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06-30-2019, 01:43 PM
Hi All! I am in the process of ripping out the crappy tile job and redoing the bathroom floor.

I have already ripped out the old tile and dry pack. I replaced half the subfloor due to water damage and rebraced the new and existing subfloor. I put down 1/2 inch plywood to match the existing re-usable subfloor. Here is where I start to have questions.

I have roughly a 1.5 inch height difference between the subfloor and the hallway transition. I plan to put down another layer of 3/8 inch ply wood using liquid nails and then screwing every 6 inches down the joist lines.

After that I plan to put down a layer of modified thinset to be topped with Ditra underlayment.

Then I'm not sure if I need modified or unmodified thinset for the layer that goes on top of the ditra for the tile?

I am using 12x12 ceramic tiles. I was directed over here from reddit and seem to be getting conflicting advice there.

My original plan was to put 1/4" hardieback down instead of another layer of 3/8n plywood and then put the ditra and other layers however, that apparently provides no rigidity which makes sense. The current 1/2 inch plywood subfloor is pretty sturdy but I understand the need for thicker subfloor.

So my question is, is the proper layering? And can I use the same thinset for underneath and on top of the ditra?

Layers from top to bottom:

-3/8 inch ply
-1/2inch ply subfloor

The 1/2 inch plywood floor is there to stay, I'm not ripping it up again so I need to work with what i have.

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06-30-2019, 02:55 PM
As you were advised, nothing to be gained structurally with the Hardi. Go with the 3/8 or 1/2 and the Ditra. Forget the liquid nails.

Use a modified both under the Ditra and under the tile. Versabond should be fine... :neesie:

06-30-2019, 02:56 PM
Welcome, Jake. :)

Your half-inch plywood subfloor is useless for any floor covering, 'specially for ceramic tile. I think jail time is too good for any builder who installs such a product as a first layer of subflooring.

First thing you wanna do is evaluate your joist structure to see if that qualifies for a ceramic tile installation. You can get a good first go/no-go reading by using our Deflectometer in the dark blue bar near the top of this page.

Then to your subfloor. If, and that's a big if, your current half-inch layer is in near pristine condition, it's possible to glue-laminate another layer of minimum half-inch exterior glue plywood with no face of grade lower than C to what you have. To do that, you would need a full spread of wood glue, such as Titebond II or III and screws no more than 6" on center across the entire panels. A bit closer would be better. You want to orient your panels such that they overlap the existing joints (none of which are T&G) as far as possible and be sure to screw close to those joints. You won't find that method in the tile industry standards and it's actually even more complicated than it sounds. But, done correctly, it may give you a suitable subfloor.

You do not want to use Liquid Nails or anything similar that comes in a tube.

Second choice, if you won't remove the existing half-inch layer, which would be by far the best option, is to pretend that existing layer doesn't exist and put a new subfloor of a minimum nominal 3/4" exterior glue plywood, fastened to the joists as though the first layer were not there. You'll need fasteners that will penetrate a minimum of 3/4" into the joist tops. Again, a bit more is better.

Then we can talk about your tile installation. For starters there, I'd recommend you download the Ditra Installation Handbook from the Schluter Ditra website and read their recommendations on the appropriate thinset mortars. I think you'll find that no, according to the product manufacturer, you cannot use the same mortar to install both the Ditra and the tile in your application despite what you've read elsewhere.

My opinion; worth price charged.

06-30-2019, 03:37 PM
Go with Laticrete StrataMat and you can use modified thinset under it over it to set the tile.

I'm in agreement with CX that you need to forget the 1/2" is even there. Do it the way he described and you will have a rock solid subfloor.