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Old 06-01-2019, 03:25 PM   #16
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If your current subfloor is in good shape, add a layer of 1/2" plywood using the guidelines in this article.. (link in post #18)

Which mortars are the best? Ask ten people here and you'll get ten different answers.

Mapei Ultraflex 2 and the uncoupling mat mortar will work.
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Old 06-01-2019, 03:32 PM   #17
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Are the tile ceramic or natural stone? That would make the first consideration of subflooring.

As big of a deal is what is the spacing and depth of the joists along with their unsupported length. This is NOT the size of the room, but the supports that are underneath it. If that quality is not sufficient, nothing you do to the subflooring will prevent further cracking problems.

Cracks occur because of movement. That could be caused by insufficient joist structure, improper subflooring, or improper tile installation. There are many ways to mess up a tile installation. Even if the physical part of laying the tile is done properly, if the structure is not adequate, you can still have issues.

The actual bond of Ditra to the plywood is the ultimate strength of the fleece on the underside and the bond to the plywood. Because plywood moves with humidity level changes and temperature, you need a modified thinset mortar to prevent that bond from breaking over time. The strength of the actual fleece must meet industry standards of a minimum of 50psi. Most any modified mortar will greatly exceed that spec, so going to a premium product doesn't really buy you as much as you might think. You want a quality one, but it doesn't need to be super strength. Spending a bit more means a smoother mix, easier embedding of the fleece.

Thinset doesn't stick to Ditra. It does to tile, The weakest link is where it breaks, and as said, it is NOT stuck to Ditra. So, a premium unmodified thinset works there just fine holding onto the tile, but like most things, requires good workmanship. On a tile that large, you'd want to use a mortar designed for large tile. It will have less tendency for the tile to 'sink' after setting and is designed to handle more depth, often required for large tile that may not actually be perfectly flat.

WIthout identifying why the existing tile cracked, you may not solve your problem with new materials, even if they are installed properly. The structure must be up to snuff first.
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Old 06-01-2019, 07:29 PM   #18
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I do not believe that we have yet heard what the joist structure is. Until then, everything else is based on speculation.
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Old 06-01-2019, 07:38 PM   #19
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Quote:
Originally Posted by Jeff
I do not believe that we have yet heard what the joist structure is. Until then, everything else is based on speculation.
Yes, that has been cautioned and advised multiple times in this thread. I don't know if what he has is up to snuff or not, but it appears he's continuing on. So he's either checked it and it's fine, or he hasn't and is willing to gamble.
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Old 06-01-2019, 08:06 PM   #20
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Hi, i am not ignoring you guys. And also some of the lingo/terms you guys use I am not familiar with. The existing subfloor is 3/4" plywood. Here is a picture. The joist spacing is off, please see the image. I am going to have the installer add 0.5" more of plywood.

What other information do you need? Why is the joist spacing off? How do i know what the joist structure is? The exposed joist i can see is 1.5" wide, 9.5" deep, and an unknown length. This home was built in 1970.

Also i got ultraflx LFT for the plywood bond to the ditra. Then i got Kerabond T for the ditra bond to the tile. Is this okay? They also sold Ultraflex RS and Ultraflex 2. Mind you these are 12x24" tiles.

The existing tile under and around the toilet will be ripped up. This is where i will be using the Ditra. I am thinking of having him add anywhere from 0.5" to .75" worth of plywood over the whole floor to stabilize it further. I think the old tile came up because of bad labor practices - i'm saying this because they didnt cement underneath the hardibacker, in addition they used garbage bags in the shower as waterproofing membrane. Furthermore they tiled ONTOP of tile to create a better slope.

PS I APPRECIATE YOU GUYS
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Old 06-01-2019, 10:15 PM   #21
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Sometimes, it's easy to tell how long the unsupported span of your joists is...are there walls on the floor below that line up with supports in the basement?

WIthout knowing how long the span is, NOTHING you do on top of them really will resolve things. Tile is brittle...when things move, they crack. Plywood helps in BETWEEN the joists, but the joists themselves ALONG them also need to be stiff enough. They get stiffer by being deeper and from having the span shortened. The whole thing gets stronger when they are closer together. More ply can help the staggered joist spacing (up to a point), but not along them.

