Flooring – Bathroom Floor Transition under door that hasn’t been installed

Installing hardwood flooring over concrete is generally not advisable for multiple reasons. Two of these reasons are: 1) oftentimes the concrete is moisture permeable, which means that any solid wood floor installed directly to the floor will buckle irreversibly; 2) typically solid wood floors are nailed to the substrate, which is not an option with concrete. One can install 1.5" sleepers on top of the slab to provide this nailing substrate, but this affects significantly things like ceiling height, door height, countertop height, etc. So this is generally not done as a remodel project unless you're remodeling the whole thing. One thing I would recommend as a compromise is finding some way to seal the slab, say with an epoxy paint, and then gluing an engineered wood floor to it.

I know this is an old question, but here's what I did:

(1) drill holes in the concrete. Yes, this is a PITA, but with the right bit it is not too bad. The holes do not need to be huge, nor do they need to be deep if you use the right screws.

(2) Use the right screws. The ones I used are from a standard Big Box store, and are specifically designed for concrete. They specify a hole size to use, and the screws themselves seem to self-destruct as they dig into the hole.

(3) Since I am also puncturing my moisture barrier, I did apply a line of silicone around each screw as I put it into the floor. This pushed the material into the hole, under and around the hole in the moisture barrier, and under the screw itself.

So far, this seems like a good install, although it's only been a week, and they were well away from the foundation's perimeter, and above grade.

Also, while I did one this way after the floor was installed, I did the next five before installing the floor, since this is not only dusty work, a little water (and associated drying time) helps a lot with the drilling.