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ship_wreck 05-22-2011 10:54 AM

3/8" ceramic tiles over 1/4" cork over concrete
Hi all. I live in a highrise condo and I'm installing 6x6x3/8" ceramic tiles in my hallway. The subflooring is very smooth, flat concrete (building was built in 1959, it seems to be 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 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 I won't have to trim any doors, and it lines up better with the existing floors. I also won't have to cut the backer boards. When I look at the websites that sell the cork underlayment, they say you can put ceramic tile directly on it if your subflooring is concrete, and 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?

JazMan 05-22-2011 11:07 AM

Definitely no backer board. You can NOT install concrete backer to concrete. BTW, your floors may actually be Gypcrete and not concrete. Install the cork according to manufacturer's directions if it meets the condo association's requirements for sound reducing values.

Install tile over the cork with modified/enhanced thin set mortar.


ship_wreck 05-22-2011 11:23 AM

Thanks for the really quick reply. Out of curiosity, why can't you put backer over cork over concrete? I already did that in the main room, with the difference that I didn't remove the vinyl tiles that were there (would have done it if I knew how easy they were to pull up, but the floor has been there for 5 years now, no sign of trouble).

Bud Cline 05-22-2011 12:58 PM

Generally speaking backer board is never used over concrete simply because
both the concrete and the backer board are irregular. Installing backer board without using fasteners is problematic and using fasteners to install backer board to concrete is also problematic. In addition, the concrete (in most cases) offers everything the backer board offers as far as bonding the tile. Backer board is not necessary.

I said "in most cases". One instance that the concrete IS NOT suitable for bonding tile is in a condo. Such as yours. The concrete used in high-rise construction is lightweight concrete and contains additives that are not compatible with tile bonding products. Any tile being applied to light weight concrete requires special attention prior to making the installation.

What you have done in applying the cork using the organic adhesive is correct, you would not want to install the cork using thinset.

I have no idea why this tile store isn't up on these installation recommendations of the industry. This has been the case for a long long time. Maybe they just wanted to sell some backer board.:)

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