Now, not putting cement underneath the cbu is one thing that often causes an installation to fail, and, it appears that that wasn't the only shortcut they made. Fixing that may still not fix your underlying problem, though, if the joists aren't stiff enough for the tile to survive.
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Old 06-01-2019, 11:06 PM   #22
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Put a layer of 1/2" plywood down. It will be sized 15/32
Get the 4 or 5 ply stuff, don't use the 3 ply. Its junk. Screw it down with 1-1/4th inch or 1-5/8th inch screws that are designed for subfloor. Don't use your basic gold screws. Get the good ones and they will most likely have a t25 torx drive. Lay the plywood on top of what you have. Don't let any of the seams line up, try to overlap them by 8 inches. Don't let your edges of the new plywood line up with joist and try not to hit a joist when screwing through both layers of plywood. Screw the plywood off at every 6 inches on perimeter and 8 inches in the field. That's gonna be 103 screws per sheet, if my maf is right.
Cover that plywood with your choice of cbu or ditra. You should be good to go.
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Old 06-01-2019, 11:18 PM   #23
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Thanks Shady. I think i found the screws.:
https://www.homedepot.com/p/Deck-Mat...DMT5/305418739

Also the dimensions of my bathroom are attached. I think i will need 2 of the 4x8 plywood strips to be safe, but i should be able to get by with one being that my area is 30 SF and not 32 SF. I am trying to imagine what size (aka dimensions) i should cut the 4x8 piece of plywood into so that it fits nice and snug on top of the existing plywood. Am i better off doing a giant piece that covers the majority of the area then taking small cuttings and filling in the rest?

Also, how does my thinset selection look?
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Old 06-02-2019, 10:34 AM   #24
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The plywood needs to run perpendicular to the joists. I would avoid a bunch of small pieces. Just make sure the plywood joints don't line up with the first layer like Travis mentioned.
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Old 06-02-2019, 02:00 PM   #25
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UPDATE:
I ripped up the bathroom tile.

Not only did they not cement under the hardibacker, they didnt completely butter the backs OR even screw in the hardibacker!!

Okay so look at the pictures. How does it look? How do i remove the left over remains of that old floor? I dont know if its lineoleum or not, but it was plastic on the top. Underneath (stuff that is still stuck to the floor) feels like cardboard.

Please let me know your thoughts. ALSO, did i just remove asbestos tiles?
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Old 06-02-2019, 04:13 PM   #26
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Yes your thinsets are ok to use.
Here is a website to identify asbestos flooring.

https://inspectapedia.com/hazmat/Arm...st_1974-79.php

I worry I have demoed a 1929 kitchen with asbestos square tiles 32 years ago. I remember not seeing that much dust but feeling like it was hard to breath. I didn't know better. I have lost lots of sleep from worry about this.
I would use good resperator from now on. Looks like black mold.
it looks to me like you had a leaking toilet that ruined the floor installation that was done in a terrible manner. It looks like they basically set the hardibacker down and tiled on top of it and look at the imprint of hardibacker in that tile with no adhesion at all. so they did not wet down the Hardy before they put thinset in and they did not burn the thinset into the HardieBacker. And they did not back butter or get good coverage. But a leaking toilet seal was the final straw and probably why it failed so quickly. You are going to have to remove that floor and replace it.
I still don't think you have measured the span length of those joist can you stick a tape measure in that hole and extend it until you hit the next support in each direction?
Keep going you are going to fix this problem correctly and feel good in the end.
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Old 06-02-2019, 06:40 PM   #27
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the black stuff was only underneath where the linoleum was. I reviewed those pictures from the Asbestos link you provided and none of them looked like my linoleum.

Anyone with experience know if that was probably asbestos? I just circular sawed it all out for the most part. Here is what it looks like now.

edit: joist length is 12.5 feet, 9.5 inches deep, and 1.5" wide.

game plan is to put down another 5 ply 3/4" plywood. Then on top of that put another 1/2" of plywood to cover the entire thing.
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Old 06-02-2019, 08:01 PM   #28
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Linoleum by itself does not contain asbestos...the adhesive that held it down could.
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Old 06-03-2019, 12:39 AM   #29
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How should I stabilize the joists while adding the new plywood? There arenít visible joists on either side
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Old 06-03-2019, 06:01 AM   #30
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Mike - can you post a picture of the joists from the stairwell side? Trying to see if you can get a tape in the to measure span of joists.
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