Video
Preservation Brief 16: The Use of Substitute Materials on Historic Building Exteriors
Transcript
Preservation Brief 16: The Use of Substitute Materials on Historic Building Exteriors
David Trayte
Thank you for joining us as part of the National Park Service webinar series related to the interpretation and application of the Secretary of the Interior’s Standards for Rehabilitation. My name is David Trayte, and with my colleagues John Sandor and Amy Elizabeth Uebel, we will focus specifically on the updates to Preservation Brief 16, the Use of Substitute Materials on Historic Building Exteriors. The Brief was revised to better reflect current practice, the broader range of substitute materials available and in use today, and how these materials can sometimes provide more flexibility and less costly alternatives in meeting the Standards in many instances. The Brief provides a framework for evaluating the appropriateness of the use of a substitute material in the context of a rehabilitation project. The full guidance document is available for download in digital format on the Technical Preservation Services website.
Preservation briefs and other technical preservation publications and information issued by the National Park Service are intended to help property owners and others involved in historic preservation to formulate plans for the preservation, rehabilitation, and continued use of historic properties consistent with the intent of the Rehabilitation Standards. The Standards take precedence in decision-making and, together with the accompanying Guidelines should always be consulted first. In using the information contained in the preservation briefs, it should be considered in its entirety and portions should not be taken out of context. The Technical Preservation Services website should always be consulted to make sure you're using the most current version of any published guidance or technical preservation information. We have approximately one hour of content that we look forward to sharing with you.
So let's get started. We're going to break this down into three sections today. First, I will cover the basics of substitute materials, including the evolution of NPS guidance, the history and general concept of substitutes, and the various considerations for use. Amy Elizabeth will take us on a deeper dive into understanding materials, talking about the physical properties, visual appearance, and performance of materials. Then John will present a series of case study examples and how to apply the guidance to assess the use of substitutes for common building features. Before we get into the details of the current preservation brief, this timeline illustrates the evolution of NPS guidance related to substitute materials and how we got to where we are today. The original preservation brief 16 was published in 1988 and established guidance on the use of substitute materials on historic building exteriors. This was a valuable guidance document with general concepts and principles that have certainly stood the test of time. In 2007, following recommendations from the National Park System Advisory Board, additional guidance was developed to provide greater clarity to program users and implement program improvements. One of the resulting efforts was the evaluating substitute materials in historic buildings guidance, serving as a companion piece to the original 1988 preservation brief using the same general concepts that could be applied to building interiors. More recently the treatment guidelines were revied in 2017 to incorporate additional building types and materials reflecting contemporary and current best practices. The guidelines on flood adaptation for rehabilitating historic buildings were issued in 2021. Both of these documents, referenced here in the middle of the screen, utilized a recommended versus not recommended treatment format with more content related to the use of substitute materials and the Secretary Standards.
That brings us to the latest effort of guidance updates published in October of 2023, which included the preservation brief as well as the 2007 NPSAB guidance. Our intention was to bring the substitute material specific guidance in line with current practice, the 2017 treatment guidelines and other existing guidance, which had already moved the needle on this topic a bit. The 2023 updates continue to recognize that flexibility may sometimes be needed when it comes to new and replacement materials as part of historic rehabilitation project. Apart from the need to align the various existing guidance, we recognize the materials industry and the application of substitute materials has advanced significantly over the last 35 years. As you can see in the small sample here, there are many product options on the market today. Some perhaps a little bit more radical than others. Not all products have a place in historic application, but it's important to understand the materials market is continually shifting, and our intention was to better reflect the opportunities and challenges of the industry in that updated guidance. Our discussion today focuses on the preservation brief and the use of substitute materials on historic building exteriors, and we'll take a closer look at a number of these material types. Now, to be clear, the updated guidance is not a prescriptive list of materials that we're saying should be used without careful assessment.
The guidance is intended to be a framework for decision making for preservation professionals and practitioners relative to the standards. With that in mind, let’s step back and consider where the concept of substitute materials comes from. There is a long and well-documented history of using affordable and common materials to imitate more expensive or rare ones. As an example, these images show four different types of historic materials that were commonly used as a substitute for carved stone. Starting at the far left image is an example of concrete block cast to mimic quarried stone. Metal products like cast iron seen in the second image were commonly used in storefronts across the country. Scored stucco on a brick structure was another way to cheaply imitate stone facades, and lastly terra cotta with varying glazing techniques provided a lightweight alternative to stone. The words "imitative" and "substitute" are often used interchangeably when talking about materials. Whether we mean to or not, but there is an important distinction to be made.
The concept of "imitative" materials is detailed in the 2017 treatment guidelines and can be described as materials that were often used in initial construction rather than later repair and replacement purposes. Imitative materials, while evoking other materials, usually had distinctive qualities of their own and were not always a very close match in appearance to the historic material that they were meant to imitate. The images here illustrate one of the most famous uses of imitative materials. This is George Washington utilizing painted wood with sand applied to the surface to evoke rusticated stone at Mount Vernon. The term substitute materials is used to describe building materials that have the potential to match the appearance, physical properties, and related attributes of historic materials well enough to make them viable alternatives when historic materials require replacement. And the key concept here is that the substitute matches the historic when replacement is necessary.
