Building a Better Tile Job with Underlayment
May 1st, 2005


May-June 2005

No matter how you apply it—with a trowel, a spray, a roller, or as a sheet—underlayment helps build a better floor from the bottom up.

By its narrowest definition, underlayment is a leveling layer topping the subfloor and offering a flat surface for applying tile. Some manufacturers broaden the definition to include underlayment materials that also suppress cracks, provide a moisture barrier or even reduce noise. But most importantly, the right underlayment for the installation can keep the tile and/or grout from cracking and failing. The wrong application or the lack of one can ruin the best tile installation.

Patricia Bohnert, president of National Applied Construction Products, says using the right underlayment for the substrate and the tile above it makes for a better installation and one that is less likely to fail from subsurface cracks or movement. Leigh Hightower, National Business Development Manager for C-Cure, says “underlayment should level the floor to provide a flat surface” for tiling. Because tile is set on top of it, underlayment is often the overlooked layer in any installation. It also adds to the cost of materials, time and installation. What the customer can’t see, he or she doesn’t necessarily want to pay for. So, how necessary is it?

Bob Pritchard, of Southern Grouts, points out that surface preparation varies significantly based on what’s under that surface. Construction in southern regions of the country is predominantly over slab on grade. Every slab cracks and virtually every slab is steel trowel finished, says Pritchard. Both conditions require the right surface preparation.

In other parts of the country, the installation goes over a subloor installed over a basement or crawl space. Even if the joists are on 12-inch centers, the span of the joists may not be stiff enough for a particular tile installation, especially large format tiles. Charlie Martin of Halex, which makes plywood underlayments, says current residential trends calling for larger rooms finished with larger-format materials, simply require a stronger base.

Hightower agrees that although a flat surface has always been important to a successful tile installation, they are even more critical as tile size gets bigger. This can be especially troublesome in commercial and multi-family construction where the installation is often on concrete. Architects don’t specify leveling compounds, because that would imply that the cement contractor is not leveling the floor. However, the TCA Handbook specifies that the substrate should be level to ¼-inch within 10-feet. This almost never happens, says Hightower. The result, however, is a debate over who levels the floor—the builder or the tile contractor.

Choosing the right material or what goes down first?

Some underlayments go only over cement; others are designed for use over wood. The surface you’re laying tile on determines the underlayment choice. Hightower points out that trowel-applied underlayments are dependent on the installer’s ability to get the material flat. Self-leveling underlayments have a pourable consistency that allows them to flow and seek their own level. Because self-leveling underlayments contract, then expand in the curing process, they must be applied to a rigid substrate like wood or cement. In either case, the underlayments should be Portland-cement based. Gypsum-based materials are weaker. The stress of ceramic contraction and expansion can shear the gypsum-based underlayment, resulting in a failed installation.

There are other considerations. Additional underlayment layers such as waterproofing or sound-reduction materials are often applied separately. Self-leveling underlayments can’t stop a crack. Crack isolation membranes need to go on top of self-leveling materials.

Consider what else may be under the tile installation. Pritchard points out that a multitude of contractors are typically on the job between the installation of the subfloor and the installation of ceramic tile. Plasters, painters, electricians, etc. leave a trail of dirt and debris—much of it stuck to the floor—before the tile contractor even arrives to install. Tile contractors are left terrible installation conditions, ones in which the underlayment will not bond.

But, says Pritchard, not every builder or tile contractor makes underlayment part of the package—even if it costs just pennies. However, if the tile fails, the builder wants the installer/contractor to assume responsibility—financial and physically—for the repairs.

Bohnert advocates full floor coverage of underlayment—as opposed to applying it just over existing cracks as some contractors do—because she believes it limits the contractors’ liability for installation failures.

In the end, most experts agree that the contractor has to educate the builder about the role underlayment plays. Underlayment, Bohnert says, is like insurance—done right it’s going to go a long way to guarantee that the tile installation won’t fail.

What’s under the underlayment?

Underlayments are a wonderful thing but without exception they all need one thing first and foremost, an underlying supporting structure. Backer boards are designed to provide a solid and stable backing for ceramic tile. Membrane systems are designed to provide not only backing, but in an underlayment scenario, may also contribute crack suppression and waterproofing capabilities not otherwise possible. None provide structural value, which is not their purpose.

There are many instances where the subfloor is not adequate to the task due to lack of thickness (rigidity), improper installation, and weathered or abused conditions. In such cases, given a choice, and there is always a choice, either the addition of a layer of plywood or in cases of height or budget constraints, a layer of plywood in lieu of a backer board or membrane underlayment may be appropriate.

Another common misperception is the use of 5/8″ square edge plywood where a 5/8″ thickness is an acceptable to the manufacturer. All tile underlayments recommend a tongue and groove floor system. Maintaining the subfloor manufacturer’s recommendation of 1/8″ spacing between panels is also important to prevent telegraphing of panels through the underlayment system. All subfloor panel manufacturers publish specific recommendations based on type of finished flooring selected.

For a generic recommendation on wood floor systems you can go to and click on their publications page for a wealth of free information that can assist you in understanding proper subfloor installation for various wood flooring systems.

Special thanks to Dave Gobis, Executive Director of the Ceramic Tile Education Foundation, for this information. Mr. Gobis is a member of the NTCA Technical Committee and the American National Standards for Ceramic Tile Installation (ANSI A108) and the TCA Installation Handbook Committees. He can be reached at 864-222-2131 or

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