Perfect stone slabs: Do they really exist?

January 25, 2006
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Figure 1: Gauged edges to eliminate lippage at underside of overhang.

A few large companies exist in the countertop industry with the capacity to perform both the sawing/face finishing of the stone slabs and the final fabrication of the finished countertop material. This scenario is indeed the exception to the normal course of business. The typical arrangement is for the fabricator to purchase the slabs, already sawn to nominal thickness, with one surface finished. The fabricator then continues the refinement of the product by sawing to finished sizes, applying the appropriate edge treatments, and making necessary accommodations for mechanical equipment and anchorage. The fabricator, in effect, is then the party in the contractual chain that insulates the sawyer of the slabs from the final customer. When different importing, brokerage or wholesaling companies are involved, there can be several companies contractually between the slab sawyer and the end user of the countertops.

How then, does one address the end user's complaints when these complaints are the result of slab quality? Variation in slab thickness and variation from true plane (warpage) are two common causes of homeowner complaints. Neither of these issues are under the control of the countertop fabricator. We then look for an industry-adopted standard - one that has been based on consensus opinion - to establish the limits of variation allowable for this product.

Click image for larger view. Figure 2: All slab surface profiles shown above would be compliant with the NBGQA's Flatness Tolerance.

Because we do not live in a perfect world, all dimensional standards must include tolerances, or allowable deviation from the targeted dimension. In the case of granite slabs, the National Building Granite Quarries Association (NBQGA) establishes these tolerances. The NBGQA's current edition of their Specifications for Architectural Granite lists the allowable variation in panel thickness for thicknesses between 3⁄4 and 15⁄8 inches (20 to 41 mm) as +/- 1⁄8 inch (+/- 3 mm). The Marble Institute of America Dimension Stone Design Manual adopts this same tolerance. Considering the accuracy capability of modern shot-sawing equipment, this tolerance seems extreme. And one could easily note that this tolerance was established for slabs to be used as granite building panels, and not granite countertops. Yet, this is the only industry-established tolerance for this particular condition. Unless more stringent dimensional control was established and agreed to in the purchase documents, all slabs delivered with +/- 1⁄8 inch (+/- 3 mm) of the nominal dimension would be deemed acceptable by industry adopted standards.

Will the homeowner accept this degree of variation in the slabs used in their countertops? Not very likely, at least not at exposed edge conditions. Additionally, the Marble Institute's recently revised Residential Stone Countertop Installation guide limits the slab-to-slab variation on the underside of an exposed edge to 1⁄32 inch. To the fabricator, the only means of making these slabs acceptable to the homeowner and compliant with the MIA's installation guide is by machining the underside of the overhanging portion of the slabs to a uniform thickness (see Figure 1). All slabs must be accurately measured to determine the thinnest slab in the project, and the exposed edges would need to be ground down to this thickness. When the fabricator does not have the capability to do this, or if this is simply cost prohibitive, the only options involve modifying the standard. As with any standard, the involved parties have the right to agree to a more or less stringent tolerance, provided the agreement is properly documented. In this case, that agreement can occur between the seller of the slabs and the fabricator, where the seller agrees that the sawing tolerance will be less than the +/- 1⁄8 inch (+/- 3 mm) prescribed by the NBGQA. Or the agreement can occur between the fabricator and the homeowner, where the homeowner can agree to accept greater than 1⁄32 inch of variation between adjacent slabs.

Figure 3: MIA's Residential Countertop Installation Guide limits lippage to 1⁄32 inch at center of countertop.

A similar issue occurs with the flatness of the slabs. The NBGQA establishes allowable variation from a true plane to 1⁄16 inch (1.5 mm) when checked with a 4-foot (1.2 m) straight edge, placed in any direction on the slab (see Figure 2). In the MIA's recently revised Residential Stone Countertop Installation guide, the maximum lippage allowed between adjacent slabs is 1⁄32 inch (0.8 mm) (see Figure 3). A 1⁄16-inch warp in 4 feet would generally be less than 1⁄32 inch in a standard countertop width, but this tolerance is applicable to both concave and convex warps. It is entirely possible to have contra-directional warps adjacent to each other in the installation. This problem is magnified when the slab width is of a greater dimension, as in the case of an island top. When using 20-mm slabs, it is sometimes possible to bend the material sufficiently to eliminate, or at least reduce, the warp. In 30 mm materials, this is unlikely, and in such cases the only available remedy would be to grind and refinish the slab surface to eliminate the lippage. In some materials, it is not possible to replicate the same gloss levels as the original line polish on the slab, so this remedy may actually make the problem more noticeable than if left untreated.

While the issue of thickness tolerance in slabs is one that can be addressed with improved equipment, the warpage of slabs cannot be so addressed. Warpage of stone slabs is a naturally occurring condition in the slabs. Often times, the force required to eliminate the warp of a slab approaches the stress at which fracture occurs. A technology that controls warps would be welcome, but I don't see it arriving any time soon, if at all. Diligent customer communication and education will go a long way in reducing complaints due to warps. As a last resort, some stock culling may be necessary to produce acceptable results in the finished installation.

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