfloor) and while the home was built most likely got wet and pressure from walking on it has caused the cupping. In this case, if the joists are improperly space you will need to get additional supports built in to account for this.
In the case where the material is bowing because it is not up to spec, you will want to replace this with new 3/4″ plywood that fits spec for a subfloor. After you have installed the new subfloor you will be set to install your floor.
In regards to your underlayment question, using both 6mm cork and the pre-attached underlayment would not be over kill. With the pre-attached sitting over the cork (which will act as a subfloor) you should have a floor that is both quiet and feels very good under your feet. Now you could avoid using both and stick just to the 6mm cork with your floor once your subfloor is fixed, but again, both will not be over kill, just account for it when installing transitions and any other trim.