Prep uneven concrete for marble tile?
I’ve purchased a bunch of marble tile at a low price and intended to use it to replace the carpeting in the finished basement of the 15 yr old house we just bought. The concrete floor turns out to have some cracks and high spots (and I’m getting conflicting suggestions on how to flatten the floor and adhere the marble). One of the cracks occured at a high spot, both sides of the crack are at the same level but I’m wondering how to determine if this is an active crack or a one time event? Must I use a special mortar with added flexibility to protect the tile? I’m looking at Laticrete products (Lowe’s) and Conproco. Also considering Superseal membrane between layers on thinset to assure water impermeability. This is an L-shaped room 12″ wide, 28′ x 24′ L. Any suggestions?
Replies
First you need to determine if the cracks are minor enough and old enough to allow the use of stone tile, and then you need to get the floor flat. If the floor is relatively level with humps and dips then I suggest you make it flat using self-leveling cement (SLC). use expansion material such as foam or sill-seal around the perimeter of the room. Use the primer to bond the SLC.
After the floor is flat then I suggest you use a crack isolation membrane such as Ditra on the slab and then apply the tile.
Go over to http://www.johnbridge.com and talk with folks there -- they will have lots of good advice. And show them pictures.
Billy
How bad are the dips and bumps? Are you talking 1/8" or 1"? Small variations can be handled with a bigger trowel notch. I would lay down a sheet of ditra first to help deal with the crack.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
The floor height varies about an inch overall. The most significant (worrisome) crack is "Y" shaped, is about 1/8" across and has isolated a couple of small "islands" of loose concrete no more than 1/2" x 2", so some repair is needed. It may be significant that this crack occurs along the top of a crown in the floor. Each side of the crack ramps up to the crack so that a straight edge following the slope on one side will be 3/4" in the air at about 2' beyond the crack. The edges of the crack are at the same height. I'm thinking about cutting out that area, checking the substrate, doweling in some rebar and repouring. Other than waiting until winter (even with global warming I suspect we'll have winter in Maine) to see if things move, I don't know how to evaluate the status of this crack. I got a great deal on a couple of palets of 12" marble tile ($1.00 / tile) which, as I learn more, may not be the best floor for a finished basement game room. It would look great for a while, but I might be better off giving someone else a "great deal" and switching to vinyl. Your comments are appreciated.
"Experience is what teaches us to recognize a mistake the next time we make it."
You might have some serious problems. It sounds like either something is pushing up the middle iof the slab, like a tree root. Or the edges are dropping, like the footings are being washed away by groundwater.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Understood. But, how do I find the problem?
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I've read about, (but never tried) a method to check out active cracks. IIRC it involved epoxy glueing a piece of single strength window glass across the suspect crack. By bonding the glass to the concrete on both sides of the crack, any crack movement breaks the glass. Apparently, even minute movement is detected this way.
It would seem to me that you would want to cover the glass to avoid any accidental breakage, and monitor the glass over a long enough time to be sure the crack is not still moving.
Hope this helps.