Imitative materials are not necessarily appropriate for use as substitute materials, because they may not sufficiently match as required by the standards. Now we want to briefly touch on a concern that often comes up with substitute materials, and that is the concept of integrity. It's important to note that any replacement of a historic material with a new material can diminish the integrity of a building to some degree, even if it's an in-kind replacement. Mount Vernon is a good example to help illustrate this concept. We know that over time some of the wood siding has been replaced as part of a restoration effort. Even though the replacement wood matches exactly with sand applied to the surface, it's still new wood with new sand, and any new material can affect integrity. When considering substitute materials in context of the rehabilitation standards, we're talking about a greater material change with focus on the preservation of a building's historic character. Rarely would we expect to see a substitute material used in a restoration project.
The original preservation brief noted four distinct considerations for using substitute materials while also referencing general ideas of several others. In the update we have included four additional considerations. These are not new concepts but the result of formally bringing current practice and prior guidance in line with each other. Combined we've illustrated eight considerations or situations where substitute materials could potentially be appropriate for use. We recognize there may be other unique situations that warrant consideration of a substitute, but these eight are the most typical in our experience. Let's take a look at examples related to each of these considerations.
The first consideration is the unavailability of the historic material. Some manufactured materials that were used historically on buildings are just simply no longer made. A common example of this is asbestos roof shingles or siding. This 1924 apartment building had the original asbestos shingle roof on steep gables, a highly visible and character defining roof form as we can see in the lower left image. The asbestos had reached the end of its serviceable life and required replacement. As asbestos products are unavailable efforts were made to find a substitute that could appropriately match the historic material that we see in the middle image. Unfortunately at the time of the project there were very few options available that could come close to matching the visual and physical characteristics of the original asbestos.
If we think about this a little bit more carefully asbestos shingle was a historic substitute for slate so the visual qualities could be achieved by using the traditional material, which was feasible given the size of the roof from this building. The dimensions, the profile, and the color of the new slate were a good match to the historic asbestos. And given the unavailability of the historic roof material, an appropriate substitute could be considered in this case. The next consideration is the unavailability of skilled artisans or historic craft techniques. These issues are more likely when working with detailed elements like carved wood or carved stone. Modern technologies like 3D laser scanning have helped fill the gap of lost craft, but they can be difficult to implement on larger scale projects. As you see in these images, high-profile projects like the US Capitol exterior restoration, deteriorated marble cornice brackets were replaced in kind, utilizing professional stone carvers. In reality, most rehabilitation projects don't have the luxury of time to undertake this level of work. We're typically faced with the opposite end of the project spectrum, and the result is a consideration for the use of a substitute material.
Some historic building materials were of inherently poor quality or were not durable. Common examples could be soft sandstone or brown stone or perhaps more modern wood products. Many of you have certainly seen this type of comparison in the left image before. This is the stark 100-year difference between old growth lumber and the modern equivalent resulting in decreased decay resistance. The image in the middle shows a wood storefront where a piece of replacement wood frame is rotting at the base, likely from a lack of paint on the end grain before the board was installed. The surrounding historic wood elements outside of the red box remain in good condition.
The image on the right shows advanced wood decay at a window frame and surrounding elements. This isn't to say that we shouldn't ever use wood. In-kind replacement may still be the appropriate treatment, but installation details, the exposure, and location should be carefully evaluated in making the decision of how best to replace these materials when needed. I should note this particular example starts to blur a little bit with the consideration of unavailable material. We can certainly get wood for an in-kind replacement, but depending on the historic feature and the means in which an in-kind replacement would be achieved, durability may be a genuine concern that leads to the consideration of a substitute material.
Again, this consideration extends beyond wood and could be applicable to a number of traditional building materials, but this is just a common example that many of us have been faced with. A less distinctive secondary feature may have more flexibility in the replacement material and required match. However, the overall character should still be preserved. In this example of a late 19th century residential property, the new and existing porches and stairs received cellular PVC railing systems as secondary features on a rear elevation as we see in the image on the right. Since these features are not readily visible from the street elevations that we see in the image on the left, the overall character of the building is well preserved. When constructing a new addition using materials that evoke the historic material without exactly matching it can be an effective means of achieving the balance between compatibility and differentiation for new construction. In the wood frame residential building shown here, a new addition was constructed at the rear of the historic building. The use of fiber cement siding gives the addition a compatible character without having to replicate the exact lap exposure or board thickness of the historic siding. The standards for rehabilitation do not require a missing feature to be reconstructed. However, the use of a more cost -effective substitute material can often create the opportunity to undertake such work. As long as the work is substantiated by historic documentation, reconstructing a missing feature in a substitute material may offer a positive impact to the overall character of the building.
