Substitute materials: Weighing pros and cons in exterior restoration

by brittney_cutler | July 4, 2022 9:00 am

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By Richard W. Off, AIA

While repairing and salvaging existing materials is generally regarded as the first choice when addressing deterioration in exterior restoration projects, replacing some amount of material is often unavoidable. When replacement becomes necessary, design professionals are confronted with a critical decision, to match existing materials “in-kind” or to use substitute materials. Using “in-kind” materials is almost always considered preferable, especially amongst preservationists with respect to historically significant buildings. However, the choice of whether to use substitutes can be more complex and nuanced than it might first appear.

Substitute materials are not only used within contemporary restoration projects but also have been employed in construction for many centuries. Additionally, although traditional construction materials are time-tested and there are good reasons for their continued use in many applications, material technology is ever evolving and often improving, as are the codes, regulations, and industry standards which govern their use. Most importantly, there can be situations where the use of substitutes might be the most logical and beneficial choice, resulting from various design and construction circumstances and criteria.

Terminologies

An evaluation of material use within exterior restoration projects requires an understanding of commonly used terminologies, and their synonyms, which may not necessarily mean the same thing. The first is “substitute” versus “alternative.” Both are often used within the industry but there is no officially recognized difference between them. Therefore, for the sake of this article, they shall be considered interchangeable, but substitute shall be used throughout. As for their meaning, using a substitute implies one type of material, product, or system is used in place of another, within a building assembly. This could involve replacement of select units, comprehensive reconstruction of entire areas and, in some cases, building additions, especially those in historic districts.

The next important terms are “existing” versus “original” versus “historic.” Although there are instances where they all mean the same thing, these words are not inherently interchangeable. Existing is the material that is currently present, original is the material used when the building was constructed. Historic is a more ambiguous term which could imply there is cultural or aesthetic value to the original building or material, or it might simply refer to the fact the original construction is dated to a certain era or is no longer employed the same way in contemporary practice. These are important conditions to understand because material replacement always involves existing conditions but does not necessarily involve original or historic materials, as the latter two might have already been replaced in a prior project.

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If some original material has been replaced with different materials, the question of which is to be matched must also be answered. It also begs the question, was the previously replaced material appropriate in the first place? Further, it is important to note historic does not necessarily mean traditional, such as stone and wood, as it could be in reference to industrial materials used in 20^th century modernist style buildings. Further, traditional construction does not necessarily mean the structure is inherently old. The Basilica of Sagrada Familia in Barcelona, Spain, designed by famed architect Antoni Gaudi, has been under construction for 140 years, and new stone masonry is still being laid per the architect’s original plans. Not to mention, many thousands of vernacular structures around the world are still built using traditional materials.

The next term is not a word, but the phrase “in-kind,” which is frequently employed within preservation projects. Matching “in-kind” is often the desired approach when replacing materials, but what “in-kind” means can be unclear. Does it mean matching original or existing material? Is it matching general appearance like size, shape, color, and texture? Or is it matching physical properties as per ASTM testing or other officially recognized standards? Must the material come from the original source, such as a particular quarry or shop, and does it need to be fabricated the same way?

If materials must match the same type, it is critical to understand not all stone, wood, metal, etc., is created equal. Not only because limestone is not the same as sandstone (for example) but also because even the sourcing, extraction, processing, and classification can affect its quality. Even if replacement material has the same origins, adjacent existing material to remain may have weathered and become altered over time from exposure, so it may be impossible to truly match the properties and appearance of the original or existing material. These questions demonstrate the importance of not relying too much on the phrase “in-kind” without developing detailed criteria for its project-related meaning and providing thorough technical specifications that clarify these ambiguities.

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Other important terminologies to consider include those officially recognized within The Secretary of The Interior’s Standards for the Treatment of Historic Properties, established under the National Historic Preservation Act (NHPA). This document defines four different approaches to take when treating historic properties:

1. Preservation—focuses on the maintenance and repair of existing historic materials and retention of a property’s form as it has evolved over time.
2. Rehabilitation—acknowledges the need to alter or add to a historic property to meet continuing or changing uses while retaining the property’s historic character.
3. Restoration—depicts a property at a particular period of time in its history, while removing evidence of other periods.
4. Reconstruction—recreates vanished or non-surviving portions of a property for interpretive purposes. (National Park Service, “Four Approaches to the Treatment of Historic Properties”)

Many projects involve one or more aspects of each of these approaches, and the use of substitutes could potentially fall under any of them. However, substitutes are more likely to be entertained when performing the three latter approaches.

