Do I need an underlayment?

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Old 01-04-11, 10:00 AM
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Do I need an underlayment?

So I am about to lay tile in a newly constructed bathroom on the second floor of my 1927 farmhouse. I know there was some sloping because the previous owner put a bathtub in there (on 2x6 joists) so I removed the tub and the flooring/subflooring and laminated some new 2x6's to the old ones to do what I could to straighten out the floor (I know this doesn't do much to increase the load limits but it made it flat). I then put down some high grade 3x4 plywood.

Now when I lay my tile do I need to use one of the special underlayments that protect against movement (such as ditra) or can I just use normal greenboard and be safe? The floor feels very solid but most of my remodeling experience is from AZ where we didnt have many 2 story houses.
 
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Old 01-04-11, 10:15 AM
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Just to clarify something....greenboard is for walls..not floors, which I am assuming we are talking about. And even now, greenboard is not recommended for any wet wall areas (around showers or tubs). Finally, there is no advantage to using greenboard even in non-wet areas...except to the people selling it.

Hardieboard or Wonderboard is recommended for tile. Either that or the Ditra as you mentioned.

Ok...so..you have doubled 2x6's. Are they supported somehow or just screwed to the old ones? Whats the span and spacing of your joists?

And then you put down 3/4 ply?

What kind and size of tile do you want?

I can't answer your questions, but the Pro's will need the above info...
 
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Old 01-04-11, 12:06 PM
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Originally Posted by Nurumkin View Post
Now when I lay my tile do I need to use one of the special underlayments that protect against movement (such as ditra) or can I just use normal greenboard and be safe? The floor feels very solid but most of my remodeling experience is from AZ where we didnt have many 2 story houses.
1. How smooth is your new subfloor? I'm talking about crowns and valleys. I trust thinset mortar to handle up to about 1/8" deviations under a 12" tile, but I'm very conservative about this because callbacks cost money. Otherwise, I start with a layer of floor leveling compound right on the subfloor when remodeling older homes. Because you just installed new subfloor, you can possibly shim low spots without a lot of trouble, but the shims have to go between the subfloor and the joists.
2. Install 1/4" cement backer board on top of however far you went in 1. Use backer board screws in all the indicated locations. I prefer the square drive screws because they perform better and the bits last longer than Phillips.
3. Next fiberglass tape and mortar the seams using your thinset mortar.

Now you're ready to lay your tile with the expectation that it will provide good service for as long as you want.
 
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Old 01-04-11, 03:44 PM
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You need thinset under the CBU.
 
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Old 01-08-11, 11:12 AM
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re

Well to clarify a couple things. When I say greenboard I mean a backerboard made for floors, in MN we actually have a brand called greenboard (not because its green but because its supposed to be environmentally friendly) i've used it before with good results. My subfloor is brand new with really no gap other then at the edges of the wall where I cut away the old subfloor. There is a little bit of flex in the floor maybe 1/8" in some spots. I will of course be thinsetting the backerboard to the floor. I am just wondering if it is necessary to use one of the fancy underlayments along with this procedure?
 
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Old 01-08-11, 11:27 AM
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Well...I'd have to say...1/8" of flex anywhere in the field (where it might be walked on) has to be corrected. Maybe if its out near a wall or something it might be ok..but I think you need more stiffness.
 
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Old 01-08-11, 12:51 PM
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bah I was afraid you would say that, Guess i will have to pull up what I have done and laminate a couple more 2x6's to it
 
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