This turn-of-the-century office building had been victim of a modern renovation that included recladding the entire facade and, unfortunately, demolishing the original decorative terracotta elements in the process, and we can see the aftermath of that in the upper right -hand image. Rehabilitation efforts removed the nonhistorical façade and replicated the well-documented missing features, utilizing a combination of traditional and substitute materials. Given the quantity and location, glass fiber reinforced polymer, or what we refer to as GFRP, was used for elements at the upper floors to restore the original character of the building, and we can see the result of that in the two post-rehabilitation photos here. Rehabilitation projects often trigger compliance with code requirements that were not in place when a building was constructed. Although building codes may often allow for the retention of historic materials and assemblies, substitute materials can offer an alternative in situations when the historic materials are non-compliant and cannot otherwise be reasonably retained. And this is an example of a dense residential district consisting of late 19th century wood frame buildings on very tight lots. Non-combustible cladding materials may be required due to the proximity of wood frame structure to the lot line and other buildings. And this is an example of a situation where a change in materials on the side elevations, which have limited visibility, may be appropriate to meet code requirements. Depending on the building, this could impact a number of different features including the sighting, soffits, eaves, or perhaps other roofline details.
Code requirements may differ for building types and locations, so the documentation and a thorough understanding of those requirements is important when assessing the use of a substitute material under this consideration. The final consideration is enhanced resilience and sustainability. You may find yourself with a project where it is necessary to try to minimize impacts on a building's historic character as much as possible while still adapting it to be more resilient. We typically see this situation in projects that are faced with wildfires, flooding, projects in parts of the country that are prone to hurricanes or other extreme weather events.
This industrial mill sits on the bank of a river that is prone to flooding, located within the established floodplain. The existing condition of the building prior to rehabilitation had historic window openings that had been previously infilled. As part of the rehabilitation for mixed residential and commercial use, those windows were proposed to be reopened. One of the unique challenges with this building is the fact that ground floor windows are located within base flood elevation, meaning the anticipated height of a flood could potentially be higher than the sill of the windows. The solution was to construct the ground floor window assemblies with aquarium glass and cellular PVC trim in order to withstand repeat flood inundation, which in turn allows for occupiable spaces within the historic building. The key here is finding a balanced approach that maintains historic character while meeting resilience and sustainability goals.
As I mentioned previously, these are eight considerations that we use both in practice and outlined in previous guidance that we have formally aligned here in the updated preservation brief. But that's not to say you couldn't find yourself with a unique situation in your particular project that would warrant discussion and consideration of something different than what we have listed here. Economic factors are often cited as justification for the use of a substitute material. The standards are inherently flexible and are applied taking into consideration the economic and technical feasibility of each project. However, cost alone should not be the only factor in assessing the use of a substitute. The historic character of a building must still be retained. But there are other factors that play into duration, such as the cost of maintaining the historic material, because it is difficult or costly to access, or perhaps the frequency of maintenance the historic material requires. Maintenance is not the reason to replace, but it can be the reason to choose something different when you do have to replace it. The example on the left is an early 19th century church, with many changes over time, to the point where it's not quite clear what material is historic. Proposed work to the steeple includes reusing any material that is determined to be historic, and the rest will be rebuilt with a mineral-polymer composite. The use of a substitute in this instance will offer greater dimensional stability and therefore a longer finish life, which means reducing the frequency that repainting will be needed, which comes at considerable expense because of the need to scaffold a building like this for access.
Another factor in feasibility can be when an in-kind replacement material is found to be prohibitively expensive. In this instance, it may be reasonable to consider a substitute that offers an alternative and is a good visual and physical match. The example on the right as a 1930s nurse's dormitory as part of a larger hospital complex. The large masonry building had a historic slate roof that was not particularly distinctive in detail, but it was expansive in size. The cost of an in-kind slate replacement was determined to be prohibitively expensive, and a polymer slate product was able to accurately match the visual and physical characteristics of the historic slate. The overall character of the building is maintained, as is the character of the multiple building complex. Lastly, not all substitute materials are cost-effective replacements. Long-term durability and repairability are other important factors to consider in conjunction with the initial cost. And now that we've covered the considerations for when you might use a substitute material. I'll turn this over to Amy Elizabeth to take a closer look at the characteristics of materials.
Amy Elizabeth Uebel
The options for potential substitute materials can be overwhelming with the sheer number and variety of available architectural products. Furthermore, products change over time as materials develop and the market changes. What wasn't an acceptable substitute material 10 years ago might be acceptable today. Additionally, there's no list of materials that always meet the standards. Products have advantages and disadvantages and must be evaluated based on the unique needs of the project. The search for a substitute material can be made even more difficult when you try to decipher the cut sheets provided by manufacturers used to market their products. In order to help, Preservation Brief 16 provides an overview of commonly seen contemporary materials that may be appropriate for use as a substitute material.
For each of these materials, the brief provides information on the material self, including a description of the basic composition, its use in installation characteristics, any general manufacturing characteristics, and expected physical attributes. It's important to note that the attributes listed in the preservation brief are high level and certainly don't include all possible options that can be found in the market today. Just because the material is listed and discussed does not mean it is appropriate for use in all situations. You must evaluate the suitability of any potential proposed substitute material and determine if it is appropriate for use in your project.
