kitchen floor cement board question

Hi, I am remodeling my kitchen and redoing my floor. I will be using 12x12 porcelain tiles. I want to know what needs to go under the tiles.
The contractor have removed the existing vinyl flooring with some kind of backerboard, this is what I have now:
http://fenjing.smugmug.com/gallery/4460346_n85dF
I was under the impression the contractor will put weather paper, then a layer of cement backboard, and then the tiles..
My question is, can the cement backboard (1/4 or ??) go directly on top of this? Or do I need another layer of plywood board (how thick) beneath the cement board?
I asked my dad (who is somewhat knowledgable but by no means a pro), he said the plywood board is essential because the cement board is brittle if lying directly on the wood beams.
Can someone help me? The contractor just started yesterday so I haven't talked to him about the flooring yet. He did mention weather paper and cement board. Should I insist on ? cement board if he is no plywood is involved?
Thanks!
Raymond
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There are many factors to consider, but what is that weather paper under the cement board. The cement board should be bonded with thinset directly to the floor under it with NO paper between. I would not install 1/4 cement board over wood boards as you have, you need for 12 x 12 tiles, 1/2" cement board over a layer of plywood, as thick as your installation will allow. The plywood should be glued and nailed down to the floor boards and the cement board thinsetted down to the plywood.

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What are we seeing in the pics? If that is the subfloor, and the subfloor is on the floor joists, then you will need at least another 1/2" plywood plus 1/4" backerboard. You need at least a total of 1 1/4" solid flooring for the tiles not to flex or break. That's what I did in my kitchen. I ripped up the floor down to the 3/4" subfloor, put 1/2" plywood, then 1/4" backer on top. Don't forget you have to thinset the backerboard as well as screw it to the plywood. I'm not sure why he would want to use weather paper. If that is an existing oak floor in the pics, then he probably wants to use the weather paper to prevent the oak from getting wet, which would cause the oak to expand, thus cracking the tile
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So I talked to him, he said he'll use 1/4 hardy board. He said it's like a cement board but the particles are finer. I asked him if he can do 1/2 hardy (or cement) board and he insist those are used on walls and not on floors.
He did not talk about a layer of plywood under the bardy board, and I didn't ask him either since I am just reading this now. Is a 1/4" hardyboard enough? How important is the plywood layer? He said there will be another layer of cement (thinset) on top of the board so having a 1/2" cement board is too tall.
Thanks!
Raymond

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No, the hardyboard alone is not enough. You need 3/4" structural plywood with a sanded face underneath it. A vapor barrier should go between the subfloor (shown in your pictures) and the plywood. The plywood should be screwed down or nailed down with ring shank nails every 8 inches in all directions. The hardy board 3/8" or prefereably, 1/2", not 1/4", is then screwed to the plywood (nails are not used here). Then the tile is set with the proper setting material on top of the hardy board.
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I should add that since floor height is an issue for you, you can get away with a combination of vapor barrier, covered by 5/8" structural plywood with a sanded face, with 3/8" hardyboard over that. That would be a minimum spec installation. Anything less will fail in short order, guaranteed.
Given that the height of the finished tiles floor is an issue, you should look into getting a nice transition or edge piece from the adjoining space(s). My guess is that the contractor is trying to address your concern about floor height by appeasing your concern instead of insisting on a structurally sound floor. If I were building the floor on a vapor barrier and 1/4" hardyboard alone, I would make you sign an informed consent waiver of structural inadequacy before I started the job.
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You left out a decoupling membrane ;) How can you know what he needs if you dont know the joist spacing?
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I'm assuming standard 24" O.C. joists. You're right, I shouldn't assume.
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Thanks. Thats explains why I thought most of the floor advice I saw was way thicker than needed. My joists are 18".
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On Mar 5, 2:06 pm, snipped-for-privacy@none.com wrote:

Yes. According to the instructions, 1/4 is stronger than 1/2.

The subfloor is the most important part of the job. The tiles will crack if it's not right.
Like Mike said, 1 1/4" solid wood with any layers glued and screwed together. Thinset and nails/screws securing the 1/4" hardiebacker. And that's a minimum if the joists are solid.
And yes, all that makes for a high floor when all is done. There are ways to deal with the transitions.
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The contractor insist that a plywood layer is not suitable for my case. He said when he does new constructions he uses 3/4" plywood because the sub-floor is not as strong as older homes so the plywood is needed for structural needs. The wood subfloor you see in the pictures are pretty thick and these types of homes don't need a layer of plywood (mine was built in 1960). On the adjacent rooms where there was hardwood floor, the hardwood sits directly on top of the subfloor, there were no plywood in between... but hardwood is probably differentn from hardyboard and can flex somewhat.
ah crap.
Raymond
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How thick is the subfloor in your picture? Most subfloors are 3/4". Hardwood flooring is usually put right on top of the subfloor, there is no plywood underneath.
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It looks like 2x6 cardeck which would be very solid 1/4 inch hardy right over the top.
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The subfloor is about 1.5" thick. This is how thick the subfloor is on the cover to the craw space (on top of the 1.5" is the actual hardwood floor on the cover).
Now my neighbor told me that the kitchen subfloor has been replaced before due to termite damage, so it may not be as thick as the rest of the house..
It's all done now. Subfloor + 1/4" hardyback + thinset + US made 13" porcelean tile. It feels fine. The cabinets, frig, stove and everything else is in, and the tiles didn't crack during the install.
With just the 1/4" hardyback installed, I walked on it and it feels soft. I was quite upset thinking that it won't be enough. But after the thinset and tiles are layed down, it feels much better now.
I didn't know the subfloor were not the original ones when I started the job, had I know that, I would've insisted on either 1/2 hardyback or a layer of plywood. Oh well. I am pretty careful anyway and walking on it wearing slipper it feels pretty sturdy, for now. ;)
Thanks everyone!
Raymond
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No need for the additional ply- what you have is definetly 2x6 if it's really 1.5inch. Good solid floor. Don't worry about it- it'll outlast you!
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