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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
Stuart
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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
Stuart
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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 Yesterday, 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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