A properly prepared substrate is crucial for the successful installation of ceramic tile or stone. Tom Plaskota, technical support manager for SCB, explains the procedures for properly preparing a substrate prior to tile or stone installation.

Every successful ceramic tile or natural stone flooring installation starts with a properly prepared substrate. Tile installed over an inferior surface is subject to cracks, loose tile, cracked grout and expensive callbacks.

The key to proper surface preparation is to devote the necessary time and effort on the front end of the project. Fortunately, advanced technologies can make quick work of the process.

Starting off on the right foot

Ceramic tile and stone can be installed over virtually any sturdy surface, barring a few general guidelines.

For starters, the tile needs to be installed on a suitable substrate. Suitable substrates include cured concrete, APA Grade Trademarked Exposure I plywood, cementitious backer units, and existing tile, to name a few. With questions about suitable substrates, consult the Tile Council of America (TCA) and the organization's Handbook for Ceramic Tile Installation (commonly called the TCA Handbook) or the Marble Institute of America's Dimensions Stone Design Manual. And always follow manufacturer's installation recommendations.

The substrate also needs to be structurally sound. For example, water-damaged plywood might not support tile - let alone allow a proper bond to form. Of particular importance is deflection, which is often associated with wood floors. Installation failure can result if deflection is more than L/360 of the span when measured under a 300 lb. concentrated load (L/720 for natural stone). One example of a residential ceramic tile installation over a wood floor consists of two layers of APA Grade Trademarked Exposure I Plywood (underlayment-grade or better) with a total thickness of at least 1 1/8 inches. Floor joists in this case should be spaced no wider than 16 inches on center.

Address substrate issues, as well as any structural concerns, with the facility architect and/or builder.

Bond breakers take many forms

Starting with a structurally sound and suitable substrate is the obvious first step. Common bond breakers, however, take a variety of different forms. These include:
  • Improper coverage: An insufficient amount of tile-setting material will not allow for a proper bond.
  • Contaminated substrates: A substrate with contaminants spells trouble. These include oil, grease, dust, paint, concrete sealers and/or curing compounds. Also check for laitance, which is a weak, top layer of concrete that can form during the concrete curing process. Surface voids, seems and depressions, as well as protrusions, are also bond breakers. The same is true for glazes found on the surfaces of existing tiles.
  • Un-level surfaces: An un-level surface can make proper installation of the bonding materials difficult, inhibiting the ability to create a strong bond. The TCA Handbook requires a substrate tolerance of 1/4 inch in 10 feet and 1/16 inch in one foot.
  • Concrete cracking: Concrete is typically the best substrate. The downside is that naturally occurring cracks can cause rigidly bonded tile to crack. Additionally, cracks might already exist. Both situations can spell trouble for bonding, unless properly treated.
  • Tile over green concrete: Increasingly tighter timelines make it difficult to schedule the traditional 28-day wait to install tile over newly poured concrete. Installing before the 28-day period can lead to bonding issues, which is why it requires special attention.
  • Wet areas: The TCA defines wet areas as tile surfaces that are soaked, saturated, or regularly and frequently subjected to moisture or liquids. Precautionary measures are necessary.

The art and science

Avoiding common bond breakers requires a balance of art and science. The art involves the technique; the science is in the products. Tips to ensure surface preparation success include:
  • Proper coverage: A general guideline is to achieve a minimum of 80 percent coverage for interior and/or dry areas and 95 to 100 percent for wet or exterior applications. Also strive to get the air out from under the tiles, which is more challenging with larger tiles. Do so with the proper-notched trowel, and by pushing the tiles in a direction perpendicular to the combed ridges when setting them.
  • Clean substrates: Test for contaminants by placing a few drops of water on the surface. If the water beads, it might signal the presence of a contaminant. The best method to ensure the substrate is contaminant-free is to employ a mechanical cleaning method, such as sandblasting, shotblasting or scarifying. Also consider a strong degreasing agent, such as TSP, when removing soap or wax residue.
  • Level surfaces - small areas: Trowel-applied, latex-modified patches make quick work of filling cracks, voids, seams and depressions up to a 1/2 inch in most floors surfaces. Some patches dry in as little as 45 minutes.
  • Level surfaces - large areas: It might be necessary to create a large level layer with an underlayment. Standard guidelines require that concrete substrates in most cases have a m aximum variation of 1/4 inch in 10 feet to 1/16 inch in 1 foot. One option is a pourable self-leveling underlayment, which creates a flat, level surface. Advanced self-leveling underlayments dry to a level, walkable surface in as little as two hours. Prime the surface before applying a self-level underlayment.
  • Isolating cracks: The solution in some cases is to isolate the substrate from the ceramic tile or stone to prevent cracks from transferring upward. Crack-isolation systems are available in several formats. One-step mortars represent the latest technology, which allows installers to isolate cracks and set tile in a single step. It's important to ensure the surface is flat before the crack-isolation system is installed because the systems are not intended to correct irregular substrate surfaces.
  • Tile over green concrete: Advanced technology exists that cuts 25 days off of the normal waiting period for tile installation to begin. The liquid membrane, with crack isolation properties, can be applied over new concrete in just three days. Follow up with a quick-setting mortar allows tile or stone to be installed the same day.
  • Keep wet areas dry: Some liquid-applied waterproofing membranes incorporate crack-isolation properties and can be roll-, trowel-, or spray-applied. Some advanced membranes are also mold- and mildew-resistant and ready for tile within two to three hours after they're applied. Additionally, they can be used over green concrete that is only three days old. A ready-to-use membrane eliminates the need for a mesh unless a substrate plane change is involved, which saves time and materials.

The bottom line in surface preparation is not to cut corners. It's also a good practice to take advantage of the latest technology. Time and effort, combined with the right surface preparation products, will pay dividends for years to come.