Just a warning to anyone building with the glass floors - you may find that you won't be able to return to edit town after building with them. In which case you will have lost all of your hard work
Like I just did
I'm not sure if it's all the time, or just restricted to CAW. I found further information on the official forum confirming its brokenness.
http://forum.thesims3.com/jforum/posts/list/724818.page