Subfloor and Ditra [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-01-2010, 05:40 PM
I am planning to tile my kitchen floor with 18" porcelain. I have no experience and I have done some research, but I am not sure how to deal with the subfloor. I intend to use Ditra-XL so that I can install the tile so it will be flush with adjacent oak T&G floors. The current floor is on 9.25" x 2" joists, 16" OC, spanning 12'. There are solid wood (some kind of fir) floor boards on top of the joists - 8" x 3/4" and they are running diagonally across the floor (the house was built in 1930). There is 3/4" T&G flooring on top of that. I am planning to remove the T&G boards, but I am not sure if I will need to replace the layer below that with plywood or OSB. I am hoping I can install the Ditra on top of the floor boards. Does anyone have any advice for me?

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02-01-2010, 06:02 PM

You'll need to install at least a layer of 1/2" ply on top of the planks. Planks are not a approved substrate.

02-01-2010, 06:17 PM
Thanks Mark - that is what I was afraid of :(

Do you think if I use 1/8" Ditra I will be able to transition from the adjacent floors - I have 3/4" to play with, so the 1/2" ply + Ditra + tile will bring the tile maybe 1/4" higher than the oak flooring - is that a reasonable difference to do a transition with say a 4" wide sloping hardwood board?

Dave Taylor
02-01-2010, 06:50 PM
Like Mark says.....
welcome to Tile Your World forums.

1/4" is a easy height to cover using a wood "T" or other type of transition.

I am yet (however) unsure of just what your flooring sandwich consists of.... and it is important there be no question about this as it is vital to our advice.

What are the dimensions, composition and condition of the 3/4 T&G boards up top your diagonal 3/4" x 8" subfloor?


02-01-2010, 07:16 PM
the T&G flooring is some kind of fir, 3 1/4" wide, 3/4" thick and 6' to 8' lengths. What I have seen so far is in good condition (it is still mostly covered with hardboard and linoleoum). But I am planning to remove that layer of T&G, leaving only the 3/4" diagonal planks on top of the joists. I was hoping I could use 5/8" Ditra directly on the planks, but Mark says I should put in 1/2inch ply on top, but then I would have to use the thin Ditra (1/8" I think) so as to keep the height difference to a minimum.

Any ideas/suggestions would be most welcome, as this is my first DIY project and I need all the advise I can get :)

Dave Taylor
02-01-2010, 08:06 PM
Mark is right then Eamonn.

Use a minimum 1/2" thick ply underlayment with no face grade greater than "C" with exterior rated type I gluing..... installed as per THESE ( recommendations.

Be sure your 3/4" subfloor boards are flat and well affixed to the joists. Sand or replace them as necessary and affix them to the joists as required using galvanized deck screws.

Hope this helps.

02-02-2010, 08:10 AM
Dave's got you very much on the right path. You're counting on those ancient planks to do a lot of work - they alone will be holding the new 1/2" ply you put down, thus critical to eliminating fatal motion in the tile work.

If the boards wind up being really checked, cracked, loose, etc - you would be better off tearing them out, replacing w/ new 3/4" T&G ply or OSB, and then a new layer of 1/2" over that to build up.

There's variations on that theme, of course. You could add some new joist sisters to present a perfectly virgin, uniform, flat surface and then a single layer of 3/4" over that, plus Ditra and tile. Might make everything flush to the neighboring room, even.

(there's also the old way of adding a long, gradual shim under the adjacent hardwood to bring it up ever so slightly. then you'd essentially have more height in the neighboring space)

Hope this helps - good luck on your first big project!

02-02-2010, 11:55 AM
Thanks to everyone for the advise. Based on what you have told me, I am considering removing the T&G flooring and the original planks, then install high grade 3/4" ply on top of the joists, topped with Ditra-XL as the underlayment. It will be a big job to remove all the old floor!

I am curious though about Ed's suggestion that I could 'shim' the surrounding floors to bring them up to the level of the new tile. The height of the tile floor would be: 3/4" planks + 1/2" ply + 3/16" thinset + 1/8" Ditra + 1/4" thinset + 5/16" tile = total 1 3/8". This would make the differential between the surrounding floors and the tile equal to 7/8" - is there any way that shimming (or some other method) could be used to equalize this difference?

I really want to use tile, but another option of course is to simply replace the T&G with new oak flooring :(

02-02-2010, 12:19 PM
You're not gonna know what you have til you dig into it, Eamonn.

Old joists sometimes relinquish their grip on the old planking with minimal objection. other times, they shred all to heck. Planking that's been nailed into repeatedly and walked over for 100 years is almost certain to give you less post-demo support than when it was new - it'll be essentially a spacer in there.

You wanna good floor? make it good. You wanna get by with what you got, that often times works too. Some of us are routinely shocked to hear how well things work in the face of general understandings...

I'll whip up a sketch of what I'm proposing.

02-02-2010, 12:42 PM

Here's my thoughts - presumably I understand a little about what you've explained. If not, feel free to comment.

"Option 1" is how I envision your description, using Ditra-XL and a new layer of 1/2" ply over the existing boards. "Option 2" shows 3/4" OSB on the joists, w/ the planks removed, still using Ditra-XL.

Hopefully "Option 3" describes my joist-sistering idea. Dunno why you'd need XL in this case but you seem bent on getting and using it :)

Hope these help.