Evaluation/assessment

Prior to replacing any material, either “in-kind” or with a substitute, an investigation should be conducted to assess why the material is failing in the first place. Thoroughly assessing the underlying causes of distress allows for a better determination of an appropriate corrective action, which should be performed by a design professional experienced in exterior restoration. This may involve investigation techniques such as visual and hands-on inspection, sounding with a hammer, material sampling and laboratory analysis to determine chemical composition or microscopic deterioration, exploratory probes, or non-destructive testing. Assessment could also include structural, foundational, seismic, or water intrusion investigations, as the cause of distress within materials might be the result of deficiencies in other areas of the building, such as below-grade, concealed, roof, or flashing conditions.

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Following the investigation, the existing materials should then be evaluated to assess whether they can be repaired and salvaged. Repairs might involve removal and reinstallation or pinning in-situ if the unit is displaced or anchorage has failed, and patching, grouting, or dutchman (partial replacement of the unit’s material “in-kind” if it is cracked or spalled). It can also include recoating to repair damaged finishes or enhance waterproofing capacity, and cleaning if it is soiled or has biological growth. If repairs are insufficient, and it is decided the best course of action is replacement for cost, aesthetic, safety, or performance reasons, the design professional should only then consider whether it can be replaced “in-kind” or if a substitute should be used.

Why substitutes might be considered

One of the foremost reasons a substitute may be used is the limited availability of the original material. This can be the case for traditional materials found in pre-war construction, as well as industrial materials for modernist and contemporary buildings. Availability issues can include such things as quarries being closed when an exact stone match is desired, or when an old growth wood is no longer harvested.

Availability can also be affected by the reduced quantity of skilled craftsman. Prior to mass production, many historic structures with ornamental components were produced by specialized individuals, such as carvers, metalsmiths, stonemasons, and carpenters. Although these trades still exist, most remaining workers within these professions are typically educated to produce more standardized contemporary assemblies. So, there are fewer trained people available to reproduce historic conditions, and this is especially applicable to complex, handcrafted pieces.

Constructability issues can also generate the need for substitutes, even if the material or trade is readily available. Sometimes, the size, weight, or configuration of original materials might be too cumbersome to replace with a matching material, such as monolithic stone blocks in load-bearing masonry construction. Repair in lieu of replacement of these elements is often preferred.

Availability can be an issue even for historic materials which were once mass produced, such as terra cotta. A popular material in the late 19^th and early 20^th centuries that once had many sources throughout the United States, architectural terra cotta now only has two major manufacturers in the country. Limited availability not only affects lead time, as it can take longer to source, but it also makes the product more costly. These conditions can also be exacerbated by the amount of material that is needed to be replaced, but how this plays out depends on multiple factors. Less material does not necessarily take less time to generate, as it may not be beneficial to the fabricator to interrupt steady production of standard units if only a handful of unique units need to be replicated. However, if there is a large number, and especially if they are repetitive, the supplier may be motivated to prioritize the order.

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When cost and lead time are a problem, it can be especially concerning when deteriorated material is being replaced to address a safety issue. This is common with masonry conditions on urban high-rise buildings, especially those municipalities with stringent facade inspection requirements, like New York and Chicago. In these scenarios, when owners are under pressure to correct safety issues within strict timeframes set forth by local laws, they may be inclined to replace with a substitute, especially if it costs less or is readily available. This problem can be exaggerated by the compounding costs of protection, scaffolding, and stabilization that might be needed to address the unsafe conditions, not to mention the fines incurred from local building departments. This can further sway building owners toward substitutes, at least as a temporary solution.

There can also be performance reasons for selecting a substitute material. A design professional may determine the original or existing materials have fundamental deficiencies, or they were inappropriately implemented in an assembly, climate, or exposure that makes them prone to deterioration. Some examples include:

Thin marble panels

Used in many modernist buildings, thin marble is susceptible to hysteresis when exposed to heat, a type of permanent deformation that causes it to progressively bow, crack, and eventually dislodge from its attachments. Several of these buildings have required multiple replacements of the original material, and some have been reclad with a different material due to the ongoing issue.

