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Old 05-07-2019, 03:51 PM   #31
BasicDIY
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Well, we took up the cbu and put plywood down. I did several things for the first time this past weekend for example, I unscrewed 500 screws.
Here are pictures from this weekend and a selfie of me picking up screws and plywood.

I know I will make some of you cringe, but I don't want to use thinset under the cbu. I don't really have an explanation besides not being fully convinced. There is an argument on whether to use the mortar that bonds or the mortar that does not have a polymer...or something like that. The manufacturer says to do it and I get they are the pros but they are also in business to sell more product. Feel free to change my mind. We plan to tape and mortar the cbu.

If we do use the thinset cushion then do we caulk the joints in the subfloor as to not get thinset in them?
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Old 05-07-2019, 04:03 PM   #32
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While it is critical to have mortar under the CBU, the type of mortar and quality is not. It functions purely as a incompressible filled under the mechanically attached CBU.

Think of the CBU as more like lasagna noodles and less like ridged boards. In general they appear perfectly flat and strong but in particular they move and flex unless fully supported. The thinset is what provides the support by filling any gaps or voids between the CBU and the plywood.

Personally, at this point I’d write off the CBU and use Ditra, much easier to do a quality install and you’ll save 3/8” of height.
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Old 05-07-2019, 04:05 PM   #33
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By the way, nice work on the second layer of plywood.

You certainly did a tidy job spacing the screws.

And don’t worry about calling the gaps. Technically it’s supposed to be clear but realistically it never happens.
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Old 05-07-2019, 04:40 PM   #34
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Catherine, some years back one major manufacturer of CBU, actually a well known Fiber/Cement board manufacturer, did extensive testing on the need for thinset mortar under their panels on floor applications as well as the need to fill and tape the joints.

To paraphrase their result, if you don't fill and tape the seams your chances of a failed tile installation increase a good deal; if you don't bed the panels in thinset mortar before fastening your chances of a tile installation failure increase dramatically. It's just not the least bit optional and all the CBU manufacturers agree on that.

I think you'll find that the manufacturers, even those who specify an A118.4 mortar for the bedding, generally will agree that any thinset mortar at all with serve the intended purpose. The mortar holds the panels up, the mechanical fasteners hold it down.

Your floor, of course. Your decision.

My opinion; worth price charged.
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Old 05-07-2019, 04:54 PM   #35
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RE: not using mortar under the CBU

Think about it this way: If a CBU manufacturer could make a panel that would allow for its installation without mortar, they would corner the market on CBU, because they'd be the only one.

Nobody likes having to spread mortar, it's messy and takes extra time and money. So if it wasn't necessary, why would the manufacturer require it?

A bag of dry-set mortar at my local Lowe's is $7.28. That 50-pound bag would cover about 100 square feet of floor (give or take) underneath the CBU, assuming you use a 1/4" trowel. So you end up spending an additional seven cents per square foot for this step in the process.

To me, it's just not worth the risk to save seven cents a foot and maybe an extra hour or two of time.
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Old 05-08-2019, 07:03 AM   #36
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Ok, using thinset between cbu and plywood. Starting today.

While we looked at the ditra and would love to save the height, we already have the 1/2" cbu and screws. Ditra is pricey to start over at this point I think.
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Old 05-08-2019, 07:48 AM   #37
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Yes, since you've already got it and can't return it, I would do the same and use the CBU. Don't forget to gap the boards 1/8", and to not have four corners together.

However, while Ditra costs more, it probably comes out the same when you figure that you don't have to buy screws and mesh tape, and that doesn't even figure in the ease of use.
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Old 05-20-2019, 10:59 AM   #38
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ouch - update

We have finally started. It is sloooooow going and boy is it tough on every body part! As hard as I try to stay clean and neat, mud everywhere.

Because of the variance in the tiles, we decided to use a leveling system. We started with the screw kind and quickly hated it. Now we are using the lash system and it is more to our liking. We are learning so much and improving our "system". We are using a 1/2" tool thing and also back buttering. I still feel the lash system is pulling some tiles out of the mud too much and will cause weak spots.

At this speed, it will be July before we reach the bathroom. Anyway, that's my update.
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Old 05-20-2019, 12:45 PM   #39
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I can sympathize. If it makes you feel better, I’m on the other end of the project lifecycle. Just sent off one 30 yard dumpster and loading up another.

Good luck and keep us posted.
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