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Unread 12-02-2018, 02:25 PM   #1
zse4xdr5cft6
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Tiling Over Basement Slab

Hi, I'm new here and would appreciate some advice before I create potential problems.

I would like to tile my basement slab and wondering the best approach. There are three aspects that I see.

1. I know there is seasonal water under the slab ( spring, big rain events, etc. ). I will tile in the winter, when the slab is dry. However, I've read a few posts about moisture under the slab and it sounds like that could be a problem. I'm wondering why that would be a problem and whether it should influence my decision on whether to use a decoupling membrane ( see # 2 ).

2. There are a few hairline cracks in the concrete. As close as I can figure, they are "dead". The slab has been there since 1958. Some patches I did to the cracks a few months ago have not cracked, so I'm thinking they may not be a problem. However, I'm also doing in-floor heating, so this might be a good opportunity to use Ditra Heat, plus get the insurance of the membrane.

3. There are some floor irregularities that I need to address. If I don't go with the membrane, I could do dry pack mortar at 3/4" over everything, including the heating coils. If I go membrane, I have to smooth out the floor first. Very expensive with leveling compound. Otherwise I could try doing a skim coat, but this would be my first try at something like that and likely to screw it up.

So, thanks for an experienced opinion on this.
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Unread 12-02-2018, 04:30 PM   #2
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Ted,

Welcome to the forum.

Where are you located? If you add this to your user CP, this will help our answering. The User CP is linked in the dark blue bar above.

Is one side of the crack higher than the other?
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Unread 12-02-2018, 06:15 PM   #3
zse4xdr5cft6
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Hi, thanks for your reply. I'm in Ottawa Canada. I have updated my profile.

The cracks do not appear to be of different heights. I'm pretty sure they are shrinkage cracks from the original pour. The footings are well below the frost line.
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Unread 12-03-2018, 09:22 AM   #4
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Hi Ted,

Although self-leveling compound is somewhat expensive, it's probably the way you need to go. Then use the Ditra-Heat or at least a tiling membrane of some sort. How much variation in height do you have throughout the floor?
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Unread 12-04-2018, 08:45 AM   #5
zse4xdr5cft6
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Thanks for your input. From zero, it drops off quite quickly to 3/8 and then more or less 3/8 throughout, execpt for 1 corner which is 5/8. So, I guess an average of 3/8 for 107 sqft. The TEC online calulator tells me I would need about (7) 50lb bags. So, a few hundred bucks for sure.

Does it feather to nothing, or do I need to cover the high area with at least 1/8 ? Really it's that initial drop off that is consuming the volume. Maybe i could level the lower part and then taper to the high area to it with skim coat ?
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Unread 12-04-2018, 08:54 AM   #6
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Floors to receive tile need to be flat; they don't necessarily need to be "level."

Yes, you can work SLC to just about a feather edge. Don't forget that you need to first use a primer.
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Unread 01-13-2019, 07:29 AM   #7
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Ditra Curling Up - Not Sticking

Hi,

My very first time putting down any sort of membrane.

I used Keribond unmodified thinset. I put down the thinset and pressed down the ditra, then peeled it back to check coverage. Just like in the videos, some thinset came up and it looked like 100% coverage. I was very happy.

Then after 20 minutes or so, I noticed the ditra curling back up from the memory of being in a roll. So I peeled back the ditra again and this time, no thinset came back up with the ditra. I thought something was wrong, so I removed the ditra and scraped the thinset off the floor.

Thinking back, I suppose the reason the thinset didn't come back up with the ditra is because it had begun to set and probably all I needed to do was just place some weight on the part where it was curling up.

Can someone confirm this ?

Also, I am now left with a haze of thinset on the concrete floor and on the fleece of the ditra. Can I just reapply thinset on the floor and reuse the ditra ?

Thanks.
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Unread 01-13-2019, 07:37 AM   #8
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Hey ted -

If I pull Ditra back and check for thinset coverage immediately after pressing it in, I’ll get thinset transfer to the fleece.

If I wait more than 5 minutes or so, then peel back to check, the fleece will just be grey (assuming grey thinset) without much transfer. Almost all the thinset will be on the floor with the square imprints from the bottom of the Ditra.

That being said, 20 minutes is a long time to try and re-stick it. I would’ve pulled it up like you did.

If it curls up from memory, stack bags of thinset, boxes of tile, whatever to hold it down.

And yes - if you just have a thinset haze on the ditra and the floor, you can re-apply thinset and re-use the ditra. Wipe down the slab with a damp sponge immediately before setting a section to help prevent too much moisture being sucked out too soon.
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Unread 01-13-2019, 07:50 AM   #9
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When I put ditra down I really had to sponge the floor wet at first. The thinset also had to be mixed wetter than normal, but able to hold notches. I only mixed like a third of a bag at a time as well.

Did you go over it all to press the ditra down?
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Unread 01-13-2019, 08:05 AM   #10
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I usually just backroll the membrane beforehand so it'll lay down on it's own.

But at this point it looks like you may have to redo part of it anyway, so weigh it down this time.
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Unread 01-13-2019, 08:10 AM   #11
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Hi, thanks for the info. This is a relief.

Yes, I always wet the concrete before and press the ditra down as much as possible.
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Unread 01-13-2019, 10:28 AM   #12
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Ted, you do not want that concrete floor wet when you install the Ditra. Sponging it off before installing is good, and having it damp is OK, but it should not be wet.

My opinion; worth price charged.
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Unread 01-20-2019, 11:43 AM   #13
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Thanks, that sounds like a fine distinction between wet and damp that I was not aware of.

Also, for anyone in a similar situation that is thinking of reusing ditra that has had some thinset on it, ... I called Shluter. They recommended using sandpaper to scrape off as much dried thinset as possible and then just reuse.
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Unread 01-26-2019, 11:36 AM   #14
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Ditra Lifting Up

I was checking some Ditra Heat that I put down a couple of weeks ago. I wanted to make sure it was stuck down so I went around lifting up on the dimples with my fingers. Everything seems fine except for a couple of places here and there along one edge. I was able to lift it up 1/16 in a few spots. Not sure if this is something to be concerned about.
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Unread 01-26-2019, 02:08 PM   #15
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There will probably always be places you can pull it up since unmodified was used. I'd stop tugging at it.
I don't go by Schluter's rules, I use Flexbond under Ditra.
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