Leveling Issues

• The biggest issue with a rough and broken concrete floor is that it is not fully level. If the subfloor over which tile sits does not lay completely level, the tile itself will not sit level. Because tile is so brittle, uneven pressure from behind due to an uneven surface will cause cracking, and can even cause whole tiles to snap. An uneven surface will also ensure that the grout lines between tiles don't run straight. This will leave gaps in the grout through which moisture and contaminants can penetrate the tile surface, causing potential cracking and mold issues.
  
  

Cleaning Broken Concrete

• The other serious problem with broken concrete is that the surface is contaminated with dust and debris from the breaking and cracking. If you try to apply tile directly to this surface, the dust and debris will mix with the adhesive for tile, causing uneven drying time and creating bumps and gaps behind the tile that will cause cracking and other damage. The first step in preparing a broken concrete surface for tile installation is to use a chisel, hammer and wet/dry vacuum to remove all loose debris and dust. This will clear out the area for repair so that you can later install the tile.
  
  

Repairing Broken Concrete

• For cracks, chips and gouges, concrete patching compound can be your best friend. This material mixes up like regular concrete when combined with water, and will dry within 24 to 48 hours of application. Fill in all damage, as well as any low points in the concrete surface. For extremely rough and broken surfaces, enough concrete patch can be applied to cover the entire floor, essentially creating a whole new surface to support your tile. If you take this route, be sure that the layer of patching compound that you apply fills in all gaps fully, so that the surface is completely flat once it dries.
  
  

Dealing With Rough Surfaces

• If the surface of your concrete only features small breaks but is very rough, you can fix this problem with a similar resurfacing technique. Once the damaged areas are completely cleaned out and patched, apply a scratch coat of the tile's thinset mortar adhesive. This layer of adhesive, generally between 3/4 and 1 inch thick, will cover the rough surface and provide a smooth, level base for the tile installation. This scratch coat tends to work best in smaller areas, since it is generally applied by hand with a notched trowel to ensure a level thickness throughout. You can also coat the surface in the patching compound as mentioned before.