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.
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.
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.
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.
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.
- 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