Slab Prep Disaster


Old 08-29-07, 08:17 AM
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Unhappy Slab Prep Disaster

Following some questionable advice, I bedded hardi-backer board in thinset over a concrete slab to prep for natural stone tile. After reading these forums I now know better. Before laying any tile, I noticed backer coming up in some spots and corresponding hollow sound when walking in some areas. The flawed purpose in using the backer was to level any unevenness in the slab and cover any black mastic (asbestos containing) to create good bond between the slab and the tile.

Needless to say, after several days of chiseling away at the thinset and backer I removed all the backer, but it left behind a more uneven surface for tile (much of the thinset remains). Now I am left with the problem of how to prep the slab, and I want to get it right this time. As I understand it, my options are to do a "mud" job over the whole floor, use thinset and tile right over it, or use a product like level quik to level the whole thing out. Any suggestions/advice greatly appreciated!
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Old 08-29-07, 09:56 AM
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Ouch, sounds like a mess you got there. You'll need to chisel or chip away as much of the thinset as possible. Grinding wouldnt be good idea with the cutback adhesive containing asbestos and would gum up the wheel anyway. You'll then need to scrape up all the cutback adhesive. You can do so safely by keeping it wet while scraping. Im assuming that you'll still have some leveling to do since the floor was not flat in the first place. If thats not the case, you can tile away.

SLC over cutback generally wont work unless your able to get all the cutback scraped of the concrete slab so that the slab will absorb water. Unless you can chip off all the thinset and scrape the cutback to a transparant stain I would not go with the slc.

A mud bed may be an option and would be cheaper than slc. You may be able to bond a thin mud bed (mix of 1 part portland cement 4 parts sand no less than 3/4" thick) to the existing slab/thinset/cutback with a thinset that will bond to cutback. If you could go minimum 1 1/4" with the mud you wouldnt have to bond it to the slab. You wouldnt have to get the slab cleaned up as good for the mud bed as you would slc.

We have had this discussion of cement board over a slab here before. Its not recommended by any of the cbu manufacturers as there is no way to mechanically bond it to the slab. Thinset under cement board is not meant to bond it to a subfloor. Its purpose is to fully support the cement board so there are no voids between the subfloor and the cement board. Screws or nails secure the cement board to the subfloor. Cement board shouldnt be used as a leveler either. Trying to use it as a leveler will create voids and cause tile failure.

Maybe somebody else here has some other ideas.
Old 08-30-07, 12:30 PM
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Re: Slab Prep Disaster

Thanks for the suggestions, HJ.

Some further checking of the slab reveals it is generally level, with the exeption of the remaining thinset that I put under the backer (which really means that I caused this entire problem myself -- should have come here first). Most of the black cutback adhesive was gone before I put the backer down and some more of it has come up with the chiseling at the backer and thinset (an air chisel comes in very handy for this job).

After considering the options you suggested, I'm probably just going to chisel away as much of the thinset as I can and get the surface relatively smooth and just tile over (what I probably should have done to begin with). Spending an additional $250 for slc seems like a waste that will probably not work and doing the mud bed may raise the floor too much and is probably not necessary from what you're saying unless you have very out-of-whack floor.

Thanks again for the help/advice.
Old 09-04-07, 07:01 AM
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You need a floor buffer, with dust control, with a scrape-away attachment.

You may be able to rent it, or you may have to buy it and e-bay it when your done.

E-bay has become the rental source for many hard to find speciallty tools. Buy and resell, and it is less costly then renting, and you can take your time, instead of cutting corners, getting in a big hurry.

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