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Old 12-20-2009, 01:02 PM   #1
swingthought
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A solution for cutback black mastic on a large floor area…will it work?

Read everything about the cutback adhesive removal/scraping solutions on John Bridge.com and agree and like the idea of using a thinset that is approved to go over the thin film left after scraping (that can be done wet, if need be) but in this large floor area that is 17’ x 15’ it may not be required, nor desired for the reasons below:

1. Concrete slab floor is not flat enough for large format tile and has a couple of cracks, so leveling and using isolation membrane is necessary
2. The slab is this room is 1/2 – 5/8” lower than the room next to it, because that room has been troweled with a stained topping mortar. Floating wood will be going over that, so taking up that 5/8” would be better before tiling and help the two different rooms floors line up later.
3. This cutback is not wet scraping off.

My idea is to:

a. Do very minimum wet scraping of the cutback, as this stuff is not easily coming off, so just basically leaving most of it there.
b. Use approved for over a cutback film thinset and first level the slab going right over the old cutback and filling in low spots.
c. After overnight, 1/2” notch trowel the same cutback approved thinset (or maybe even a bigger notch, if advised) and setting back buttered 1/2" durock into the thinset. Use no mechanical attachements. Then, after another overnight also tape and thinset the joints on top.


My thinking is that this 5/8” thick surface will be unified, and will not only solve the black mastic removal problem, but would also serve as a crack isolation solution for the slab and raise the level to a similar level of the room next door. Of course my question is will it work? I think I remember reading that pros have used durock without any mechanical attachment. Even if there was no bond between the cutback and the 5/8” surface on top, It only needs to be strong in compression, correct? It seems like a “maybe”, no?

Thanks for your opinions!
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Old 12-20-2009, 02:30 PM   #2
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I think Wedi is the only board that is approved by the manufacturer for what you're wanting to do.

If you go that route, hang on to your checkbook, it's not cheap.
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Old 12-20-2009, 03:29 PM   #3
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Yep, you can't use durarock like that. Now you could use the Mud as you go method to level the floor, or an unbonded mudbed. If you are going to thinset anything over top of it I would as get as much as the cutback off as possible and then scarify the floor.
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Old 12-20-2009, 03:42 PM   #4
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Maybe over slab

Not familiar with Wedi, but worth looking into maybe, as gaining 1/2" is a good thing here


Okay, it is not an approved installation, but has anyone used durock without mechanical fastening over a slab?

Why can't you use durock over a slab, or am I missing something?

So the question is, what chance does my idea have of working with something like durock?

Anything is better than giving in to cutback removal!
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Old 12-20-2009, 04:33 PM   #5
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Is this your house you're workin on? If so, you can do whatever you want - you're dime and all. We're just trying to tell you what has been tested and proven to work if done correctly.
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Old 12-20-2009, 06:00 PM   #6
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Why not use Self leveling and membrane over that? If bond is the issue ( SLC to cutback) I bet it can be solved..Call Laticrete..They will likely have a solution.

I'm thinking something like 254 "bond coat" over remaining cutback..prime and Self level??
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Old 12-20-2009, 06:04 PM   #7
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Trask- right on. Then membrane for movement- then set tile.

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Old 12-20-2009, 06:11 PM   #8
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My thoughts exactly Trask ! I did 2200 sq ft of Travertine in this almost exact situation & used 254 on top of my scraped cutback primed it 2 times & mixed Ardex`s K-15 then Ditra then stone & that was 3 years ago & I see the customer once a year @ a BB-Q fair & no mention of problems!
I didnt ,But Givin Latacrete a buzz aint gona hurt .
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Old 12-20-2009, 10:10 PM   #9
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One caution here.. scarifying is the best way to go IF THIS IS NON-ASBESTOS CUTBACK. There's a good chance it is, most of the older black cutback had asbestos in it. If you're going to do anything other than wet scraping, I'd definitely get it tested.
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Old 12-21-2009, 10:44 AM   #10
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Thanks for good ideas.

Yes, this if my own home, but don't want problems either.

I apologize for not being clear about the project and the cracks in the crete. I think that the cracks are all just stress cracks without any lipage and very small gapping. Everyone is informing me that setting thinset down will not give any crack isolation. But will it stick down fine and not give any isolation?

I'm thinking that I may not need any crack isolation here.

Sorry, that I was not clear about that. So, if the cracks are not a problem and I make the decision that it's solid and no cracks are coming through, could what I propose be a solid substrate to tile over?

I've never used self leveling in this kind of quantity, and it's a given that coming up 5/8" approx. would be good in this situation, so I can get my mind around laying in 1/2" sheet goods more so than a large pour of self leveling mortar.

Also, it might be backwards, but an isolation membrane could be used over the durock, too, if the durock would stick.

Thanks for all the ideas, think a call about the best products to use is a great idea
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Old 12-21-2009, 12:10 PM   #11
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Lee pouring a 1/2" or 3/8" of SLC would be the easiest and best approach from my perspective. The Cement board is a very bad idea. I've seen it done and seen it fail.
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Old 12-22-2009, 09:48 AM   #12
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SLC: can it be done right without experience?

Quote:
Lee pouring a 1/2" or 3/8" of SLC would be the easiest and best approach from my perspective. The Cement board is a very bad idea. I've seen it done and seen it fail.
Is this possible to do well, or at least acceptable without the experience that tile setters have?

With two guys, what is the easiest mixing and application for almost 400 sq. ft.? Is one bag/one pail at a time the way to go? This is 17 cu ft. or 2/3 of a yard. Is that considered a lot?

Of course, you're saying that going over the cutback with SLC is not a problem, correct? Basically, bury it like I wanted to do with the durock.

As far as cracks, there is a 4 footer that crosses one corner, I've used the new durock membrane and was satisfied, but would want to try an easier isolator here, or go without. Is there a roll on that would work over SLC?

Thanks for the help!
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Old 12-22-2009, 10:40 AM   #13
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You get about 1/2 a cubic foot of SLC per 50 pound bag. Your pour would be 34-35 bags. Most pros would call that "a lot." It would need to be mixed so that you were always pouring. 2 or 3 bags being mixed at once, such that you have about a minute between batches to get it in place would do it. You'd still be mixing (at 2 minutes per batch) when the first batch started to cook off.
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Old 12-28-2009, 12:20 PM   #14
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Yes, it seems that the SLC pour is a fairly difficult task, if not familiar with it. My Daltile contact told me that Mapei (sp?) ultra prime WE is spec'd for going over cutback.

Then Ultra Plan SLC

Also, Aqua defense (2 coats) would make an isolation membrane.

Are there any tricks (for example, renting a mixer) to help with a big pour of SLC like this?
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Old 12-28-2009, 12:42 PM   #15
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Get lots of help, several garbage cans to mix in and a couple of 1/2" drills and mixing paddles. Do a dress rehearsal so everyone knows what to do. You don't need to actuall mix the SLC at that time, just walk everyone through the process so they'll know what's expected and to discover potential traffic jams in your house.
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