Cement Board [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-24-2008, 08:19 PM
Hi " Just joined up and my first post. I have 7/8" OSB on the floor of my entry ( 70 SQ') and I am going to install porcelan tile. Under the OSB there is two layers of 1'' boards on 2X10 on 20 " centers. (Real old house) First I was going to install 1/2" cement board with unmodified thinset under and modified thinset over to install the tiles. Am I on the right line of thinking? Do I need expansion joints when I install the cement board? thanks

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02-24-2008, 08:24 PM
Two things, unless you have a need for the height, you only need 1/4" cbu on the floor. Then, have you determined the joist deflection numbers? Use the "deflecto" in the blue bar above. Unsupported lengths are what's under the floor, not what is above the floor.

How long an area are you going to tile? That and whether it gets a lot of sun will determine if you need expansion joints.

02-24-2008, 08:30 PM
That floor structure sounds questionable. I'd check deflection here.
Check the specs on the cbu you use and follow the installation guide for the spacing of the sheets. You also need to tape the seems on most of them, excluding wonderboard, I think. They have the tapeless installation. Unless they've changed that.