Substitute Materials in Historic Restoration

Evaluation/Assessment

Prior to replacing any material, either in-kind or with a substitute, an investigation should be conducted to determine why the material is failing in the first place. Thoroughly assessing the underlying causes of distress allows for appropriate corrective action. The evaluation, which should be performed by a design professional experienced in exterior restoration, may involve investigation techniques such as visual and hands-on inspection, sounding, material sampling and laboratory analysis, exploratory probes, or non-destructive testing. Assessment could also include structural, foundational, seismic, or water intrusion investigations, as the cause of distress might be the result of deficiencies in other areas of the building.

Following the investigation, the existing materials should 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. Restoration can also include re-coating to repair damaged finishes or enhance waterproofing capacity, and cleaning of stains or biological growth. If repairs are insufficient, and the best course of action is replacement for cost, aesthetic, safety, or performance reasons, the design professional should only then consider whether in-kind materials or a substitute should be used.

When to Consider Substitute Materials

One of the foremost reasons a substitute may be used is the limited availability of the original material. Examples include stone from a quarry no longer in service, or old-growth wood that is no longer harvested.

Availability can be an issue even for historic materials which were once mass-produced, such as terra cotta. A popular material in the late 19th and early 20th century that once had many sources throughout the United States, architectural terra cotta now has just two major manufacturers in the country. Limited availability not only affects lead time, as it can take longer to source, but also makes the product more costly.

Availability can also be affected by the reduced quantity of skilled craftspeople. 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, hand-crafted 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.

When material sourcing is a problem, it can be especially concerning when deteriorated material is being replaced to address a safety issue. This is common on urban high-rise buildings, especially those municipalities with stringent facade inspection requirements, such as New York and Chicago. 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 and is readily available. The problem can be exacerbated 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. These expenses 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 that 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. For example, concealed anchorage and structural systems may need to be replaced with substitutes not only because they lack corrosion-resistance, but also because they may have fatigued or were not engineered to contemporary standards. Sometimes, supporting metal has deteriorated but cannot be replaced. Instead, the design team might replace exterior claddings that rely on these metal components with substitutes that are lighter weight, such as cast composite resin in lieu of stone masonry, or asphalt shingles in place of slate roofing.

Other backup and underlayment materials that might need to be replaced with substitutes include felt paper and coal tar pitch, historically used as waterproofing. Contemporary advancements, such as self-adhered rubberized or fluid-applied membranes, permit easier and safer installation than these hot-applied materials, and they are more waterproof, too.

Substitutes might also be used when there is a new addition or major reconstruction. For designs intentionally made to look like an intervention to the surrounding fabric, substitutes assist with creating an aesthetic contrast. Additionally, these new or reconstructed assemblies may need to comply with contemporary standards or 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 increased structural capacity, lighter weight, or improved moisture management.

Concerns with Using Substitute Materials

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 to 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 at building interiors. Substitute material insertions should be analyzed to verify they do not introduce excessive moisture into the assembly. They should also allow water to escape, where appropriate. Historic masonry walls can be especially susceptible, 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.

Mitigating moisture infiltration might involve 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 to increase weather durability and waterproofing capacity or to conceal differences between new and old materials.

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, whereas concrete materials tend to shrink as they continually cure. In addition to this elastic deformation, some materials can 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.

Substitute materials can also create problems if strength and weight are not properly evaluated. Not all masonry is created equal and, depending on type, origin, and applications, can have different degrees of compressive, tensile, and 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, and spacing may need to be adjusted to accommodate these differences.

Addressing compatibility concerns is not only a matter of selecting the right materials, but also may require alterations to the unit or assembly. This could involve supplemental reinforcements, anchorage, and support augmentation, or redirecting load paths to address structural concerns.

Substitute 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 does adhere to sealant. However, the less porous cast composite increases the potential for trapping moisture, especially when combined with watertight sealant. When new metal types are introduced, they might require a separation, such as a gasket or coating, to avoid contact with a dissimilar metal. These examples demonstrate the need to consider how substitute materials impact the behavior of the entire enclosure. Moreover, what proportion of the envelope is replaced affects whether 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. 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 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. Colorfastness is also an important aesthetic consideration, as traditional materials tend to exhibit better ability to hold color over time. Some substitutes, especially those with coatings or thin veneers, are not dyed throughout their cross-sectional thickness, and can be subject to fading from ultraviolet (UV) degradation and weathering.

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. Substitute materials are more likely to be approved for components on upper floors, parapets, roofs, or non-street-facing facades, or for isolated elements less likely to contrast with adjacent materials. Whether the building is part of a historic district, or a prominent individual landmark, can affect where, how, and which substitutes may 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.