If installed properly, natural stone will often outlast the structure of your home itself. Often, professionals are called upon to install natural stone. This is usually the case for specialty applications like granite countertops or marble steam showers.
The first and foremost factor to consider when installing natural stone is support. Stone is more brittle than ceramic or porcelain, and will break under far less stress. It is therefore critical that the support material beneath the stone have enough strength to hold the weight of the stone, and everyone and everything on top of the stone, without bowing or deflecting enough to cause the stone to break.
Stone varies in strength from type to type (i.e. granite is much stronger than marble, marble is stronger than slate) and even from variety to variety (i.e. Paradiso granite is generally stronger than Absolute Black granite). There is little practical way for the consumer to know the specific strength of the stone he has purchased and is going to install. Therefore, the Marble Institute of America recommends that all natural stone be installed over a subfloor that meets a minimum deflection standard of L/720 (meaning the floor will not deflect more than 0.02 inches or half of a millimeter when a heavy load is placed on it).
Most homes built after 1985 have floors which are well above the minimum deflection for natural stone, though some do not. Older homes often do not meet the minimum standard. Do NOT install natural stone without making sure your floor is strong enough, even if it means cutting a hole in the floor to look at your joists. Skipping this step may cause you to lose the hundreds or thousands of dollars you spent on stone tiles when they start to break as you walk over them.
Once you are confident that your joists will support your new stone floor, you will need at least 1” of plywood or OSB on top of your joists. Many professional contractors prefer 1 1/2”. If you have the standard ¾” plywood floor, simply add another layer of 3/8” plywood, making sure the borders do not match up with the layer beneath. On top of this, you will mortar ¼” cement backer board, and secure it to the plywood subfloor with moisture-resistant screws.
Before choosing a size for your grout lines, consider practicality as well as aesthetics. In the past, many stone installations involved laying the tiles directly against each other, with no grout lines. While this may look attractive, most modern professionals recommend at least 1/16” grout lines in order to help secure the tiles and ultimately to provide a stronger installation.
Natural stone cannot be scored and broken like ceramic tile. Also, due to natural faults and flaws in stone, it often cannot be “nipped” like manmade tile without severely damaging the stone. A diamond-blade tile saw is almost a necessity, and these can be rented by the hour or the day. In instances where holes must be cut in the center of tiles for shower pipes or electrical outlets, trace the hole on the tile with a pencil and drill small holes around the pencil line with a masonry bit. On slippery polished tile, a piece of duct tape over the area to be drilled will help keep the bit from wandering. Once you've drilled around the perimeter of the hole, gently tap on the inside of the hole until the waste piece breaks out. Then smooth the edges of the hole gently with a file or sandpaper.
Due to unevenness in the thickness and size of natural stone tiles, it is often advisable to use a medium bed mortar, instead of the more readily available thinset mortar. Medium bed mortars can be laid thicker without losing strength, and thus accommodate differences in tile thickness. Using the trowel size recommended by the mortar manufacturer, spread the mortar onto the cement board. Set the stone tile into the mortar and press down, rotating back and forth, until the tile is set firmly into the mortar. Then pry it up and check for even coverage of mortar on the back of the tile. If the coverage is good, comb back through the mortar and replace the tile.
As you continue through the installation, you may find it necessary to “back-butter” some tiles which are thinner than others. The use of a small spirit level from tile to tile will indicate if your installation is level, and will show if a tile needs an additional layer of mortar back-buttered onto it to raise it enough to be level with the adjacent tiles.
Likewise, if a tile is particularly thick and seems to sit above the rest, a 2x4 wrapped in a towel can be rested atop the tile and hammered down to force the tile deeper into the mortar below.
Natural stones with dramatic differences in thickness, like slate or tumbled marble, may require the use of a 50- or 75-pound carpet or tile roller. Install the tiles as recommended above, then roll over the tiles carefully with the roller to flatten and level the entire installation. This procedure takes an extreme level of caution to prevent tile breakage. Ask at your equipment rental facility for specific suggestions.
If the stone you are installing is porous, like slate or travertine, it is highly recommended that you seal the stone before grouting, to make cleanup easier. Two coats of sealant may be necessary with extremely porous tiles.
Choosing a grout may be the most difficult part of your installation. Polished stones like granite and marble necessitate the use of an un-sanded grout, to prevent scratching the finish. This will also require that the grout lines be 1/8” or less. Honed tiles and slate may be installed with sanded grout, and the grout lines can be larger if desired. Epoxy grouts are also becoming increasingly popular. These waterproof, stain-proof grouts are much more difficult to install, but make an excellent choice for countertops and showers.
Follow the grout manufacturer's instructions religiously to ensure a proper installation with even color. The use of distilled water to mix and clean grout will dramatically improve your chances of ending up with perfect coloration. Cleaning off grout haze from natural stone can be more challenging than removing it from ceramic tile. Vinegar may be used for stubborn grout haze, but NEVER on calcareous stones like marble or travertine. If your haze is much harder to remove, or if efflorescence (salts leaching out from the grout and causing discoloration) appears, it may be desirable to use a stronger acid, such as sulfamic acid. However, you must carefully read the manufacturer's directions, and test the acid on an extra piece of tile to ensure that the acid does not etch polished finishes. And remember that acid and marble, travertine, or limestone DO NOT MIX!
Once the grout lines have cured, it is advisable in almost every application to seal both the stone and grout lines. There are dozens of sealers on the market, and most contractors agree that you get what you pay for. A good sealer will keep your stone beautiful and stain-free, as well as make it easier to clean, so don't skimp here. For countertop applications, make sure the sealant is food-safe. In outdoor, shower, and countertop applications, you will likely need to reseal once or twice a year, if not more. Floor and wall installations may not need such frequent re-sealings, depending on traffic and manufacturer recommendations.
For basic floor and wall applications, installing natural stone is not too different from ceramic tile. It takes a little more care, but is certainly not beyond the capability of the weekend warrior. You may feel that your stone tiles are very delicate while you are installing them, but once they are down, they will provide an extremely durable, resilient flooring or surfacing that will add far more value to your home than manmade materials.