Subfloor thickness question for 19.2" I joists using 24x48 porcelain tile tile

[truthisone]

I am redoing the bath where I am looking to place 24x48x9mm porcelain tile. I have I joists spaced at 19.2". with 23/32" OSB glued and nailed. Previously bathroom was done with 12x12 tile over 1/4 backer board. Floor needs some spot feather leveling to address low spots.
My plan is to use ditra heat for heated floors.

I was hoping to hear advise on the following options:
option 1: place ditra heat directly over existing subfloor after addressing low/high spots and tile over it. Final thickness above subfloor (1/8 thinset+1/4 ditra heat+1/4 tile thinset +9mm tile.) =0.979in

option 2: 3/8 underlayment over existing subfloor screwed and following guidelines for placing joints. +ditra heat+tile. this option will add 3/8 additional thickness for total of 1.354".

My concerns:
Option 1: -possibility of cracked tile/grout using large tile 24x48?
Option 2: -large thickness of final tiled floor 1.354-3/4"=0.6" of transition to deal with if placing hardwood in adjacent room (Master Bedroom) or placing a thick carpet pad and carpet. (that probably will endup with even higher transition)

My personal preference is to deal with transition if there is any change of tile/grout cracking but if option 2 overkill then I would rather stay with option 1.

Please let me know what you pros think...

[cx]

Welcome, David.

Presuming you have already evaluated your joist structure and found it to meet at least the minimum L/360 deflection requirement:

1. Schluter allows the installation of Ditra Heat over single layer nominal 3/4" OSB over 19.2" joist spacing. Keep in mind that the testing for that was done using new materials, in near perfect condition, near perfectly installed over joists with zero deflection. And the test need pass only once. If you're comfortable with your installation meeting the subfloor deflection requirements, use that method.

Your estimates of mortar thickness under the membrane and under the tiles are more than I think you'll actually have.

Potential for cracked tile or grout is no greater than if you were using smaller tiles.

2. I'm always in favor of a second layer of subflooring (it's not really an underlayment), but I find that nominal 1/2" is my minimum thickness because the available nominal 3/8ths" material is seldom flat enough to be useful. Up to you.

Transitions to adjacent hardwood are among the easiest to make, and I don't think you would find a half-inch rise to be troublesome in use.

Transitions to padded carpet are also rather simple, using cedar shingles or similar tapered products to ramp the carpet up to the tile. Done regularly.

My opinion; worth price charged.

[truthisone]

Thank you. you confirmed my thoughts about going with option 2. I'd rather deal with transition than cracked tile. Thank you

[truthisone]

CX, do you think Exterior grade OSB sheathing 7/16 would work as underlayment before going Ditra?

[Kman]

David if you have a link to the product you plan to use, that would be helpful.

But it can't be pressure treated plywood, if that's what you're looking at.

[truthisone]

here is the link :https://www.lowes.com/pd/7-16-CAT-PS...4-x-8/50382768

[cx]

While that would meet the structural requirement, David, I would not use it in your application. While Schluter is happy to have their sheet products bonded with thinset mortar to OSB, some manufacturers of thinset mortars don't particularly want their mortars used in that application. I personally don't care for bonding of any kind to OSB, but that's a personal preference.

Schluter does make their own thinset mortar, which does meet the necessary specification (ANSI A118.11), and I understand they have given verbal assurances that it can be used in that application.

I'd want plywood, and I'd use a less expensive mortar. Entirely up to you.

My opinion; worth price charged.

[truthisone]

This the quality of 1/2 plywood Georgia Pacific brand. Need to go to a builder supply store to find something better

[cx]

David, you need an exterior glue plywood with no face of grade lower than C. That's not what you've got there.

[truthisone]

This is rated Grade Georgia Pacific in Lowe’s and HD. The bottom side . I looked at 6 sheets in 4 different stores - very bad quality wood! Edges missing chunks of middle ply . Bunch of knots . Even the top face . It is 3 ply total product
Here is the link to product : https://www.homedepot.com/p/15-32-in...2411/100067329

Something happened with quality of plywood . It wasn’t like this before

[AR]

David,
Lowe's carries BCX plywood. Pricey but suitable. Take a look to see if it is available near you.
https://www.lowes.com/pd/Plytanium-1...-4-x-8/3010108

[truthisone]

Thank you Andrew, it is the same Product that HD carries. However I am going to check another Lowes in the area and get back to you. Something really happened with the quality of wood in the last 2 years.

[truthisone]

Ok was able to find a decent quality BC plywood at Builder supply. Lowes was not good. DO you all think is it better to level the subfloor (some low spots)via feather finish before or after screwing 3/8 underlayment plywood?

[cx]

You'll need to do any flattening or leveling on top of your second layer of subflooring and before your Ditra installation, if you still plan to use the Ditra.

My opinion; worth price charged.

[truthisone]

CX , my thoughts were is to level before to avoid any possible voids under 3/8 underpayment. Or the underpayment will conform to uneven subfloor mimicking it’s shape?