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Old 10-24-2008, 08:11 PM   #1
waj
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Floor over sweating concrete slab

I am finishing off a breezeway and would like to install 12"x12" slate tile floor. The space has not been heated/cooled but we are planning on doing this and using this as living space.

I have removed the previous flooring which was very old and brittle square tiles. The adhesive looked "tar like" and I scraped a lot of it off with the tiles- but there is some remaining.

The floor is an uninsulated concrete slab. We are in the Midwest (weather ranges from below freezing to 95F) and at times this floor sweats pretty bad. I have added gutters to the outside as well as proper grade slope. I'm hoping this will take care of the majority of the sweating.

The floor is uneven in places from poorly poured concrete- valleys, hills, etc. There is also a small crack running from one side to the other, approx 12' from some settling. This crack doesn't appear to have enlarged over the last few years.

My questions...

With what I have said, will the slate floor work for me?

Assuming the slate will work-

(a) How do I prepare the slab for slate if it is doable?
I was going to use pressure treated 2x4 sleepers laid on their side, 16" OC and directly on the slab. Then add 1.5" extruded polystyrene between the sleepers for insulation. I was then going to use 3/4" exterior grade plywood over the top. I was not going to use plastic at all. I guess I'm not sure if this is the best way to do this.

(b) Where I need to level the floor- Do I level the sleepers as best as possible by shimming, cutting, etc. and then finish any leveling on top the subfloor or should I be leveling the concrete floor?

(c) I am also going to be replacing a couple of exterior doors that are presently sitting directly on the concrete slab. My plan was to raise the headers and install the new doors on the new subfloor giving me approx 1.25" of usable height for the finished floor. This should be sufficient, right?

If the slate will not work, is the answer also no for any other type of ceramic, porcelain, etc. tile? Suggestions for other floors? How to prepare slab for flooring?

Thanks for your time.
Walt
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Old 10-25-2008, 02:01 AM   #2
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Welcome, Walt - this is a bit of a tough one. The pros will be along later I'm sure, but a couple of thoughts to get you started. I think the PT lumber is a bad idea as it's going to want to twist up on you as it dries - not very stable. If you can get the floor reasonably flat, Ditra might work. It's been know to work where there is some moisture coming through. I would call Schluter to get a better answer.
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Old 10-25-2008, 09:17 PM   #3
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Welcome, Walt.

First we gotta agree on what "sweating" means to you on this slab.

Is this condensation on the slab surface during very humid (as in raining) periods, perhaps? This unconditioned space is fully exposed to the outside, like a screened in breezeway, or no?

Do you know if there was a vapor barrier installed under that part of the slab before it was poured?

It sounds like you've mostly eliminated the potential for hydrostatic moisture problems, so what you have must be either surface condensation or serious water vapor migration "issues."

That being the case, conditioning the space would eliminate the condensation. The water vapor migration might be controllable using Ditra over the surface, as Art is suggesting.

I'd do just about anything to keep from laying wood sleepers and a double plywood subfloor (required for slate), though. If you have that much height available, a reinforced mud bed is clearly the first choice as far as I'm concerned.

My opinion; worth price charged.
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Old 10-26-2008, 12:41 PM   #4
waj
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Quote:
Is this condensation on the slab surface during very humid (as in raining) periods, perhaps?
It seems like the majority of the condensation is in the late winter/early spring when we are changing from moderately cold to moderately warm temperatures. BTW, the attached unconditioned garage floor does the same thing during the same time period.

Quote:
This unconditioned space is fully exposed to the outside, like a screened in breezeway, or no?
That is correct- the space is unconditioned with very old and bad fitting jalousie windows on two sides, unconditioned garage on one and home on the other.

Quote:
Do you know if there was a vapor barrier installed under that part of the slab before it was poured?
Not from what I can see- the slab is 40 years old and I'm not seeing anything but concrete and then soil below grade.

Quote:
It sounds like you've mostly eliminated the potential for hydrostatic moisture problems, so what you have must be either surface condensation or serious water vapor migration "issues."
The work I did to help eliminate the problem was just recently finished. So I wouldn't know how much it helps until spring- but obviously don't want to wait to finish the space.

Quote:
That being the case, conditioning the space would eliminate the condensation.
Sounds good to me.

Quote:
The water vapor migration might be controllable using Ditra over the surface, as Art is suggesting.
Art suggests getting the floor reasonably flat before applying Ditra. How flat is reasonably flat? I'm not sure how tolerable tile is.

I would guess I have hills and valleys as small as 1/16" to as much as 1/2" in some areas. One example of the worst is an area approx 12"x12" right up against one of the walls- this is probably 1/2" higher than the surrounding area.

Quote:
I'd do just about anything to keep from laying wood sleepers and a double plywood subfloor (required for slate), though. If you have that much height available, a reinforced mud bed is clearly the first choice as far as I'm concerned.
What do you mean by reinforced mud bed? I like the sound of no sleepers and no plywood.

Then, how do I insulate the floor?

Do you think the crack I mentioned will cause a problem or will the procedure we're discussing take care of that?
My quote: "There is also a small crack running from one side to the other, approx 12' long from some settling."

Quote:
My opinion; worth price charged.
Worth more than you'll know. (It's my first tile job if you haven't already noticed!!)

Thanks Art & CX !!
Walt
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Old 10-26-2008, 09:10 PM   #5
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ask the laticrete rep about the 2 part epoxy moisture control system they have its pretty new and easy to install..... ardex has one as well
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Old 10-26-2008, 09:50 PM   #6
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Quote:
Originally Posted by Walt
What do you mean by reinforced mud bed? I like the sound of no sleepers and no plywood.
You would lay down a cleavage membrane (six mil poly will do) then add a minimum of 1 1/4" of deck mud with welded wire mesh in the center of the mud. Allows you to make a really flat surface over which to lay your tile while providing isolation from the slab.

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Originally Posted by Walt
Then, how do I insulate the floor?
You don't. The only effective slab insulation for a tile installation I know of requires the insulation to be installed before the concrete is poured.
Quote:
Originally Posted by Walt
Do you think the crack I mentioned will cause a problem or will the procedure we're discussing take care of that?
If there is no vertical component to the crack, the "floating" mud bed should be sufficiently isolated from small cracks. If there is a vertical component (one side higher than the other) to the crack, ain't nobody gonna promise to save your tile installation.

My opinion; worth price charged.
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