Do you need underlayment for a new vinyl plank flooring?

I'm putting down vinyl plank flooring over a fairly new OSB sub floor. Because of the weather elements during construction of the house the floor must have gotten wet, the butt joints swelled a little. I took a grinder and ran it down the joints getting them as flat as I could, but still have a slight rock in my level going from one sheet to the other, not all the sheets are like that, just a few here and there. I know the vinyl planking has a underlayment already on the bottom, but should I also put wood underlayment or that fan fold over my sub floor first to help with any unevenness?

 

Your installation instructions will say how flat the subfloor needs to be. Typically I see 1/8" in 6 ft or 3/16" in 10 ft. So you be the judge if its flat enough.

Simply adding another layer of underlayment doesn't really fix humps, it just eases them. Your installation instructions will also say whether or not you can add foam underlayment or not. Typically if your flooring has an underlayment attached they do not allow another layer of foam underlayment under it.