When is Close, Close Enough?

An earlier version of this article originally appeared in the Spring 2023 edition of Building Stone Magazine.

Precision stone floor and wall jointing of the lobby of 7 Bryant Park, New York, New York designed by Pei Cobb Freed & Partners Architects. Photo by Fernando Guerra

GATHERING DIMENSION STONE TOLERANCES ALL IN ONE PLACE HELPS INDUSTRY PROS FIND GREATER ACCURACY.

“We live in an inexact world,” says Charles “Chuck” Muehlbauer, technical director for the Natural Stone Institute (NSI). “The stone industry is part of this inexact world.” Being not quite exact when it comes to bowling might mean you roll a spare or even a gutter ball instead of a strike, but being not quite exact when it comes to building and construction could at best look horrible or at worst cause injury. It’s Muehlbauer’s job to help fabricators, design professionals, installers, and others in the stone and building industries do their jobs accurately and safely. It’s all about tolerances, which define the acceptable limits of inexactness.

To that end, Muehlbauer recently helped revise and update the Dimension Stone Design Manual, a 500-page document that he calls, “our ‘Great Big Book of Everything.’” In earlier versions, information about tolerances was filtered throughout the book. Now, thanks to Muehlbauer and his team, tolerances have their own chapter, “Tolerances in the Dimension Stone Industry,” nine pages that explain how tolerances are written and interpreted. The workhorse of the chapter is the four pages of tables.

Muehlbauer, with the guidance of a 16-person volunteer committee including quarriers, fabricators, installers, and design professionals, worked hard to make the information consistent with what was already published by the Indiana Limestone Institute (ILI) and the European Committee for Standardization (CEN). They looked at the American Concrete Institute (ACI) standards as well as the ASTM International standards and information from the National Building Granite Quarries Association (NBGQA). Prior to publishing, the chapter was reviewed by 21 different professionals from the United States and abroad. “It is one of the most extensively peer reviewed and thoroughly vetted of any document that we’ve ever done. It’s also one of the most frequently cited,” Muehlbauer says, adding, “I know of quite a number of companies that will actually attach a copy of it to their purchase order as part of their agreement when they’re purchasing stone.”

In fact, Muehlbauer says, the document is often used for dispute resolution. “If a specification or a purchase order or a contract lists tolerances different than what is in this document, then those tolerances take precedence. But if a project doesn’t cite the document or list its own specs, the document will usually prevail if there’s a dispute.”

STONE INDUSTRY TOLERANCES

WAYS TO EXPRESS TOLERANCES

The “extremely comprehensive” chapter, as Muehlbauer describes it, covers types of tolerances, how to comply, applicability of the document, and charts of stone tolerances divided by sector—fabrication, installation, and substrates.

The chapter outlines five ways to express tolerances. It should be noted that all five expressions can be written to describe the same limits of acceptance, just with a different syntax and perhaps a slightly different emphasis as to what is important. The two types of bilateral tolerances are the most frequently used in the stone industry, Muehlbauer says.

Symmetrical bilateral tolerance. One of two types of bilateral tolerance, allows the same amount of overage and underage from the target. It might be written as +3 mm.

Asymmetrical bilateral tolerance. The second of the two bilateral tolerances, allows the overage and underage to be off in both directions and by different amounts, i.e., the target can be 1.5 mm shallower and 3 mm deeper (-1.5, +3 mm).

Unilateral tolerances. Here, the deviation is allowed in only one direction.

Limit tolerances. These prescribe the minimum and maximum dimensions allowable without defining an actual target dimension, e.g., a panel must be at least 27 mm thick but not more than 33 mm thick.

Percentage tolerances. They can be bilateral or unilateral and simply state the percentage of variation that’s accepted; e.g., a panel should be 30 mm plus or minus 10%. The percentage tolerance is a little different in that as the stone gets thicker, the tolerance would grow if the percentage were kept the same. Expressing tolerances this way is more common in Europe than in the United States.

CUMULATIVE, NON-CUMULATIVE OR DEPENDENT TOLERANCES

Cumulative, non-cumulative or dependent tolerances.

There are other tolerances to consider such as cumulative, non-cumulative or dependent.

Panel size tolerance and panel squareness are not cumulative so both the sizing error and the squareness error must be contained within the one allowable deviation.

But sizing error and installation positioning error are cumulative by necessity. As Muehlbauer says, “You can’t ask a stone setter to hold a 1/32″ joint tolerance when he or she is given +/- 1/16″ panels.”

