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Tile Job: 1/2" plywood plus Ditra plus tile?

MichDoug's picture

Hello all, and thanks for the great info here.

I am getting ready to install tile in a room that has 3/4" OSB over 16" OC joists.  I have just over an inch of space unless I modify one of the doors.  I am wondering if my best approach is to add 1/2 inch plywood (with adhesive and screws), then Ditra, then tile.  The other alternative is backerboard then tile, but seems like the plywood plus Ditra would give me a stiffer subfloor.

The tile is 1/4" thick, 12x12.  I am also wonder how much total space for Ditra with mortar above and below.  I am assuming 1/4", but any guidance would be appreciated.

Finally, how hard is it to get the Ditra flat?  This is a DIY job.


Doug (post #206041, reply #1 of 7)

What are the size of the joists and their span?


I've opted for plywood when support was in question.  Never used Ditra.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


I am not sure on the joists - (post #206041, reply #2 of 7)

I am not sure on the joists - I would need to get underneath and measure.  But, I believe they are 2x10 wth 10 foot span. 

Well then, (post #206041, reply #3 of 7)

The frame is sufficiant for tile.

The Tile Council of America recommends 1-1/4" wood subfloor/sheeting thickness.  Wonderboard adds no structural strength.  The ditra as a decoupling membrane I would guess would be fine over that underlay you add.

Again, never used ditra-hope someone comes by that has to give you some good advice.  My goal was for you to expand on your information.


A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


so it sounds like adding the (post #206041, reply #4 of 7)

so it sounds like adding the 1/2" plywood is key to getting to 1-1/4" total

Cement board does not count towards the 1 1/4", correct?

Can someone tell me the total thickness for Ditra plus mastic above and below, and how challenging it is to install properly.

I guess I should ask if there are some products equivalent to Ditra.  It is just the one I have seen.

Ditra (post #206041, reply #5 of 7)

Ditra is about 1/8" thick. Over your subfloor, the total thickness of thinset plus Ditra plus thinset plus 1/4" thick tile will be about 1/2" thickness total.

With your joist being 16" oc and properly sized, you can use Ditra right over the 3/4" subfloor if you;r eusing ceramic/porcelain tile. If using natural stone, you need more plywood. But to Ditra over the existing ply, obviously you;d want it clean to ensure a good bond. I'm not sure if this is new construction of remodeling.

If you are trying to match the thickness of a 3/4" strip hardwood floor, there's also a Ditra-XL product. Ditra-XL is designed so the XL plus tile will be flush with a 3/4" hardwood strip floor.

If your existing ply is sketchy, sure, put down another layer. Offset the seams, and don't fasten the second layer to the joists, just the subfloor.

There are 10 kinds of people in this world; those who understand binary and those who do not.

What's the reason for not (post #206041, reply #6 of 7)

What's the reason for not fastening the second layer of ply straight through into the joists?  I'm about to put a second layer down on a bathroom remodel also planning to use Ditra, and I'd assumed I should screw into the joists.  I was going for the second sheet of ply exactly for the reason you mentioned, the original subfloor is a bit sketchy in spots and I need the additional support.


 I know this is a super old (post #206041, reply #7 of 7)

 I know this is a super old post, but just in case anyone sees this and is wondering...

If the floor is made up of joists, subfloor layer 1, subfloor layer 2, then layer 2 is only screwed to layer 1.  This allows any movement in the joists to be somewhat independant of layer 2.  In other woods the very bottom of the floor can move a little bit, and it will affect layer 1 some, but layer 2 even less.  It is just another means of decoupling layers before your tile

I'm no expert, but pretty sure that is correct.