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Old 06-01-2019, 03:32 PM   #17
jadnashua
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

 
Join Date: Jul 2004
Location: Nashua, NH
Posts: 12,944
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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Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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