How to Prepare Concrete Slabs for Outdoor Tile Patios

Preparation Counts in Forming a Solid Base for Your Patio

Woman holding leaf outside tent on concrete slab.
Concrete slabs make great foundations for patios. But one might question how comfortable they are to sleep on under tents. Picturenet/Getty Images

For an outdoor tile patio installation to be successful, it needs a solid base, and a concrete slab is an excellent choice. If you do not already have such a base, read this article on building a patio with this material. If you already have a slab in place, 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 is a natural characteristic of the material. Small, hairline cracks will not interfere with your new patio. But 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 to learn more.
  • 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: If you see 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 has not rained in your area lately? If so, then 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. But avoid using paint-removal products that contain harsh chemicals. The chemicals may remain in the concrete slab. A better alternative is using a pressure washer, a grinding wheel, or a wire-brush.
  • 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 4 feet long, it should be an inch higher on one end.

What Are Crack Isolation Membranes?

As the Tile Council of North America, Inc., states, "Whenever tile is bonded to concrete, cracks occurring in the concrete can cause cracks in the tile layer - this is often called 'reflective cracking'."

Crack isolation membranes keep imperfections in the concrete slab from spreading up to the tile. Some also keep water out of the installation area. Masons often use a crack isolation 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.