Brownstone

Commonly used in 19^th century townhouses in the northeastern United States, this soft and porous stone is subject to exfoliation and spalling, which is worsened by the northeast climate, where freeze/thaw cycles occur every winter. It is worse yet in urban areas with heavy pollution, as surface soiling traps moisture which further wreaks havoc during freeze/thaw cycles.

Metals prone to corrosion

Corrosion is often found in early high-rise buildings, which began to use ferrous structural and anchorage components in a substantial way but did not necessarily incorporate coatings, membranes, or adequate solid masonry infill to protect the metal. Additionally, certain combinations of metal can make them prone to galvanic action (corrosion resulting from adjacent dissimilar metals, such as aluminum and copper).

Inappropriate use of wood

Wood can also be expressive of this condition, as not all species and types are equally suitable to exterior exposure. This is more prevalent in post-war wood constructions that no longer used old growth lumber, which was more decay resistant.

These examples show substitutes do not necessarily have to involve more contemporary synthetic materials. Instead, they might also include traditional materials that are more suitable to the application than the original material. The example of metal also shows how substitute materials can be equally relevant to concealed anchorage and structural systems, as they are surface materials. These components may need to be replaced with substitutes, not only because they lack corrosion-resistance, but because they may have fatigued or were not engineered to contemporary standards. Sometimes supporting metal has deteriorated but cannot be replaced, or its structural capacity cannot be confirmed. These can be additional reasons to replace exterior claddings that rely on these metal components with substitutes that are lighter weight, such as cast composite resin replacing stone or brick masonry, or asphalt shingles replacing slate or terra cotta roofing.

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Other backup and underlayment materials might also need to be replaced with substitutes for both facade and roofing projects, such as waterproofing materials. Historically, felt paper and coal tar pitch were used. However, contemporary advancements such as self-adhered rubberized membranes or fluid-applied membranes can allow for greater ease of installation than hot-applied materials. These membrane systems are more waterproof than traditional felt paper, too.

Substitutes might also be used when there is a new addition to an existing building, or a portion of a building is being largely reconstructed. In these scenarios, the new addition or reconstruction might intentionally be made to look like an intervention to the surrounding fabric and the substitutes could assist with creating an aesthetic contrast, whereby new material colors and finishes are purposefully different than the old. Additionally, these new or reconstructed assemblies may need to comply with contemporary standards or be reconfigured to correct deficiencies that prompted replacement or augmentation in the first place. These upgrades might demand physical and chemical properties that differ from those of the original assembly, such as structural strength, lighter weight, or improved moisture management.

What are the concerns with using substitute materials?

While there may be situations where substitute materials can be used, there are considerations design professionals and owners should thoroughly evaluate before implementing them. The most common and significant concern with substitutes is compatibility, especially when select materials are being replaced in an otherwise existing or original assembly. The new material may have properties that differ from those of adjacent materials, which could make it prone to deterioration or liable to cause damage or accelerate decay within the surrounding substrates. Differences in coefficients of thermal expansion, for example, can cause adjacent materials to crack or displace as they move differentially over time.

Differences in vapor permeability, watertightness, and porosity are also a concern, as they all play a role in moisture mitigation, both from external precipitation and condensation of warm, moist air from building interiors. Substitute material insertions should be carefully analyzed to verify they do not admit excessive moisture into the assembly. They should also allow moisture to escape where appropriate. Interventions within historic masonry assemblies can be especially sensitive to these issues, as they manage moisture differently than contemporary cavity walls or rainscreens. Condensation can also be a concern in historic roofing assemblies that are replaced with less porous cladding, underlayments, and/or insulation, which can cause moisture to accumulate at the underside of the roof deck, within attic spaces, or within the roof assembly. Not only can condensation cause water-related deterioration, but it also could potentially generate mold.

