I have slate tile flooring in my greenhouse/sunroom. It's been installed at least 20 years now with no problems. It's an air-conditioned/heated room (as much as you can air condition or heat a glass room). In the last year, it's started to peel with the top layers coming off and virtually turning to powder if you walk on the broken pieces. Constantly having to clean it up. Nothing seems wet, so I have no idea what is causing this. The floor is not sealed. Any ideas are appreciated.