Leveling the Concrete

• The first step to any tile installation is ensuring that the underfloor is perfectly level. The concrete floor in a basement makes for a good tile underfloor as long as it doesn't have any dips or ridges. Place an 8-foot-long board on the concrete and mark any area where the height difference is more than 3/16 inch over the length of the board. Sand down high areas with a power sander or sanding block. Build up low areas with leveling compound.
  
  

Insulation

• Since a concrete basement floor sits right on top of soil, it tends to be both cold and damp, although not every basement floor is this way. If the basement floor gets damp, a moisture barrier above the concrete will prevent the coolness below the concrete to cause condensation on the tile. If the floor is exceptionally cold, in-floor heating can make the area livable. This heating membrane will go above the concrete and below the tile.
  
  

Crack Isolation Membrane

• Basement concrete slabs may look solid, but the truth is that they can crack over time. If the slab cracks with tile installed right on top of it, the tile will almost certainly shatter. To prevent this, place a crack isolation membrane between the concrete and the tile. This membrane will allow the concrete to shift slightly without disturbing the tile.
  
  

Mortar and Tile

• Once the subfloor is prepared, installing the actual tile on a concrete basement floor is much like installing tile anywhere. Apply thinset mortar to the subfloor and scrape it with a notched trowel to create ridges. Place the tiles into the thinset and twist them slightly once they are in, to ensure they get set all the way into the thinset. Plastic spacers placed on the corner or sides of the tile ensure that the grout lines remain even.
  
  

Grout and Sealing

• Once the tile has been set in place, wait about 24 hours before you install the grout. If the basement is either hot or wet, extend this time to 48 hours. Apply the grout to the floor and scrape the grout float diagonally across the tiles to ensure the grout reaches to the tops of the grout lines. Let the tile sit for a few days before using the area heavily. After a few weeks, apply a grout sealer to the grout lines and stone sealer to any stone tiles.