Subfloors and Underlayment for Ceramic Tile Floors

Ceramic tile can transform the look and feel of any room. With proper subfloor and underlayment preparation, your tile installation will be beautiful, durable and long-lasting. Here is an overview of subfloor and underlayment options for ceramic tile floors.

What is a Subfloor?

The subfloor is the underlying surface that provides structural support for the floor covering. For ceramic tile, the subfloor needs to be rigid and stable to prevent cracks in the tile and grout. Common subfloor materials include:

  • Concrete slabs
  • Plywood
  • Oriented strand board (OSB)
  • Cement backer board

The subfloor must be clean, flat, dry and structurally sound before installing tile. Any imperfections in the subfloor will transfer to the tile, so take time to properly prepare the surface.

Underlayment for Tile Floors

An underlayment is an intermediate layer installed over the subfloor to create a smooth, uniform surface for applying tile. Underlayments improve adhesion, prevent moisture damage and bridge minor imperfections.

Common underlayment options include:

Cement Backer Board

Cement backer board, such as Durock or HardieBacker, is the preferred underlayment for most tile installations. It is water-resistant, prevents cracks and provides extra strength. Thinner 1/4″ backer board is used for walls, while thicker 1/2″ board is recommended for floors.

Fiber-Cement Underlayment

Fiber-cement underlayment, such as Hardibacker 500, is applied as a skim coat directly over the subfloor. At just 1/8″ thick, it smoothes irregularities while adding strength. Fiber-cement has less expansion and contraction than plywood.

Plywood Underlayment

Plywood provides a smooth, stable layer over subfloors. Use at least 1/2″ exterior-grade plywood and ensure proper fastening into the subfloor. Coat plywood with thinset mortar to minimize swelling and shrinkage.

Self-Leveling Underlayment

Self-leveling underlayments, such as Ardex and Mapei, produce an extra-smooth surface for tile installation. Pour the liquid mixture over the subfloor where it spreads evenly and cures to a durable, bonded layer.

How to Prepare Subfloors for Tile

Proper subfloor prep is crucial for successful tile installation. Follow these key steps:

  • Clean – Remove all debris, wax, grease, paint and other contaminants so the thinset mortar bonds directly to the subfloor.
  • Make repairs – Fill any cracks, holes and low spots with floor leveling compound. Sand down high spots.
  • Moisture test – Check for excess moisture which can damage tile. Conduct calcium chloride, humidity and plastic sheet tests.
  • Prime – Apply primer to increase adhesion, especially for concrete, OSB and plywood subfloors.

With the subfloor smooth, clean, dry and ready, you can proceed with installing underlayment and ceramic tile.

FAQs About Subfloors and Underlayments for Tile

What is the best subfloor for ceramic tile?

Plywood and cement backer board provide the most stable, rigid subfloors for thinset ceramic tile. Concrete is prone to cracking unless properly reinforced.

Should I use an underlayment for ceramic tile?

Yes, underlayments are highly recommended. They prevent moisture issues, smooth imperfections and strengthen the subsurface before tiling.

What underlayment is best for tile in bathrooms?

Cement backer board is ideal for wet areas like bathrooms. Its moisture-resistant properties prevent damage from leaks. Use 1/2″ cement board for floors and 1/4″ for shower walls.

Can you put tile directly over OSB?

Tile should not be applied directly to OSB subfloors. The surface is unstable and prone to swelling from moisture. Install plywood or cement backer board over OSB before tiling.

How do you prepare concrete for tile?

For concrete subfloors, mechanically etch the surface, apply a concrete bonding agent, fill cracks with epoxy filler, and prime before laying tile. This improves adhesion.

Can you put tile over vinyl flooring?

Vinyl should be completely removed before installing ceramic tile. Use an underlayment over the subfloor for a smooth surface. The thinset mortar will not bond correctly to vinyl.


A sturdy, properly prepared subfloor is the foundation of every successful tile installation. Take the time to assess and fix any subfloor imperfections, choose the right underlayment, and carefully follow each preparation step. This ensures your tile flooring stands the test of time and looks beautiful for decades to come. With sound subfloor and underlayment selection, you’re on the path to a flawless ceramic tile floor.