In order to help your decision making, the brief provides a list of properties for the potential substitute material. This list details the various properties that the material is known for, including characteristics regarding the composition, water absorption, flame spread, and insect resistance among others. It also lists typical finishes that can be found on these materials and identifies areas where the material is commonly installed and any specific physical characteristics related to that usage. While the preservation brief offers information on potential substitute materials, the attempt here was merely to provide a vocabulary and framework that provides guidance. Just because we discussed the material does not mean it is appropriate for use in all cases.
In order to choose a suitable substitute material, you must understand how any material and its potential substitute relates to and interacts with each other. What is the material made of? What advantage does it give? How does it absorb or retain water? Does it expand with heat or moisture? These and more are all valid questions. The amount and types of architectural materials available for purchase today can be dizzying. When looking to better understand the properties of architectural materials, it's easiest to think of materials as a long, slow, steady march towards a more sophisticated material. That's not to say a better material, but rather a material that imitates, decreases cost, adds resilience, or otherwise provides a quality that doesn't exist with another material. This puts the burden on the user to better understand the materials they have or may want to use. Sometimes a professional firm is needed to come in and give research, but a rudimentary understanding of basic material properties can shed light on how materials will relate to each unique situation, environment, and the surrounding materials.
The timeline provided on this slide is intended to illustrate how materials developed over time. Earlier, simpler materials such as wood, stone, and iron occasionally needed additional properties to achieve structural or aesthetic needs. Concrete and terra cotta were developed as alternatives. By the early 20th century, plywood, asbestos products, cast stone, and aluminum exploded into the market as lighter, less corrosive, and cheaper materials that required less maintenance. These materials were able to achieve these properties by being a more processed version of their natural simpler counterparts. They introduced the use of adhesives and additives to achieve these new properties and to create a unique material. By this point in time, many of these materials are now considered historic materials in their own right and occasionally require the use of a substitute. But by the late 20th century and 21st century, knowledge and technology continued to hone in and develop how basic materials, such as wood, stone, and metals, could be processed and enhanced to create new alternatives to traditional building materials. Most of these new materials are considered to be composites.
Composite materials are a combination of two or more materials with different physical and chemical properties that are combined to make a unique material. To make a composite material, base materials or fillers are combined with various binders and additives. Often the base material is a form of wood chips, cellulose fiber, or a mineral. These binders can be cementitious or polymeric using adhesives or a plastic resin. This often creates a homogenous material without the distinctive planes, textures, and features that are characteristic of stones, woods, and early metals. Usually, composite materials will be processed into a highly refined form, then heated and extruded or cast into specific shapes. This results in a more uniform appearance than what is found in more traditional materials. However, developing technology is able to create different forms, more nuanced colors, and colors that permeate the entire material or are veneered with other more desirable materials.
You don't need to be a material scientist to understand the basic properties of materials. Materials have three basic categories that define how they will react in an architectural setting. Physical, visual, installation and performance. Physical characteristics, such as permeability and stone that you see on the right-hand side of the screen, finds how far a moisture can be absorbed into a material, but it will also describe if a product will swell in the heat, or if it will be prone to biological growth. Visual characteristics shed light on how something will appear to passers-by, and the installation and performance guide when something can be used in specific circumstances.
When comparing a material and its potential substitute, it is helpful to understand how the materials relate to each other in each of these three categories. How are the dimensions similar or different? Is the texture the same? Do you need it to be the same? Will the materials shrink and swell with water, or heat, or time? What is the service life of the proposed replacement material and how does that relate to the one it might replace? It's generally recommended that any potential substitute material has similar physical characteristics to the material it is replacing.
Physical characteristics that should be compared include dimension and scale. Is a material orthotropic? Meaning does its behavior vary differently depending on the direction, or is the material isotropic, meaning it will behave the same in all directions. Porosity, or how much water can be absorbed in a material, and permeability, or how far moisture travels in that material, will vary greatly depending on if it is an older or newer material type. Most materials will swell and shrink with moisture, heat and time. How will two unique materials work together or against each other? Some modern composites have different strengths and support requirements. This can require extra or lessen additional support for the replacement material. Lastly, additives are intended to increase performance, but they don't always increase performance in perpetuity. Some, such as antimicrobials, only work for a short period of time. Others, such as UV stabilizers, may require replenishment.
Visual characteristics can be some of the hardest to replicate. Is this surface to be painted or left unpainted? What was the finish historically? How close will observers be to the material. If something is on a cornice 20 ft from the ground, it likely will not require as close of a textural match than a feature that is at eye level. Similarly, color and the subtle color variations that are prominent features of stone, cast stone and wood can be difficult, though not always impossible to match with homogenous manufactured materials. Though physical attributes dictate this last set of characteristics, installation and performance characteristics are important enough that they should be understood on a higher systematic level. The expansion and contraction of materials will require planning to ensure that bowing, sagging, or gaps do not appear where they are not intended to be. Some composites may increase durability in some physical aspects, but at the sacrifice of others. Does the material change how something should be adhered or supported?
