is backer board always necessary to put ceramic tile over cork?
I'm installing 6x6x3/8" ceramic tiles in the hallway of my condo. The subflooring is very smooth, flat concrete (building was built in 1959, seems very well put together), and the condo association has a rule that I need sound-proofing. I have rolls of 1/4" acousticork cork underlayment here, and a bunch of 1/4" backer boards. The guys at the flooring store said I should not put tile directly on the cork, I should put backer boards in between. Now, after doing the main room myself, I called a couple of contractors to give estimates for the hallway and kitchen. One of them said that since the subflooring is concrete, he can just put the tiles straight down on the cork, I don't need the backer board at all. (The other, I don't remember what he said, I think he was going to not use the backer boards either, but his English wasn't that good and we didn't have much of a conversation...)
Now, because I am broke these days, I am doing the hallway and kitchen myself. Was this contractor wrong to say you can put tile straight down on 1/4" cork underlayment?
The advantage to not using backer is it lines up better with the existing floors. I also won't have to trim doors and cut the backer boards. When I look at the websites that sell the cork underlayment, they say you can put ceramic tile directly on cork if your subflooring is concrete, and eHow.com says the same thing--attach the cork to the subflooring with organic adhesive, then thinset over the cork, then tile.
Would it help to put thinset under the cork instead of organic adhesive--could that make the cork more rigid?