StoneDimensions

Q: We’ve been asked to rout out the bottoms of a granite overhang at an outdoor kitchen and epoxy in a 3” x 3/8” strip of steel to reinforce the stone. We’re nervous about routing out so much of the stone in that we think it could make it more fragile – has this been done before?

A: A variety of embedded reinforcement measures have been attempted in natural stone, some of which have been successful and others which haven’t. The particular description that you provide makes me share in your concern. First, it is unlikely that the 3” x 3/8” deep flat stock will actually prevent fracture of the stone. It may prevent a catastrophe, in that it would keep the overhanging stone portion from falling to the ground if it did fracture. But it wouldn’t be rigid enough to make a sufficient support contribution prior to fracture of the stone. My second concern, similar to yours, is that routing out a large, rectangular cross-section of the stone does compromise the strength. In a brittle material, any abrupt geometrical discontinuity creates what is called a “stress riser”.

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Last week, the MIA and Stone World held 2 successful installation seminars in California. Both were well attended and well received. Here is a recap of the topics covered and some pictures from the events:

Daltile's Danny MacNair highlights several points about selecting the right stone during the first phase of the MIA/Stone World Installer Seminars.

He really pushed several key points:
- with each finish option available, know the pros and cons
- understand that any backing on a stone will influence the materials needed for the install -know the stone's absorption rate if the stone is being used in a wet area
- the selection of larger format or mixed stones will impact installation techniques
- presealing light color stones may be necessary when considering the pigments from the grout used

He concluded his section of the seminar by stressing the need to discuss installation techniques and care/maintenance issues during the stone selection process.

Mike Micalizzi (Custom Building Products) led the second section of the seminars. He focused on a number of fundamentals, as well as best practices and problem resolution. A number of take aways further showcased the value of this seminar:

- substrates must be sound, true and smooth
- importance of rigid L/720
- value of a high performance mortar
- use of rapid set mortars for some stone types
- give your stone a break (eg expansion joints)

He fielded many questions and kept the attendees engaged. MIA vice president Jim Hieb said, "it is great to watch a speaker gain immediate respect, based on their vast knowledge and connection with an audience ---- Mike did a great job today."

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Q: We’re having trouble interpreting the tolerances for stone panels. In the thickness tolerance, it says “± 1/8” – is this both directions so that the total tolerance is actually ¼”? And then in the face tolerance, it says ”non-cumulative” – what exactly does that mean?

A: Dimensional tolerance is indeed one of the most common areas of questions that we field. Interpretation of them is far from uniform, and trying to reconcile them between different associations and standards writing bodies makes it even more complicated. Then add the conversion factor required because the United States rarely uses SI (International System of Units) units of measure, and we’ve got the perfect recipe for confusion.

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Q: We’re installing a mesh-backed stone tile interior wall project. The architect is insisting that we use epoxy thinset, since the MIA Design Manual says that is required. We think this is overkill – is there any way around using the epoxy?

A: We have addressed this on a case-by-case basis on a number of occasions. The mesh is applied using an adhesive that is most commonly epoxy-based, but can also be of polyester or other adhesive families. As a general rule, portland cement based adhesives do not bond well to mesh-backed stone units. Within any industry, there is a constant race between standards and technology, in that when technology advances, the standards must be adjusted to address the performance advantages of the new, technologically improved products. This is exactly such a case, in that a few of the modern thinset adhesives actually do produce a serviceable bond to the resin adhesive coated mesh-backed surfaces. Particularly in the case of a wall, when there are no traffic loads on the tile, there may in fact be a portland-based thinset available that achieves adequate bond without having to use epoxy based thinsets. Unfortunately, the testing and evaluation needs to be done on an individual project level, since we have no control over what mesh is used or what adhesive was used to apply it. So while a combination may exist that is workable using a given thinset and a stone from a given supplier, the same thinset may not produce the same results on stone from another supplier.

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Q: We recently submitted shop drawings on a small cladding job. We didn’t specifically locate the anchors. We just showed them on the detail and noted “anchors located at 1/4 points, typical”. When the shops came back, the note was changed to 1/5th points. We have no problem doing this, but we’ve always used 1/4 points as a rule of thumb – is there some reason that fifths would be better?

