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Old 04-03-2011, 10:17 AM   #1
tonymnace
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Cement Board Subfloor Fastening

I am ready to do some floor tile. I am going to use 1/4" cement board subfloor. After talking to a few people, it is my understanding that one uses thinset underneath this as a leveling agent. However, I have heard a couple different ways to then fasten the cement boards. One person told me to use roofing nails as if I used screws, they would bring the cement board down in any low areas. Someone else told me to use screws. Which is the proper way to go as I won't have the chance to do this again once the tile is installed? How thick should the thinset be? Do I want to avoid walking on the cement boards until the thinset has had a chance to dry?
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Old 04-03-2011, 10:20 AM   #2
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Roofing nails will work, but screws would be better. Use a 1/4" notch trowel for thinset under the cement board, Use a 3/8" notch trowel for thinset under tile. You can walk on it and set tile on it right away.
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Old 04-03-2011, 11:14 AM   #3
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As chuck said either roofing nails or backer screws are fine. I prefer to shoot nails from my roofing nailer on larger jobs.

Quote:
it is my understanding that one uses thinset underneath this as a leveling agent.
It's not really for leveling, the level is not gonna change. It's a supporting agent, so that every square inch is supported and so will not move vertically. Fasten from center out.

1/4" square notch for the CBU, the trowel for the tiles depends on the tiles, which we don't know yet. However the 1/4x3/8x1/4 is best for standard 12-13".

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Old 04-04-2011, 10:23 AM   #4
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Thanks for the info guys.
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Old 04-06-2011, 08:41 AM   #5
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Cement Subfloor Thickness

I am doing a tile floor in my kitchen. I was all set to use 1/4" cement board as a subfloor, but after talking to a friend who does tile work, he said I need a minimum thickness of 1 1/4" underneath the tiles. I have 3/4 X 5 1/2 boards on top of the floor joists, so according to that, I would need 1/2" cement board, which would give me the correct 1 1/4, but put me a little higher than I want to be. My question is, if I am using thinset underneath the cement boards, wouldn't that give it enough support to use 1/4 boards as it would keep it from flexing? My friend also says that if I use the 1/2", I don't need the thinset.
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Old 04-06-2011, 08:53 AM   #6
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Tony, please keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered. Thanks.

I'd also recommend you stop asking that particular friend for tile installation advice.

There is no rule of total thickness for subflooring. And where layers of subflooring are specified, it does not include the tiling substrate such as your CBU.

The CBU manufacturer will specify the minimum thickness of subflooring for installation of their product and the required method of installation.

The TCNA also publishes various tested methods of floor installation with the requirements for the subflooring and underlayment called out.

For your board subfloor, the minimum additional subflooring you require is nominal half-inch plywood. On top of that you can use any thickness CBU recommended by the manufacturer, which will virtually always be their thinner product.

The only reason for half-inch CBU in a floor application is to increase finished floor height if that is desirable.

And all CBU manufacturers, to my knowledge, require a bed of thinset mortar under their product during installation.

My opinion; worth price charged.
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Old 04-06-2011, 10:16 AM   #7
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I only made it another thread because I thought people would stop looking at my original post once it was answered. I'm asking lots of questions because it seems I get a different answer depending on who I ask. As far my thickness under the CBU, according to the James Hardie site, the instructions for their board state:

Floor Installation

We recommend 1/4” HardieBacker™ board for floor applications, unless 1/2” thickness
is needed for transition.
1. Ensure subfloor is structurally sound
On existing structures:
• Ensure subfloor is not damaged. Replace any loose, warped, uneven or damaged sections
of floor.
• Make certain subfloor is a clean and flat surface.
For all floors:
• Use minimum 5/8 exterior grade plywood or 23/32 OSB with Exposure 1
classification or better, complying with local building codes and ANSI A108.11.
• Joist spacing not to exceed 24” on center.
• The floor must be engineered not to exceed the L/360 deflection criteria (L/720 for
natural stone), including live and dead design loads, for the specific joist spacing used

Seems that they only require 5/8" underneath, so it would seem my 3/4 flooring would be adequate or that's at least how I read it. Is that how you read it or are you saying that I would need to put 1/2" plywood on top of what I already have?
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Old 04-06-2011, 10:29 AM   #8
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Tony,

Hardie specified 5/8" plywood. That's a different animal than 3/4" planks. Planks move a LOT more than plywood.
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Old 04-06-2011, 11:39 AM   #9
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Quote:
Originally Posted by Tony
I'm asking lots of questions because it seems I get a different answer depending on who I ask.
You're barely getting started, Tony. This site was created by one John Bridge years ago for the express purpose of answering DIYers' tile project questions. You just axe away.

But let me caution you about taking information from people not known to you who you find on an Internet site, eh? In this case I assure you we're correct, but verify whatever you are told with the product manufacturer's recommendations and the applicable industry standards. Sometimes it's real important.

And read carefully, as Paul is pointing out.
Quote:
• Use minimum 5/8 exterior grade plywood or 23/32 OSB with Exposure 1 classification or better, complying with local building codes and ANSI A108.11.
It's not easy sometimes, but don't assume anything that's not in there. If they wanted you to install over a sawn board subfloor, I assure you they'd say that. They don't.

And what they do say represents their minimum requirement. Don't lose sight of that, either.

My opinion; worth price charged.
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Old 04-14-2011, 07:55 AM   #10
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Subfloor Outgassing

I was going to use 1/2" OSB as a subfloor over 3/4 planks for my tile job. I then started reading about outgassing of formaldehyde from these products. Is this going to be a problem and should I use plywood instead? Is that any better, as they use formaldehyde in plywood as well, I believe? Would the fact that its going to be covered by tile make a difference?
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Old 04-14-2011, 08:00 AM   #11
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Unless you're particularly sensitive to formaldehyde, Tony, you're not gonna know the difference even if you don't tile.

More important reason for using plywood is the restriction of many setting product manufacturers against use of their products over OSB.

Is this the same floor for which you have another thread indicating you intend to use CBU as your tiling substrate?

My opinion; worth price charged.
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Old 04-14-2011, 08:05 AM   #12
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Yes it is. I didn't know they had a restriction against OSB. I got it because it was cheaper and lighter to carry, but I'll go get the plywood if it's going to be better. Could I get by with 3/8 plywood or do I need the 1/2?
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Old 04-14-2011, 08:17 AM   #13
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Let me ask you once again to keep all your project questions on one thread, Tony. Will prevent confusion and duplication of effort, as you see here with your most recent question. We can give it a more generic title any time you'd like to suggest one.
Quote:
Originally Posted by Tony
Could I get by with 3/8 plywood or do I need the 1/2?
Please see post #6 for the answer to that.

I suppose a fella could use OSB, but the plywood would be my first choice in that installation.

My opinion; worth price charged.
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Old 04-14-2011, 08:21 AM   #14
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Thank you sir!
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Old 04-15-2011, 03:44 PM   #15
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In putting down my plywood, I find I have a 1/4" gap underneath one section near an adjoining room. Do I shim underneath it or do I screw it down tight to conform with the existing plank subfloor?
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