Thinset and Schluter Membranes

Discussion in 'Shower & bathtub Forum & Blog' started by jadnashua, Mar 18, 2014.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Many people do not understand the difference in the relationship between bonding with thinset with the use of a membrane such as Kerdi or Ditra from Schluter systems, and a thinset when used over something like a cement board, or even plywood. Very different mechanical things are going on.

    First, thinset does not stick to either Kerdi or Ditra. What, you say, how does it hold anything together? With Kerdi, and the fleece on the bottom of the Ditra, it bonds because, when properly installed, the fleece (which IS bonded to the membrane) is embedded by the semi-fluid thinset - IOW, the fleece is encapsulated by the mortar, and because the fleece is bonded to the sheet membrane, once the thinset cures, it's all locked together. Sort of like the difference between getting gum in your hair verses a wax job...the gum sticks, the wax encapsulates and then hardens...they both create a bond, but do it in different ways. So, the bond strength is limited by the strength of the fleece, not by the ability of the thinset to actually stick to the membrane. Knowing this, it should become clear why it is VERY important to use the right thinset and to mix it up properly...it must be viscous enough so that when you press the membrane into the stuff, the thinset actually flows around all of the fleece...this is CRITICAL to the proper bonding. Letting your thinset skin over, or get its moisture sucked out by a dry substrate before embedding the membrane is the major user error. Knowing how it works, and it's easy to check (pull a section back off and look for full saturation and coverage of the membrane!), and you can avoid that pitfall.

    Kerdi has fleece on both sides, and the same action is what is bonding the membrane to both the wall, floor, or the tile attached...the thinset surrounds the fleece, but it does need to bond in the traditional way to the tile you install.

    As to Ditra, the top surface does not have any fleece, and as was mentioned earlier, thinset doesn't stick to it. So, how can a tile be anchored to a surface that thinset does not stick to or bond to? The shape of the Ditra membrane has square dovetailed pockets in it...when you spread the thinset, you pack it into the pockets, and then comb out enough over the top of it to provide a bond to the tile. The thinset to tile bond is 'normal', but the bond to the membrane itself is entirely mechanical...the pillars of thinset that hold the tile up are locked into the dovetails. That the tile is actually supported by those pillars to the bottom of the pockets (and the whole membrane is bonded to the floor) and the fact that there are gaps between those pillars of thinset is the reason why there is a minimum size requirement when using Ditra - you must have enough pillars holding it up so a point load won't have a chance to tip the tile...that works out to a minimum of 2" square tile. Anything smaller and you MIGHT dislodge things with a point load (it might work fine, but Schluter doesn't like to take chances - they're conservative).

    The cement in thinset gets it strength, mechanical bond strength, by the crystalline structure of the cement when it cures...this creates interlocking spikes of cement spires, or crystals. The sand and other fillers minimize the amount of cement needed and provide density and strength to hold the tile up while the thinset cures (i.e., so it doesn't sink). It is the water you use to mix the thinset with that activates the chemical process that starts the curing process, and maintaining excess moisture means that all of the cement can become fully hydrated and cure, creating the maximum number and longest crystals possible. The environment between the waterproof membrane and the potentially impervious tile provides the optimum environment for the mortar to reach its highest possible strength. Those cement crystals actually grow into the microscopic depressions and pores of the tile, and lock it in place.

    So, given that the actual bond to the membrane is relying on a different mechanism than say tile over a cement board (cbu), it is a logical conclusion that the use of a dryset mortar (unmodified) to hold the tile in place is entirely adequate, and preferred. Without getting into the full chemical characteristics, a modified thinset must not only cure, but it must also DRY the modifiers for it to reach its full capability. When used over a waterproof membrane, that can take literally MONTHS to happen, where a dryset (unmodified) will cure at a predictable rate, and actually works best when it is kept wet during that process.

    A very different scenario exists when trying to bond a tile to cbu or plywood, or nearly anything else, and a modified thinset can often provide a stronger bond than an unmodified. With Ditra over a plywood subfloor, because the wood moves MUCH more than the thinset or the membrane with the tile on top, they specify a modified thinset to help maintain the bond to the plywood. The wood also allows any excess moisture to be wicked away, so drying isn't a factor like it would be on top of Ditra with a dense tile on top. The differences in expansion/contraction between the thinset and plywood means the modifiers used also allow a little more flex than when an unmodified is used.
  2. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    What's complicated about: modified to hold Ditra to wood, and unmodified everywhere else?

