I live in a fairly upscale apartment complex and last month my kitchen was remodeled. I received new cabinetry, marble counter tops and stainless steel sinks. A few days later, the tile was laid on the floor - a beautiful dark gray slate with lighter gray and tan marbling. I was surprised that the new tiles were laid directly on top of the old without any prepping. I even asked the installer if this was the normal procedure not to remove the old tiles first. He told me that there was no need to remove the present tiles, and that the new tiles used were made to be placed directly over the old tiles. The entire floor was done in one morning, including new cove base. The floor looks fantastic. The job turned out to be an easier process than I ever thought. which was tearing up the old floor, applying an adhesive, letting that set and then putting the tiles down.
My son manages one of the largest commercial tile companies in the US so I asked him this question and it mirrors pretty much the responses. One thing about tiling over, if you have any subsidence to your property, it will buckle any new layover time and therefore, it is never recommended. He suggests, taking the tile down completely, getting rid of the mold, then putting your membrane down and retiling, from scratch.
I suggest that you bleach the hell out of the tile (try it on one little area first, who knows what's gonna happen), clean it up, and seal it.
Since you cannot clean the old tiles of their nasties, you will just be sealing them in and encouraging the same problem on the new tiles. Can't they be removed? What were they attached with? If they were self-stick, you might be able to remove them with heat gun and some elbow grease.
Hi the problem might be mold --if you cover over it. Maybe if you had a way to insure the floor was completely dry so no mold would grow. It is technically possible. I saw a new product on tv today. It looked like a thin soft plastic / foam. It is laid down first, cut (it looked easy to do). then the new tile is laid on top. I think the softness helps fill in the grout lines of the old tile. It didn't give the name of the product. Good luck!
Tile needs a strong cement board/green board type backing underneath for support. While I know it will save you time to put tile on tile, don't do it. In the end, it will cost you in repairs and aggravation.
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