04:32AM | 07/18/06
Member Since: 03/04/03
44 lifetime posts
Site conditions are 1/2" cement board over 1/2" plywood subfloor.

Application is ceramic floor tile.

I bought pre-mixed thinset at HD - against my own judgement but I was tired and wanted to save some time.

Used 1/4" x 1/4" trowel, and installed about 50sf of tile.

48 hours later, the thinset still wasn't dry.

The tiles lifted easily (there was adequate thiset applied, the stuff just didn't dry). Threw tiles away and bought more. Scraped the thinset off the cement board.

Returned the used bucket of pre-mixed thinset to HD for full refund (they advised a number of ppl are having the same experience with the stuff).

Anyway, what was left on the cement board was only a "film" of the pre-mixed thinset which within 2 hours was thoroughly dry. If you run your fingers across a new cemnet board (off the shelf) it feels smooth. With this "film" they now feel somewhat "rubbery" - not sure how elese to describe it. They are still smooth (flat), the film is dry, but you can defintely feel a "traction", as if you troweled some mastic onto the board and then scraped it off before it dried.

Anyway, sorry fo the long winded description, but my question is - do you think I need to replace the cement board, or can I trowel over it with thin-set (which I will mix)?




07:03PM | 07/18/06
Member Since: 07/03/05
283 lifetime posts
You can use the thinset. What I am concerned about is the 1/2" ply under the cement board. Is there anything else there too? If not, that is not enough support. You need a minimum of 5/8" tongue and groove ply of if straight edge it has to be 3/4" BCX grade, and that is only if the joists are 16" OC or less.


04:14AM | 07/19/06
Member Since: 03/04/03
44 lifetime posts
The 1/2" subfloor is over 2"x8"s 16" oc, below that is my basement with a 7 1/2" ceiling height. The subfloor is standard/typical throughout the house, as well as througout the neighborhood for that matter. The cemnet board stiffens it up sufficiently. The 2"x8"s are covering a 12' span, once the cement board was laid down and screwed through the subfloor into the beams, there is no flex/creaks/softness determinable.

Not sure what else I can do at this point outside of scrappng the durock and laying another subfloor and then new tile backer. But then I have to contned with door and sill heights.



05:37PM | 07/19/06
Member Since: 07/03/05
283 lifetime posts

The above is a website with a helpful little thing that tells you if your floor is up to snuff or not.


02:55PM | 07/22/06
Member Since: 10/28/05
311 lifetime posts
According to the American National Standards Institute (ANSI ) as adopted and distributed by The Tile Council of North America (TCNA/TCA ), the minimum acceptable standards for any subfloor system is 5/8" plywood over 2"x8" joists, spaced 16"o.c., with a maximum unsupported span of 12', which gives us a deflection of L/389. The minimum deflection for ceramic tile is L/360.

Please keep in mind these are minimums.

Of course, decresing spacing/area/span measurements or increasing dimension of the materials will decrease deflection to considerably above the requirements.

Deflection is movement in the subfloor system which is the primary cause of ceramic installtion failure.

The 5/8" requirement for the subfloor thickness adds rigidity between the joists along with any cement board backer unit or membrane.

Your current 1/2" thickness currently does not fall within those standards for rigidity, even with the cement backer board, and could further end up causing tile bond failure and/or excessive grout joint cracking in the future.

As to your pre-mixed thin set, there are MANY MANY MANY MANY various complaints about this throughout the industry and on DYI boards.

We do not recommend use of any pre-mixed thin set mortar except those in powder form where you need to add water.

Please use a latex modified thin set mortar mix (bought in a bag).


There are two ways to do any job. The right way and the wrong way. Do it right everytime.


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