A: Quarter-point anchor positions seem to be a rule of thumb for many in the industry, and they likely stem from the traditional, classic building designs of years ago. If one looks at the stone cladding in these classic buildings, one will note that it was common, if not standard, to use a staggered jointery pattern (similar to a running bond in brick masonry). In this case, quarter-point anchor positions make sense, because when an anchor is shared between two courses which have 50% offset, quarter-point positioning in one course is also quarter-point positioning in the next course. And because the stone panels used at that time were generally thick, cubic stones, bending stresses were a nonissue.

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Q: We’re proposing the supply of slate pavers for exterior walkways. These will receive pedestrian traffic only. We’re planning on suggesting ¾” thickness, but in the MIA’s Dimension Stone Design Manual it calls for 1¼” minimum thickness for any exterior pavement. Is this an absolute minimum, or simply a rule of thumb?

A: It’s more of a rule of thumb. There are probably many more examples where an exterior paver needs to be thicker, but there are some cases where thinner is acceptable, and the installation that you describe is perhaps one of them. Paver thickness is influenced by the traffic loads, the paver unit size, the stone’s bending strength, and the type and rigidity of the bedding.

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Q: At what point is a stone no longer “natural stone”. We’re working with more and more slabs that have backers and fillers and resined faces. It seems as if we no longer have the right to call them “natural”. Where do we draw the line here?

A: There probably isn’t going to be consensus on this, although I know where I draw the line in defining “natural stone”. At StonExpo, and possibly a few other venues during a typical year, I teach a very basic class covering rudimentary information on stone types, formation, mineralogy, properties, and definitions.

The class can serve as either a very informative primer for those relatively new to the industry, or it can serve as a sure cure for insomnia, depending on the student. At the start of that class, I offer a definition of natural stone as “Stones which have been harvested from their in-situ position in the earth, then cut and machined into final products without alteration to the internal fabric of the material.”

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Q: I’m an engineer that designs floor frame systems for ultra-high end custom homes. When I’m designing the floor frame, the finished surfaces usually haven’t been picked yet, but just by looking at the floor plan, I usually have a good idea which areas of the home are likely to receive tile or stone floor finishes. Where can I get a general idea of the deadloads that these floor finishes typically add? The second question I have is that I have an older version of your Dimension Stone Design Manual which limits deflection to L/720, but not to exceed 7/32”. In the current version, I no longer see the 7/32” limit; has this been dropped?

A: The best source to find assembly unit weights for a variety of residential finish floor systems is the TCNA Handbook for Ceramic, Glass, and Stone Tile Installation. In the 2012 version, Appendix B contains this information and can be found on pages 281 through 287. You will note that stone tile assemblies are typically a couple of pounds per square foot heavier than a comparable method using ceramic tile. Another adjustment that might be necessary is that some stones will be cut to slightly greater thicknesses due to fragility or unit size requirements. Certain stones, particularly black igneous rock types, have much greater densities than other stone varieties, so this will also influence the deadload. Of course, there are deflection limits and subfloor/underlayment requirements for stone that are more stringent than those for tile. So the safe thing to do would be design the floor system for stone finish, and then it will be adequate for either stone or ceramic, albeit a bit over-designed for the latter.

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Dick Laliberte, Ripano Stoneworks Ltd., Nashua, NH, visited MIA offices May 22, 2012.

He donated original 1957 and 1964 business appraisals of The Taylor Stone Company, which was established in 1898 as McDermott Stone Co., McDermott, Ohio. 

Ripano, an avid geocacher, was en route to Louisville, KY and the 10th annual GeoWoodstock event.

Geocaching is a high tech treasure hunting game, employing a GPS to hide and seek containers with other participants in the activity. 

Q: We specified stone pavers for the interior of our project. Our specification, which is largely taken from a boiler plate spec that we use for most projects, reads, “all stone units to have an eased edge.” The supplied pavers are nearly sharp, with a barely detectable blunting of the edge. The contractor says that this is the industry standard for eased. We thought the term “eased” implied a much larger chamfer, something in the range of at least 1/16”. Which is correct?

A: Unfortunately, the industry doesn’t know either, or we just can’t agree. In our last revision of the MIA’s Dimension Stone Design Manual Glossary, I called numerous industry members in an attempt to establish a consensus of the term’s definition, and I found that there were nearly as many answers as there were people to ask.

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