Journal of architectural technology published by Hoffmann Architects, specialists in exterior rehabilitation Third Issue 1997 Volume 15 Number 3 Thin-Stone Veneers Offer Unique Design and Performance Demands relatively new product known as rein- Theodore F. Babbitt, AIA forced stone veneer is also available. Made of very thin stone veneers N ew stone-cutting technologies laminated with epoxy to a reinforcing have allowed for ever-thinner stone expanded-steel mesh, these reinforced veneers to be used as a curtain-wall stone panels come in 5/16" and 3/16" component on high-rise and other thicknesses and offer the strength of buildings, a radical departure from the stone veneers twice their thickness. traditional use of dimension stone as This new technology was developed in a load-bearing element or a heavy- Turin, Italy and to date has been used in weight, non-load bearing facade. But Europe and New York City. while these veneers offer an excellent design opportunity at a cost lower An Intrinsic Danger than dimension stone, relatively little While the engineering principles and data is available on their long-term physical properties of dimension stone performance. And problems are are well understood and tested by starting to crop up after just two time, thin-stone veneers offer a new decades of use. Owners, architects, realm of design, engineering, fabrica- and builders should be forewarned tion, transport, and installation when choosing thin-stone veneers: demands. Testing prior to installation precise detailing is essential to may show no problems, but under prevent displacement, cracking, real-world conditions, veneers may spalling, water intrusion, and, ulti- begin to display unexpected and mately, failure. serious potential for failure. Actual failure has occurred in many cases. Thin-stone veneers are commonly defined as dimension stone cut to The very thinness which makes thicknesses of 2" or less (dimension veneers so economically attractive is stone is typically more than 2" thick). also the instrument of their downfall. As with dimension stone, veneers are That is because stone’s relative available in granite, limestone, marble, thickness has a direct effect on its sandstone, slate, and travertine. A performance. Thin veneers suffer more than dimension stone — and Thin-stone veneer being prepared for face- pinning. perform differently — under the Mr. Babbitt is an expert with more than onslaught of thermal movement, 45 years of experience in design and gravity, wind, seismic, and handling technical issues for exterior rehabilitation. loads, and with the settlement of As Director of Architectural Services, he surrounding building materials. oversees stone, curtain wall, and roof Veneers are also more subject to wear rehabilitation services for Hoffmann from environmental conditions. Even Architects. the building interior becomes more 1 Copyright Hoffmann Architects 1997 Hoffmann Architects / Journal Common Problems creating weak points. The most common source of prob- lems in thin-stone cladding comes Stone texture can also play a role in from: performance. A thermal finish on • Anchorage failure due to placement granite veneer, for example, reduces of the kerf, slot, or hole too close to its thickness by up to 1/8th of an inch. the edge of stone. This will cause the In turn, that loss can reduce bending stone to break and lead to loss of strength by up to 30%, increasing the securement to the back-up structure. risk of cracking under seismic or wind • Anchorage failure due to corrosion, loads. Thermal finishes also increase material failure, incorrect installation, the risk of water infiltration, as the or unanticipated shear force (e.g. dramatic temperature differential An anchor set in a slot in the top of a deflection). created in applying the finish can cause thin-stone panel. • Anchorage failure due to warping or micro-cracking within the crystalline bending of the stone. structure of the granite. Those micro- prone to moisture damage when a • Cracking and spalling due to cracks can allow water to penetrate up thin-stone cladding is used. There is insufficient joint width or the use of to 1/4" deep into the stone, leading to simply that much less between the rigid grout or epoxy where a flexible cracking and spalling through the stress building interior and the elements. sealant should have been used. Both of freeze-thaw cycles. A similar Dimension stone is also far better able situations fail to accommodate thermal weakening effect occurs when brown- to handle the wide variances in or dynamic movement. stone is traumatized by bush-hammer- tolerances of companion building • Cracking and spalling due to water ing, a type of decorative finish. materials. The connections between infiltration behind the stone, which will panels require new and complex also lead to damage to the back-up Handling and fabrication concerns techniques. Maintenance demands