The Secretary of the Interior's Standards for Rehabilitation generally require that deteriorated distinctive architectural features of a historic property be repaired rather than replaced. Standard 6 of the Standards for Rehabilitation further states that when replacement of a distinctive feature is necessary, the new feature must “match the old in composition, design, color, texture, and other visual properties, and, where possible, materials” (emphasis added). While the use of matching materials to replace historic ones is always preferred under the Standards for Rehabilitation, the Standards also purposely recognize that flexibility may sometimes be needed when it comes to new and replacement materials as part of a historic rehabilitation project. Substitute materials that closely match the visual and physical properties of historic materials can be successfully used on many rehabilitation projects in ways that are consistent with the Standards.
Any proposed use of substitute materials should be reviewed within the framework of the following general issues:
- First, the need for replacing historic material is assessed,
- Second, the amount and location of replacement material is evaluated in relation to the building’s historic character, and
- Third, the appropriateness of a particular substitute material is considered regarding its appearance and other factors, such as the location of the application, the known physical compatibility of the substitute material relative to the historic material, and the performance of the material over time.
While the goal may be to achieve an exact match when replacing a historic material, most replacement, whether a matching historic material or a substitute material, involves some measure of change, even if only minor. For example, new marble available today — even from the same quarry — will not be exactly the same as historic marble panels that require replacement. Thus, the evaluation of any replacement material needs to take into account the quality of the match needed in terms of both appearance and performance for a given situation.
Need for substitute materials
According to the Standards for Rehabilitation, deterioration should generally be addressed through repair if in repairable condition. When the level of deterioration makes repair infeasible, or the feature or a component of it is missing entirely, there are circumstances in which the use of substitute materials may generally be considered appropriate, taking into consideration technical and economic feasibility reasons, including:
- the unavailability of historic materials,
- the unavailability of skilled artisans or historic craft techniques,
- inadequate durability of the original materials,
- the replacement of a secondary feature; construction of a new addition,
- the reconstruction of a missing feature;
- code-required performance; and
- for enhanced resilience and sustainability.
Economic feasibility is inevitably a concern when choosing a material for any part of a project, whether a historic or substitute material, but it should not be the sole determinant factor at the expense of maintaining the historic character and historic integrity of a building. Other factors may prompt the consideration of a substitute material, however, such as the cost of maintaining the historic material, because it is comparatively difficult or costly to reach or access, or the frequency of required maintenance the historic material needs. Additionally, where in-kind replacement material is found to be prohibitively expensive, it may be reasonable to consider a substitute that offers an alternative and is a good physical and visual match.
Amount and location of proposed application of substitute materials
Such factors as the design of a building, its history, the materials used, and the degree of craftsmanship combine to give a building its historic character. Different materials and features play different roles in the building’s historic appearance. Where a particular feature contributes significantly to the historic character of a building, the material or materials that make up that feature are likely to require a closer replacement match than materials making up a feature of lesser importance. A careful evaluation of the building and an understanding of the historic significance of its various materials and features will assist in determining the degree to which the use of substitute materials may be acceptable.
All replacement work reduces to some degree the historic character and integrity of a building. While the limited use of substitute material on a historic building is acceptable, there is a point where the amount of replacement material becomes excessive, when the overall sense of the building as a historic structure is lost, and when the building’s integrity is diminished to an unacceptable degree.
The overall visibility of a character-defining material or feature is an important determinant in whether substitute materials will be appropriate. Generally, the more visible a feature is and the more important that feature is to the building’s historic character, the more likely any change will negatively affect that character. For example, a replacement cornice using a substitute material proposed for a two-story building would have to match more closely the historic feature than one intended for a ten-story building. Materials on the rear elevation or side elevations partially obscured by adjacent construction may be of secondary importance to a building’s character.
Visual and other matches for the historic material to be replaced
Substitute materials, like all replacements, must closely match the design, color, surface texture, reflectivity, finish, details, and other visual qualities of the material or feature to be replaced. For example, the defining characteristics of a historic roofing material usually include its size and shape, as well as its thickness, color, and reflectivity. An asphalt shingle may be available that matches the size of a particular roofing slate, but its thin profile and granular surface may bear little resemblance to slate. A polymer–based slate substitute may match the thickness and surface texture but only be available in a larger size than the historic slate. Before one can evaluate the appropriateness of either substitute, one has to first identify the characteristics of the historic roof that are most important to how it is perceived on the particular building. This may lead to choosing one substitute over another, or rejecting all if the resulting differences appear to be too great.
While visual qualities are an important component, other factors should also be considered when evaluating the appropriateness of a particular substitute material. In some cases, if the surface texture of a substitute material differ markedly from the historic material, the building’s character could be diminished by its use.
Use of a substitute material should also take into account any differences in the physical properties of the new material and adjacent or related existing materials. For example, proposed substitute materials may have rates of thermal expansion and contraction and rates of vapor permeability that differ significantly from the adjacent historic material. In some cases, a substitute material may be so new that there is little information on how it will perform over time. When information on the durability, performance over time, and physical compatibility of a substitute material relative to adjacent historic materials does exist, it should be evaluated. Repair or replacement using physically incompatible substitute materials could damage surviving historic fabric and should be avoided.
Many modern materials used as substitutes are promoted as “maintenance-free.” Historic materials that require maintenance offer the possibility for indefinite life spans sustained by the renewal of maintenance. Materials that are maintenance-free may have more limited life spans and may not be repairable, with replacement being the only response to deterioration.
Substitute materials and applying the Standards for Rehabilitation
The Standards for Rehabilitation require that the replacement of a distinctive feature match the old in physical and visual properties and, “where possible," materials. While the use of matching materials is always preferred, the Standards purposely allow for the use of substitute materials when the use of original materials is not reasonably possible, such as in consideration of economic and technical feasibility. They also provide additional flexibility in the treatment of secondary, less distinctive features that are less important in defining the historic character of the property as well as missing features, new additions, and new construction. The Standards recognize that flexibility is appropriate to facilitate “a compatible use for a property… while preserving those portions or features which convey its historical, cultural, or architectural values” (definition of “Rehabilitation,” 36 CFR 67.2(b)).
Additional guidance »
- Preservation Brief 16: The Use of Substitute Materials on Historic Building Exteriors (rev. October 2023), provides more detailed information on this topic as well as information on some of the more commonly used, currently available materials that may have some applications as substitute materials and the properties of each that affect their suitability for use as substitutes.
- The Secretary of the Interior’s Standards for the Treatment of Historic Properties with Guidelines for Preserving, Rehabilitating, Restoring & Reconstructing Historic Buildings (2017).
2027; rev. October 2023
Last updated: February 13, 2024