    Some people like to know why, some don't care, some just like to throw jabs.

    I'm signed up to take a 2-day course at Laticrete's Bethany, CT headquarters, but that's not until June. My goal is to get better, up-to-date from the manufacturer information, so I know how THEY intend their products to work, rather than someone who thinks they know so I can provide a counterpoint to the sometimes inaccurate info out there. As time allows, I hope to extend that info gathering to other manufactures.
  4. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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    Restrictive.
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  5. Justadrip

    Justadrip New Member

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    Jim....that Laticrete course....how much does it cost to take? What do they cover?
  6. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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  7. eurob

    eurob In the trades

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    Hey John , I guess you did take the crab picture with a well tested waterproof camera , right ?

    Soon we will probably have waterproofing sheet membranes from all manufacturers .....just of a different color , specific to each one of them . Or just use the styrofoam boards , covered with different products -- not only the Kerdi boards , but others --.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    It costs $459, covers room, board, and if needed, pickup from the airport, and they have several different ones. They have some course notes on their website, it would be easier to look there than try to repeat them here.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Some products have a bigger learning curve than others. While many people can wield a paint brush or roller, few of them can do it as well as someone who does it for a living. When you are installing something that has to be even, and a specific thickness to be waterproof, it is much trickier than pleasing your spouse when painting a wall - IOW, a pretty looking paint on waterproofing may NOT be waterproof - it's not how it looks, it's how it performs. A sheet membrane is already the proper thickness, and if you can mix thinset up properly, embedding it on the surface is pretty simple.

    My goal in further training is to be able to try to see through the BS that is sometimes posted, and help people. Knowing more about more products, their benefits and ease of use should help me guide people, should they ask. Many people use one thing, and do not want to learn others, or if they do, apply the same rules as the original when they may not apply at all. The adage "but I've been doing that like that for years and never had a problem" is BS - they may have been lucky, or just never checked.
  10. Vegas_sparky

    Vegas_sparky Eat kitty

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    My next bathroom project starts in a couple weeks at my Vegas place. Tub to shower conversion on slab. The wet wall is caving in around the control knobs. I might try some variety of sheet membrane this time, but liquid sure is easy.

    I'm having King Crab for dinner this weekend. They better not taste like cat food.
  11. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    No, I'm talking about trying to do a bonded mudbed over a decoupling membrane, or that it's okay to level OVER a decoupling membrane, or, to try to remove some foam from a toilet by pouring boiling water then ice water over it; or saying that Kerdi won't work unless you use Ardex 8+9 over it, or that the mortar on a shower curb must be at least 1.5" thick.

    If you do not trust a manufacturer's products, don't use them, but some here just seem to think they know better than the engineering group that developed it, many years in the business with thousands of successful installations, and some common sense.

    And, if you can't add something useful to the thread, please don't dilute it with more of the off-topic BS that is totally irrelevant to the subject.
  13. Vegas_sparky

    Vegas_sparky Eat kitty

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    Jim, you, yourself didn't know what the thickness of mortar on a curb should be until you asked the question at John Bridge. Throwing that out in your last post really discredited you, IMO.

    What is the source for the information in your first post in this thread?

    Working in the trades I've learned to respect information from people that have knowledge AND experience.
  14. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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  15. Vegas_sparky

    Vegas_sparky Eat kitty

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    I'm not knocking the John Bridge forum at all. There's a lot of knowledgable, talented, and experienced people on that site. I enjoy participating there also.
  16. DougB

    DougB Member

    I didn't know you could get a certificate in paint rollering?

    Well all you need to do is look at the coverage. The Hydroban that I just applied says it covers 70 sq/gallon. It goes on like paint, and I gave it two even coats, and had just a tad left over.

    Consider, even there were 'pin holes' - considering the area of the pin holes as a percentage of the total area to be waterproofed (a shower) it would be something like 99.9999999999% covered. Additionally, there's a great proability that the pin hole will becovered by thinset and tile. Then add to that it's cement board - well you'd have to be paranoid to think it's not waterproof enough.