are structure through subsequent freeze- Thin-stone veneers are particularly more critical. All of which requires thaw cycles. subject to handling damage, which can expert knowledge on the part of the • Cracking and spalling due to occur at the quarry, at the panelization design and construction team. improper installation, including over- drilling, or micro-cracking caused by Image and Endurance dry-sawing and dry-drilling. So why use thin-stone veneers at all? Simply put — aesthetics, performance, These common problem areas, and and economics. Stone, whether thin others, are looked at in more detail Kerf or thick, is the hallmark of classical below. design — timeless, monumental, and enduring. As a design medium, it Fabrication issues offers the architect and owner a stellar Stone is essentially a brittle material array of color, texture, and finish that will crack, spall, or shatter under choices, and can be used for exterior stress. How well stone performs over cladding, interior flooring and walls, time is as much a question of its Slot and ornamentation. Yet it’s not all interior composition as it is of its beauty. Stone provides a stable intended use. Strength, density, and thermal mass which is energy-efficient, composition can vary widely, even relatively resistant to environmental with the same type of stone. Natural damage, and easy to maintain. With weak points within the stone must be the advent of thin-stone veneers, all analyzed and addressed in both this becomes much more affordable to fabrication and design to reduce the Hole the building owner, as the lighter risk of cracking along rift planes. The weight of the stone will mean savings stone must be cut at the proper angle on structural costs. and to the correct dimension to avoid Illustration A: Anchorage Types 2 Volume 15 Number 3 or cutting plant, during transport to on high-rise buildings, as the panels Anchorage the job site, or during installation. The can be installed from the interior. Anchors connect the thin stone to the risk of cracking increases when the These panels frequently serve as structure of the building — the steel stone is transported flat or when lifted spandrels or in-fill panels on curtain or concrete spandrels and columns from a horizontal position. Even walls. The stone is attached to the which are the ultimate support of any vehicular vibration can cause damage. back-up (the structure of the panel) facade. Choosing the right anchorage Proper packing materials should be using stainless steel pins set in epoxy. is vital to the performance of thin- used to cushion the stone during stone veneers. Adhesives alone are transport. When a pre-cast concrete back-up is not enough to secure the stone to the used, the pins can be set in the stone, building. Even in the case of panelized Hysteresis with the concrete then cast against stone, the pins create a positive Most thin-veneer stones remain the stone to create a monolithic mechanical anchorage to the back-up dimensionally stable, even with thermal panel. For aesthetic reasons, “false” panel. In general, anchors should be expansion and contraction. Some joints are often created between made of stainless steel to avoid relatively pure, fine-grained marble stones that are attached to the same corrosive interaction with any metallic veneers with uniform texture, how- panel. In most situations, the panels elements that may be present in the ever, may gradually expand with each span one structural bay, and are stone. thermal change, a condition know as secured at the column lines to allow hysteresis. This “growth” occurs in the columns to directly carry the There are two types of anchors used minuscule increments, which nonethe- gravity load. with thin-stone veneers: anchors which less add up over time, causing the are inserted into kerfs and anchors veneer to cup or warp. That in turn Less commonly used are stone panels which are inserted into holes drilled increases the stress on both the “glazed” directly into mullions (such into the sides or rear of the stone anchorage system which holds the as the technology from Turin, Italy panel. (A kerf is a narrow slot cut veneer in place and on the supporting described earlier) or exterior stone longitudinally into the edge of the structure. If analysis of the selected tile set with tile techniques. The stone panel, and is used to hold an stone panel shows a propensity to latter is not recommended due to its anchor which secures the stone slab to hysteresis, specifying a thicker panel lack of durability. its support backing. Illustration A on can help mitigate its effects. Design and Installation Issues There are three basic installation systems used in thin-stone cladding: 1. Hand-set stone with mortar joints and anchors that connect to the structure. This system is more typical of thicker dimension stone systems, and more likely to be found with a concrete or masonry back-up structure.