Another underlayment quesstion

I recently extended my 9'x 15'concrete, stained lanai to 13x15.
I now plan to tile,ceramic.
It is screened and under roof.
It is in Florida so freezing not a problem.
A professional tile setter said I needed underlayment, because of potential cracking but others said not.
I plan to do it myself having some experience in laying tile but have never used underlayment when laying directly on concrete.
Any thoughts on the need for underlayment?
If needed, any suggestions on what to use?

DC

You don't need an underlayment, you need a "membrane" but even that is questionable. Tile has been going over concrete for a long time and the bulk of the membranes weren't even available until just a few years back.

The decision is yours. You could also use a "flexible thinset" and maybe achieve the same end. MegaFlex from Custom Building Products is one I like.

Even with a membrane or flexible thinset keep in mind there is no guarantee against cracks from a slab moving vertically.

[Edited by Bud Cline Tile Contr on Sun, Aug 24, 2008 at 1:33 pm CST]