Can you use extra foam underlayment to fill in voids in my cement or plywood subfloor when installing a floating hardwood or laminate

Here is a transcription, if you prefer to read instead:

“Hi, Don Lovato, back again. So this is a really good question that I get asked from time to time. It’s something that people should ask more often but they don’t. And I find that they’d find some clarity if they did. So here it is. The question is if the slab or the plywood underneath is very wavy and we’re going to install a very flat hardwood floor in a floating style or a laminate, Pergo-type flooring in a floating style, can’t we just go ahead and fill in the voids with padding?

There’s a padding that goes underneath. It’s called underlayment. It goes underneath Pergo and laminate floors and some floating hardwood floors, and they asked, “Can’t we just go ahead and fill those in?” Unfortunately, you cannot because what it does is it offers too much deflection, and you’ll break the tongue and groove off and you’ll have a defective floor. So, unfortunately, you cannot.

But the best way to do that and the good news is that it can be fixed really easily and somewhat cost effective with floor leveler. Liquid floor leveler is mixed up like a very liquified cement, if you will, and we do it in five gallon buckets. And we pour it out, and it seeks its own path. We mark the high areas. We mark the low areas. We pour it out. We kind of help trowel it in the right direction, but for the most part it’s self-leveling. And so it gets the floor pretty flat. So if you have any questions about that, feel free to give us a call. We’d be glad to help you with that. Thank you very much.“