Wood – Subfloor tolerance for engineered hardwood

I have placed Dricore or its equivalent on 2 different jobs. Both are holding up fine, neither felt hollow or creaky. One was in a basement slab, the other was placed on a slab on grade such as yours. The only difference is one had 4" to 10" wide plank flooring from recovered barn wood, the other 2 1/4" strip.

While the subfloor was being laid, I glued the edges of all the pieces before placing. even though it is not required. Also the perimeter had a gap to the wall of at least 1/2". It is laid as a floating floor. I really don't see any problem stapling down your engineered floor to that. When the strip floor went down, the staples where checked for length so they did not go all the way through and pierce the plastic matting.

If you're not wedded to a nail-down install, the best route to go with here is a floating floor with nothing but a soft underlayment between the flooring and the slab. This is an accepted install method for engineered hardwood over a slab, and your flooring manufacturer/seller should be able to provide you with instructions for how to do it and not void the warranty.

As for vapor barriers, you only need to worry about that if your concrete slab doesn't already have a vapor barrier underneath it. Obviously there's no way to easily check, but if the slab was poured in the last 20 or 25 years, there's a very good chance it already has one. In any event, once the current flooring is off, you should have a calcium chloride moisture test performed on the slab. Where you live matters, too. I live in the desert, so soil moisture is a non-issue here, and my slab's lack of a vapor barrier underneath it is irrelevant. If you're in the pacific northwest, it's a different story.

If the moisture test shows that you need a vapor barrier, choose a closed-cell foam underlayment, not something water-sensitive like cork.