Lastly, many new materials may appear to have specific performance advantages over their traditional counterparts. But these characteristics may be only designed to work for a short amount of time. It can be difficult to determine the durability of any material, but especially so for new materials with a shorter track record. Warranties are not the same as service life, and the repairability and durability of both the historic and potential replacement should be understood prior to any decision making. For example, while tongue and groove flooring in a pre-finished style may be a cost -effective option, it can only be replaced, which may create larger and more costly maintenance cycles if placed in a high -traffic area.
Additionally, many of you may remember that some earlier substitute materials from 30 or 40 years ago may have chalked, degraded, faded, or otherwise proved to be a less than ideal long-term solution. However, just because one product did that does not mean that all products will do the same into the future as materials continue to evolve. Now that you know everything about modern materials, you're ready to go. Well, if not, the preservation brief has pre-digested many of the physical and visual characteristics of potential substitute materials. The chart on the left is not comprehensive, nor is it a list of always acceptable materials. It's merely intended to provide a framework for you to make educated decisions on the appropriate use of materials in your unique project.
Furthermore, the chart only reflects materials commonly considered for use as a substitute. The market will continue to change and develop in the upcoming years, and it is expected that materials may improve, disappear, or change. The preservation brief also provides a quick reference to the types of materials on the market today and where you might find them used. Lastly, we've provided a list of questions relating to the specific building features that are intended to act as a starting point for the evaluation of a potential substitute material in its particular application. These questions should help narrow down and eliminate what materials are and are not acceptable for use as a substitute material. We now have looked at the various circumstances where it might be appropriate to consider using a substitute material.
John Sandor
And we have now looked at the various circumstances where it might be appropriate to consider using a substitute material, and then we’ve had an introduction to the properties affecting both behavior and appearance of materials that we need to take into account, as we explore what, if any, of the substitute materials may be appropriate to use. While the chart does match various classes of materials to the building features where they may have some applicability. It should not be misconstrued as a list of materials that can always be considered appropriate for those features, and is not the appropriate starting point for considering replacement. The process of exploring use of substitute material needs to begin with an assessment of the feature where the material is to be used. What has triggered the need for the replacement? How much replacement is needed? What are the physical performance and visual characteristics the new material needs to have?
Now let's go through the various building features from the chart, looking at some specific substitutes, their limitations, and the issues that affect their potential for success. When architectural features that are metal need to be replaced, it's often just a matter of turning to a different metal. If the feature is a painted one, such as standing seam metal roofing, the issues with the change will be less about appearance and more about the performance of the material. Thus, turn metal, not now available, goes to galvanized steel, enameled aluminum or even copper. Where different metals intersect, coatings or membranes need to be used to prevent galvanic corrosion. And, of course, expansion and contraction rates need to be taken into account especially when different metals are in the same assembly. A typical metal for metal substitute is cast aluminum for cast iron as illustrated by the cast eagle depicted. An example of the use of a completely different material for metal is the replacement of missing cast ornamental elements that are part of the grill work inside the waiting room windows of the Michigan Central Station. Here, 3D printing proved to be a cost effective way to reproduce the needed repetitive elements. They are supported within the surviving metal frame and painted a match and probably could have been a good solution even in exterior location.
The soft red sandstone at the base of this 19th Century building had not fared very well and it was most likely a local stone no longer quarried and thus hard to match with stone from another source. The needed new pieces needed to be inserted incrementally into an existing loadbearing assembly making cast stone the appropriate choice. The similarity of physical properties of cast stone to natural stone are quite important for this type of installation. Unique molds were used for each stone, capturing the randomness of rock-faced tooling. It's hard to distinguish the new material from the original without close inspection. Color, which is an integral part of the material, not surprisingly has remained a good match over the 40 years since this work was done. Glass fiber reinforced polymer was used to replace the top element of the terra cotta corners of the Michigan Central Station on the left, and glass fiber reinforced concrete was used for missing spandrels and other elements on the shipper block in Peoria on the right. Both materials offer great advantage for replacing terra cotta or stone because of their typically shorter lead times, and often importantly, their reduced weight. The decay of the steel supporting terra cotta is a big part of the deterioration of these features. With less substantial steel is needed to support the new feature, the potential need to disrupt the existing backup masonry to make major structural modifications is reduced. Such lighter materials can make it easier also to achieve needed seismic requirements. These materials are particularly successful where an entire feature is being replaced in the expansion and contraction of the materials, which are different from stone or terra cotta, can therefore readily be accommodated. Because glass is the fiber most often used to reinforce concrete products GFRC is the shorthand most often used for it. It has the advantage over GFRP or glass fiber reinforced polymer in some situations, because it can be fire rated and has an expansion and contraction a little bit closer to that of stone. GFRP will always have a coating to create the finish of the material it is imitating. The color and finish can be part of the surface of the material built into the manufacturing process or may be applied postproduction. Fiber reinforced concrete, on the other hand, may have color integral to the material, but can also be coated, which is especially likely when a glaze surface is being reproduced. Though coatings can last quite a long time, they do not have the permanence of color that is an integral part of the material.
