A surface-applied membrane creates ideal conditions for a tiled shower stall. Here, the author uses a short straightedge to flatten a two-inch overlapping seam on a backer board floor.

Like the two cement boards mentioned in the first part of this article series (TILE Magazine, March/April 2009 issue), Coated Glass Mat Water-Resistant Gypsum Backer Board (WRG) is available in a range of sizes and thickness. When properly specified and installed, and covered with an appropriate tile installation, WRG boards offer good value combined with relatively easy cutting and fabrication.

Cutting is done as you would any other type of gypsum board, using score-and-snap methods. Though the cutting method may be the same, the core of WRG boards is made from waterproofing agent-treated gypsum filler. Unlike regular gypsum, or the brown stuffing found in typical MR boards, the core of the board will not deteriorate when exposed to moisture, and its surface is protected with a waterproof coating. When installed according to the manufacturer’s instructions, this type of board may carry a significant warranty.

WRG boards are easy to handle, cut, and install, requiring only hot-dipped, galvanized roofing nails, but in my opinion, WRG boards should be confined to residential or dry commercial applications only. In addition, because of their relatively low compressive strength, tile size is an important consideration since larger tiles are capable of distributing loads over a wider area: one-, two-, or three-inch tiles may not even be allowed. For an extra margin of safety, 8-, 10-, and 12-inch tiles provide much higher compressive strength and longer useful life for a WRG installation.

It is good that WRG panels have a waterproof coating, but like behind-the-board membrane systems, this waterproofing is punctured with every fastener (and every mis-hit!), and because it has been treated to resist moisture, the core’s gypsum filling is no longer able to swell and seal off around the fasteners. For years, I have prodded WRG manufacturers to market an accompanying material – a patch or a paste – to fix the inevitable jobsite problem of sinking the nail head too deep, or dinging the board with a mis-hit, but so far, the manufacturer continues to recommend an approved tile membrane to cover boo-boo’s, or when a WRG board is specified for use in a steam shower.

Because of their low density, coated foam backer boards require washered fasteners to prevent pull-through.

Cementitious Coated Foam Boards

The core of this type of board (CFB) is a layer of machined foam skinned with a reinforcing mesh and parget, or coated, with a thin layer of mortar, applied as an attachment layer for the tile adhesive. These boards first gained popularity in Germany, where they were designed for use over concrete structures as insulated attachment planes for ceramic and stone tiles and other hard surface finishes. CFB boards are available in all the usual sizes and in a surprising range of thickness – up to two inches; the thicker boards are used to fabricate countertops, partition walls, and benches. CFB panels can be cut using score-and-snap methods, but because the foam core has such a low density, it’s easy enough to slit through the entire board with a sharp blade. The core is so soft that the board requires fastening with large diameter (about an inch) washers to increase the board’s rather low pull-through resistance.

A distinct advantage of the better grades of CFB panels is that their foam cores are both water-resistant and waterproof; nevertheless, this advantage is punctured each time a fastener punches through. The resulting pathway allows moisture seeping past the unsealed washer to reach the structure. For this reason, I recommend that once ready for tile, a CFB setting bed should be completely covered with a waterproofing/crack isolating/sound reduction membrane system suitable for use in residential wet areas. A vital issue here is compatibility: some sealants, including silicone, can dissolve certain types of expanded foam used in construction. Check for compatibility with both manufacturers (sealant and board).

CFB panels have great potential when properly specified and applied, but in my opinion, they are better suited for light duty installations and always require the addition of a tile membrane system. As well, these boards require rather high minimum tile sizes – 3- or 4-inches. Twelve-inch tiles or larger are recommended for greater load dispersal and load carrying. The selection of a membrane is an important consideration since some of these boards give off a hollow sound – even when the tiles installed above have been properly embedded in thinset mortar with 95- to 100% adhesive contact. Because of their rather high sound transmission values, CFB panels used on walls– showers or tub surrounds – should be waterproofed with a membrane system that also has sound reduction properties. One property unique to this type of board is its ability to isolate tiles from movement in the substrate, and that property, alone, makes this type of board a reliable performer. Nevertheless, every aspect of its installation must include – like every other type of tile setting bed – a system of perimeter movement joints as described in TCA Handbook under EJ171.

All backer board edges must be supported by framing members: fasteners too close to the edge can split some tile backers.

Installing Tile Backer Boards

For all wall installations mounted with nail- or screw-on tile backer boards, the same structural, support, and tolerance criteria apply, with surfaces above the minimum standard of ¼-inch in 10-feet being easier to tile and yielding more attractive finishes, and surfaces below the standard more difficult to cover and yielding an unattractive and unprofessional finished appearance.

Of particular importance is a network of movement joints – especially the joint between the walls and floor of a shower. For best results, and because fasteners always leave a pathway for moisture to reach the structure, all wet-area backer board installations should be covered with an appropriate surface-applied membrane system.

On floors, backer board fasteners should be long enough to grip the backer board to the subflooring. Fasteners should not be long enough to tie the backer boards to joists, and should be the type that is made specifically for the brand of board being installed – not drywall screws or ring shank nails. Fasteners may vary from board to board, but an essential part of the floor installation for all types of board is the support bed of thinset mortar spread on the subfloor. Not to be confused with a leveling plane – which provides a level surface for attaching a membrane or tile – the support bed of thinset is required to minimize air space beneath the boards that can significantly reduce a floor’s compressive strength. Ideally, a board’s function is to act as a stable attachment plane for membrane or tile.

Backer board screws are specially heat-treated and coated for use with cement backer boards. The vanes under the screwhead are designed to countersink the board so the screwhead can be installed flush with the surface.

Installing a backer board over a bare subfloor, or over ribbons of construction adhesive or dabs of sealant ignores the advice of every tile backer manufacturer to use thinset mortar as a leveling plane. Seams and joints need to be reinforced before they are covered with tile, using compatible materials installed as flat as possible. It is highly recommended that the installation of all backer boards be delayed until just before they are needed. This approach ensures that the usual job site environment won’t contaminate the good bonding surfaces of the board.

Not all tile backer boards are alike! Each brand of board has its own unique properties, selling points, and limitations. The practical needs of the installation should dictate board selection, but except in small, dry-area interior applications, few boards, alone, can provide the tiles with a stable base without an isolation membrane, and for the best waterproofing practices, no backer board is complete without an appropriate membrane.


Additional Resources to Consider: