Installing Stone Tiles
by Michael Byrne
August 8, 2008
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To complete a shower floor, the author uses old copper
patterns to shape these granite tiles cut from 12-inch stock.
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Art
& Decorative Tile Techniques
According to many historians, people used stone tiles long
before ceramic tiles came into existence. Slate and other thin stones have been
used as stepping stones, floors, countertops, and roofing since pre-history. In
fact, stone has been a part of human life since early man first picked up a
stone tool. Today, stone in many forms is as popular as ever, due primarily to
technological improvements in stone processing and finishing machinery and
tooling. Low-cost, pre-finished stone countertops from China are flooding the
market, and premium stones from around the world are both helping to expand the
stone market. Stone has a long and enviable reputation for strength and
durability, but unless it is selected, installed, and maintained properly, it
can suffer like any other construction or finishing material.
Selecting Stone Tiles
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To provide a perfectly smooth floor with these soft, honed marble
mosaic tiles, the author uses an 80-grit carbide grit belt sander that is
normally used for wood projects.
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While it may appear that all stones are the same, nothing
could be further from the truth. In fact, compared to the known properties of
manufactured ceramic tiles, stone tiles exhibit an infinite range of
properties, not all of which are desirable for construction or tiling. For
construction, the use of stone is divided between two camps: dimensional stone
and stone tiles. Generally speaking, dimensional stone is thick – about
2-inches – and it is self-supporting; that is, it does not need additional
support other than some form of attachment to a structure. Stone tiles are
generally less than 3/4-inch thick, usually around 1/2-inch, and always require
mounting to a solid backing such as concrete, mortar bed, backer board, or
other approved setting bed. There are numerous industry standards and tests for
dimensional stone, but few are specifically focused on stone tiles. The Marble
Institute of America acknowledges that the properties of some dimensional
stones, under testing and actual use, may not translate to tiles. For example,
some tests require a 2-inch thick sample: impossible to comply with 1/2-inch
thick tiles. For this reason, when considering an untested stone for a project,
I prefer to submit stone tile samples to ASTM tests developed for use with
ceramic tiles.
Stone tiles are an internationally
traded product with voluntary standards that include ANSI, ASTM, ISO, and
numerous other designations of quality assurance. For unqualified or untested
stones selected for any wet-area, exterior, or commercial project, stone tiles
should be lab-tested. The following is
not a complete list of tests appropriate for stone tiles, but it is the first
group that I turn to whenever performance is an issue: Moisture absorption,
ASTM C373; Chemical resistance, ASTM C650; Stain Resistance, ASTM C1378;
Freeze/thaw resistance, ASTM C1026; Coefficient of friction, ASTM C1028; Break
strength, ASTM C648; and Bond strength, ASTM C482. Some stone tiles are tested according to the following
dimensional stone tests: Water absorption, ASTM C97 (ASTM C121, for slate);
Compressive strength, ASTM C170; Abrasion resistance, ASTM C241; Coefficient of
friction, ASTM C1028; Modulus of rupture, ASTM C99. Ask your supplier to
explain.
For interior, residential, dry-area
installations, practically any stone tile can be used with good success, but
for exterior, commercial, or wet-area applications, only stones that have been
tested to withstand the rigors of a particular use should be selected.
Limestone, marble, slate, and granite are the four major types of stone sold
today and each type has it own range of properties. As well, among any given
batch, each stone tile will exhibit an appearance and a range of properties
that is unique to that tile alone. Many stones can be damaged by exposure to
water, direct sunlight, freezing conditions, pollution, food acids and other
conditions, so if an installation you are contemplating has special
requirements, make certain the stone tiles you select have been tested and are
rated for the intended use. It is always possible to find stones at a lower
price, but it is foolish to install any stones not robust enough for the
application because repair and replacement costs often amount to three or four
times the cost of the original installation.
Preparing the Setting Bed
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This corner detail, made by cutting 12-inch tile stock into
slices and radially-cutting the floor tiles, gives an idea of the creative
possibilities of stone tiles. All photos courtesy of Mike Mesikep.
