Selecting and Installing Tile Backer Boards, Part 2
by Michael Byrne
June 2, 2009
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| 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. |
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Art
& Decorative Tile Techniques
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.
Cementitious Coated Foam Boards
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| Because
of their low density, coated foam backer boards require washered fasteners to
prevent pull-through. |
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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.
Installing Tile Backer Boards
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| All
backer board edges must be supported by framing members: fasteners too close to
the edge can split some tile backers. |
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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.
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| 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. |
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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.
Resources:
Additional Resources to Consider:
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