The document also considers the instruments necessary to comply with tolerances. While a common tape measure may be adequate for most dimension checks, “it’s the poorest system,” Muehlbauer says. “It can get knocked out of tolerance pretty easily at the end of its hook. A fixed rule, like a metal ruler is both more durable and more precise, but it’s not as easy to read as a caliper or micrometer, which would be the most accurate. Those instruments also have a tolerance, and they should be checked and recalibrated at the manufacturer’s suggested interval.”

It’s the charts that Muehlbauer says are probably the most useful aspect. For example, in the fabrication section, the tolerance issues covered are thickness, face size, squareness, flatness, edge conditions, and anchor preps. In installation it’s lippage, joint width, and alignment. Substrates covers surface plane tolerances.

PUTTING THE STANDARDS TO THE TEST

“We probably get most of our calls about joint widths and lippage,” Muehlbauer says. Lippage, when one tile, for example, has a planar offset from the adjacent tile, “is commonly disputed from a quality-control standpoint. And there are 11 different types of lippage tolerances depending on the product and the installation.”

Joint widths. There is a separate tolerance for stone-to-stone joints and for stone-to-non-stone joints. In a countertop, if the countertop is stone to stone, the allowance is plus or minus 25% of the joint width, and not less than 1/64.” But if it’s stone abutting another product, the tolerance allowance is ±1/16.”

Lippage. The image above, center, shows a textured surface lippage measurement, and due to the profilometry of the textured surface, the tolerance is by necessity greater than that of a smooth surface. “Textured surface lippage tolerance gets to be complicated,” Muehlbauer says.

Alignment. There is huge joint misalignment on these textured stones, bush hammered and cleft surface. “Not only does it look like hell, there are anchors in these joints. Wide joints can reduce the anchors’ capacity, while tight joints can restrict accommodation for dynamic building movements.” Muehlbauer says. “In most cases, this kind of work just looks sloppy.”

ADHESIVE COVERAGE

The images show where a stone paver has been removed from its setting bed, photographed, imported into CAD, then stretched to scale and traced around the voids. The resulting voids shown in the black and white images confirm the installed paver met the 80% required adhesive contact spec, but showed a lack of adhesive in the corners, that ultimately did not meet spec.

One of the most common issues in adhered stonework is the coverage of the bedding adhesive. Muehlbauer begins by removing some stone units, then taking digital photos from a normal (perpendicular) angle. The photos are then imported into CAD software and stretched to squareness and scale. From that point, the software does the rest of the work with Mass Properties calculations resulting in an extremely accurate measurement of the percent of void space area in the bedding.

The above images show Muehlbauer’s methodology of verifying compliance with adhesive coverage tolerances. In this particular case, two installed stone pavers were removed from the floor. Muehlbauer took a digital photo of the setting bed, imported that photo into CAD software, corrected it for squareness, stretched it to scale, and traced around the voids. At that point the CAD software takes over and calculates the area of the void regions using a Mass Properties function.

There are different specifications for these installations. The photos above were of a kitchen installation, which is not considered to be a “wet” area, and the stone was nominally 20 mm thick. The specification for this type of installation would be for a minimum of 80% contact area with no individual void greater than 4 in². Had this installation been done in stone tile thickness (<20 mm) or in a “wet” area, the specification would be more stringent, requiring a minimum adhesive contact area of 95% with no individual void greater than 2 in². In either case, voids are not allowed to occur at the corners or edges of the stone units. Muehlbauer likes putting things into practical terms to visualize the prescribed limits more easily, as he points out that for voids which are roughly circular in shape, an area of 4 in² or 2 in² would represent the size of a racquetball or golf ball, respectively.

THE STANDARDS IN ACTION

A stone industry veteran and full-time NSI employee, Muehlbauer has experience in fabrication, installation, anchorage design, stone selection, industrial stone applications, laboratory testing, stone forensics, and both domestic and international trade. Muehlbauer has a methodology and process for when he’s called to examine a project, often one in which work is being disputed.

Muehlbauer, or any of the technical team personnel at the NSI, can be retained by any of the partners on a project. (This service is available to anyone, not just NSI members, but the fee structure is different.) “The standards are the standards,” he says, “and our work may bolster the argument for an installer a designer or owner.” In some instances, Muehlbauer is asked to defend the work of his client, only to discover that the work is substandard and not worthy of such defense. In these cases, after discussing the issue with the client, the client will often request that no report be written, at least until the work can be corrected.

Once Muehlbauer comes to a decision, the resolutions can be binding, but they’re “usually not,” he says. “It can get messy. Our goal is to settle the dispute, without things ending up in court.”

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