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Unread 10-27-2018, 12:14 PM   #1
JamesR
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Hardie backer 500 or not...

Hello all,
I am in the process of re-doing a small 6' x 6' powder room on our first floor. It is completely empty now. It was 12" x 12" tile and I had ripped all of that out as well as the 1/2" plywood that was under it. Just left with a clean plywood subfloor now. The wife wants to re-tile it so I had bought 3 sheets of hardie board (.42" thick stuff) I was reading about the proper way to install it and that looks to be 1/4" troweled thinset and screw it into place. That makes me wonder about future bathroom remodeling if that were to happen. How would I be able to remove the floor? Should I even be using hardie board in the 1st place? I used it about 20 years ago on a counter top tile job for a bar in a previous home but never for a floor tile job.
What is the recommended way to prep a floor for tile in a powder room these days? Am I ok using the hardie that I have purchased? Here is my blank slate so to speak...
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Unread 10-27-2018, 12:22 PM   #2
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Welcome back, James.

Nothing wrong with using that product on the floor. However, it's traditionally used on walls, and the 1/4" version is used on the floor. The only difference is that you'll add almost 1/4" of height to the floor with what you have.

Again, nothing wrong with it, but there are other options if you want to save the extra height.
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Unread 10-27-2018, 12:28 PM   #3
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Kman, I forgot to mention, I went with the .42 vs the 1/4" because I need to build the height back up to meet the transition from the 2 rooms that have doors to this bathroom. I ripped out everything on the first floor and installed 3/4" solid hardwood. So i need to build this floor back up 3/4". I figured the .42 CBU and new tile would be pretty close.
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Unread 10-27-2018, 02:25 PM   #4
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James, don't worry about future remodel jobs. The tile and Hardi can always be removed. If you install it so it's easy to remove later, you may have problems before you decide to remodel.

In the dark blue bar above, you'll see "Deflecto". You might want to check the floor structure before moving forward just to be safe. How thick is the plywood you now have? Sometimes I will drill a 3/4 inch hole close to the wall to check the thickness.
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Unread 10-28-2018, 06:01 AM   #5
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Davy,
The joists are neither syp or fir. They are the "Silent Floor" joists. I think they are also called Trus Joints. 10.5" tall. Not sure how that factors in to the deflecto calculation. Spaced 19.2" throughout most of the house and 12" on center under the bathroom. The subfloor on the joists is 3/4" thick. The floor feels rock solid. Based on that, the thought is to go ahead and apply thinset to my existing subfloor and screw down the hardi board?
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Unread 10-28-2018, 06:28 AM   #6
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If the floor feels rock solid then it's probably good.

Yeah, get the sheets set in place and then pick one up at a time, spread the thinset and screw it down. Then move to the next sheet.
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Unread 10-28-2018, 07:49 AM   #7
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OK, that sounds good, but now I am rethinking the .42" thick backer board ( it actually measure .38" BTW) vs the 1/4" backer board. I am supposed to use a 1/4" square trowel to install the backer board. Can I assume that will compress to 1/8" thick?. That would make my height of thinset + backer board about 1/2" and then when I install 1/4" thick tile and thinset on top of that I would be about 7/8" thick. That puts me about an 1/8" above where I want to be to transition to the existing hardwood floor. Is my assumptions on the added thickness of 2 layers of thinset correct here?
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Unread 10-28-2018, 12:45 PM   #8
Tool Guy - Kg
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Hi, James.

You're close. But that 1/4" square notched trowel will result in a slightly thinner layer of mortar. Because the trowel is run at something like a 45 degree angle, the resulting square notches deposited when you are spreading mortar are lower than 1/4" tall. In laboratory-perfect conditions: Each layer of mortar, fully compressed will be really close to 3/32". So, the two layers will be really close to 3/16". That should help you get closer to your goal thickness using the measurements you gave.

But realize, that's laboratory conditions. If the substrate or tile aren't flat and the tile is doing a little "teeter-tottering", the thickness will change. Or if you don't fully compress the ribbons of mortar, the thickness will be a little more.

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Unread 10-28-2018, 01:01 PM   #9
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What size are the tile you're installing? You may need to use a slightly larger trowel.
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Unread 10-28-2018, 02:12 PM   #10
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Thanks for the explanation Tool Guy. I did not take into consideration the angle of the trowel. Shame on me being an engineer type guy.
Kman,
Tile has not been picked out yet, but something along the lines of a 12" x 12"
Maybe 16" max.
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Unread 10-28-2018, 05:00 PM   #11
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Nobody can give you a definitive trowel notch size based on the size of the tile. It's dependant on the tile flatness and substrate flatness.

Best we can say from this side of the screen is something like this:
On average......with 12" tiles, you're likely to need at least a 1/4" notch, but could easily go bigger if they're a little wavy.
With 16" tiles, you're likely to need at least a 3/8" notch, but could easily go bigger if they're a little wavy.
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Unread 10-29-2018, 01:56 AM   #12
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OK, I guess I will wait to see what tile she picks out and then plan on the hardie install from there. Whether it be the .42" or the .25"
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Unread 10-29-2018, 04:12 AM   #13
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Don't count on the tile being 1/4" thick either. They can vary quite a bit.

Best to already have the tile, or at least have a sample of it, and test it on site to find out the total thickness of the assembly.
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Unread 10-29-2018, 12:46 PM   #14
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I don't see any mention on doubling the sub-floor.

From what I've read over the years in this forum, I believe that it's usually desired (if not in industry standard) that the sub-floor be made of 1st a 3/4" layer of plywood, then a 2nd layer of 1/2" plywood oriented in the other direction (perhaps) and covering the joints of the 1st layer (for sure). Then ONLY use thin set to attach the 1/4" backer board to the 1/2" plywood.

(And if I recall correctly, the 2nd layer of plywood should only be attached to the underlying plywood rather than trying to screw thru the underlying plywood back into the floor joists.)

With such a setup, if you ever needed to remodel again, you'd also be able to rip up the 2nd layer of plywood and again start over from a clean 3/4" sub-floor.
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Unread 10-29-2018, 02:12 PM   #15
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Quote:
Originally Posted by Joseph
I believe that it's usually desired (if not in industry standard) that the sub-floor be made of 1st a 3/4" layer of plywood, then a 2nd layer of 1/2" plywood oriented in the other direction (perhaps) and covering the joints of the 1st layer (for sure). Then ONLY use thin set to attach the 1/4" backer board to the 1/2" plywood.
Whoa, there, Joseph!

All layers of structural subflooring need to be oriented with the strength axis perpendicular to the joist structure. And all CBUs must be installed with mechanical fasteners over a bed of still wet thinset mortar, per the manufacturer's instructions.

But you got the part about fastening the second layer of plywood subflooring only to the first layer of subflooring right.

My opinion; worth price charged.
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