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I S S U E 2/2014  VOLUME 31  NUMBER 2

Journal of architectural published by Hoffmann Architects, Inc., specialists in the rehabilitation of building exteriors. Historic Stone 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 . proficient execution, restoration can As the number of new build- preserve architecturally and culturally ings constructed of 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 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 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 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 should Drip edges on the undersides of ment. Laboratory analysis of stone and consider performance features and window lintels and sills prevent water 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, characteristics, wall assembly. Throats, which may be question due to age, observed dete- durability, , 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 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 , 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 and , is generally ability of the masonry wall to shed masonry is characterized by massive, better suited to these high-traffic, sen- moisture. Some 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

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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 stones are naturally lar in texture to granulated sugar, the into the wall assembly through im- prone to delamination, which occurs stone may be succumbing to surface properly installed or missing flashings along the bedding planes when the disintegration, or sugaring. Cohesive at the top of a parapet. is set perpendicular to the direc- in the stone dissolve, and the Where efflorescence is observed, tion in which it was originally formed. remaining deposits are easily brushed subflorescence also may be present. As Like delamination, exfoliation is a type from the surface. Carbonate stones, trapped moisture migrates through of surface disintegration in which the especially fine-grained , are stone, it may leave behind a potentially stone sloughs off in very fine layers. particularly susceptible to this type of harmful accumulation of crystallized Although the two terms are often granular separation. salts along veins or internal cracks, used interchangeably, delamination As the building ages, the natural dis- which can damage the stone’s internal tends to refer to along natural integration and erosion of stone due structure. In some cases, these salts bedding planes, whereas exfoliation is to wind and rain leads to weathering, may accelerate corrosion of naturally usually due to thermal stress, im- which results in worn and rounded occurring ferrous compounds in the peded moisture movement, or other surfaces. Exposure to acid rain tends

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to drastically increase the rate of gentlest method possible to remove its acidity can cause dissolution of the weathering and produce a noticeable staining, to avoid damage to the stone stone surface. color or carved detail. Cleaning can softening or loss of detail. Any wet cleaning method should be reveal the natural color of the stone, Selecting a Preservation carried out only when there is no while exposing damage that may re- danger of frost or freezing. If the wall Treatment quire repair. remains saturated during cold tem- Once the design professional has Water washing may be the most ver- peratures, absorbed water may freeze catalogued the location and extent of satile and gentle technique for remov- and expand inside the stone, causing deterioration, along with the probable ing dirt and stains from historic stone cracks and spalls. causes – whether normal wear, exces- masonry. Methods include prolonged Chemical cleaning agents, including sive contaminants, flaws in the original spraying with fine mist, low-pressure organic surfactants, should be selected design or construction, inappropriate washes, and water used in combina- based on the type of stain, the pres- previous repairs, or other such condi- tion with chemical agents. Take note, ence of biological growth, atmospheric tions – he or she can identify and test however, that even simple water wash- pollutants, and, most importantly, methods for restoration to address ing should be carried out with caution. the stone substrate. Alkaline clean- the deficiencies. Permanent discoloration may result ers, which are used on and should dissolved minerals or impurities Cleaning marble, should not come in contact leach out of the stone, or if absorbed with polished granite or windows, as While it may sound simple enough, water corrodes metal elements. Soft the high pH may cause etching. Acidic cleaning of historic stone is a delicate water should not be used on cleaners, formulated for granite, , process. As a general rule, use the carbonate stone, such as limestone, as , and other non-calcareous stone, must be removed from ma- Stone Consolidation: sonry by a water rinse to neutralize Benefits and Risks the acid following application. It is cru- cial to protect facade elements, such Consolidants may be applied to stone as light fixtures and ornaments, that when its composition is no longer stable, may be damaged during the cleaning as occurs with crumbling or sugaring. process. A consolidant is a chemical, typically Abrasive cleaning may be used, in lieu composed of polymers, that is applied to of or in addition to chemical clean- the stone surface in cycles over a speci- ing, to remove soil, stains, and coatings fied length of time. The end result is a by abrading the surface of the stone substrate whose microstructural stability through impact. Sandblasting, grit, and/ is restored. Consolidants require reap- or pressure washing are all common plication over time, so there is scheduled forms of abrasive cleaning, as is the maintenance involved in this repair. use of power sanders, wire brushes, Consolidants alter the physical character and other hand tools. Due to the high and color of the stone. The liquid polymers applied to the surface are drawn potential for damage, abrasive clean- into the stone via capillary action, and subsequently solidify within the stone ing should only be used for historic . The application area is transformed permanently into a combination of masonry in special circumstances, and stone and polymer. As such, the use of a consolidant to stabilize historic stone then only under direction of an expe- must be carefully considered, because the result is a new substrate. rienced architect or engineer. The extent to which a consolidant will darken the stone can vary from slightly Stone Repair/Restoration to markedly along a facade composed of the same stone exhibiting the same Repointing is used to repair dete- levels of deterioration. Performing a mockup of the application will produce riorated mortar joints. The existing the variety of results that can be expected though the course of the project. mortar is removed by hand to a depth A mockup is always recommended to determine the appropriateness of con- of two times the joint width, or until solidation, because it is an irreversible repair. sound mortar is reached (whichever is greater), and new mortar is set in