02-02-2010, 09:26 PM
Thanks for the explanation Ed. Now I understand what you meant about the sistering, and I see how it would work. I think Option 2 is going to be my choice if the joists are in really good condition. That would allow me to bring the tile flush with other floors using a single layer of high quality 3/4 ply if I use Ditra XL. Option 1 would present a height problem, even using standard Ditra. Option 3 would allow me to use standard Ditra if I raise the joist height with sisters, but there is more engineering to do with the sister joists.

Since Option 1 is ruled out, and I have to replace the planks with ply in both other cases, then Option 2 seems the easier approach - but please let me know if I am missing something?

Thanks again - this has been very helpful

02-02-2010, 11:59 PM
You're not missing anything. You're just counting largely on those joists being receptive framing members for what you have planned.

06-09-2010, 05:17 PM
I am a bit confused over the resulting thickness of the tile system using Ditra. According to what I have read, there is Ditra and Ditra-XL. Ditra-XL apparently is used to allow a complete system that is 3/4" thick, to bring it up to the same level as 3/4" hardwood floors adjacent to the tile floor. My confusion arises from my calculation of the thickness of a standard (1/8")Ditra system... that is: 3/32 thinset under Ditra + 4/32 Ditra + 6/32 thinset under tile + 12/32 tile. That all adds up to 25/32, or just over 3/4". So it seems to me that 'standard' Ditra is fine - but if so, then what is Ditra-XL for?

Can anyone see where I went wrong in my calculation, or what is the source of my confusion :(


06-09-2010, 06:15 PM
The thinset won't build up as much as you are thinking. With 3/8 tile, regular Ditra builds up to about 5/8. Just a little extra buttering gets it up to 3/4 hardwood.

Houston Remodeler
06-09-2010, 06:28 PM
the ditra xl just makes it easier and a bit faster to get to 3/4"

06-09-2010, 08:40 PM
Thank you Davy and Paul for the fast response! So Paul, I think you are agreeing with Davy in that you seem to be implying that XL eliminates the need for buttering up to the thickness - is my assumption correct?

I think I will do a test installation on a piece of plywood and measure up the thickness, is that a reasonable thing to do, or are there better/easier ways to come up with a good evaluation?

Houston Remodeler
06-09-2010, 08:52 PM

We LOVE when readers/posters do test runs !! They are a great idea for a lot of reasons.

Yes your assumption is correct, that's exactly why they made the stuff, as most tiles are 3/8" thick.

06-10-2010, 05:35 PM

I removed a wall between my kitchen and the den as part of my remodel, and I am planning to lay tile, hopefully slate if I can get a solid subfloor. One of the problems is the floor of the den is concrete slab, and it is 1" lower than the adjacent floor joists in the kitchen. The picture above shows how I think I can deal with the difference, by installing 2x4 boards on the concrete, topped with 3/4" ply. Then 3/4" ply on the kitchen joists and 1/2" ply on top of that. This will bring both floors to the same height. I am concerned about the line where the two floors meet, because I would expect different movement profiles in the 2 areas. Can I deal with this by arranging for the tiles to meet over the join in the floors, and maybe use flexible caulk or some kind of expansion joint at that location? Does anyone see any other potential problems in this approach, or any better way to get the floors to the same height?


06-10-2010, 06:14 PM
Here is another view of the floor model - it shows the transition a bit more clearly...

06-10-2010, 08:00 PM
Welcome, Eamonn. :)

It helps if you'll keep all your project questions on one thread so folks can see the history and what's been previously asked and answered.

I'm not a big fan of attaching wood over a SOG for tile installations, but what you describe (excellent drawings, by the way) could be successful if you are absolutely sure you have no moisture vapor issues with the slab and if you are able to get the "sleepers" firmly attached to the concrete with no movement.

For mine own part, I'd rather use deck mud to raise the level of the concrete to match the adjacent floor.

And your thought about requiring a movement accommodation joint in your tile surface over that transition between subfloors is right on the money, regardless how you do the floors.

My opinion; worth price charged.

06-11-2010, 09:33 AM
Thanks CX, I understand your concern about the vapor barrier and securing the sleepers to the slab. I will pay attention to those issues. What does SOG mean (Slab On Grade perhaps)?

Regarding the 'new thread', I really wanted to start a new one because the topic of this is rather specialized, even though it is the same project, but I don't think my previous questions are particulary relevant now, and I didn't want people to have to read through all of those old posts and going to a second page to see what I am asking about. I imagine many readers would not bother, especially if they read the earlier posts and saw that my questions were answered, or that they had no additiional input to give on the those questions. I would consider myself a 'responsible citizen' and I would not have started a new thread if I didn't think it was necessary.

btw, I used the free version of sketchup 7 to create those drawings. If you need a drawing tool I would recommend it. I am using it to design my whole kitchen, including detailed construction drawings for the cabinets I will be building. The tool is awesome :)

06-11-2010, 01:13 PM
Eamon, part of the beauty of our system is that your thread becomes a project file for your project. Everything is in there. Readers can follow your trials and tribulations and appreciate the efforts you go through to get to the "beauty shot" at the end. One of the things some folks don't realize is how the choice of techniques and materials is affected by things you did earlier and forgot about.

Yes, it's cumbersome, and we do understand that we do things differently here than at other forums. But we feel we can help you, and others, better if we can easily find all the details of your project in one place.

BTW, SOG means Slab on Grade, as you guessed.