To help eliminate air pockets between the boards and setting bed, a support layer of thinset is required.

Tile backer boards were invented as a response to a dwindling supply of tile installers who could produce mortar beds, but unlike properly built beds of thick, reinforced mortar, backer boards do not provide structural strength. Instead, they are attachment planes to which tiles can be securely bonded.

Not all backer boards are the same. There are five basic types of backer board: Fiber-Cement Underlayments (FCU); Cementitious Backer Units (CBU); Coated Glass Mat Water-Resistant Gypsum Backer Board (CGB); Fiber-Reinforced Water-Resistant Gypsum Backer Board (FGB); and Cementitious Coated Foam Boards (CFB). All of the above boards have different compositions and properties, but all can be used in wet or dry areas and all are suitable for use on floors, walls, and ceilings. Generally, FCU and CBU boards have the highest compressive strength and can be used with tiles of any size, while the other boards have lower compressive strengths and may not be suitable for use with smaller tiles.

Floors must be clean, with screw or nail heads flush with surface.

Substrate Tolerances

One misconception about backer boards is that they can level a floor surface. Quite the contrary, backer boards follow dips, high spots, and undulations, and are incapable of leveling an out-of-level floor or flattening a floor that is not flat. The flat/level industry minimum standard for subfloor surfaces is 1/4-inch in 10-feet. This is acceptable for tiles up to 10-inches, but for larger tiles up to 16-inches, a tolerance of 1/8-inch in 10-feet is better. Some very large tiles, 16-inches or larger, may require an even tighter tolerance. Instead of expecting any type of backer board to level or flatten a subfloor, an installer should use other means, such as carpentry or the use of a self-leveling underlayment (SLU). Often, on a floor where backer boards have been specified but an SLU is required to level or flatten, backer boards may not be needed.

After combing the thinset, the installer drops the boards into place and positions them 1/8- to 1/4-inch apart.

Cutting Backer Boards

Cutting backer boards ranges from relatively simple to difficult depending on the composition of the board. Some CBUs are somewhat soft and may be cut with score-and-snap methods using special scoring tools, while others are quite hard and are more practically cut with a dry-cutting diamond blade, but this practice creates a tremendous amount of dust. An alternative to a diamond blade for cutting cement backer boards is a scoring tool specifically designed for use with cement boards. Unlike using a utility knife which require four, five, or more scoring passes, a backer board scoring tool requires only two passes on fiber cement boards and three for CBUs. Foam and gypsum boards can be easily cut with a sharp knife and score-and-snap techniques.

Positioning and Installing Backer Boards

The placement of backer boards used for floor installations often determines whether or not an installation will last. By that, I mean that spacing between boards and between boards and abutting surfaces is critical. Most boards require 1/8- to 1/4-inch spacing between boards, and 1/4-inch where the boards meet surrounding walls or other hard surfaces. When boards are butted together with little or no spacing, tiles installed above are highly likely to crack. When boards are butted up against walls or other restraining surfaces, tiles often shear off the surface of the boards.

Installing backer boards over a floor that does not meet minimum standards affects the appearance of a floor, but installing backer boards without a support plane of thinset mortar will greatly affect a floor’s performance. Most backer board manufacturers specify dry-set thinset mortar for the support plane, but others may specify a latex-modified thinset mortar. With either thinset, it’s important to lay down an even layer. For most boards, this is done with a 1/4-x1/4-x1/4-inch square notch trowel.

Unlike spreading thinset mortar as a tile adhesive, which requires an installer to use the flat side of the trowel first, then comb out ridges of mortar, thinset used as the support plane for backer boards can be spread with the notched side only. The reason? The support plane is not used to laminate or glue most boards to the subfloor, but rather eliminate air between the board and the subfloor that can cause tiles to collapse from loading once the tiles are installed. Since dry-set thinset mortar will not remain attached to the subflooring, the tile/backer board assembly - including movement joints - is able to accommodate some deflection without damaging the tiles. The best way to achieve an even layer is to spread and comb the thinset, and comb it again to remove any excess that could cause bumps or high spots to appear.

Fastening Backer Boards

Cement backer boards can be fastened with hot-dipped galvanized roofing nails or approved backer board screws only. Electro-galvanized roofing nails, ring-shank nails and drywall screws should never be used and are never permitted by board manufacturers: the very thin electro-galvanized coating can be damaged by contact with cement boards, the small heads of ring-shank nails have little or no holding power, and drywall screws made specifically for use with drywall, can rust when exposed to water or the alkali materials found in thinset mortar.

As well, drywall screws do not have cutting vanes that cut a countersink in cement boards like backer board screws. This means that the hard composition of cement boards will prevent the heads of the screws from being flush with the surface. As a result, in an attempt to force the screws, about 25% of them will break off. With this much visible breakage, an installer using drywall screws should think about the condition of the screws that do not appear damaged.

Screws can be installed one at a time, but this is tedious work and screws are hard to start on cement boards. If you have to install them one-by-one, you can save considerable time and energy by having your helper set the points with a hammer. The idea is not to drive the screws home, but rather sink the point into the surface of the board, and finish up with a screw gun. The quickest way to install backer board screws, though, is to use a screw gun designed for the purpose and loaded with collated screws. I use a stand-up model made by the Simpson StrongTie Co. The Quik Drive PROCGB combination system contains a screwdriver motor plus two attachments that drive cement boards and gypsum boards. An extension in the kit allows stand-up fastening and ease of use on floor installations. The collated screw strips are easy to load, and they fasten boards with incredible speed. Labor savings on even small floors will more than pay for itself.

Backer board joints are reinforced with alkali-resistant mesh tape.

Reinforcing Backer Boards

Finally, backer board joints require reinforcing. Fiberglass mesh tape used for drywall will rot out quickly when exposed to thinset mortar. Backer board joints should only be installed with alkali-resistant mesh tape. Reinforcing tape can be installed as soon as the boards are installed, or just before the tiles are set. When installed before, the self-adhesive tape is laid down over each joint, and then covered with thinset mortar. When installed with the tiles, thinset for the tiles is first troweled and combed, the tape is laid over the thinset, followed by the tiles.