Stone Cladding as Preservation Challenge
By Sarah Ripple
The architectural use of stone changed dramatically with the development of modern steel frame technology in the late nineteenth century, which resulted in the transition from stone as a load bearing material to a thin veneer. As with every many technology shifts in architecture, thin stone veneer experienced a period of trial and error from 1949-1980, and it was applied without real precedence or a comprehensive understanding of how it would perform (as per my own definition stone veneer is less than 3" in thickness). This reduction in stone thickness for cladding purposes required a rethinking of the systems for attachment to the structural framework, materials and detailing, and the stone selection process. Consequently, the modern architecture movement parallels the period of greatest technological change within stone veneer development.
Modern thin stone veneer buildings, although now of appropriate age for landmarking or preservation efforts are not necessarily held in such high historical regard as Modern glass curtain wall structures. The preservation of these buildings has increasingly become an issue as the structures have reached or neared the end of their service lives. Sensitive repair or restoration of such a complex system requires a thoughtful, informed approach, and should include an understanding of the historic technology and aesthetic.
It is difficult to determine which first prompted the use of stone as a thin veneer - the technology or the aesthetic - as they seemed to have occurred concomitantly. Aesthetic preferences shifted from an architectural tradition based on historic, European architectural styles to those of the Modern movement. Heavy ornamentation gave way to a very monolithic, spare language, utilizing manmade, repetitive features; the craft aesthetic gave way to the machine. The book by Philip Johnson and Henry-Russell Hitchcock The International Style presented a new idea of the wall where "the skin or veneer of a structure should be detailed to express a thin, continuous surface," and materials such as "painted stucco or tile…aluminum, thin slabs of marble or granite, and glass, opaque and transparent" were called out specifically for use. This desire for clean, sleek facades devoid of any classical or traditional ornamentation, allowed stone, a conventional building material, to reemerge for a new, modern purpose.
While stone is one of the oldest building materials known to man, the increased complexity of the thin stone veneer system necessitates an extensive evaluation of the material as well as its intended application during the stone selection process. The Modern architecture movement was the first time period when stone was consistently used in 1 ½" - 3" thicknesses, and when the application of stone changed wholly from a load bearing to a thin veneer use, the desired natural characteristics for stone were also changed. Traditional, bearing masonry typically cubic in form could accommodate weathering and deterioration to an extent simply due to the sheer size; for this reason, soft materials such as sandstone and limestone, which were relatively easy to quarry, were commonly utilized. The impact of weathering is significantly greater, and the durability of stone is significantly more important. As thin stone veneer emerged, since it no longer carried any load, the important performance criteria transferred from compressive strength to the durability of stone as defined by the level of absorption, density, and flexural strength. Granite and marble succeeded as thin veneer due to their higher density and level of absorption characteristics while sandstone and limestone limited to thickness over three inches.
The decade of the 1960s represents a seminal point in the development of thin stone veneer systems when enough of the system's limitations were understood to have implemented many standards that still stand today, but much had yet to be fully resolved. The elemental issues of thermal movement, moisture intrusion, and structural loading, were understood fully in theory, and building designs from that period illustrated this by incorporating expansion joints, plastic jointing materials and configuration, and various types of moisture control. The materials and suggested locations for expansion joints have changed over time, but the concept of using plastic joints to accommodate thermal movement is a method still used in contemporary construction. Improvements were made on the early jointing materials, and the means for moisture control underwent revisions that continue today. The steady progress sealants have made since the 1960s has most likely prompted the greatest change in wall efficiency; the ability to both prevent water infiltration and maintain a plastic joint was a huge achievement. The anchorage systems, another technology that underwent great improvement, experienced a distinct increase in the level of adjustability between 1950 and 1980. Movement from thermal expansion and structural loading could be accommodated through anchors with adjustability, and veneer walls were made much more efficient when on-site adjustments could be made during construction. The progress in thin stone veneer technology from the 1960s to today was in the form of ongoing and incremental refinement, rather than total transformation, because many of the key aspects of thin stone veneer detailing and construction were established at that time.
However, missing at this time was empirical performance data due to the relative novelty of the system. Performance issues with early thin stone veneer wall systems arose in the decades following completion. Certain problems occurred due to inclinations inherent in stone. For instance, thermal hysteresis, a catastrophic stone phenomena where the stone shows visible signs of flexure and bowing, prompted the total recladding of Finlandia Hall (Helsinki, Finland; Alvar Aalto, 1971) and the Amoco Building (Chicago, IL; Edward Durell Stone, 1974), both of which were clad in a fine-grained, white marble. Other thin stone veneer structures have required less dramatic means for repair.
The buildings of Lincoln Center, clad in travertine, underwent a repair program not twenty years after completion due to cracking, soiling and decay, bedding plane delamination, and outward displacement of the stone. Many of the issues were a direct result of vertically orienting the travertine bedding plane, which allowed water infiltration through the length of the stone causing cracking and delamination from the interior; the decision to vertically orient the stone was made wholly out of the desire for a certain aesthetic. Fortunately, preservation efforts allowed much of the travertine to be saved using methods such as Dutchmen repairs for panels having only small pockets of decay and epoxy-injected stainless steel threaded rods and cramps to secure cracked panels. Some replacement was necessary for panels with large amounts of decay. The repair program began with a close up investigation of the buildings, careful examination of selected travertine anchorages, and laboratory analyses of the stone. The investigation was thorough and took every measure to explore the integrity of the system in its entirety. Other non-destructive methods of investigation are available and may be used when possible to assess thin stone veneer in a way sympathetic to the materials.
Cracking or outward displacement of panels are common occurrences and can be caused by thermal movement or improper structural loading. However, these problems are also potentially attributed to use of ferrous materials, inadequate quality control during construction, or lack of maintenance. Failure issues occur also simply because materials, especially caulking and damp proofing, have relatively short service lives. The caulking and weatherproofing will eventually weather and fail, and if the materials are not maintained or replaced accordingly, moisture will infiltrate and detrimentally affect the system. Eventual replacement of caulking and sealants is necessary and expected; a lack of diligence to maintain the material integrity can lead to much larger repairs and great preservation concerns. However, routine maintenance can also diminish the material integrity if employed without knowledge of the historic value and technology. Insensitive or uninformed repairs have the potential to harm historic buildings as much as inaction. Unfortunately, preservation standards from the Secretary of the Interior do not differentiate Modern masonry or thin stone veneer buildings from traditional, bearing masonry, and leave much room for inappropriate repair or alteration. Understanding the design and construction of Modern thin stone veneer structures is essential to appropriate repair and preservation.
The lessons learned from several generations of thin stone veneer buildings is that their clean, contemporary lines belie the sophisticated technologies addressing structural loads, thermal movement, fastener detailing and water infiltration. If this technology is not fully understood, or if the systems are not properly maintained over time, failures will occur. However, the abundance of successfully performing thin stone structures in use today proves that with sensitive detailing, construction, and maintenance, combined with appropriate preservation standards, these structures can contribute to our architectural heritage.
Sarah Ripple graduated from Columbia University this past May with a master of science in Historic Preservation. Her thesis, entitled "The Evolution of Modern Thin Stone Veneer Systems, 1950-1980,” covers in greater depth the information presented here. She will be presenting at the Association for Preservation Technology International 2012 Conference in Charleston as part of the session Knowledge in Practice: Three Traditional Materials and Assemblies…Up Close on Monday, October 1.