Clean the concrete to remove any grease, oil, paint or "cutback adhesive" residue -- such as the residue after you have removed a prior layer of glued-down sheet vinyl, for example. Do not use chemical strippers on the floor. Remove dirt or dust with clean water only, and scrape off paint or other layers manually if possible. To test for any substances on the floor, pour a few drops of water on it; if it puddles, there is still a layer of residue on the slab.
According to the Hometime website, concrete is one of the best underlayments for ceramic tile. However, it is important that the concrete be clean, level and smooth. This may take some time, but it is worth it to avoid problems down the road.
Clean the Concrete
Remove Hills and Valleys
The concrete floor must be flat. If it is not, the tiles may rock instead of lying flat, or in the case of glossy tiles, they may reflect light in different directions -- which makes the uneven substrate obvious even after laying the tile. Check for imperfections by running a metal straightedge over the floor and noting any hills and valleys. The thinset layer beneath the tiles will fill in tiny cracks and holes, but you should not depend on thinset to even out larger holes. Fill these in, before starting to lay tile, with a floor leveling product, which is usually cement based. This product needs to cure for at least 24 hours. Grind down high spots with an electric grinder, which you can rent at any home improvement store, or use a rubbing stone if the bumps are small.
Apply Crack Isolation Membrane
In new construction, sometimes cracks that appear in the concrete slab will transfer stress to the surface and cause cracks in your grout or tile. There are a couple of ways to prevent this phenomenon. One way is to install a thin rubbery material under the tile by gluing it to the slab. This is called a "crack isolation membrane" or an "anti-fracture membrane." Another way is to roll on a thick, rubbery substance and let it dry. In either case, this flexible layer absorbs any settling or horizontal movement in the substrate.
Apply Thinset Properly
To further keep the new tiles flat, apply thinset not only to the floor but also to the back of each tile, a process known as "back buttering." This will ensure that each tile is perfectly flat and will fill in any holes you might have missed with the leveling compound.
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