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place and tooled to match the original profile. Inappropriate Repairs For the majority of older buildings, As holds the principle of bioethics, so should the approach to treating his- a historically accurate mortar mix toric stone: “First, do no harm.” If the degree of deterioration does little to should contain only , , and detract from the architectural character of the building or to call into ques- water. White Portland may be tion its structural stability or performance, no repair may be needed. substituted for a portion of the lime to improve workability and plasticity. Too often, application of a A high-lime-content mortar is softer “protective” waterproof coating, than the stone and prevents damage aimed at preventing hypotheti- to the stone masonry by acting as the cal moisture-related deteriora- sacrificial element in the wall assembly. tion, has the unintended effect of sealing moisture inside the To achieve a mortar mix that approxi- masonry surface and leading, in mates the character of the original, many cases, to real and irrevers- it may be necessary to add pigment, ible damage. Stone masonry crushed shells, or colored sand. Testing buildings of traditional construc- and analysis of the original mortar tion were designed to “breathe;” may facilitate accurate identification of Blistering waterproof coating. mortar components. to be permeable to air and moisture, allowing water to leave Mechanical stitching stabilizes and the wall assembly through the arrests wide cracks that extend deep natural process of evaporation. into or through a stone unit. The stitch Introducing impervious coat- pins are usually stainless steel, to resist ings and vapor barriers disrupts corrosion and to accommodate the the balance of moisture across tensile forces that caused the stone to the building envelope, which can crack. To execute the repair, the face result in dire consequences for of the crack is routed in a “V” shape, the stone, at both a macro- and and the ends of the stitch pins are micro-structural level. embedded in the stone surface. Then, As a shortcut to address deterio- the pin holes and the crack surface are Deteriorated face-pointed mortar. patched. rated mortar joints, a thin layer of mortar is sometimes added Patching with cementitious mate- without first removing the exist- rial may be used to treat small areas ing mortar to the appropriate of deteriorated or spalled stone. If depth. While quicker and cheaper carried out by a skilled worker, patch than full repointing, face-pointing repair may blend into the facade is inadvisable, as it does not pro- more than a stone replacement or vide suitable stability and tends Dutchman repair, and it is usually less to crumble out of the joint. expensive. Patch materials vary ac- cording to the type of stone masonry Introduction of pointing mortars being repaired, but they should be of incorrect composition may re- strain the natural expansion and weaker than the stone substrate. If the Incorrect use of sealant. patching compound is too hard, it may contraction of stone masonry accelerate deterioration of the sur- subjected to moisture absorption and drastic temperature swing cycles, rounding stone. leading to cracking and spalling. Faulty patching mortars and ill-conceived pinning methods may have similar detrimental consequences. Even stone To match the color and texture of the cleaning, which sounds innocuous enough, is fraught with hazards, as using patching material to the existing stone, abrasive or chemically incompatible cleaning methods can abrade or other- colored sand is often used, along with wise damage the stone surface. . Pressing stone dust (continued on page 6)

