I have been doing polished concrete throughout my house. My wife loves it more than I thought she would and now wants me to do it upstairs. I am afraid that the subfloor flexes too much, which wasn't a problem on the slab.
I don't want to put down anything too thick over it or the profile will be too much against the adjacent carpet. What is the thinnest way to stiffen the floor? I am going to use lathe so I don't really care about the cement board. I just want to get the flex out.