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Historic Stone Masonry Restoration

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Historic Stone Masonry Restoration I S S U E 2/2014 VOLUME 31 NUMBER 2 Journal of architectural technology published by Hoffmann Architects, Inc., specialists in the rehabilitation of building exteriors. Historic Stone Masonry Restoration Elizabeth A. Campbell, AIA, LEED AP BD+C and Christopher M. DeRosa Delicately carved ornament, dis- best ways to arrest and, to whatever tinctive tooling and bonding patterns, degree possible, undo the damage and rich surface textures, along with caused by poorly conceived repairs. soaring arches, towers, and buttresses, To best treat a historic stone struc- inspire awe and give historic ture, consider ongoing concerns, such stone masonry a sense of as leaks and deterioration, in the the sublime. Yet it is those context of past repair work, as well as details we most admire that properties and features of the stone are often the most vulner- itself. Through stone condition evalu- able, succumbing not only ation, laboratory testing and analysis, to the effects of time and and historical records review, the weather, but to the con- design professional can determine the sequences of insensitive deterioration mechanisms at work and repairs, from inappropri- prepare an appropriate restoration ate coatings to careless program. With thoughtful design and repointing. proficient execution, restoration can As the number of new build- preserve architecturally and culturally ings constructed of solid significant details, while reestablishing stone masonry has dimin- the resiliency that has enabled the his- ished over the past century, toric stone to last through the years. so too has the number of skilled artisans available to Evaluating Masonry Construction restore these structures as Before developing a stone treatment, they age. With few crafts- it is necessary to develop an under- people proficient in the standing of the history and condition treatment of traditional of the existing building. The design stone structures, building professional should consider the origi- To ensure the legacy of historic stone structures endures, owners may unwittingly em- nal design intent, particularly in terms it is important to apply appropriate conservation strategies. ploy inexperienced trades- of how the stone type, surface dress- men, whose repair efforts ing, bond pattern, pointing, and other may cause more damage than they attributes contribute to the function resolve. Too often, that damage is ir- and appearance of the exterior enve- reversible. As such, discussions about lope. Stone is generally chosen based the restoration of historic stone must upon both its structural properties address not only the treatment of age- and its aesthetic qualities, so the initial related wear and decay, but also the investigation should identify those Elizabeth A. Campbell, AIA LEED AP BD+C, Project Architect with Hoffmann Architects, develops historic stone restoration strategies that combine material science and preservation. Project Coordinator Christopher M. DeRosa has experience designing stone cleaning and repair programs for traditional masonry buildings. JOURNAL Efflorescence. Iron subflorescence. Exfoliation. properties of the stone that are inte- interior. Repair programs that fail to Just as the stone’s inherent proper- gral to the design and performance consider the distinction between tradi- ties affect the performance of the wall the building. tional stone construction and modern assembly, so too does the style of con- struction influence the longevity of the In the field, the investigation begins cavity walls can adversely impact the stone. Traditional stone detailing often with a visual survey, at times involving moisture balance in the assembly, re- served to direct water away from po- a stone-by-stone evaluation of existing sulting in poor indoor air quality, leaks, tentially vulnerable areas and to pre- conditions. To identify concealed dete- and damage to the stone. vent concentrated streams that cause rioration conditions and locate points When selecting stone, whether for staining and freeze-thaw damage. of water infiltration, wall openings may reconstruction or for isolated replace- be performed as part of the assess- ment, the architect or engineer should Drip edges on the undersides of ment. Laboratory analysis of stone and consider performance features and window lintels and sills prevent water mortar samples may yield additional known properties, as well as texture from passing along the length of the useful data, particularly where the and color. Strength, coefficient of protruding element back into the resiliency of the stone is called into expansion, weathering characteristics, wall assembly. Throats, which may be question due to age, observed dete- durability, porosity, appearance, and present as gargoyles or other troughs, also channel rainwater away from the rioration, or, more often, due to the ill workability are all key considerations. building, as do stoolings, a sloped por- effects of inappropriate prior repairs. Sound knowledge of the harmful tion of the sill built into the surround- The investigation should incorporate effects of salt contamination, ero- ing wall to shed water. The slope, or an inventory of previous treatments sion, chemical attack, frost action, and weathering, of the sill is usually set at and repairs, as well as a recommended vegetative growth is also important to an angle pitched to direct water away program for restoration. predicting the resiliency of stone to from the wall. At roof lines, coping Characteristics of Stone Masonry environmental factors. stones, which are large, sloped cap Thermal and moisture management For vulnerable areas, subject to severe stones, are set atop parapet walls to in a traditional stone masonry building exposure, atmospheric pollution, and serve a similar function. Unfortunately, is quite different from that in modern repeated wetting, the choice of stone today’s climate of acid rain, airborne construction, which uses impervious is especially important. Steps, curbs, pollutants, and soluble chemicals tends materials, vapor barriers, and cav- pavers, and base courses near grade to accelerate damage to stone mason- ity space to prevent the migration of and adjacent to hardscapes are all ry despite these traditional deterrents moisture across the building enve- prone to degrade at a greater rate to moisture-induced deterioration. lope. In contrast to the thin exterior than are smooth, vertically-oriented The size and tooling profile of mor- envelope of a contemporary veneer surfaces. Strong and resilient stone, tar joints has a direct impact on the assembly, load-bearing historic stone such as granite and quartz, is generally ability of the masonry wall to shed masonry is characterized by massive, better suited to these high-traffic, sen- moisture. Some joint profiles can ef- thick walls, which absorb moisture sitive areas. The softer the stone, the fectively double the surface area of at the outer surface and release it more it will react to impact damage the joint and, consequently, increase gradually, before it reaches the building and be subject to erosion. the absorption capability of the wall 2 VOLUME 31 NUMBER 2 Delamination. Cracking. Sugaring. assembly. In cases where an inappro- stone, leading to characteristic rust- environmental factors. priate tooling profile or deteriorated colored stains. Where a crack in the masonry unit mortar allows water to pool in the Where a portion of the stone surface leads to a clean break, the frac- joint, the mortar and adjacent stone has broken away, the problem could ture is referred to as detachment. can become severely deteriorated. be delamination, exfoliation, or spalling, Detachment may be due to failure of Forms of Deterioration in depending upon the type of stone and an original construction joint, or it may be the result of a weakened plane Natural Stone the nature of the break. Often caused by freeze-thaw cycling, spalling is the within the stone. Depending upon the type of stone, result of trapped moisture and salts Tracing the source of cracking in his- the climate, exposure, orientation, that expand beneath the surface of toric stone masonry can be difficult, building use, provision for moisture the stone, forcing off a piece of the as it can originate in a wide variety of management, and type of construction, outer face. As stone is exposed to sources, ranging from structural settle- the causes and manifestations of stone age and wear, the surface layer may ment to a repointing mortar that is deterioration can be diverse. separate from the body along a vein, incompatible with the stone. If cracks As moisture within the wall evapo- through processes resulting from salt are narrow and short or confined to rates from the exterior face, it leaves crystallization. Spalling can also result within a single stone unit, the issue behind waterborne salts, which remain from poor repointing techniques may be relatively minor; cracks that on the stone surface. This characteris- and from using too hard a repointing are wider and longer or those that tic white stain, or efflorescence, is often mortar. extend over large areas may be indica- a result of rising damp, which occurs Delamination takes place when the tive of systemic problems. when groundwater is drawn up into outer surface of the stone splits into If, with light rubbing, the surface of the the base of the wall. Efflorescence may thin layers, which then peel off the stone falls away in small granules, simi- also be caused by moisture introduced face. Sedimentary
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