A Flat Surface
The concrete surface does not have to be perfectly level in order to accept ceramic tiles. It does need to be flat -- devoid of raised areas, noticeable dips, and large cracks. Any of these flaws will eventually transfer to the tiles, which will then break or crack. Sometimes, these flaws are difficult to see. Run a long level over the floor in several directions to help spot them before laying tiles. Fill dips with a self-leveling compound, a product available at home centers. Grind or sand down any high spots, and lay a crack isolation membrane over cracks that are wider than 1/8 inch. The membrane spans the crack and eliminates the possibility of it transferring to the tile. Find crack isolation membranes in home improvement or home flooring stores.
Clean the Concrete Subfloor
Clean the concrete surface thoroughly after completing all repairs. Sweep and vacuum any dust or debris and clean any grease or old paint that may be present. Determine whether the floor has any wax, oil, sealer, or adhesive by sprinkling a few drops of water on the surface. If the water soaks in, the concrete is clean of these materials. If the water beads, the floor needs a thorough cleaning with a tough product such as trisodium phosphate (TSP), which breaks through old films. Mix TSP powder with water as directed on the packaging.
Laying the Tile
Once the concrete is clean and flat, lay out a few tiles without applying adhesive in order to get a feel for the look you want (this is called a dry fit). Typical layouts keep the tiles parallel to the walls, or at a 45-degree angle to them. Once you are satisfied with how the tiles look, take them up and mix a batch of thinset adhesive as directed on the packaging. Work in small sections at a time -- about 3 feet by 3 feet -- to avoid having the thinset dry too quickly. Comb thinset with a notched trowel, and set the tiles in place, leaving just enough space between tiles for grout lines. For standard tiles, which measure 12 inches by 12 inches, that space is about 1/4 inch. Eliminate the guesswork by using tile spacers designed to be put in place temporarily while thinset bonds the tile in place.
Expansion Joints and Gaps
Ceramic tiles move with changes in temperature and humidity. An expansion gap allows for this movement without buckling tiles. When tiling over interior concrete subfloors, leave a 1/4 inch expansion gap between the tiles and the walls. This gap must be present around the perimeter of the room, or anywhere the ceramic tiles meet a structure that is both permanent and vertical. Outdoor applications, where humidity and temperature swings are greater, require a gap of 1/2 inch wherever the tiles meet a permanent vertical structure, such as an exterior wall or a barbecue island. In addition, outdoor tiles need a 3/8 inch expansion joint every 12 to 16 feet. That gap is slightly larger than the standard 1/4 inch grout line between tiles, and it provides additional expansion space for tiles applied outdoors. Expansion gaps disappear from view once base molding goes up in interior rooms, or when joints get filled with tinted polyurethane caulk.