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Substrate Considerations

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.

Transitions

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.

Setting Material

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

Lippage is the height relationship from one tile to the other. Lippage shall be specified not to exceed 1⁄32 in.

About the author:
This article is an excerpt from NTC’s new guide Selection and Design Criteria for Stone Tile in Commercial Applications. Frederick M. Hueston runs the National Training Center for Stone & Masonry Trades, Asheville, NC 28806,
www.ntc-stone.com, and can be reached at Fhueston@aol.com.