Solid Hardwood, Engineered and Laminate Flooring - Subfloor Leveling

I'm "leveling" 500 s.f. of 3/4" gypcrete subfloor in a 2nd-story condo. The gypcrete is over what appears to be old wood plank flooring. I'm sanding and applying Ardex feather finish according to the process described in earlier threads.

The gypcrete is in pretty good condition except for a thin crack in one area that runs the width of the room. The crack is at the top of a ridge which is about 1/2" higher than anywhere else, forming a kind of "continental divide". The crack/ridge also corresponds to a squeaky area of woodplank underneath. Probably the wood has warped or buckled.

Anyway, to level things out, I'm thinking about:
1) sanding down the 1/2" high crack/ridge as much as possible and smoothing with the Ardex; or
2) breaking through and removing the gypcrete along the crack, perhaps fix the squeak, then fill the fissure.

I'm not sure (2) is worth the trouble and mess. Anyone have any thoughts or other suggestions?

You pretty much wasted some good ARDEX, if you used the SD-F.!!!!!!

Gypcrete and portland based patching compounds don't mix. The SD-F will blow off the gypcrete.


Over gypsum based lightweight poured substrate, your going to need ARDEX: GS-4.

Stop what your doing, unless you want to bust even more of it up.

At first, I thought that too, but that's not what the Ardex people said. I called the company's PA phone number and spoke to several helpful tech guys a few weeks back. They all said that with primer, you're ok using the feather finish on gypcrete. Then I tested out some in a corner and it's held up fine to my jumping up and down on it while I cursed this humpback floor I'm dealing with.

Of course, I might add that I'm not sure what I have is gypcrete. It may well be a portland-based, light-weight cement. The place is 35 years old. No telling what they were pouring back then...

Did you ask them why they make GS-4, then??


It is a known fact that portland and gypsum, don't mix.

No, it is not going to blow today or tomorrow, it takes a little longer for the high alkaline in the gypsum, to spawl the Ardex right off of it.

Minimum application thickness on the GS-4 is 1/4", according to Ardex. Not all the duck ponds in my floor are that deep, though, so that's what led me to the Feather Finish. But do you think GS-4 can be used <1/4"? Or should I just go with felt for the shallow depressions?

All I can say is, I hope your floating the floor and not direct gluing.

You may be able to deal with the crunchies, later. They may drive you crazy, though.

Can thinset be used over gypcrete. I had some float work done by a professional to get my floor flat and he used a thinset mortar compound. He said it would be fine. Any disagreements?

No portland cement based products casn be used over gypcrete, period. Portland based products like thinset or Ardex sd-f will cause the gypcrete to release from the floor. It will blow the gypcrete off the floor, not the portland off the gypcrete.