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Unread 08-01-2020, 05:05 AM   #1
Stuart
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Hardiboard over this subfloor?

Hi,

I removed the tile in my bathroom. The top photo shows what it looks like below the removed tile. The checkered pattern is small amounts of paper left from the bottom of the tiles. The removed tile was small in nature and I'm guessing the paper was used to hold a bunch of small tiles together, maybe 12" x 12" to make installation a bit easier.

It appears that below the tile motar layer is 1 inch thick mud bed. You can see the x-section in the other photo. I know a crack is there (leading to toilet flange), but overall the entire floor is very solid, stiff. That is one of the few cracks. Only in a few places is the top motor chipped away during tile removal.

My question is:

Can I tile over this? I'm thinking of placing a layer of hardibacker over the raised mud bed. I would screw it down into the wood subfloor/joists along with thinset motar. And then place new tile on hardiboard.

Why would this be different than laying hardiboard over concrete (following man'f recommendations??

Thank you

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Unread 08-01-2020, 06:35 AM   #2
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Welcome, Stuart.

You're certainly free to try that if you like, but I don't think it's a good idea and I'm certain it doesn't comply with the manufacturer's installation recommendations. James Hardie does not indicate his backerboard for use over concrete at all, only for use over wood framed subfloors.

You might get by with just grinding off that paper backing and bonding your new tiles to the existing mortar bed, but the safest method is probably to remove the mortar bed, evaluate the existing subfloor structure, and install a new mortar bed or alternate substrate.

How large is the floor in question?

My opinion; worth price charged.
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Unread 08-01-2020, 12:14 PM   #3
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Thank your the reply.

I was just trying to avoid and very dusty and messy solution.

What would be some of the alternates you mention to build it up?

Why would they need a 1 inch build up?

The room is 5 x 8 , not including the bathtub, which I 'm also certain it is not under (its my last thing to remove for complete demo). It also was not under the vanity (included in the 5 x 8 measurement).

Thank you
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Unread 08-01-2020, 01:04 PM   #4
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Removing the mortar bed shouldn't be very dusty, Stuart. Grinding the surface of the mortar bed would be, though.

Alternatives would be additional plywood and a tile substrate.

There is no technical reason for the one inch build-up unless you do another mortar bed. The minimum thickness for that is 3/4ths of an inch. You may need the height to match up with adjacent flooring or to prevent having to remove and lower the toilet drain flange or similar issues.

My opinion; worth price charged.
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Unread 08-01-2020, 02:43 PM   #5
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Hi,

How would you recommend to remove the mud base? Just chip sections out with a chisel?

To get my one inch thickness back (to mate up with toilet flange), would a combo of 3/4 " plywood and 1/4" cement backer board be ok?

Also, I assuming it would be best to put the one inch thickness where the vanity will go (is not there now). But not needed under bathtub.

Thank you again for the help and quick responses. Very helpful and appreciated. This forum is great...
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Unread 08-01-2020, 03:09 PM   #6
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A layer of 3/4" plywood and 1/4" cement board would work fine, but if you're trying to just mate up to the flange, you could possibly use 1/2" plywood, then 1/4" cement board, and the tile and mortar would make up the rest.

The flange should really be on top of the tile anyway.

What is the thickness of the existing subfloor?
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Unread 08-01-2020, 04:00 PM   #7
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The existing sub floor is 3/4 inch.

TY
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Unread 08-02-2020, 07:52 PM   #8
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Then I'd use 1/2" plywood and 1/4" cement board. See if that allows enough room to get the tile underneath the flange.

If not, you could go with a tile membrane like Ditra, which is only 1/8" thick, and that may get you there.
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Unread 08-18-2020, 03:29 AM   #9
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12 x 24 Tile

Hi,

First time DIYer. Our tile store recommended 12 x 24 tiles. They are bigger than I expected to use. Do they pose any type of unusual challenges for install? He also recommended a 1/16 inch grout line.

The floor is 5 x 8 and the tub/shower area is 5 ' high with 3 ' sides walls and 5 ' back wall.

I found some info and it was suggested to use a medium bed motar, back butter all tiles and check the level for the floor tiles.

I am using kerdi board in the shower/tub area and uncoupled membrane on the floor (3/4 subfloor with a 1/2 plywood underlayment)

Any other suggestions?

Thank you,
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Unread 08-18-2020, 07:27 AM   #10
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Welcome back, Stewart,

Any suggestions? From this lot? Yeah, maybe. The first might be to not take advice from the tile store. Or, at the very least, trust but verify.

First off, the minimum recommended grout line depends on the tile itself. Specifically, how uniform in size the tile is. If each tile is exactly the same size as all the others you can probably get away with a 1/16th grout line. If they are not, you need to figure out the difference between the smallest one and the largest one, and multiply that number by 3 to derive the minimum recommended grout joint size. For instance, of the difference (length or width) is, say, 1/16th then the recommended grout joint is 3/16th. Then there's the matter of tile thickness; if the tiles are not of uniform thickness then narrow joints exacerbate the resulting lippage.

Mortar wise, "medium bed" mortar seems to not exist any longer, that term being replaced by something like "LFT", large format tile, which those 12X24's are considered. Yes, you'd want to back butter the tiles and, yes, as a DIY'er you'd want to use some kind of leveling system.

Those large tiles are going to want very flat walls and floor. And speaking of floor, that you ensure that the structure is stout enough for ceramic tile? At this point is sounds like you have enough plywood, if it was installed correctly, but what about the joists under the ply?
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Unread 08-18-2020, 10:22 AM   #11
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Thank you for the reply.

The joist seem very solid. I had a 1 inch mud bed that I removed. My previous tiled floor was very solid with no cracks in tiles/grout, etc.

Just so I'm clear...you mean flatness and not level.

The studs are very flat & plumb when I check with a 4' level. The floor (subfloor) is flat and level on the width dimension. It is slightly out of level for the length dimension. See photo. Is this ok?

Thank you
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Unread 08-18-2020, 11:47 AM   #12
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Comparing a mud bed on a floor's reliability versus say cbu or a membrane is like day and night. You cannot, or should not rely on that fact alone as to whether your floor structure is sufficient for tile unless you decide to do another mud bed on the floor. An unbonded mud bed on a floor is very forgiving as the whole slab can move a little on its own, protecting the tile from structural potential deficiencies. So, you need a bit more info. Deflection has two components:
- along the joists (which you don't know)
- in between the joists, which, given what you indicated, should be fine assuming the sheets were installed properly.
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Unread 08-18-2020, 12:13 PM   #13
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How do I measure joist deflection?

Does the Deflectolator calculator on this site work?


My joists are 2 x 10s, 16" on center, wood in good condition. ..I'm not sure of the length. The ones in my basement are 12' If so, it says ok for tile.



Thank you
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Unread 08-18-2020, 12:29 PM   #14
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Yes, the Deflecto utility works...it differs somewhat from load tables in that it is more conservative since most people doing work on this site are remodeling, and the materials may not be perfect.

The required length info, only you can tell. Typical supports can include load bearing partition wall(s), beams, or rim joists. It is the distance under your floor between supports that is critical, along with the spacing and their depth.
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Unread 08-18-2020, 01:05 PM   #15
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Thank you.

What about level vs flatness? I know its hard to tell, but when I put the new plywood underlayment, the floor will be flat, but slightly out of level as shown by the picture

seems ok for 12 x 24 size tile.

Thank you for the help.
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