A successful outdoor tile patio installation needs a solid base. If you don't already have a concrete slab, read this article on building a patio with this material. If you already have a concrete slab, that will save you some work; but before you can start tiling over this base, you need to examine it more closely. Check for the following characteristics:

• Cracks. All concrete cracks. It's a natural characteristic of the material. Small, hairline cracks won't interfere with your new patio. Long, wide cracks can pose a risk to the tile. These cracks can cause the tile to crack as well. See the section below on using a crack isolation membrane.

• Holes. Are there any large holes in your concrete? These need to be filled with new concrete before tiling. After the holes are filled, consider using a crack isolation membrane.
• Crumbling. This is not a good sign. If your concrete slab is crumbling, it may not be structurally sound. Covering it with tile will not make it any stronger and your new patio will deteriorate over time. Consider replacing the concrete slab altogether.
• Constant wetness. Is the concrete slab wet even when it hasn't rained? It may be absorbing water from the ground. If this water freezes, it may pop your tiles loose. Definitely consider using a crack isolation membrane that doubles as waterproofing.
• Paint. In order to get a good bond (using a tile adhesive) between this base and your new tiles, any paint that may be present needs to be removed. Avoid chemicals that may remain in the concrete slab. A pressure washer, a grinding wheel, or a wire-brush are good places to start.
• Pitch. If your patio will be near the house, make sure the concrete slab is pitched away from the house. The rule of thumb is a quarter of an inch of slope per foot. So, if your base is four feet long, it should be an inch higher on one end.

Crack isolation membranes keep imperfections in the concrete slab from spreading up to the tile. Some also keep water out of the installation area. I always use a membrane when setting tile outdoors. There are different kinds to choose from. Some are applied with a paint roller. Others come in rolls and are installed with tile adhesive. Talk to the people at your tile store about which products may be right for your project.