Why, When and How to Use Backer Boards

By Steve Rausch

FinPan Utilcrete Cement Board
FinPan Utilcrete Cement Board

Today’s technology and consumer demands have created a huge variety of tiles and tiling installation materials, each with their own specific properties to match specific applications.

There seems to be no one single set of any materials that will be suitable for every application. Therefore, tile and installation materials need to be carefully selected. All ceramic tile installations are influenced by the stability, permanence resistance to moisture, and precision of the installation of the backer materials. The skilled labor required to produce traditional mortar beds has grown increasingly scarce. As such, this discussion is specifically about backer board units and how they function.

We should begin with several basic questions that need to be answered. Where is the installation? Is it residential or commercial? Is it an interior or exterior installation? What type of installation is it ‒ artistic or functional? Will this be a wet area or dry? I do not believe you can have a successful installation if you don’t understand the physical properties needed from the materials selected.

What is a backer board and why are they used? Most tile industry documents do not have any definition of what a backer board actually is or does. Most widely accepted wording is: “A nailable/screwable panel which is composed of stable material and reinforcements that have a significant ability to remain unaffected by prolonged exposure to setting materials and moisture.”

Backer boards are made in various types, with four major categories and they include a wide range of ingredients. The basic categories are cementitious, fiber, gypsum and foam-based products. Each of these basic categories has physical properties and performance standards that must be met.

The industry started to use backer boards in the 1950s when tile was being installed over concrete and wood substrates. Today, many concrete slabs are no longer fit for installation of tile due to being too slick, being covered with curing compounds or other bond-breaking materials. The slabs are usually inadequately prepared, placed, cured, reinforced, nowhere near level, and often have extreme cracks.

Successfully installing tile directly over a concrete slab requires a high level of skill from the installer, and proper choices of crack isolation materials as well as their proper installation. Backer boards are not approved for installations directly over concrete slabs so the installer needs to be aware of the many options for this type of installation.

With wood subfloors and underlayments, the flatness and level of the floor depend upon the straightness of the joists ‒ both at the time of the installation as well as down the road. Plywood expands and contracts far more than tile, and will break any bond between the tile and the subsurface. Let’s focus upon backer board units as a different and better choice.

The following is a basic look at the four main categories of backer boards:

Cementitious Based Units (CBUs) are the first generation of sand and cement backer boards. They are generally grey in color and have a partially visible skin of woven fiberglass mesh. CBUs are unaffected by exposure to moisture, but will absorb and wick moisture. A waterproof membrane is usually required for wet areas. This board can be used for both residential and commercial applications in floors, walls, and countertops, as well as interior and exterior applications.

Fiber Based Units (FB) are fiber-cement and/or fiber-gypsum boards made of chopped wood, paper, or mineral fibers along with sand and gypsum or cement as the bonding agent. These are the second generation of boards that have impressive strength. They will absorb and wick moisture and require a waterproof membrane in wet areas. This board can be used for both residential and light commercial applications. This board is not recommended for exterior applications.

USG Durock Glass-Mat Tile Backerboard
USG Durock Glass-Mat Tile Backerboard

Gypsum Based Units are the third generation of backer boards. These boards are generally known as gypsum core boards or coated gypsum-core boards to be held in a different light as the older gypsum boards known as Green Boards. This category of boards has a water resistant core and a waterproof covering on the face of the board. This waterproof skin can be damaged during shipping or installation and must be treated properly before tile is installed. Also the gypsum-core boards do not have the compressive strength of other backer board units and therefore generally have a limitation of tiles to be 2″ x 2″ or larger. This category of board is typically only used on residential interior applications. Do not use this board on passive solar or radiant heating installations; and only after checking with the manufacturer on residential steam rooms.

Foam Based Units are the latest generation of backer boards, and are still being developed with newly discovered technologies. These boards are extremely lightweight, very easy to work with, generally have water resistance built into the core. Some of the newer foam-based boards claim noise reduction properties and some have minimum tile-size restrictions. This category of boards can be made from polystyrene and polyurethane foams in various colors and use expanding foam and extruding foam. Also many manufacturers of these boards add cementitious coatings or other coatings for additional performance properties.

Other Units. There are other technologies and types of boards being made and sold as backer units for ceramic tile installations. These include wood-based, cork-based, and magnesium oxide-based products. While various manufacturers market these products into the ceramic tile industry it is generally accepted that none of these boards meet any current performance or material standards. The only warranty available is from the importer or manufacturer, who is sometimes based in other countries and difficult to enforce in case of a failure.

Care should be given when installing all backer boards as moisture can sometimes travel through fastener holes and pass through the board.

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