Hey everyone, I've encountered a strange problem this board might be able to help me out with.

I've been using Ditra on my floor tiling jobs for the past 6 months with, what I thought was, good success. When I install I follow these steps:

1) For ceramics I ensure the substrate is at least 5/8" plywood, then I screw it down with 2 1/2" floor screws every 6 inches along all joists which are 16" OC. For natural stone I expect 3/4" ply with joists 16" OC.
2) Mix up soupy Versabond fortified/modified thinset, pour it out and comb it using a 1/4 x 5/16 v-notch trowel.
3) Lay the Ditra and, using a wooden float, ensure there are no air bubbles, etc.
4) Mix up more Versabond to notch-holding consistency and use it to bed in the tile. I do not make a point of filling all the cavities in the Ditra, as I understand that would prevent it from "floating" as it's designed to do. As a matter of course though, many of the holes will be filled, probably about 2/3 of them.
5) Sanded grout fills all the gaps, which I usually size to about 3/16".

I was back in a house where a I did a floor a few months ago and noticed, to my alarm, some grout joints had cracked. Now I'm worried that all my jobs might be so compromised. What did I do wrong?

If I can figure out where I went wrong I'll be out there re-checking all the jobs for the past 6 months, when I started using Ditra, and fixing anything I see. Right now I'm afraid to keep using it!

Mike.

Hi Mike.... you've made a few mistakes.
It should be mixed slightly wetter ... but not too runny.

This is a very serious mistake. You should make absolutely certain that the square dove-tails are filled. This is what creates the mechanical bond .... nothing else bonds the tile unless they are filled.
You should also be using an unmodified thinset mortar to set your tiles.

My history with schluter systems is as follows- I have seen 2 large job failures using Ditra mat in Denver.I dont truly know the cause. The ditra rep sent me to Anderson SC to TCA and CTEF. The products failed during the classes because 1-thinset was too runny, 2- thinset was too stiff, 3-didnt properly get out the bubbles. I have never used it because A( cost. B( can only use nomod motar. C( they say can use on 5/8 plywood or osb (but must meet l/360)I have never seen a single layer of 5/8 anything that is not a trampoline. Guys -i know im the odd one-i know alot of guys that are sold on it. PLEASE-Id love to get all feedback

Originally Schluter wanted us to use a 1/4 x 3/16 V trowel, since last year they added or a 5/16 x 5/16 V trowel. Excuse me, but there is a tremendous difference between the two isn't there? I never thought the smaller trowel left enough thinset, so I would always cut bigger grooves in my trowels. I think the 5/16 V is best. We do NOT use anything that is mixed soupy.

The squares need to be filled as Harry said. Those dove-tail squares are the 'columns' that hold up the structure above.

The 5/8" plywood over joists at 16 o.c. I think is garbage. Once one manufacturer claimed it's ok, the others jumped on the bandwagon. However, no matter what they say, you'll always be referred to the disclaimer that states the subfloor has to meet L360 max deflection. You are always on your own.

Jaz

Thanks for the input guys.

Harry and Jaz: Filling the dove-tails makes sense when you explain it that way. When I was introduced to Ditra I was told filling didn't matter, so I haven't paid attention to it. Also, the modified thinset appeared to set hard and fast so I haven't worried about unmodified, but I'll use it from now on.

Rob and Jaz: There is no deflection on the substrate. These are small bathroom floors less than 50 sq ft, which is why I was so puzzled about grout cracking.

One last thing, re getting the air bubbles out of the Ditra. Using a wooden float sort of works but it's a ton of work. Is this because the thinset is too thick? I'm mixing the thinset to pourable consistency... should it be thinner? If so it might not hold a notch.

Mike.

Mike .... you're in the Newmarket area, why don't you visit Tilemaster in Aurora. I know they would be happy to help you with proper spreaders and procedures. They also supply a good quality unmodified mortar highly recommended by Schluter for setting tiles over Ditra.

I have an old 100 lb roller I use for my Ditra installation. I mix the mortar slightly thinner than normal (but not pourable) and then roll the heck out of it. It CANNOT be removed in pieces bigger than 1 inch after it sets up, so you'd better hope you don't have to do a repair.
For smaller jobs I use a 2"x4" about 14" long .... with both hands I push the membrane into the mortar. Some people use a short piece of PVC pipe. You basically use whatever gets the job done.