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Stone tiles are sensitive to excess deflection, and
therefore need a setting bed whose surface meets L/720 deflection limits. This
is half the acceptable deflection for ceramic tile installations, and accounts
for the non-homogenous nature of many stones, plus the presence of cracks,
fissures, and other features that give stone tiles there distinctive look and
appeal. Usually, stones with an abundance of such desirable features are
considered to be more fragile, while stones with a homogenous composition tend
– but not always – to be more robust and more appropriate for exterior or
wet-area interior installations. There are a few stones that can meet the L/360
limits, some that need L/1080 setting beds, and some that need L/480 – it’s all
over the map with stone tiles, and each stone will have its own unique
properties and structural requirements.
The typical wood framed floor, made
with 2x10’s installed on 16-inch centers and covered with a 5/8-inch plywood
sub-floor and a properly installed tile backer board, makes a minimal base for
ceramic tiles, and is inadequate for most stone tiles. I prefer to glue and
screw (100% glue coverage with Type I or II glue with 1 5/8-inch
corrosion-resistant screws every 6-inches) a layer of 3/4-inch plywood over the
subflooring, instead of thinset mortar and a backer board, and cover the
plywood underlayment with a crack isolation/waterproofing membrane system. Of
course, a perimeter movement joint is required whether or not a membrane system
is used. A nominal 2-inch thick, reinforced mortar bed, also with perimeter
movement joint and installed over 3/4-inch plywood subflooring should also
provide good support for stone tiles. Some self-leveling underlayments can be
used as long as the deflection limits are suited for the stone tile being
installed.
Fabricating and Installing Stone Tiles
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The contoured and bullnose edge of this BBQ top is made from
12-inch slate tiles that were cut on a regular wet saw and bullnosed with a
sander using 120-grit discs.
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Stone tiles have one advantage over most ceramic tiles in
that with a small investment in tooling, an installer can make bullnose and
other surface trim on-site. Bullnose can be roughed out with a profile wheel
mounted in a wet saw, or by eye, using dry or wet-cutting blades, and then
finished with wet or dry polishing pads. If the stone tiles are particularly
soft with a honed, rather than a polished finish, power sanding tools and discs
or belts made for wood or metal can sometimes be used to shape and smooth stone
tiles.
Stone tiles must be cut with a power
saw and diamond blade. A dry blade can be used to cut tiles to a specific size
or shape, but wet-cutting blades and saws last longer and impart far less
thermal shock. For straight cuts, there are blades for both hard and soft
stones. I avoid multi-purpose blades, and instead, use a specific blade to suit
the material at hand, and ensure that the coolant water is exchanged frequently
to avoid excess blade and pump wear. For large on-site installations, some
municipalities require a system of settling tanks to capture water-born stone
dust and chips and separate it from the coolant water.
For floor-only installers, no other
equipment, besides blades, a profile wheel, and polishing tools, are needed for
most installations. If you are lucky to be close to a trustworthy stone
fabricator, you can contract with them to do bullnose or other profiling work.
Working with stone tiles, only, is relatively simple for most installers, but
making the jump to working with slabs and panels requires significantly more
skill, and a greater investment in tools, facilities, and support.
As for the actual installation of
most stone tiles – unless you plan to grind and polish the surface – you will
get the best results if you use a medium bed thinset mortar. This can be
applied in a much thicker layer than regular thinset mortar, it helps lift the
tiles above irregularities in the floor, and it helps account for slight
differences in tile thickness. For best results, there should be 100% contact
between the tile and the setting bed, with no visible ridges of adhesive
remaining after the tiles are beat in. To achieve this, I recommend that
installers spread an even amount of thinset mortar on the setting bed, and use
a mortar box to back-butter each tile with another even layer of adhesive. For
12- to 16-inch stone tiles, I use a 1/4x3/4 U-notch trowel to apply thinset to
the setting bed, and a 1/4 by 1/4, 3/8, or 1/2-inch square notch trowel to
back-butter the tile (larger, if needed).
Because the granite tiles I prefer
to use in wet or exterior areas inhibits the curing of the adhesive layer, I
give these installations an extra day or two to dry and cure before grouting
the joints, and another day or two before the movement joints can be filled
with a resilient sealant. If the stone tiles are to receive a penetrating
sealer or impregnator, make certain that enough time elapses so that all
installation materials are fully cured and dry before the treatment is applied.
Resources:
- For ANSI and TCNA Handbooks in both English and
Spanish, contact the Tile Council of North America at: www.tileusa.com.
- For more information about working with
dimensional stone and stone tiles, refer to the "Dimensional Stone Design Manual," available from the Marble Institute of America at:
www.marble-institute.com.
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