Traditional porch decks were nearly always constructed of tongue and groove boards, producing a continuous surface that was always painted. Subject to severe weather exposure, even the best constructed and finished wood porch floors have a finite lifespan, especially when constructed with the wood that is readily available currently. Four different materials are being used to produce tongue and groove porch decking, cellular PVC, mineral polymer composite, cellulose fiber polymer composite, and cell lead polymer or PVC. All are currently available as actual tongue and groove decking. Each material offers slightly different properties. Some are available only pre-finished with a capping surface material eliminating the need to paint. Though long-lasting these factory applied surfaces are not usually a suitable base for paint which can be a limitation in terms of renewing surface when it is worn or repairing one that is damaged. Other products are designed to accept paint making their surface renewable and visually more similar to painted wood. For safety reasons, pre-finished material can reasonably expect to have a little bit of a texture for slip resistance.
These materials and more are used to produce square edge boards as well. Historically such boards would have been limited to very utilitarian applications, but they should be no less appropriate for use on a new porch or a deck added to a historic -- as a of a new feature to a historic building. And for use in secondary locations such as the porch on the right where standard pressure treated decking coated with an opaque stain or paint might also be considered appropriate to use. If a wood deck floor would need to have a finish to make it compatible, then any composite material should not be one with a bold grain pattern imitating a natural wood finish. This second-story porch is a pre-finished material that has held up well for the several years that has been in place at the Southern House Museum.
What is significant about this installation is that the product chosen was acceptable to install directly atop a water roof membrane, making the deck effectively a roof and protecting the ceiling below as well as the structural framing itself from water in its effects. Other tongue and groove products can also be installed over a membrane but require intervening sleepers as part of the assembly to facilitate drainage. For any wood frame building, siding as a major contributor to its character. And where historic wood siding is in place, any needed replacement will need to be a good visual match. Though there were a variety of profiles used particularly in the late 19th Century, basic lap siding or clapboard could be found throughout the country from the 18th through the 20th Centuries. Its thickness and exposure might vary with the date and style of the building, but a four-inch exposure was fairly typical at 19th Century buildings, whereas a sighted with a wider exposure might be more commonly found on an early 20th Century craftsman-style house. The thickness of the butt edge of a board could vary as well, though clapboard was generally no less than 3/8 in. thick and no thicker than 3/4 in., with the wider exposures generally being at the thicker end of the spectrum.
The photo on the left shows wood siding on the front of this house with a narrow exposure and a butt edge that is between 3/8 in. - 1 /2 in. thick. The siding depicted on the side wall of the house is fiber cement with both a wider exposure and a thinner butt. Though the exposure could have been matched by increasing the lap. Even if the exposure was the same, the effect would not be a match here because the shadow line at the butt edge is so obviously less. The shadow line is a function of the thickness of the material, which is limited by what is available from the manufacturer, and cannot be customized or adjusted. For fiber cement and wood fiber composite, that dimension is typically 7/16 in., though 5 /8 in. material is also available in cement board. Both of the widely available substitute materials are thinner than any traditional wood siding, and the thicker version of the cement board option is too thick to accurately replicate wood siding traditionally used for narrow exposures such as 4 in. When the thickness differential of the substitutes is discernible, it limits their appropriate use to very secondary elevations where a less accurate match could be acceptable. Some siding materials have heavily embossed surfaces, looking like a piece of wood that's been sandblasted, as in the middle photograph. Some are available only with a smooth surface and some offer both options by flipping the board.
Since traditional wood siding was planed, only a smooth option is appropriate for replacing historic wood siding. Though one might create a board and batten siding from materials available as trim boards, the only other historic siding profiles available in a substitute material at present are beaded edge in fiber cement and cove or drop siding available in the mineral-polymer composite. None of these materials have the same properties as wood. Cement board is noticeably heavier and the polymer in mineral one has less tensile strength, both of these factors being ones that affect the handling of the material in installation. But they can both be affected less than wood by moisture, and some can offer better fire resistance, which can be critical in some locations.
