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Most
manufacturers recommend a vapor membrane behind their products in wet area
applications to avoid excessive vapor accumulation in the wall cavity. However,
in some areas of the country, building code may provide other guidance due to
geographical differences. |
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There are numerous types of backer boards available. What
makes one backer board better than the other is the varying feature each
provides. Some are inexpensive, a few are very forgiving of installation error,
there are those that are waterproof, others lightweight, and a few made in a
very environmentally friendly manner. The following are some of the more
commonly known product types using their industry definitions and appropriate
reference standards. The American Society for Testing Materials (ASTM)
designations are included for reference as they are increasingly referred to by
building code officials.
Cementitious Backer Unit: Some of
the better-known examples of this type of product are Durock, Permabase,
Util-A-Crete, and Wonderboard. They are defined as a cement substrate designed
for use with ceramic tile in wet or dry areas. Most are available in various
lengths, widths, and thickness. This material can be applied over studs and
subflooring. Ceramic tile can be bonded to it with modified and unmodified
thinset, organic adhesives and epoxies. There are specific use recommendations
numerous installation details contained in the Tile Council of America
handbook. Industry specifications for interior installation and material specifications
are contained in ANSI A108.11 and ANSI A118.9 or ASTM C-1325. For exterior
applications, consult manufacturer’s written instructions.
Coated Glass Mat Water-Resistant
Gypsum Backer board: The better-known example of this type of product is
DensShield. It is designed for use on floors, walls, and ceilings in wet or dry
areas. This material is applied both vertically and horizontally, directly to
wood or metal studs or a suitable subfloor. Ceramic tile can be bonded with
modified, unmodified, or organic adhesive. Follow the manufacturer’s
instructions for installation and joint treatment. This is critical where
waterproofing applications are desired.
Fiber-Cement Underlayments: Some
specific examples of this type of product are made by Hardibacker, and
CertainTeed. This panel is a dispersed fiber-reinforced cement backer board and
underlayment designed for use with ceramic tile in wet or dry areas. Available
in various lengths, widths, and thicknesses, this material can be applied
vertically over studs and over code-compliant subflooring. Ceramic tile can be
bonded to it with modified or unmodified thinset and organic adhesives. General
interior installation and material specifications are contained in ANSI A108.11
and ASTM C-1288. Consult the manufacturer’s written literature for more
specific application details.
Cementitious-Coated Foam Backer
board: Some specific examples of the type of product are Easy Board and Wedi.
This particular product category has a wide range of products. Some are
constructed from extruded polystyrene and coated with fiberglass mesh and
cementitious coating and offer waterproofing properties. They are designed as a
substrate for ceramic tile in wet or dry areas on floors or walls. They are
available in various lengths and thickness. Extruded material can be applied
over studs and sub flooring and bonded with modified or unmodified thinset or
epoxies. Other types of foam boards are expanded polystyrene with a cement
coating which does not provide waterproof properties and is limited to wall
only applications. As there are no current standards for these products,
manufacturer’s instructions should be strictly followed. There are several
details for this product in the TCA handbook.
Fiber-Reinforced Water-Resistant Gypsum Backer
Board/Underlayment: is backer board/underlayment conforming to ASTM C1278. Some
specific examples of the type of product are USG Fiberock and CertainTeed
ProRoc. Designed for use on floors, walls, and ceilings in dry or limited water
exposure areas, this board is applied directly to wood or metal wall studs or
over wood subfloors. Ceramic tile can be bonded to it with latex/polymer
modified portland cement mortar, organic adhesive, or epoxy by following the backer
board manufacturer’s instructions.
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Yes,
there is a reason you should pay more for backer board tape. Regular drywall
tape is not alkali resistant and will not provide adequate reinforcement if
subjected to movement. Cement is very alkaline. |
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While there are additional backer boards on the market
available for various applications, they currently are either proprietary or
single source in nature or too new to warrant inclusion in the Tile Council of
America Handbook or American National Standards. Strict adherence to
instructions is well advised, as no tile industry performance standards exist
for products not meeting the above descriptions. Plywood panels, while not
considered a backer board due to their inherent lack of dimensional stability,
can also provide a suitable surface when properly selected and installed. OSB
of any type is not considered a suitable surface for tile.
When it comes to backer board
installation, all panels share some basics but many other recommendations are
panel specific. Manufactures offer warranties based on their specific
instructions, not industry recommendations. Industry recommendations only
provide for a basic overview. There can be no opinions what works if you wish a
warranty to apply. If you desire to apply your own installation
recommendations, you do so at your own risk. Let’s explore some basic concerns
and recommendations that nearly all backer board products share.
In floor applications all panels require a bedding coat.
Recommendations may vary on what to use but universally everyone agrees the
panels need to be fully supported, hence the term bedding. This not a leveling
coat! A level coat would mean when a level is placed on a floor, the bubble is
in the middle. Many end users mistakenly assume that the floor will be level
often after hearing misused terminology. When you speak of tile floors, the
preferred term is flat, not level, there is no level requirement for ceramic tile
floors. Thinset is the product of choice for bedding the panel by all
manufacturers with few exceptions. A common but misguided practice is to glue
the panel down with a tube of construction adhesive. This is not a bedding coat
and a poor choice for a bonding coat. Coat means covered with a thin layer.
Long-term: this coat for bedding the panel is important to the performance of
the installation. The benefit of full support is not realized in the
short-term. There will always be inconsistencies in wood floor construction
resulting in a less than flat surface. While these irregularities may be minor,
they can and will result in cracked grout joints, which may cause bonding loss
or possibly cracked tile.
