Effects of Improper Ditra Installation

by Roger

Improper coverage on tile / Ditra not filled correctly

Photo 1

I am not writing this to tell you why your tile is cracking or why your grout is cracking – I have other posts that may tell you that. (Click on the pretty little links :D ) If you happen to have Schluter Ditra as your substrate, this post will tell you why either one of the above may be happening.

While Ditra is my preferred membrane for floor tile installation (as well as countertops and tub decks) it absolutely needs to be installed correctly. The two main techniques for this are fairly simple:

  • Make sure the cavities (waffles) are filled correctly
  • Install it over an approved substrate (and with the correct type of thinset mortar)

Improper coverage on tile / Ditra not filled correctly

Photo 2

There is a lot more to ditra than those two items but if either one is incorrect I can nearly guarantee a failure. See photos 1 and 2 there? The tile was cracked and it was a direct result of a) not getting the waffles filled correctly and b) improper coverage on the tile. Now b may be due to not backbuttering the tile, an improperly-sized trowel, letting the thinset skim over or set too long before installing the tile or simply incorrectly mixing the thinset. All three of those things will cause any tile installation to fail – whether you use ditra or not.

Not filling the waffles correctly, though, will cause the tile to not be fully supported and/or not ‘locking’ the tile into the ditra. Because it is not correctly locked into the ditra you will lose the mechanical bonding properties of ditra and you may as well install it directly to particle board at that point (That was sarcasm – don’t do that!). For more specifics about exactly how ditra works you can check out Provaflex vs. Ditra wherein I describe exactly how the mechanical bonding process works – and rant about a particular jackass. But the mechanical thing – that’s what you want to concentrate on. :D

You need to use the flat side of your trowel and spread thinset in every direction over the ditra to ensure that all the little waffles are full. Since the cavities are dovetailed (that means they go down and away from the opening) you need to ‘force’ thinset into the bottom corners of the cavities. Simply running the trowel over the ditra will not do this. Simply running the trowel over the ditra did that (photos 1 and 2).

Improper substrate for Ditra

Photo 3

Installing ditra over an approved substrate is much, much easier. In fact, nearly every bare substrate you find in a modern house would be considered an approved substrate – shiny linoleum is not one of them (Photo 3). While there are thinsets that ‘say’ they will bond to linoleum (and some of them will) apparently the jackass who installed that particular floor was not aware of that. :guedo:

See photo 4? I lifted that up with my pinkie – literally! It was not attached at all. He may have had correct coverage beneath the tile and all the little waffles filled – I have no idea. There was not enough stuck to get enough leverage to tear one off and find out.

Improper substrate for Ditra

Photo 4

Most any plywood (even osb :whistle: ) is an approved substrate for ditra. And  if you use a thinset approved for that substrate, there are no problems at all. Photos 3 and 4 had an unapproved substrate and, apparently, incorrect thinset (and a shitty tile job, but that’s a whole other post). It was nearly guaranteed to fail.

When you buy ditra for your installation every roll comes with a handy little instruction booklet. You can go to Schluter’s Ditra Page on their website and access the instruction booklet (This link is a PDF!). They even have a flash video about the proper installation technique. You can leave a comment below and ask. You can email me. You can send up smoke signals – I’ll answer.

Given the 17 ways to acquire correct ditra installation information above there is absolutely never a reason to do it incorrectly. Ditra, in my opinion, is the best membrane for most floor tile installations. The only time I’ve seen it fail is due to incorrect installation. And that isn’t just the common BS everyone accuses failures on. Me, personally, every one I’ve seen fail is incorrectly installed.

If you use ditra, and if you have an approved substrate, and if you have the correct thinset mortar, and if you fill the waffles correctly, and if you use the proper trowel and get proper coverage it will not fail. Yes, that’s a lot of ifs – when you read it. In practice it really is not that many things to get right. It’s just common sense, mostly.

So here’s one more if: If you have any questions at all about correctly installing ditra and using it for your tile installation please, for the love of all the marble in the Sistine Chapel, ask me below in the comments. I WILL answer you. I’m just super-cool like that 8)

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Steven J

I’m going to tile my brothers bathroom. He wants to use a mosaic tile on the floor. I was reading the specs on ditra and saw that 2″ by 2″ is the minimum tile size. This tile comes on a 12″ by 12″ sheet but the individual tiles are smaller than 2″ by 2″. Can you explain the risk I would be taking if I installed this tile? Should I have him pick out a different tile?
Also, I’m going to use the same tile for his shower. I’m getting the 72″ by 72″ shower base and he wants to use the tileable drain. I’ve seen pics of this and it looks really nice. However, the only pics I have found have a full piece of tile covering the drain (no grout line on the tileable drain). If I used this mosaic tile there would be grout lines because the drain is 4″ by 4″. I’m guessing that is ok since it is covered with kerdi? I just thought I would check since I never saw I pic.