The landmark home of Charles and Ray Eames and Pacific Palisades utilized a painted asbestos cement board to face an insulated panel used instead of glass in some locations in the steel frame. Panels that had previously failed, probably due in part to failing to coat the edges before installation, were replaced previously with a wood fiber-based panel, which has also failed. The original Cemesto panel as well as the earlier replacements are both depicted in the center photo. Replacements currently underway are using a fiber cement panel closer to the material of the original. Given the importance of the property, sample installations have been done to understand the weathering and predicts the long-term suitability of the new replacements. Polymer-based roofing shingles have become a popular replacement for slate roofs on historic properties and have great potential to create much of the visual effect of a slate roof with both durability and possibly some cost savings. Unfortunately, there are limitations to what is currently being manufactured that limit the appropriate application of this material. These polymer products are molded to mimic the irregular profiles created at the edges of real slate shingles when they are cut. Different manufacturers offer different shapes, but only one manufacturer offers narrower alternatives to the 12 in. wide shingle. And then all the widths are the same length, creating some inefficiency for matching smaller slates that have a proportionately shorter exposure. Others offer multiple widths only as a blend intended for creating a picturesque or rough stick effect. The visual effect of this size limitation is apparent in the photo of these adjacent row houses. The one on the left has its original slate roof, though now painted. The historic roof consists of both square edges and point shingles, not much more than 6 in. wide, and with a similarly short exposure. The synthetic slate roof on the right pretty closely matches the exposure of the shingles. It's easy to reduce the exposure of overlapping material, whether it be siding or roofing, but it is a little inefficient. If wasting material was not the issue, though, one might wonder why the polymer slate was not just cut to width and shape as real slate would be.
The inset photo on the bottom left illustrates what might be seen as a problem with cutting these shingles to achieve custom sizes and shapes. The material is easy enough to cut, but a cut yields a very crisp edge, which is in contrast to the molded edges that are designed to mimic the way natural slate breaks when cut. It may, however, be worth considering just how big a factor this edge appearance is at the distance from which a roof is typically scene. Would the edge profile really matter more than the overall texture of the roof created by the size of the shingle? And I might say there are some of these polymer slates that have a kind of hollow back and therefore you don't want to expose the cut edge of those no matter what. For all the effort to achieve a matching effect, the larger scale of the new material creates a significantly different appearance, giving the surface an overall texture that is inconsistent with the scale of the building. For this small amount of roofing needed for this prominent feature of the house, the savings resulting from using a substitute slate would not likely have been sufficient to warrant so great a change in character, especially in a high value neighborhood in a larger area where there is ready access to real slate and the skills to install it. For a large building like the block long school on the left, a small difference in the size of a shingle has a lesser visual impact than on a smaller building. And these larger buildings generally have larger slates in the first place. In such a case, there can be little change in the historic character with the use of this substitute material. But the close-up inset of the school roof illustrates an issue that does affect its appearance, that being slightly lifted edges that some brands of this product have repeatedly faced. Though the manufacturer attributes it to failing to follow recommended installation procedures. It does at any rate not seem to affect the weatherproofing function of the roof, however much it may be a little disconcerting to look at. Both concrete and tile also are materials that attempt to mimic slate, but for issues that can include cost and weight, and for some products appearance as well that's not all that convincing, they are much less frequently considered. While we do not usually consider fiberglass-based asphalt shingles, a substitute material for slate.
There are situations where some of the fiberglass products that are designed to evoke the appearance of slate actually do a better job of approximating the size and proportion of smaller slates than the larger polymer products. On the right-hand image, the right plane of this roof retains its historic slate, whereas the plane of the roof with the dormer has a multi-tab fiberglass-based asphalt shingle with a tab size and proportion that is very similar to that of the retained slate on the other side. The granular texture of the new shingle is not the material's most salient aspect at the distance that it's typically viewed. The overall texture created by the size of the shingle is far more critical to maintaining the existing character than the surface texture of any individual element. Wood shingles have too short a lifespan to survive long enough to often become historic. Even when a building still has a wood roof, it's likely to be a second-generation one.
Many projects are looking to reestablish the appearance of a wood roof long gone when it's known that wood was the historic roofing material. Wood shingle roofs present a significant challenge for matching with any substitute material, though wood shingle substitutes are produced in concrete and polymer composites. Because wood is a weathering natural material that tends to have a good bit of subtle color variation, choosing a substitute that is consistent with the weathered color of the wood without being too uniform in color will probably have the most convincing effect. Because of the fire risk of wood shingles, even if treated, the justification for using a substitute may be more readily made in a high fire risk area, especially if other risk factors to the building are also being addressed. Most substitutes available can have a class A flame-spread rating, if not being outright non-combustible. Just as fiberglass-based asphalt roofing is not similar enough to slate to be considered a substitute for it, neither are those fiberglass shingles that attempt to evoke wood shingles considered a substitute for replacing an existing historic wood roof. When the color is carefully selected, some products in this class of material may approximate the scale and texture of a wood roof adequately to be a good choice for a roof that was historically wood, but has long been lost to other asphalt shingles. The image on the right juxtaposes whether wood shingles on the sidewall and a typical small tab fiberglass shingle on the roof, illustrating just at best how close in appearance the two materials can be.
Wood trim is typically a painted feature with a smooth surface, making many substitute materials with a smooth, paintable, or pre-finished surface appropriate to consider. None of the several possibilities behave exactly like wood though and understanding those differences is critical to using them successfully. In fact, the failure to use real wood correctly is responsible for its failure in many applications, as an image David showed earlier illustrated. Much historic trim was not designed to shed water the way most wood siding does, making it a feature frequently prone to decay. Moreover, trim typically involves a lot of joints with material oriented in different directions.
