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average 09-24-2007 08:42 AM

Installing Backerboard for Ceramic Tile Floor
I am just curious why it is necessary to tape and thinset the joints of backerboard as opposed to just butting the backerboard sections up against each other and moving on.

JJC 09-24-2007 08:18 PM

The main reason is because without the tape it cracks due to movement and those cracks will transmit up through your expensive floor. Actually the main reason to tape the seams is because the "manufacturer says so". They have years of testing data available and of course it was not cheap. This testing was not only by their labs but also done by independent labs. Another reason is if you don't follow their written instructions and if fails, they are under no obligation to warranty their products because it was not installed per their instructions. Another mistake is when all the edges of the sheets line up. Be sure and stagger them. The reasoning behind this is due to the fact that seams are the weakest points of the system. If you are installing on a floor, the 3' seams are not inline with the next row of 3' seams. This principle goes back to the American Plywood Assoc. for sheathing of walls and floors.

skymaster 09-24-2007 09:52 PM

Also, by taping and thinset over, you create one contiguous water tight mass

average 09-24-2007 10:07 PM

thanks for the quick answers guys.

skymaster, i'm a little confused why it would make a difference if the backboard was a "water tight mass." it seems to me that if water is getting under the ceramic tile, then you're already in trouble. maybe i'm just missing the point altogether.

JazMan 09-24-2007 11:06 PM

It's not to make it a water tight mass, a floor with cement board is not going to hold water. The board its self may not be affected by water, but it's not water tight.

The purpose of tape is as mentioned before, to strengthen to joints and have a contiguous layer of concrete to bond tile on to.

You would not be in trouble when water gets under or behind your tiles. That's one of the reason we use CBU's in the first place. This of course as long as you don't use anything pre-mixed in a can.


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