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

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.

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

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.

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

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.

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.