Understanding the relative movement characteristics of the chosen material is critical to its successful use. Most of the substitute materials suitable for use as trim, unlike wood, are isotropic, regardless of whether their expansion and contraction is driven primarily by heat or moisture. They will move proportionate to their dimensions, that is, equally in all directions. For example, moisture will have no effect on the movement of a piece of cellular PVC. It will expand the contract with temperature change, though. But for a short section, that temperature-driven movement will typically be much less than the movement of an equal-sized piece of wood will have across its grain as it wets and dries. By contrast, a 16 ft. long wood board will not change in length noticeably from either heat or moisture, whereas an equivalent long section of cellular PVC will change dimension enough with normal solar heat gain to require accommodation at the ends for the change in dimension or fasteners to resist it. The base of a bay window at the center of the slide makes good use of cellular PVC by its design. Most of the components are short pieces and the longest ones terminate at corners where some expansion can be accommodated. Also, it is painted white, reducing the potential for solar heat gain.
All of the materials listed as potential substitutes for trim can be painted, and in fact, except for the cellular PVC, must be painted if not available pre-finished. The tracery in the steeple on the far right illustrates a potential for cellular PVC to be shaped with woodworking tools. Most of these materials are available as molding, typical of what one can find in wood, as seen in the bead board and molding in the mineral-polymer composite used for the soffit on the left. The isotropic structure of cellulose polymer composites can make them ideal where wide pieces are needed. And wood boards, wood comp., and plywood would require many steps of priming and sanding to yield a uniformly smooth surface. While not prone to movement with heat like PVC, it is subject to moisture-related movement. And being isotropic, it can move more than wood over a long dimension. Such was the case illustrated by the storefront base on the left. The material must not have been thoroughly dry when it was installed because it clearly shrank enough to merit some remedial caulking, done very poorly and ineffectively. Previous versions of this composite material by various manufacturers have not always performed well, so using material from a reputable manufacturer would be advised, and most importantly, following the manufacturer's advice about finishing and fastening. The need to coat ends before assembly was not understood in the otherwise successful use of the material for the porch post illustrated on the right, where one can see evidence of failure already emerging at the base of this column in the close -up image. This is damage one might have expected at the base of a wood column that was similarly installed without sealing or painting its end. Cement board may be less frequently used for trim than other materials we've just looked at. Perhaps because it's not as easily cut and cannot be shaped or worked with woodworking tools as most of the others can. It can, though, be successfully used, as it was in this example, where it faces the exterior surface of a wall configured to create the appearance of an early vehicular door in this historic firehouse. Simple flat boards over panels were all that were needed to execute the design. Without the moisture -driven movement of wood or cellulose composites or the temperature -driven movement of cellular PVC, it should provide a relatively stable assembly that will not be prone to paint failures. The material is not recommended for ground contact. So one might hope that the small concrete curb on which it sits is minimally adequate to prevent any moisture problem at the base of the assembly. Well, much of our focus in the brief is about when it is acceptable to use a substitute material and how to evaluate what makes an adequate match. I hope some of these examples illustrate that there are no ideal materials and what might be appropriate. One application could be unacceptable than another. The successful use of any of them when it's decided they can appropriately meet the applicable standards is dependent upon understanding their properties and following installation and finishing recommendations. If you are in a position to make decisions about the use of substitute materials, your life would be a lot easier if you just had a list of approvable materials.
But I hope our presentation has illustrated why creating such a list is not an effective approach. Using a list to replace situation specific evaluation would miss achieving some good replacement solutions while allowing others that either perform poorly or diminish historic character. Thank you.
David Trayte
I'll jump back in to help wrap us up here today. So, to recap our discussion, we've talked about material types, material properties, situations we might consider the use of a substitute material. We also looked at the examples of applying the guidance in context of common building features. Our intention in updating the preservation brief was to create a framework for decision making in addressing the material challenges all of us are faced with. So, if you find yourself in the position of having to ask the basic questions, is a substitute material appropriate, doesn't meet the standards. There are three important points we hope you take with you today to help make that determination. First is the need to assess whether replacement of historic material is necessary? If so, then evaluate the amount and location of a replacement material in relation to the building's historic character. Lastly, consider the appropriateness of a particular substitute material regarding its ability to match appearance, and other factors such as the location of the application, the known physical properties and compatibility of the substitute material relative to the historic material, and the performance of that substitute material over time. We want to be clear that the updated guidance is not a blanket approval for the use of substitute materials. But if we carefully evaluate all the factors that we've talked about here, as well as the written guidance, we can make well-informed decisions in context of the standards. And with that, we thank you for joining us for the webinar presentation. For access to all National Park Service guidance publications and to subscribe to updates from technical preservation services, please visit our website at www.nps.gov/tps.
Description
This webinar discusses Preservation Brief 16: The Use of Substitute Materials on Historic Building Exteriors and the application and interpretation of the Secretary of the Interior's Standards for Rehabilitation, including considerations for the appropriate use of substitute materials and case-study examples.
Duration
1 hour, 3 minutes, 3 seconds
Date Created
04/15/2025
Copyright and Usage Info