Underlayments for Ceramic Tile Installation
January 2nd, 2009

First Quarter, 2009
By Steve Rausch

Ceramic tile provides one of the most desirable types of treatments for walls and floors. There are many ways of doing a ceramic tile installation. Thick bed, or mortar bed, is one method which has been around for centuries; however, thin-set installation is the most common method used today.

When using a thin-set installation, you have various choices of underlayments, depending on many site factors. Many times you will want a backer board unit as an underlayment material. Backer boards are designed to provide a solid surface for bonding ceramic tile, to minimize transmission of movement from the subfloor to the tile surface, to minimize level variations with other finish materials, to increase water resistance in wet tiled areas, to insure solid bond to the substrate, and some backer boards (CBU’s) to protect combustible walls from room heaters or wood burning stoves.

Backer boards are typically a 3′ by 5′ panel, usually ½" or ¼" thick on residential applications and 4′ by 8′ panel, usually 5/8" thick on commercial applications. Backer boards are typically used on floors, walls, countertops, as a heat shield, or on the building exterior.

Backer boards may be installed over a wood subfloor (never over concrete, use a self-leveling underlayment for that application) or directly to the stud walls to function as the tile backer or substrate. There are also limitations for tile backer boards such as: they are not structural, they must be mechanically fastened, all joints and seams must be taped, and they need a leveling bed of setting material under the board if used on the floor.

According to the 2008 TCNA Handbook, there are various types of backer boards and each has unique installation requirements and suitable applications. The TCNA Handbook as well as the ANSI and ASTM standards refers to backer boards by their generic names rather than by their market names. In order to use the correct standard and method you must know both the generic name(s) for the board(s) as well as the brand names.

Different brands of backer board that fall into the same generic category will perform similarly, have similar, but not exactly the same installation requirements, and will be similar to work with. For example, DUROCK and Utilicrete are two different brands of the same generic type of board, cementitious backer units (CBUs) and the same installation methods will apply. The manufacturer’s installation requirements may differ and ultimately that is what you must follow for any installation.

Categories of Backer Units

According to the 2008 TCNA Handbook, the following categories comprise most of the common backer board units:
Cementitious Backer Unit (CBU)
Coated Glass Mat Water-Resistant Gypsum Backer Board
Fiber-Cement Underlayments
Fiber-Reinforced Water-Resistant Gypsum Backer Board/Underlayments
Cementitious Coated Foam Boards

There are other products produced that do not fall into these basic categories, but they are not generally recognized by the NTCA or TCNA and therefore have no industry guidelines or methods for installation. If you choose to use one of these other products, be certain to follow the manufacturer’s written instructions closely. Some products such as water-resistant gypsum wallboard (Green Board) and standard white gypsum wallboard are also sometimes used behind ceramic tile, but each of these products has many different limitations that need to be considered before installation.

Let’s break each category of backer boards down and discuss them individually and specific areas where they can be used.


Comply with ANSI A-118.0 and ASTM C 1325
Use on walls, floors, ceilings, and countertops
Use in wet or dry applications
Use with glass tile or large format tile
Use in steam rooms or saunas
Use on interior or exterior applications
Noncombustible, will not swell, soften, decay,
delaminate, or disintegrate.
The most widely accepted backer units
The only backerboard able to be used as a heat shield


Conforms to ASTM C 1178
Use on walls, floors, ceilings, and countertops
Use in wet or dry applications
Interior use only
Cannot be subjected to continuous high heat for long periods of time
Cuts with utility knife using score and snap method


Conforms to ASTM C-1288
Use on walls, floors, ceilings, and countertops
Use in wet or dry applications
Interior use only
Carbide-tipped knife or power shears are recommended for cutting


Conforms to ASTM C-1278
Use on walls, floors, ceilings, and countertops
Use in wet or dry applications
Interior use only
Lighter weight than cement underlayments
Cuts with utility knife using score and snap method
95% recycled content


Conforms to ASTM C-578—ONLY Extruded Polystyrene with cementitious coating designed for ceramic tile assemblies can be used.
Use on walls, ceilings, and some floor applications
Use in wet or dry applications
Interior use only
Lightweight, cuts with utility knife


Conforms to ASTM C-1396
Moisture resistant paper and core
Use on walls and ceilings only
Use in dry areas only—excluded for wet area applications by International Residential Code
Interior use only


Conforms to ASTM C-1396
Paper face
Not treated for moisture resistance
Use on walls and ceilings only
Use in dry areas only
Interior use only

Pourable and self-leveling floor systems
Ceramic tile can also be installed over pourable, self-leveling systems that can be poured, pumped, or trowel applied and would include current products that are cementitious, gypsum, or epoxy based. There are currently new composition designs being developed in this pourable category as well. This type of system is generally installed over previously installed concrete (although some are used over existing wood subfloors) where preparation of the substrate is required. There are many considerations to this type of installation, including service use requirements and environmental conditions. There are products for both interior and exterior use and generally all require primers or bonding agents. You also need to consider the thickness of the underlayment, minimum and maximum tile sizes, compressive and/or flexural strength and deflection considerations plus local building code compliance requirements. As with the backer boards mentioned previously there are many different manufacturers, and you will need to follow their specific written installation instructions for a successful application.

Using Membranes on floors and walls
Membranes generally come in several varieties, flat flexible sheet products, and uncoupling membranes. In addition to providing a bondable surface for ceramic tile, these membranes can also provide other characteristics such as waterproofing and crack isolation in the tile installation. Because the flat membranes and uncoupling membrane systems have significant variations in application, installation requirements, and performance, they have separate TCNA handbook installation methods.

Membranes are most frequently installed over concrete slabs for waterproofing, uncoupling, and/or relocation of movement joints in the concrete slab. Because of the various functions that membranes can offer, membranes use can be a challenging topic due to the variations in use, performance, and installation requirements.

When membranes are used on wall applications they generally are used to provide waterproofing of the area to protect the wall cavity behind the installation. In some applications, like steam rooms, the membrane is required to also be a vapor-proof membrane as well as waterproof membrane.

Sorting through the options
The preceding information is an overview of the industry accepted basic thin-bed installation methods of using different types of underlayments. Professional tile installers will use many of these different products at different times for different installation requirements. The important thing to know and remember is how to use the TCNA handbook installation guides and methods as well as the ASTM and ANSI standards that apply to the various methods. The TCNA handbook provides a great starting point for determining if a particular generic product category is a suitable match for the method, service requirements, and expected use of the finished installation. One final note to remember is that the TCNA handbook, ASTM, and ANSI standards are constantly being reviewed, revised, and updated, so make certain that you are using a current year edition of the publications. Everything in this article is from the 2008 publications.

Steve Rausch is with the Substrates and Specialty Products Division, USG Corp. He can be reached at 678-942-1203 or Srausch@usg.com

Leave a Reply

You must be logged in to post a comment.

Foster and Clark Real Estate
CTDA - Online Education
CTDA - Membership