How to Install Tile on a Concrete Subfloor

The Most Common Tile Substrate

Concrete is the most common substrate for tile installation, especially in Southern California where slab-on-grade construction dominates residential building. Most homes in Los Angeles sit on concrete slab foundations, making concrete-over-tile installation the bread and butter of our work at Tile Los Angeles.

The good news: concrete is an excellent substrate for tile. It's rigid, stable, and bonds well with thin-set mortar. The challenges lie in surface preparation, ensuring the concrete is clean, flat, properly cured, and free of conditions that could compromise the tile installation.

Pre-Installation Assessment

Before setting a single tile, evaluate the concrete:

Age and Curing

New concrete must cure for a minimum of 28 days before tile installation. Fresh concrete continues to shrink as it cures, and installing tile too early can lead to cracking. Most professionals prefer to wait 60 days for new slabs.

Moisture Testing

Excess moisture in concrete can prevent thin-set adhesion and cause tile failure. Two common tests:

• Calcium chloride test (ASTM F1869): Measures moisture vapor emission rate. Tile installation typically requires less than 3 lbs per 1,000 sq ft per 24 hours.
• Relative humidity test (ASTM F2170): Measures RH within the slab. Most tile adhesives require 80% RH or less.

In Southern California's dry climate, moisture is less of a concern in existing homes than in humid regions, but it should still be tested, especially in slab-on-grade construction near the coast or in areas with high water tables.

Surface Condition

Inspect for:

• Cracks: Existing cracks will telegraph through tile (more on this below)
• Contaminants: Paint, oil, adhesive residue, curing compounds, sealers
• Surface texture: Too smooth (slick troweled) or too rough
• Flatness: Per TCNA, substrate must be flat to within 1/4" in 10 feet for standard tile, 1/8" in 10 feet for large format tile

Surface Preparation Steps

Cleaning

Remove all contaminants:

• Grease and oil: Degrease with appropriate cleaners; severe contamination may require mechanical removal
• Paint and coatings: Scrape, grind, or chemically strip
• Curing compounds: Must be fully removed. They prevent thin-set bonding
• Adhesive residue: From previous flooring (carpet glue, vinyl adhesive). Grind or scrape completely

Profiling

Concrete needs a slightly rough surface profile for thin-set to bond properly. Methods include:

• Shot blasting: Professional-grade surface preparation that removes contaminants and creates ideal profile in one step
• Grinding: Diamond grinding creates a clean, profiled surface
• Scarifying: Aggressive mechanical texturing for heavily contaminated surfaces
• Acid etching: The least reliable method. Results are inconsistent and neutralization is tricky

The ideal surface profile is CSP 2-3 (Concrete Surface Profile). Think of fine sandpaper texture.

Leveling and Flatness

If the concrete isn't flat enough for your tile size, correct it:

• Self-leveling underlayment: Pourable compound that flows to a flat surface. Corrects deviations up to 1" in a single pour. This is the go-to solution for large format tile installations.
• Patch and skim coat: For spot corrections of dips and high spots
• Grinding: Remove high spots with a concrete grinder

Dealing with Cracks

Cracks in concrete are common and must be addressed before tiling:

Structural Cracks

Active structural cracks (caused by foundation settling, structural movement, or inadequate reinforcement) cannot be adequately addressed with crack isolation alone. These require structural repair before tiling. A structural engineer should evaluate significant cracks.

Shrinkage and Control Joint Cracks

Non-structural cracks from normal concrete shrinkage can be managed:

• Crack isolation membrane: A flexible membrane applied over the crack that allows movement underneath without transferring to the tile above. Products include sheet membranes (like Schluter DITRA) and liquid-applied membranes.
• Anti-fracture membrane: A full-floor membrane that isolates the entire tile installation from minor substrate movement. Increasingly popular for large format tile installations.

Control Joints

Existing control joints (intentional grooves cut into the concrete) should be honored in the tile layout:

• Continue the joint through the tile installation using a movement joint (caulk, not grout)
• Never tile over a control joint without an isolation membrane

Thin-Set Selection for Concrete

For tile on concrete, standard recommendations are:

• Ceramic tile on concrete: Unmodified (ANSI A118.1) or modified thin-set
• Porcelain tile on concrete: Modified thin-set (ANSI A118.4), required due to porcelain's low absorption
• Large format tile on concrete: Modified large format mortar (ANSI A118.15)
• Natural stone on concrete: Modified thin-set, white for light-colored stone

Installation Process

1. Clean and profile the concrete as described above
2. Apply crack isolation membrane if needed
3. Snap layout lines to establish tile placement and ensure symmetry
4. Mix thin-set per manufacturer instructions
5. Apply thin-set with the appropriate notched trowel size
6. Set tiles firmly into the mortar, using a slight twisting motion
7. Check coverage by periodically pulling a tile to verify mortar contact meets TCNA minimums
8. Use leveling system for large format tiles to prevent lippage
9. Allow to cure 24 hours before grouting (48 hours for heavy loads)
10. Grout, seal, and finish

Common Mistakes

• Skipping moisture testing: leads to adhesion failure
• Not removing curing compounds: invisible barrier that prevents bonding
• Ignoring flatness requirements: causes lippage, especially with large format tiles
• Tiling over uncovered cracks: cracks telegraph to the tile surface
• Using wrong thin-set type: especially unmodified for porcelain
• Insufficient curing time: walking on tiles or grouting too soon

The Bottom Line

Installing tile on concrete is straightforward when proper preparation is done. The substrate assessment and preparation phase is arguably more important than the tile-setting itself. Cut corners on prep, and even the best tile and mortar will fail.

At Tile Los Angeles, we invest the time in proper concrete assessment and preparation for every Los Angeles project. Contact us for professional tile installation that starts with a solid foundation.