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(continued from page 5) and chips into the surface of a newly formed patch may aid in blending Set In Stone the repair area with the surround- ing masonry. To achieve an accurate Sedimentary stone is formed match, artificial colorants may be used, through the deposition of and although these may fade or change layers over the course of slightly in appearance over time. thousands of years. Sedimentary stone, such as schist, slate, and sand- Equally important as color matching stone, is most durable if it is bed set, to the success of the repair is prepa- that is, oriented in construction, the ration of the patch area. Identifying same direction as it was formed in nature. When sedimentary stone is appropriate locations for support permanently positioned on its side, perpendicular to its natural orientation, pins, where necessary, and keeping those layers that were once horizontal are prone to shearing and delami- those pins clean and free of adhesive nation on the stone face. or epoxy is crucial to the durability of the patch, particularly for large areas. The lifespan of face-set stone is significantly shorter than that of its bed- Patching material should be applied set counterparts. There are design conditions and decorative shapes that past the stone surface, to facilitate fin- may be impossible to carve for a bed set installation. In these situations, ish carving once the patch has set. consider an alternative type of stone; otherwise, anticipate stone replace- ment well before the standard life expectancy for the material. Dutchman repair replaces the missing portion of a chipped or broken stone unit with a newly carved piece of techniques are impractical or ineffec- outward through the pores in the stone. Typically, the repair piece is natu- tive, or where stone units are missing stone, while preventing liquid water ral stone, chosen to match the hue or so severely broken or deteriorated from penetrating into the wall. Typically, and texture of the existing masonry, as to be unrepairable. Use of natural these coatings are colorless; however, and it is wedged in place, secured with stone carved to match the existing they may impart an undesirable sheen adhesive, or set with pins; larger pieces masonry is generally preferable to to the stone and may darken its color. may be set in mortar. To maintain the replacement with non-matching or Water-repellent coatings should never appearance of a continuous stone unit, synthetic material. However, accurate be applied to a damp or wet build- the joint between the repair piece and replacement may be contingent upon ing, particularly one that might have the existing stone should be as narrow locating a suitable and a satis- subflorescence under the surface. If as possible. factory color match. applied to a wet wall, the coating pre- vents moisture within the wall from Stone replacement must be con- Dampproof course installation ad- drying out, increasing the risk of - sidered where repair and patching dresses rising damp by incorporating age to the stone. a water-resistant material into the wall just above grade. Many historic One case where a water-repellent masonry buildings include some type coating may be appropriate is on a of dampproof material, such as , facade that has undergone previous— slate, bituminous felt, or metal, to pre- and often detrimental—waterproof vent the rise of ground moisture into coating application. As the water- the wall, but such a layer may also be proof coating may have permanently added as a remedial measure. altered the properties of the stone, it may be impossible to return to a Dampproofing may be used for walls stable, uncoated state. Application of that are regularly coursed and stable, a breathable coating may protect the but which are experiencing moisture porous surface of the existing stone migration originating at the portion of from further damage. Considerations the wall that is below grade. include altered appearance, which may Water-repellent coatings, unlike be evaluated through mockup testing, Mockup test of stripping agents to remove waterproof coatings, are breath- and the maintenance demand of peri- prior inappropriate coating. able, allowing water vapor to escape odic re-application. (continued on page 8)