Other compatibility issues can involve differences in dimensional stability, the tendency for a material to maintain its shape and size over time. Clay-based materials like brick and terra cotta gradually expand as they re-absorb moisture, but concrete materials tend to shrink as they continually cure. Additionally, materials not only experience elastic deformation, and regularly change size and shape when exposed to the elements, but as noted with thin marble, they can also become plastically deformed when their molecular structure changes due to fluctuations in thermal or structural loading. Over time, these dimensional changes can create microscopic openings that are avenues for moisture infiltration and freeze/thaw damage. On a macroscopic level, they can also cause cracking and displacement, as localized failures cause differential settlement.

Common Substitutes and Associated Exterior Components
Original/Existing/Historic Materials	Substitute Materials	Typical Facade and Roofing Components
Solid Stone Masonry (Limestone, Granite, Bluestone, Marble, etc.) and Structural Clay Masonry (Brick and Terra Cotta)	Cast Stone/Architectural Precast Concrete, Geotextile Fiber Reinforced Concrete (GFRC), Cast Composite Resin	Facade/Parapet Wall and Chimney Bricks, Blocks, Copings and Caps, Windowsills and Headers, Facade Ornamentation (Cornices, Brackets, Banding, Statuary), Steps, Landings, Pavement, and Site Walls
Slate and Terra Cotta (Roofing and Cladding)	Composite (Plastic/Rubber), Plastic/Rubber, Asphalt, Cast Stone/Architectural Precast Concrete	Roof Shingles, Wall and Rain Screen Tiles
Solid Wood Timber (Non-Structural)	Composite (Plastic Encased Wood Fibers), Vinyl, Cement Fiber	Facade Siding/Cladding, Window/Door Trim, Sills and Headers, Eave/Rake Soffits and Fascia, Facade Ornamentation (Cornices and Brackets), Porch Finish Decking, Posts, Steps, and Guardrails/Railings
Solid Wood Timber (Structural)	Engineered Wood (Cross-laminated Lumber, Densified Wood, Wood Byproducts (Plywood, Waferboard, Fiberboard, Laminated Lumber)	Roof and Porch Substrate Decking, Facade Sheathing, Framing Members (Studs, Posts, Beams/Joists, Rafters, Joists, Trusses, Purlins, Bracing)
Cast Iron and Wrought Iron (Structural)	Extruded Aluminum, Plain Carbon Steel, Stainless Steel, Galvanized Steel	Guardrails/Railings (Roofs, Balconies/Terraces Landings/Stairs), Window Guards, Decorative Grills, Gates/Fences, Stone Anchorage, Supporting Angles/Lintels, Framing Members (Beams/Girders/Joists, Trusses, Rafters, Columns, Bracing), Bolted Connections/Fasteners
Copper, Lead, Tin, etc., Sheet Metal and Non-Structural Cast Iron	Aluminum, Galvanized Steel, Stainless Steel, and Galvalume Sheet Metal, Polyvinyl Chloride (PVC) Sheets/Pipes/Sections, Sheet Membrane (Bituminous/Rubberized, Fluid/Liquid Applied)	Roofing Seams, Roofing Shingles/Sheets, Flashings, Fascia, Drip Edges, Exterior and Interior Roof Drainage (Scuppers, Conductor Heads, Drain Domes/Bodies, Downspouts/Leaders)
Asphalt Soaked Felt Paper, Coal Tar Pitch, Hot Mopped Asphalt, Mastic	Sheet Membrane (Bituminous/Rubberized, Fluid/Liquid Applied)	Roofing, Flashings, Underlayments
Mortar and Grout	Sealant/Caulking (Silicone, Polyurethane, Butyl, Latex), Neoprene and Rigid Foam, Epoxy and Methyl Methacrylate Adhesive	Joints and Setting Beds for Masonry/Tile Facade/Cladding and Pavement, Expansion/Control Joints, Flashing Terminations, Bearing Pads, Compressible Filler and Backer Rod, Setting Pockets for Masonry Anchors and Reinforcement Bars
Oil Paint	Acrylic, Latex, Epoxy, Fluoropolymer, Potassium Silicate	Coating/Coloring Wood, Metal and Masonry/Concrete Surfaces

Substitute materials can also create problems if strength and weight are not properly evaluated. Not all masonry is created equal, and depending on its type, origin, and applications, it can have different degrees of compressive, tensile, and/or flexural strength, as well as different densities. Similarly, for structural components, stainless steel may be significantly less prone to corrosion than plain carbon steel, but it is also weaker, so dimensions, attachments, or frequency may need to be adjusted to accommodate these differences.

