Often architects and designers choose stone based only on its color properties. Those who sell and work with stone should better understand the proper selection to aid the architect and designer. In selecting any stone for high-traffic commercial applications, there are several factors that must be considered.
This is perhaps the most important requirement for stone in high traffic areas. There is an ASTM C 241-Abrasive Resistance test designed to test the wearability of a particular stone by placing a sample into a testing machine and weighing the loss of material because of abrasion. Do not confuse this test with hardness. A stone can be very hard, but at the same time not wear well.
Certain stones have numerous veins. While this can be attractive, caution must be taken. The more veins a stone has, the more brittle it will be. The reason for this is simple geology — veins are areas where minerals have flowed and deposited. In many cases the veins are softer than the surrounding stone and are subject to erosion and wear quicker than stone with fewer veins. Examples of this are Rain Forest brown, green marble, Trani Fiorito and Rosa Perlato. Some stones have fillers that are applied at the factory to fill in veins and voids. These fillers are generally Portland cement-based and subject to wear. A good example of a filled stone is travertine, of which there are numerous types available. Some have very little fill and others have a lot. Stones with a lot of filler should be avoided in high-traffic areas.
There are several finishes available for stone, ranging from highly polished to very rough. Please note that certain finishes are not available for certain stone types.
Polished – This finish has a highly reflective surface and requires the most maintenance. To keep a high sheen, it constantly needs to be re-polished and properly maintained.
Honed – A honed finish is a matte or satin finish with little to no reflection.
Flamed or Thermal-Finished – A thermal finish is often applied to granite and some limestone. It has a rough texture created by passing a flame across the surface, causing pieces of it to pop out. Marble is not available in this finish.
Sandblasted – A sandblasted finish is a rough, dull finish achieved by literally blasting the surface with sand. Sandblasted finishes are very porous, and require a sealer to keep clean.
Hammered – Striking the surface with a hammer and imprinting it with the pattern of the hammer’s face achieves a hammered finish. Hammered finishes are commonly found on granites and limestone.
Natural Cleft – A natural cleft finish, found on the stone as it is taken from the ground, is often left on slate or sandstone.
Sawed – This is the finish produced by sawing. It is characterized by circular saw cuts on the surface.
Tumbled – This rough finish, made by tumbling the stone with abrasives, is usually only found on marble and limestone. There are also several new finishes coming out, including wet jet and antique.
Many stone tiles and slabs arrive from the factory with fiberglass or resin on the back to prevent breakage during shipping. This is an indication that you are dealing with a brittle stone that breaks easily. Extreme caution must be exercised when supervising installation. Water-based setting materials will not provide the proper bond unless a 100 percent latex additive is used. Make sure to follow the specification in this manual.
When specifying stone tile, consider the width of the grout joint. Marble and limestone are soft and scratch if sanded grout is used. Also, make sure the grout width does not exceed in. Granite can handle sanded grout and larger grout widths. Epoxy grout should be used in any food areas, restrooms, steam showers, etc. Epoxy grout is stainless and does not bar- hot bacteria. It is therefore the grout of choice in these areas.
The top 10 failures with most stone installations have to do with the substrate the stone is bonded to. It is extremely important that the substrate be prepped and designed to handle the type of stone. The following are some general tips:
1. Perform a vapor emission test to make sure there is no excess vapor emitting from the stone. Excess vapor can cause rapid deterioration of the stone as well as create efflorescence problems. Make sure That the slab does not exceed 3 to 5 lbs. of vapor over a 1,000-sq.-ft. area in a 24-hour period. The test to specify is ASTM F-1869.
2. Hairline cracks and control joints should not be bridged or covered. A fracture membrane should be specified if the slab exhibits any cracks.
3. It is recommended that an expansion joint be installed directly over any control joints or cold joint separations. If not, the stone installation will crack.
4. Many curing agents and accelerators prevent the proper bonding of stone and tile installations. It is important that all curing agents, accelerators or other topical coating be removed before the installation.
5. It is highly recommended that an independent expert be hired to monitor the installation to make sure it is being installed as specified.
6. A mock-up showing the installation technique and the range of color should be displayed before proceeding with the overall installation.
7. Wood subfloors should be reinforced. Deflection should be greater than L/360.
8. Stone tile shouldn’t be set directly to a wood substrate. Use a mortar bed or concrete backer board.
9. Wood subflooring should be at least 1¼- in.-thick.
If a stone installation meets another material such as wood or carpet, a proper transition strip should be used. This transition should be at the same height as the floor it is transitioning.
Unlike ceramic tile, set stone with white mortar or thin set, especially light colored stone. Grey mortar may stain. Use epoxy setting mortar on all green (serpentine) based marbles.
Lippage is the height relationship from one tile to the other. Lippage shall be specified not to exceed 1/32 in.