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My basement has been prone to getting water for many years before I bought the house. We're having the basement walls repaired (excavated outside) & sealed. So this problem should go away.

The basement floor currently has linoleum tiles that are all coming up from the years of constant water damage. Beneath them is what looks like a black mastic. Its hardened to the touch in most areas (there are a few sticky sections).

I understand it might have asbestos in it. We had an asbestos inspection done recently and they tested the tiles (report says 0% asbestos) but it is unclear if they tested the mastic. The room is over 1100sqft so it would be alot of work to get it all off the floor and the possibility of asbestos being in it makes removal that much less appealing.

I've read mixed things on the internet about thinset over mastic. I was thinking about laying Ditra over the mastic to accomplish a few things:

  1. I can lock the asbestos down, safe beneath the floor
  2. I dont have to kill myself trying to remove it
  3. Acting as a waterproofing material

My concern is thinset not bonding well to the mastic covered floor. I've read that if I use a high quality thinset, I should be able to secure the Ditra to the mastic pretty well. I'm considering using Laticrete 254 Platinum.

I also wonder if the mastic will hold moisture in the thinset and if so, if I should opt for an unmodified thinset instead (if both mastic and Ditra will be impervious to water).

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So you want to waterproof your whole basement tile system? –  DMoore Aug 21 '13 at 18:30
    
It's pretty straight forward to remove mastic from a concrete slab floor. Especially in a basement. You just put solvent down, wait, and then squeegee it up. You would then need to use a degreaser before you put your flooring down. It's work, but not bad. –  Edwin Aug 21 '13 at 20:47
    
I think the use of Ditra here doesn't make sense. Ditra is a waterproofing agent that is used to keep water on top. For a basement floor water could come from the bottom. I don't see how Ditra would handle this. The water has to go somewhere. –  DMoore Aug 21 '13 at 21:28
    
@DMoore Thanks, and good points but Schluter also says Ditra acts as a vapor management solution allowing moisture to escape from the substrate. It does specifically say "moisture" and not "water" and I'm not sure exactly where the line is drawn between the two. Given the situation I explained, if you have another solution, please make a suggestion! I'm open to anything - Ditra was recommended to me elsewhere. –  Bryan Migliorisi Aug 22 '13 at 3:37
    
@Edwin One thing I've read over and over is that its not recommended to use a solvent to remove the mastic because its likely that the mastic has entered and filled the holes and pores of the slab and that no amount of scraping will get that out. Without those pores exposed, the thinset wont bond correctly. Does that sound right to you? –  Bryan Migliorisi Aug 22 '13 at 3:39
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2 Answers

Schluter may not warranty Ditra if your slab does not permit moisture and you use a modified thinset.

Laticrete Stratamat has a "mortar hydration vent" that is claimed to allow either modified or unmodified thinset. I have found significant price breaks for Stratamat if ordered online. Local suppliers are a bit higher, but sometimes worth spending extra for "I-gotta-have-it-now" moments.

I would do an experiment with a modified thinset and a couple of cheap tiles... Test adhesion at 12 and 24 hrs.. Use the std. Rockford chisel-and-3-pound-maul shear test...(wear goggles).

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I am giving a +1 because I have never used the Stratamat and it has a cool name. Have you used this and how is the install? I find Ditra hard to lay super flush and not have bubbling and I have heard complaints on Ditra about water staying in those bubbles in basements. I have never laid any kind of sheeting membrane in a basement - always a "paint" or "mortar". –  DMoore Aug 22 '13 at 14:56
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Floor Leveler (optional)

Fracture Guard 7000

Thinset

Tile

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Could you explain how that answers my original question? –  Bryan Migliorisi Aug 22 '13 at 11:57
    
Don't use Ditra - overkill on basement floor - fracture guard can be laid on your mastic. Thinset can be laid on the fracture guard (I have used this product but no affiliation with company) and then you tile (don't do ceramic). –  DMoore Aug 22 '13 at 14:26
    
Do you think it'd be safe to put the fracture guard down over the mastic? I know best case would be to remove as much mastic as possible. We'll be putting porcelain down. –  Bryan Migliorisi Aug 22 '13 at 20:06
    
The fracture guard 7000 is very flexible. I don't see how installing it over the mastic or the ditra is any different. –  DMoore Aug 23 '13 at 3:41
    
Really, the biggest question here is whether or not the mastic can be covered with any kind of material - whether its thinset or FG - and still provide a strong enough bond. –  Bryan Migliorisi Aug 24 '13 at 1:12
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