    What's easier: mixing thinset, cutting and applying sheet fabric, or opening a gallon of Hydroban?

    And now everybody and his brother is making this sheet membrane!
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    As to the thickness of the curb...I wanted to check what I remembered - is there something wrong with double-checking information before posting? The thickness is correct for the material specified. IF you decide to use deckmud for a curb, THEN it needs to be that thick. The recommendation is to use brick mortar for the curb over the core, a similar mix to doing a mudded wall, and the ANSI spec calls for that to be MUCH thinner, which is what I referenced. It all comes down to trying to discredit someone trying to keep things accurate and according to the industry specifications. If you made your curb out of bricks (or studs) and then used deck mud, you'd get those huge curbs certain people complain about based on the foam ones already available. Using brick mortar, you get a stronger mix, much easier to form that doesn't need to be anywhere near as thick. Plus, assuming you're doing a conventional shower, over the curb, the brick mortar is much easier to form than trying to pack deck mud on both the vertical insides and outsides while trying to pack the top with that liner there.

    Regarding a pinhole with a membrane, on the thick stuff, it's not all that hard to get a bubble, and, while this is a personal observation and opinion, if you are not careful, you end up with an excess in places like a corner where it may run a little which can be a pain getting your tile level. Using the coverage chart won't tell you if you got more on the floor than the walls, it only tells you your average coverage, which may or may not be representative of all surfaces. Before you can put up the second coat, you have to wait...in that extra time, your Kerdi membrane would be up, and you could start tiling (at least the walls, you should flood test the pan and that needs time as do the liquid membranes). Total time, liquid and sheet probably come out about the same when you add in the drying time to apply the second coat.

    Once you embed the fleece of a membrane into the mortar, it's not going anywhere, and it's VERY easy to check if you got proper coverage...and if you follow the instructions, you'll get a simple, visual observation on how well you did (i.e., pull back a section, check the coverage, then put it back - takes all of maybe a minute to verify your thinset mix and technique). You should have a nice, even coating on the entire membrane and surface. If you do, you got your thinset mix right, and it is safe to continue.

    Maybe the reason why all of the manufacturers are trying to copy Kerdi, is that it works, and in some areas, it is much more common taking away sales of conventional materials. Kerdi has been around for decades, and has done their homework, plus has a large, established base of successful installations. Other manufacturers are beginning to notice, and to maintain market share, guess what - imitation is a very good form of flattery! Now, whether those other systems are as well executed or not is to be seen. Imitations often aren't as good as the original, nor do they have the track record. Schluter has had over 20-years to perfect both the systems and the accessories to make it all work well. The competition may or may not apply downward pressure on the materials, but you can't cheap things out without introducing potential weak points.

    As to the basis of the OP, I've used Kerdi and Ditra, been to two Schluter training courses, and have read lots of information on the materials and talked to LOTS of people that use it every day. It is a summary of my understanding of what I've been told by the manufacturer, their documentation, and my personal experience. If you can poke a hole in my OP, feel free as (as in the reason I'm taking the Laticrete course), I'm open to learning, and readily admit I can make a mistake, doesn't happen too often, but it does. I do not think there is an error in the original post. That some people feel the best offense is to offer distractions and discredit by association is in poor taste and basis.
  18. johnfrwhipple

    johnfrwhipple I love these ACO Shower Drains - Best in Class

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  19. Mariog

    Mariog New Member

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    Hi, been following this sight a couple months. Working on my master bath and going to be laying tile on Ditra. The Ditra will be over plywood which I have done before with Maipei keraset. Now I will be using Ditra-set, but the place I got it didn't have the Bostik acrylic mix. Can I use any old acrylic mix (like the Maipei mix for example) with the Ditra-set?

    I am doing the bath tiling over Kerdi, in which I will also be doing with Ditra-set.
    Thanks to anyone that can assist?

    Mario
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Generally, manufacturers only recommend their admixes for their own dry mortars. Now, whether it would work, I wouldn't feel comfortable saying one way or the other. You may want to call their tech support people, but you'd have to wait until Monday.

    Having used Ditra-Set...you'll find it a very nice thinset. The instructions on the bag give a range of water to use...when embedding the membrane, aim towards the higher number.
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