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Addressing these compatibility concerns is not only a matter of selecting the right materials but may require alterations to the material unit or assembly. This could involve supplemental reinforcements, anchorage, and support augmentation, or redirecting load paths to address structural concerns. To mitigate potential moisture infiltration issues, it might involve the introduction of waterproofing membranes or sheet metal flashings and drip edges where none currently exist or increasing their size or configuration where they do. It could also involve the use of coatings, either for the substitutes or adjacent existing materials, to increase their weather durability and waterproofing capacity or to conceal differences between new and old materials.

Different joint materials might also need to be used to address concerns with breathability, adhesive capacity, or galvanic action. For example, cast composite resin does not bond well with mortar unless primed, as it is not porous like masonry or concrete, but sealant will adhere to it. However, the potential for trapping moisture must once more be considered as the less porous cast composite increases this potential, especially when combined with watertight sealant. When new metal types are introduced, it might require a separation, such as a gasket or coating to avoid contact with a dissimilar metal. These conditions demonstrate the need to consider the behavior of the entire enclosure, and how much of it is being replaced, because that affects whether the substitute materials may present issues or require modifications.

Other concerns include the ability of a substitute to achieve a true color, texture, or profile match, which requires special consideration on landmarked historic buildings. Material technology has evolved greatly in recent decades to allow for closer matches, but as many substitutes are machine-fabricated and mass-produced, they do not always have the handcrafted or natural character of traditional materials, which often contain more surface variations. Some composite materials with external veneers or coatings have been engineered to create better matches, but they are susceptible to delamination of the veneer or coating from its substrate. Colorfastness, the ability to hold color over time, is also an important aesthetic consideration, as many natural and traditional materials tend to exhibit better colorfastness. Some substitutes, especially those with coatings and thin veneers, are not dyed throughout their cross-sectional thickness, and can be subject to fading from ultraviolet (UV) degradation and weathering, and might require recoating.

Exterior replacements, and associated material selection, can be subject to review and approval by preservation boards or the State Historic Preservation Office (SHPO), depending on landmark status and applicable regulations. In general, replacement elements with higher visibility tend to be subject to greater scrutiny and are often needed to match original or existing materials. Substitute materials are more likely to be approved for components located on the upper floors, parapets, or roofs, or for isolated elements, as their different texture might be less noticeable, or less likely to contrast with adjacent elements. Whether or not the building is part of a landmark district, or a prominent individual landmark, can also affect where, how, and what substitutes could be used. The reason for landmark status could also affect the decision to allow a substitute. A building that is landmarked because of a historical event, as opposed to architectural character, might have fewer design limitations.

Sustainability considerations

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Material selection not only influences the performance and longevity of buildings, but also affects the health of the environment. At a minimum, material choices concern the local ecology in and around a building, which is commonly affected by the issue of hazardous materials. If a component to be replaced contains hazardous material such as asbestos, polychlorinated biphenyls (PCBs), or lead, it not only concerns abatement procedures, but it also could be more reason to comprehensively replace all the existing material with a substitute material, especially if the hazardous compound is friable and will be disturbed by the project, or if it presents ongoing health issues.

Beyond the realm of the immediate building ecology, other questions emerge about the potential long-term damage or benefits in using certain materials. This should include an analysis of material lifecycle and disposal. Many traditional natural materials like stone and clay masonry and old growth wood, if carefully removed, can be repurposed. Stone and clay materials can be crushed to be used as aggregate in concrete or for roof ballast or landscaping. Wood is inherently biodegradable and can help feed other organisms as it decays. While several substitute materials might contain natural material components, like precast concrete, cement fiber board, or wood byproducts, many contemporary substitutes are synthetic. Some substitutes contain plastic, which is not biodegradable but may be recyclable, while others contain bitumen or fiberglass, which can be toxic, cause irritation, or contribute to pollution. Many traditional natural materials also benefit from having a lower carbon footprint and embodied energy. Since they are less processed, less energy is consumed in their extraction and fabrication. Moreover, the energy consumed in material production is often not “green,” as it typically pumps carbon into the atmosphere, which contributes to climate change.

