QUESTION

What is the allowable variation in thickness between two different stones? I recently sold roughly 450 square feet of material, half White Thassos and the other half Absolute Black granite, both 16x16x3/8 polished, for a checkerboard style entryway in a vintage Victorian home.

The client (a designer showroom) approved our samples as being sized and gauged the same, and the order was fulfilled from the same lots. Now, upon receipt of the materials, she is telling me the black is (I assume to the naked eye) about 1/16" thicker than the white.

My question, off the record of course, is this a "normal" issue when dealing with two different materials, from two different quarries and two different factories, and furthermore, can it be easily rectified by a competent installer, using a quality medium bed mortar, or a mud set installation?

ANSWER

ANSWER - The MIA (Marble Institute of America) has standards for tolerance of fabricating stone from one supplier or shipment. They have a category of Thin Stock (3/4” to 2”) thickness that is limited to +/- 1/32”, but don’t specifically reference 3/8” thick material. Although I would expect that gauged stone from one supplier should be within that +/- 1/32” tolerance for thickness. Sizing is generally standardized within the industry, but there are not actual specific standards for stone thickness. The stone tolerances will vary between producers depending on the equipment they use and how strong/fragile the stone might be relative to withstanding the fabrication process and shipping. They will make the stone thicker for larger and more fragile tiles. So generally speaking sizes are nominal and can vary. When combining two products from two separate suppliers it would require the due diligence to verify sizing and tolerances of the two stones are compatible. It is not uncommon to have some stones provided that are ungauged or that have more variation in its sizing. If the facial dimensions vary a lot you have to have larger grout joints. If the thickness varies a lot you have to either install it in a fresh mortar system, or if the thickness variation isn’t too excessive they can use a medium bed thin-set mortar. Using a medium bed thin-set mortar, whose name is being changed to Large and Heavy Thin-set Mortar by industry standards, it should easily compensate for the 1/16” thickness variation between the two tiles. Although the installer will incur more cost in thin-set mortar and it will take more time and effort to install the tiles in an acceptable manner to not exceed 1/32” maximum lippage between the edges of adjacent tiles.