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Historic Stone Masonry Columbia University Butler Library Before selecting a treatment for New York, New York historic stone, Hoffmann Architects Limestone Colonnade and Facade considers the ways in which the origi- Restoration nal construction responds to air and moisture, and how time, weather, and Fairfield University alterations to the natural stone have Bellarmine Hall affected its properties. Since each Fairfield, Connecticut stone building is unique, our architects New Haven Courthouse in New Haven, Granite Facade Restoration and test and evaluate resto- Connecticut. Marble Facade Restoration. Folger Shakespeare Library ration strategies to see that proposed BNY Mellon Washington, District of Columbia treatments perform as intended. One Wall Street Marble Facade Restoration Hoffmann Architects has designed New York, New York The George Washington University stone masonry solutions for historic Limestone Facade Study and Restoration Lisner Auditorium commercial, institutional, and govern- St. Luke’s Episcopal Church Washington, District of Columbia ment structures, including: Darien, Connecticut Limestone Facade Rehabilitation Swarthmore College Ashlar Granite Facade Restoration Gillette State Park Clothier Hall Old Town Hall East Haddam, Connecticut Swarthmore, Pennsylvania Stamford, Connecticut Building Envelope Schist Bell Tower Investigation and Repair Limestone Facade Restoration Investigation Lehigh University New York Exchange St. Thomas Seminary Packer Memorial Church New York, New York Bloomfield, Connecticut Bethlehem, Pennsylvania Marble Facade Restoration Ashlar Granite Bell Tower Rehabilitation Quartzite and Sandstone Master Plan and Restoration U.S. Department of the Treasury Bureau of Engraving and Printing Washington, District of Columbia Limestone Facade Restoration Walter E. Hoffman Courthouse Norfolk, Virginia Limestone and Granite Facade Consultation American University McKinley Building Washington, District of Columbia Father O’Connell Hall at The Catholic University of America in Washington, District of Marble Facade Consultation Columbia. Limestone and Granite Facade Investigation and Restoration.

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Hoffmann Architects, Inc. 2321 Whitney Avenue Hamden, CT 06518

ADDRESS SERVICE REQUESTED

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JOURNAL is a publication of Hoffmann Architects, Inc., specialists in the rehabilitation of building exteriors. The firm’s work focuses on existing struc- tures, diagnosing and resolving prob- lems within roofs, facades, windows, waterproofing materials, structural sys- tems, plazas/terraces, parking garages, and historic and landmark structures. We also provide consulting services for new building construction, as well as litigation and claim support.

For address changes, free subscriptions, or information on the topics discussed Caring for Natural Stone in this issue, contact our Marketing Since stone is not homogeneous, and Together, the owner, design profession- Department at 800-239-6665, time, weather, and stress affect each al, and restoration contractor should [email protected], or: structure in different ways, mockup use the results of mockup testing to 2321 Whitney Avenue tests should be used to evaluate the establish baseline expectations for the Hamden, CT 06518 performance of proposed repairs program of repairs. Historic stone de- 203-239-6660 prior to full implementation. Product rives its character from its age, and it is 1040 Avenue of the Americas, Ste. 14C manufacturers, stone , and neither achievable nor desirable for it New York, NY 10018 masons are useful resources; they can to emerge from a conservation effort 212-789-9915 provide test and performance data, looking like new. Realistic goals for the along with experiential advice, on the aesthetics and expected longevity of 2611 Jefferson Davis Highway, Ste. 200 restoration strategies and stone sam- repairs should take into account the Arlington, VA 22202 703-253-9800 ples under consideration. Ultimately, natural qualities of the historic stone though, each building is unique, and that give the building its personality. www.hoffarch.com even carefully formulated approaches require in situ evaluation. Editor: Alison Hoffmann Production: Cari Tate

JOURNAL offers AIA/CES Learning Units Earn Health, Safety, and Welfare (HSW) continuing credit for reading the JOURNAL. To learn more, visit www.hoffarch.com.

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