Hi Mike

I agree that the thinset should be mixed very loose,but not so wet that the combed ridges sag.

I set the tiles on the unfilled Ditra.This is better to ensure that you get a mechanical bond to the membrane and are not relying on an adhesive bond of dry thinset to wet thinset mortar.Not that it would shear.I just find that it's not necessary to add the extra step of skimming the membrane.The cavities are easily filled when setting the tile by trowelling in different directions as you're setting the tiles.I find that it is cleaner to do it this way too.

The best trowel is a 3/16 x 3/16 square notch trowel.Get one of those.Marshalltown at Tool Academy.You want to have the membrane fully bonded...100% coverage,easy to do.I use a standard cedar float to embed the membrane into the mortar...on bigger jobs I'll bring a 75 lbs linoleum roller.When the Ditra is shown on TV I've never seen them perform this procedure properly.They just run their hand or a trowel or whatever lightly over the membrane.It has to be worked into the mortar with some effort to ensure a secure bond.If a tile has to be lifted out of the fresh thinset that membrane should not debond.

The Ditra does not "float".It is not a floating floor.It is still a bonded underlayment.The fleece and thinset on the underside act together as the uncoupling system.The cavities when filled are concrete columns and can move laterally because of the space between the cavities.They will not crack or come out because they are small and locked in by the dovetail profile.

I use Ditra on every floor installation (with a few exceptions once in a while)...for many years.

The problem could be something else.Perhaps the thinset was still soft when the tile was walked on.Often I'll use quickset mortar on smaller floors.The Ditra does slow the drying time of thinset.Especially if the thinset is modified.Also good idea to place sheets of plywood or hardboard if you're gonna grout soon or need to make floor accessible early.Distributes the weight of traffic and prevents shearing of thinset between tile and membrane,because once the thinset has sheared while it's still soft even...and you can't see it or feel it...it'll never fix itself properly.

Guys... thanks very much for everything.

Harry... original advice on Ditra came from Tilemaster! They told me not to worry about filling all the dovetails. They've usually been excellent for advice but no one's perfect. I'm thinking of borrowing my wife's rolling pin... maybe that'll work.

Ron... that's pretty much the way I set the tile, by trowelling in different directions. As I said earlier, I don't make a point of filling the dovetails but many are filled just by the trowelling action. I wonder if you're not hitting the nail on the head in your last paragraph in that I do walk on the tile before it's fully set. I've always figured if it's not moving then it's ok. Maybe it's not! The plywood is an excellent idea.

Thanks again to all posters.
Mike.

I Was originally told v-notch trowel, then I heard a regular 1/4 " trowel. I also heard right from the source (when it was first introduced that modified to set the ditra then unmodified on top. Then they changed that to modified on top and bottom....what gives? No one ever told me to use a soupier mix to set the ditra either so thats scary stuff. I even read all the materials that came with it when I first saw it on the market.

The last job I used it on has a little bit of grout crack in one small area and now I am wondering who's advice I should have followed out of the many to choose from....

If you're installing over plywood .... it's always modified mortar you should use. The substrate dictates the mortar for bonding the ditra to the floor. Now I never heard Schluter changing those details.

Also ... spread your mortar ahead of the ditra but no more than 6 or seven feet (less if you're slower at spreading). A soupier mix will allow the ditra to push into the mortar forcing wet mortar into the fleece but you should always try to keep it fresh.

The bottom line is to use a membrane over plywood to allow for expansion and contraction. Ditra, and all other membranes including backer boards won't address deflection. Any movement in the floor needs to be removed by whatever means possible to prepare for your membrane and tile installation. Ditra does only 2 things .... it waterproofs and it acts as a crack isolation membrane.
Too much mortar will fill the channels under the Ditra which will prevent the product from doing its job. Not enough mortar might dry too quick before the ditra is rolled into the mud or perhaps the fleece might not push into it enough.
Sometimes I'll use whatever trowel is available to spread the mortar .... but I roll it with a 100 lb roller or I push it with anything laying around which I know will do the job.

Peel back a piece (just like you would with tile) to see the coverage and decide for yourself which spreader works best for you.