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Numerous
recommendations and details for various backer board products can be found in
the Tile Council of America Handbook. More than 25 new methods have been added
in the last three editions. It is available at www.tileusa.com. |
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Now for the most hotly debated item, should it be bedding or
bonding coat? I have done both. In the perfect world a wood floor system would
be both be flat and fully support the underlayment panel. All of this
supporting structure would meet the L/360 deflection recommendation by the tile
industry and requirement by manufacturers. The floor joist span is rarely a
problem as building code dictates it meet a minimum of L/360 under the
anticipated load. However, building code does not provide for deflection
requirements for subfloor panels unless specifically called out in the building
plans, a rare occurrence. If tile floors were not part of the design in the
original plans (think upgrade), then it is doubtful that spans were shortened
and sheathing thickness increased. Tile and backer board generally weigh about
7-8 pounds.
A basic house design typically
provides for a dead weight (weight of the structure) of 10 pounds. So the basic
support issue is the structure needs some additional rigidity such as thicker
subfloor panels and/or bigger joist, and shorter spans to accommodate the added
weight. This may be accommodated to a degree by using a material to bond as
well as bed the backer underlayment panel. Using a flexible mortar, you can
both increase the rigidity of the system and allow for minor movement that is
naturally occurring in all wood structures. Not all modified thinsets are
created equal in this instance. Some manufactures require the use of an ANSI
A118.11 EGP thinset specifically for warranty to apply. This is a very specific
product and not all modified thinsets (ANSI A118.4) will meet the requirements.
The down side to this remedial
measure is backer boards were not designed nor intended to be structural panels
and provide very little structural value, but bonding the panel can offer
provide minimal assurance in what could otherwise be a marginal installation.
The quality of thinset product used under the panel in this type of
installation will have a direct correlation with long-term performance. When
the structure is properly designed and both the sheathing and joist system
fully support the floor system, bedding, not bonding the panel will increase
the longevity of the tile installation by not subjecting the finished floor to
normal stresses of the structure.
Manufacturers are specific about what type of fasteners to use
and where to put them. Once again, you should consult the manufacturer’s
literature for the product you are using but a few things have almost universal
agreement. Fasteners should never penetrate the floor joist! The purpose of
using an underlayment is to provide a surface not subjected to the stresses of
the structure. All underlayment manufacturers recommend that the panels be
offset from the subfloor below. Recommendations vary from 2 to 8 inches
depending on the product. All fasteners should be corrosion resistant and have
a minimum of a 3/8” head diameter. Galvanized roofing nails work well in most
installations. There is a caution though on the type of galvanized fasteners.
Many of the nails readily available are known as EC roofing nails or electric
coating. The quality of this plating can vary widely and in some backer boards,
may be removed as it passes through the panel. In wet areas this can be
problematic and result in a rust stain appearing in the grout or tile if the
installation is exposed to moisture. This can be avoided by using HD or “Hot
Dipped” fasteners. Some prefer to use a roofing nailer for speed and ease of
installation. There are many pneumatic nailers out there that will do the job.
In that instance, the caution is not only the plating (HD nails are available)
but also the way the gun is used. Air nailers do not pull the sheets fully into
the thinset. The nose of the gun must be firmly applied before you pull the
trigger to firmly set the panel. When using screws, backer board screws are
specific. If you look closely you see what looks like a hat rim. This is an
integral part of the holding power. The fastener must pull down tightly without
penetrating the surface. In most instances, penetrating below the surface of
the product renders it ineffective. Occasionally staple guns are used for
fastening backer boards. History has shown this to be a mixed performance
method. Some swear by it, some swear at it. In a floor or wall application you
are on your own if that is your favored method. Taping the backer board joints
has never been a fun job but to achieve product performance, a necessary evil.
Alkali resistant tape is recommended because cement is alkaline and may cause
standard fiberglass drywall tape to separate and degrade the reinforcing value.
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In
wet area applications the general recommendation is to NOT grout in the panel.
Grout will absorb water and cause wicking of moisture into the panel. The edge
should be caulked with a sealant prior to tile installation. |
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The tile industry has a position on minimum subfloor
requirements that may or may not be shared by manufacturers of product. A
manufacturer’s product can certainly exceed the minimum performance
requirements set forth under standards, which may allow for enhanced
performance. Manufacturers offer warranties, the industry does not. That being
said, industry recommendations are 5/8 CC or better tongue-and-groove panels
for 1/2” backer board and 3/4” CC tongue-and-groove or better for 1/4” panels.
These plywood or OSB panels must be rated for subfloor application and all must
be properly gapped, fastened, and acclimated to service conditions prior to
installation. This may sound like a perfect world recommendation but real world
problems can result from excessively wet or dry installations. If the house has
crawlspace, effective ventilation or a vapor membrane must be in place. Lack of
a membrane or ventilation is a common mode to failure in areas that use
crawlspace construction. Given the structures ability to carry the load, a
double layer floor system prior to backer board application certainly will only
improve the support of the floor system but properly constructed single panel
floor systems will perform adequate support for backer board panels.
It is hard to cover all the nuances and do’s and don’ts of
backer board in an article. Each product carries specific recommendations
several pages or greater in length. We receive a fair amount of calls and
emails on backer board issues and this article attempts to cover the issues or
questions most often received. The question of what is the best backer board
cannot be answered based solely on product performance. Rather it must be based
on what the purchaser or specifier is trying to achieve. One also should have
the ability to distinguish between relevant and valid performance needs rather
than marketed performance values. Paying $25 a sheet for a waterproofing foam
board makes perfect sense in some instances. Then again, paying $10 for a
gypsum panel with a waterproofing surface makes sense in others. Just have to
sort out the overall goal of the application.