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Julian

My floor is painted aspenite. Will the mortar make a good bond to the paint. Should i put a 1/4 inch underlay overtop of the painted wood

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Jim in Idaho

Just curious. I’m installing 6 X 24″ porcelain on the floor over 7/8 OSB that is 40+ years old on 16″ centers using the thin Ditra. What will happen if I DON’T stick the Ditra to the OSB but do everything else properly? Where is it going to go? Planning on minimum grout lines (1/16″) and laying the tiles at a 45 degree angle to the joists.

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Ryan II

Roger:
I have an old house with structural plank floors. There are high and low spots in the floor, and to level it out, my tile guy is suggesting only putting plywood over half the floor and using a leveling agent on the other half. He would then install the ditra system over the entire floor. The end result would be that the high end of the floor would have the ditra system installed directly over the structural wood planks, which has me concerned. I’ve read the ditra manual, which states that a minimum 3/8″ plywood is required over the planks. He insists that the tile/grout lines won’t crack and that gluing 3/8 plywood strips to the bottom of the wood planks (basement side) will give them extra strength and prevent fluctuation. He’s the tile guy, so I don’t like to second guess him, but does this sound right to you? Thank you.

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Tim

My subfloor is 3/4″ plywood on manufactured trusses at 24″ centers. There are some squeaks, so I was going to tear up the subfloor and add reinforcement, but realized that the edges are sandwiched between the truss on the outer wall and the stud wall for the upstairs. I was going to use Ditra but squeaks mean movement, and movement means trouble. So i was thinking about adding a layer of backerboard instead to make it rigid, but would then lose the advantages of the Ditra. Should I use the backerboard or stick with the Ditra?

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Steve in Denver

Fellow DIYer here,

The first thing I’d want to check is the deflection of your trusses to make sure they are ok for tile. I think it’s a pretty good bet that they are, but you’d want to make sure they have a deflection of L/360 or better for ceramic tile. (If you are considering natural stone, the deflection requirement is double at L/720, and your trusses are far less likely to meet that requirement)

Assuming the trusses check out, I’d be inclined to keep your existing subfloor and add a second layer to it, assuming it’s an appropriate type and in good condition. You want T&G plywood with no face grade lower than C (CDX or anything called “sheathing” isn’t up to the task).

If your existing subfloor meets those requirements and you can make the squeaks go away by adding more of the correct type of fasteners and you are comfortable that the squeak fix will last), then you can then add an additional layer of plywood.

(Ditra XL (thicker) is specified for use over a single layer of 3/4″ subfloor over 24″ joists, but that scares me a bit)

If you can’t fix the subfloor you have, you’d have to remove and replace with the appropriate material – you can glue the new subfloor to the trusses with construction adhesive if you like, but be sure to screw them with the appropriate type of screw and spacing.

In either case you need a second layer of plywood to get the strength (between joist deflection) up to snuff for regular Ditra. The Ditra handbook specifies 2 layers of plywood/OSB totaling a minimum of 1-1/8″ thick (3/4 + 3/8) for using Ditra over 24″ joists. I can’t speak to it myself, buy my understanding is that you are really best to avoid the 3/8″ material that you will find readily available on account of it not being very flat.

Having said all that, a layer of 1/2″ CC grade (or better) with exterior / exposure 1 rating (needed so the plywood glue doesn’t soften up when exposed to the wet thinset) is probably the best route to go. DO follow the directions in the Ditra handbook when installing that second layer.

In particular:
– The second layer of plywood should be installed with the strength axis (long dimension) perpendicular to the joists (as with the first layer)
– The panels should be placed so seams don’t overlap with the original panels – 1/2 panel offset laterally, and the end joints should fall at appx 1/4 point of the joist spacing (6″ past a truss, in your case) (page 17 of the Ditra handbook)
– The screws should *not* penetrate into the trusses, only into the subfloor layer.
– You generally *do not* want to attempt to use glue between the two layers – it’s not that it can’t be helpful, rather it is difficult to pull off unless you know what you are doing.
– You DO want to use the appropriate type of fasteners and the appropriate spacing. Hint: it’s probably a lot more screws than you think. I recall that it is 6″ spacing on the perimeter and 8″ spacing in the field, but you’d really want to verify that.

After you are done with all of that, it’s time for the Ditra. If you chose plywood for your top layer, you just need to pick a decent quality modified thinset to adhere the Ditra (paying close attention to the Ditra instructions and the wonderful elf tutorials about such things)…if you chose OSB for your top layer, you will find yourself in the unenviable position of having to locate a modified thinset that is approved for use over OSB. I have tried and failed on a number of occassions…not saying that many won’t work, but they don’t specify it.

That’s what I have to say about it. I get some things wrong from time to time, and if I have misspoken here, hopefully Mr. Elf will step in and straighten me out.

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Steve in Denver

I meant to address the CBU issue directly. It does not add any meaningful stiffness to your floor structure. Really. An additional layer of plywood is really the only way to make the floor stiffer…Eliminating the squeak (movement) is job #1, though.

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