Types of Tiles

• Concrete floors are typically finished with a steel trowel, which makes them very smooth. Painting the concrete makes it even smoother. Self-adhesive floor tiles can attach to a painted concrete floor, but grout and other adhesives will not stick to a smooth surface. The concrete can be roughed up with an abrasive wheel, but this is time intensive and not always effective.
  
  

Concrete Floor Problems

• Concrete floors may crack over time. The tile on a concrete floor will crack if the cracks in the concrete slab shift, according to Michael Byrne in his book "Setting Tile." Moisture can collect on a concrete floor, which isn't a problem with tile adhered with thinset, but may cause self-adhesive tile to come loose. Paint that isn't completely bonded to the floor is a major problem, as the tile will peel off the floor if the paint peels. Additionally, concrete floors tend to stay cold, meaning the tile will be cold without the added insulation of a subfloor.
  
  

Preparing a Painted Concrete Floor

• Clean the floor thoroughly before installing tile. Remove dust and debris with a broom or vacuum, then mop the entire floor with clean water and wait until it dries completely. Low spots in the concrete must be filled with a concrete underlayment so the tile doesn't have waves or dips. Bumps in the concrete should be sanded smooth, as should any areas where the paint is peeling. Clean the floor again after the sanding to remove the dust that is created.
  
  

Subfloors

• Most concrete floors will require a subfloor before they can be tiled. A cement backerboard is a common subfloor to lay over concrete and is relatively easy to install. Alternately, a wooden subfloor is acceptable, but will cost significantly more than cement backerboard. Before laying a subfloor, clean the concrete of dust and debris. Dips in the concrete do not need to be removed because the subfloor will lay over them, but bumps should be sanded down to floor level.