Although there are numerous tile backers on the market, this article will focus on four types of board. This issue will focus on CBU and FCU types. In the May/June issue of TILE Magazine, we'll talk about fiber-reinforced water-resistant gypsum backer boards, and cementitious-coated foam boards.

The author uses a cut-off scrap of backer board to guide the scoring tool instead of an aluminum straightedge: the carbide bit in the scoring tool can cut into, and damage the relatively soft aluminum edge.


Snap cutting CBU panels requires several hard passes with a scoring tool. To ensure a straight cut, the author clamps a thin wood plank to the CBU to guide the scoring tool.

When tile backer boards were developed as a replacement for mortar beds, they were first made from sand and cement wrapped in a skin of fiberglass reinforcing mesh. Today, this type of board – like the boy named Sue – have to live with a rather ungainly official name: cementitious backer units. Fortunately, the industry has also adopted a nickname that rolls off the tongue a lot easier: CBU. A second type of sand and cement board, whose recipe includes various types of mineral fibers designed to replace CBU’s external wrapping of fiberglass mesh, is called fiber-cement board/underlayment. I will refer to those boards as FCB. The technology to produce this type of material (exterior shingles, for example) existed long before the process was applied to making tile boards.

For a couple decades, these two boards dominated the nail- or screw-on backer board market, but today, there are other types of tile backer available whose cores are made from a variety of materials. Although there are numerous tile backers on the market, this article will focus on four types of board. In addition to CBU and FCU types, there are also fiber-reinforced water-resistant gypsum backer boards, and cementitious-coated foam boards. I have used all four types of board, and have good and not-so-good things to say about all of them. Although there are significant differences among the four boards, they share one thing in common: all require adequate support.

Besides generating less dust than a grinder, tile biters make quick work of rough CBU edges.

Cementitious Backer Units (CBU)

This type of board is available, under numerous brand names, in sizes from 3x5- to 4x8-feet, with thickness ranging from ¼- to ½- to 5/8-inch. When properly installed, these boards have a high compressive strength, and are suitable for most residential and light commercial tile installations. Some brands offer the installer a choice of two bonding sides – one for tile, and the opposite for all other types of finish. CBUs are unaffected by exposure to moisture or water, but some may not be recommended for exterior applications.

The composition of some CBUs is extremely hard, making them difficult to snap-cut cleanly (Photo 1), but even the softest CBUs resist the kind of snap-cut finish that can be produced with gypsum drywall. A masonry rubbing stone or pair of tile biters (Photo 2) can be used to cleanup rough snap-cut edges, but for the most efficient, accurate cutting and fabrication, many installers use a dry-cutting diamond blade that generates considerable dust. When cutting CBUs with this blade, a dust-vac tool setup, and a tight-fitting filter mask are essential for safe operation. The tile industry is opposed to installing tiles over cracked surfaces, and yet some CBUs seem to be riddled with a network of cracks too small to be “repaired” when thinset mortar is applied to install the tiles. CBU panels on the hard side seem to exhibit the most cracking and fastener split-out, so I tend to specify and install the softer grades.

The author uses this stand-up, mag-load screw gun to install plywood and cement backer board underlayments. Mag-load backer board screws cost only a fraction more than loose.

CBUs can be installed with either hot-dipped galvanized roofing nails, or backer board screws. I prefer screws for their superior holding power, and use a Simpson Quik Drive PRO CGB mag-feed screw gun to significantly reduce time required for installation (Photo 3). Backer board screws have a countersink-cutting head that enables screw heads to be installed flush with the surface, and the screws are incredibly strong compared to ordinary drywall screws. But some young-growth stud lumber has such a limited grip, that when the head bottoms out against the surface, the screw threads spin like drill bits, and screw heads stand proud of the surface. When this happens, I use carbide drill bits to drill through and countersink the boards to produce a flat surface and to minimize thread tear-out (Photos 4 and 5).

Electro-galvanized roofing nails, ring shank nails, drywall screws, and narrow-head construction screws should never be used to install CBUs. All board edges must be supported and fastened according to each manufacturer’s instructions, with special consideration given to how close to the edge each brand can be safely fastened. In my opinion, fasteners should be a minimum ¾-inch from the edge: for wall installations, this requires that each edge be supported by the full width of a standard 1-½-inch wide 2x4 or 2x6.

For efficiency, the author clamps a stack of backer boards for pre-drilling with a carbide bit.

Fiber Cement Boards/Underlayment (FCB)

Like their CBU cousins, fiber cement boards are available in 3x5 and larger panels 1/4"- to 1/2"-thick. This type of board also has a high compressive strength and is appropriate for residential and light commercial tile installations. The fine-grain composition of this type of board makes it relatively easy to cut and fabricate using score-and-snap methods (Photo 6). One brand is produced with a 1-inch indented grid of lines designed to guide the typical cement backer board carbide-scoring tool. The surface of the same board has indents to suit the board’s fastening schedule. FCB panels snap-cut with less difficulty and with a cleaner edge than CBUs; nevertheless, I prefer to use power shears for greater efficiency.

FCBs have the same fastener restrictions as CBUs, but generally, FCB panels can be fastened much closer to the edge, and without fastener split-out, than CBUs. FCB panel edges must also be fully supported and fastened. I use a mag-load screw gun to install FCB panels, and find that the extra push an installer is able to generate with this tool flushes the screw head 99% of the time. Nevertheless, as with CBU panels, when working with soft woods, I may have to pre-drill and countersink.

A ¾” carbide drill bit makes an exceptionally durable countersinking tool.

The surface of this type of board does not seem to exhibit the cracking seen in hard CBUs, but its surface is a thirsty one, and requires wetting with a sponge prior to applying thinset mortar or a membrane gel, paste, or liquid. This will prevent suctioning and a possible reduction or loss of bond. Another potential problem with FCBs is a result of the board’s manufacturing process, which directionally aligns the fibers that give the board its cohesive strength: this alignment is also responsible for promoting capillary action up the long side of the board. Moisture barely penetrates this type of board when it hits face-on, but when it enters along the lower edge of an FCB fastened to a wall, special attention needs to be focused on the wall/floor movement joint to prevent absorption. At the minimum, if a membrane system will not be used, the lower edge of this and all other tile backers needs to be covered with a bonded bead of silicone or other approved sealant (a membrane system’s gel or paste can also be used).

Sealing the lower edge, alone, will not prevent this board from becoming saturated. Like CBUs, fiber cement boards require a complete, realistic waterproofing system when used in wet areas. In my opinion, behind-the-board waterproofing methods using tarpaper or thin plastic film not only are not effective, they promote absorption, saturation, structural penetration (and the damage that can cause), and they provide excellent accommodations for mold, mildew, and unwanted pathogens. In addition, behind-the-board materials are punctured with the nails or screws, along whose paths moisture finds a handy escape route to the structure. The only practical alternative is to waterproof with a made-for-tile surface-applied membrane that can flex and bend with the structure.

Resources:
  • TCNA, ANSI Handbooks available through - www.tileusa.com
  • CBU courtesy of US Gypsum - www.usg.com
  • FCB courtesy of James Hardie - www.jameshardie.com
  • Nobleseal TS courtesy Noble Company - www.noblecompany.com
  • Screw Gun courtesy Simpson Strong Tie - www.strongtie.com
  • Photos courtesy Mike Mesikep and Paul Winn