Ceramic Tile Subfloor Preparation

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  #1  
Old 09-12-10, 10:21 AM
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Ceramic Tile Subfloor Preparation

Need some advice for preparing my subfloor for ceramic tile. Existing subfloor is 23/32" plywood with a joist spacing of 24" o.c. Tiles will be 6" x 24" x 7/16" thick. Do I need to add another layer of plywood to strengthen the floor considering the joist spacing is 24", or will it be ok if I just add 1/2" cement board (Durock) to the existing floor? I really don't want to add any more height to the floor than I need to, but I also want to ensure the subfloor is solid enough that it can support these 6" x 24" tiles and prevent cracking, etc. We plan on installing the tiles parallel to the joists, which should help prevent cracking (I hope). Reinforcing the joists from below is not an option.
 

Last edited by mossman; 09-12-10 at 12:18 PM.
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Old 09-12-10, 02:07 PM
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Is a smaller tile an option or has the wife already said NO?
 
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Old 09-12-10, 03:18 PM
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We really like the wood-look tiles, and it makes sense to get the 6"x24" to give it even more of a wood look. Already ordered the tile. Floor is actually pretty solid. It's a relatively narrow bathroom so the walls give the floor even more support. The plan now is to lay 1/2" cement board over the 23/32" plywood using a thinset containing polymers which tolerates flexing more than regular thinset. I think it will be fine.

I should also mention that this tile has an 867 lb break strength and a class IV wear rating (suitable for light commercial and institutional applications.)
 
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Old 09-12-10, 05:00 PM
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Your best bet is to add a second layer of 1/2" bc exterior plywood and then use ditra instead of cement board. It will be approx the same height but will be much stiffer for your tile installation. Cement board doesnt make the floor any stronger, it merely serves as a good bonding surface for the thinset and tile.
 
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Old 09-13-10, 03:48 PM
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Just read up on the Schluter Ditra. Seems like pretty good stuff, a little expensive though. Looks like I can get away with laying the Ditra directly on the existing 3/4" subfloor without needing to add another layer of plywood, even with 24" joist spacing. Not sure if this is a good idea with the 6"x24" tiles though. I'll have to contact Schluter directly and make sure first.
 
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Old 09-13-10, 06:25 PM
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Looks like I can get away with laying the Ditra directly on the existing 3/4" subfloor without needing to add another layer of plywood, even with 24" joist spacing.
You could use ditra xl in this application, but not regular ditra. They are two different products. Ditra is 1/8" thick while ditra xl is 5/16" thick.

Adding an additional layer of plywood and using the regular ditra will make for a stronger floor in my opinion. Especially with the larger format tile.
 
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Old 09-24-10, 07:03 PM
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Think I'll either go with the Ditra XL (found someone selling it by the linear foot) or 1/2" durock.
 
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Old 09-24-10, 07:43 PM
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Have you checked the durock literature? Cementboards ive worked with want two layers of plywood for 24" oc joist spacing. Id add another layer of 1/2" plywood, then go with regular ditra.
 
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Old 09-26-10, 08:31 AM
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The primary reason for going with Ditra XL is to minimize the floor height. The tiles I have are 1/2" thick. I'm definitely going with Ditra XL
 
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