You should check with your local building inspection office where you live. We can't help you with that one. Know what I mean Vern?

Where on earth did you find 5 X 5 tile? Are you talking about ceramic tile?
I can tell you that cement backerboard should be used in my opinion.
[Edited by Bud Cline Tile on Mon, Nov 16, 2009 at 11:19 pm CST]