Although natural materials can have environmental benefits, using them may not always be the best option for a given project. Technologies are ever evolving, and production processes continue to be refined as environmental regulations gain further ground. Therefore, it falls upon the design professional to research products and include sustainability criteria when developing specifications. Choosing materials that are locally sourced is a good start, as less energy is consumed in delivery. Where possible, careful reconsideration of material color can affect a building’s energy use. Lighter color materials typically have higher albedo, meaning they reflect more sunlight and reduce solar heat gain. Increasingly, building codes are including albedo requirements by establishing a minimum Solar Reflectance Index (SRI), especially for roofing, since it typically has the most direct sun exposure. Additionally, when large portions of building enclosures are replaced, considering the insulation value of materials used, and how their connections impact thermal bridging, is also critical to evaluating energy performance.

It is also important to consider how material selection affects the type and frequency of required maintenance for the enclosure. Although the introduction of substitutes may appear to solve some availability, cost, or schedule issues, they may not be providing savings in the long-term. If materials must be replaced more often or cause adjacent materials to have to be replaced sooner than anticipated, they are only further contributing to landfills and excessive resource consumption.

Philosophical and historical conclusions

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The use of substitute materials is not a new concept. Their history shows how their implementation is part and parcel to the evolution of architecture. Although now considered historically valuable, terra cotta was commonly employed as a substitute for more expensive stone at the turn of the century. For centuries, wood has been painted with sand to look like stone and was common in American and European manor houses. Romans used concrete and brick in lieu of monolithic stone, primarily for backup materials, to achieve efficiency, as poured materials or smaller units could be installed more quickly.

During the Industrial Revolution, iron and steel replaced wood structures, and when first employed, metal structures used wood joinery techniques, like mortise and tenon, as new metal joints had not yet been invented. Even ancient Greek buildings had decorative elements which reflected the evolving history of material use. Many temples were originally constructed in wood, and the triglyphs, a projecting part of the decorative entablature above the perimeter columns, represented the rough-cut ends of wood beams used in these earlier constructions. These latter examples reinforce the notion of palimpsest within architecture, something altered over time that still bears marks of its original form.

Buildings are ever-changing, living entities that require regular repair and maintenance, regardless of the materials and assemblies used. Some degree of replacement material is unavoidable, as even repairs typically involve piecemeal removal and replacement of portions of original units, such as patching compounds and joint materials. So, the notion of replacement might be as much a matter of perspective as matching “in-kind” can be.

These arguments may only seem relevant to historic and landmark buildings, but many contemporary structures could one day be considered historically valuable, which is increasingly the case for modernist style buildings. Even structures built in the 1970s are receiving increased attention, since they are now 50 years old. The issue of substitute materials has unique challenges for buildings of this era. The industrial products they often used, considered revolutionary for their time, might now be obsolete, and they can contain hazardous materials.

Perhaps buildings should be allowed to be patchworks which reflect the passing of time and the signature of all those who worked on them. Like cathedrals that took several decades to build and incorporate multiple styles and materials, or historic cities which took centuries to accumulate, construction is an amalgam of influences. Does an edifice lose its authenticity or cultural value from too much alteration, repair, or restoration? Is Sagrada Familia less authentic because it is being constructed with modern technologies different from those available during Guadi’s lifetime? Are the building’s tectonics, its art of construction, corrupted by use of substitutes since these might compromise the original designer’s intentions?

On the other hand, design professionals should combat the common misconception that newer is necessarily better. Although substitute materials are now subject to regulated engineering and testing, in the greater history of material use, many of them are still relatively new. It is not entirely known how some of them will perform in the long-term, either independently or as part of an existing assembly. The answer to which material to use is not necessarily straightforward and should be evaluated on a case-by-case basis.

Author

Richard W. Off is a senior project manager and registered architect at Hoffmann Architects in Manhattan, New York. With a focus in historic preservation, and expertise in traditional and modern facade, roofing, and window systems, he oversees A+E teams complete numerous multimillion-dollar investigation, rehabilitation, renovation, and adaptive reuse projects throughout the New York metropolitan area. He graduated with a Master of Architecture and Urban Design from Columbia University and a Bachelor of Architecture from Rensselaer Polytechnic Institute. Off has lectured at traditional building conferences, the APT DC Symposium, and both his alma maters. An avid writer, he has also published articles with The Construction Specifier.

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