Preservation Briefs: 16 The Use Of Substitute Materials On Historic Building Exteriors
PRESERVATION BRIEFS: 16
THE USE OF SUBSTITUTE MATERIALS ON HISTORIC BUILDING EXTERIORS
The link immediately below connects to the latest version of National Park Service Preservation Brief 16:
Sharon C. Park, AIA
This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:
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Briefs 15-23 - $5.00
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THE SECRETARY OF THE INTERIOR'S STANDARDS FOR REHABILITATION
REQUIRE THAT "DETERIORATED ARCHITECTURAL FEATURES BE REPAIRED
RATHER THAN REPLACED, WHEREVER POSSIBLE. IN THE EVENT THAT
REPLACEMENT IS NECESSARY, THE NEW MATERIAL SHOULD MATCH THE
MATERIAL BEING REPLACED IN COMPOSITION, DESIGN, COLOR, TEXTURE, AND
OTHER VISUAL PROPERTIES." SUBSTITUTE MATERIALS SHOULD BE USED ONLY
ON A LIMITED BASIS AND ONLY WHEN THEY WILL MATCH THE APPEARANCE
GENERAL PROPERTIES OF THE HISTORIC MATERIAL AND WILL NOT DAMAGE THE
When deteriorated, damaged, or lost features of an historic
building need repair or replacement, it is almost always best to
use historic materials. In limited circumstances, substitute
materials that imitate historic materials may be used if the
appearance and properties of the historic materials can be matched
closely and no damage to the remaining historic fabric will result.
Great care must be taken if substitute materials are used on the
exteriors of historic buildings. Ultra-violet light, moisture
penetration behind joints, and stresses caused by changing
temperatures can greatly impair the performance of substitute
materials over time. Only after consideration of all options, in
consultation with qualified professionals, experienced fabricators
and contractors, and development of carefully written
specifications should this work be undertaken.
The practice of using substitute materials in architecture is not
new, yet it continues to pose practical problems and to raise
philosophical questions. On the practical level, the inappropriate
choice or improper installation of substitute materials can cause
a radical change in a building's appearance and can cause extensive
physical damage over time. On the more philosophical level, the
wholesale use of substitute materials can raise questions
concerning the integrity of historic buildings largely comprised of
new materials. In both cases the integrity of the historic
resource can be destroyed.
Some preservationists advocate that substitute materials should be
avoided in all but the most limited cases. The fact is, however,
that substitute materials are being used more frequently than ever
in preservation projects, and in many cases with positive results.
They can be cost-effective, can permit the accurate visual
duplication of historic materials, and last a reasonable time.
Growing evidence indicates that with proper planning, careful
specifications and supervision, substitute materials can be used
successfully in the process of restoring the visual appearance of
This Brief provides general guidance on the use of substitute
materials on the exteriors of historic buildings. While substitute
materials are frequently used on interiors, these applications are
not subject to weathering and moisture penetration, and will not be
discussed in this Brief. Given the general nature of this
publication, specifications for substitute materials are not
provided. The guidance provided should not be used in place of
consultations with qualified professionals. This Brief includes a
discussion of when to use substitute materials, cautions regarding
their expected performance, and descriptions of several substitute
materials, their advantages and disadvantages. This review of
materials is by no means comprehensive, and attitudes and findings
will change as technology develops.
***HISTORICAL USE OF SUBSTITUTE MATERIALS***
The tradition of using cheaper and more common materials in
imitation of more expensive and less available materials is a long
one. George Washington, for example, used wood painted with sand-
impregnated paint at Mount Vernon to imitate cut ashlar stone.
This technique along with scoring stucco into block patterns was
fairly common in colonial America to imitate stone.
Molded or cast masonry substitutes, such as dry-tamp cast stone and
poured concrete, became popular in place of quarried stone during
the last century. These masonry units were fabricated locally,
avoiding expensive quarrying and shipping costs, and were versatile
in representing either ornately carved blocks, plain wall stones or
rough cut textured surfaces. The end result depended on the type
of patterned or textured mold used and was particularly popular in
conjunction with mail order houses. Later, panels of cementitious
perma-stone or formstone and less expensive asphalt and sheet metal
panels were used to imitate brick or stone.
Metal (cast, stamped, or brake-formed) was used for storefronts,
canopies, railings, and other features, such as galvanized metal
cornices substituting for wood or stone, stamped metal panels for
Spanish clay roofing tiles, and cast-iron column capitals and even
entire building fronts in imitation of building stone.
Terra cotta, a molded fired clay product, was itself a substitute
material and was very popular in the late 19th and early 20th
centuries. It simulated the appearance of intricately carved
stonework, which was expensive and time-consuming to produce.
Terra cotta could be glazed to imitate a variety of natural stones,
from brownstones to limestones, or could be colored for a
Nineteenth century technology made a variety of materials readily
available that not only were able to imitate more expensive
materials, but were also cheaper to fabricate and easier to use.
Throughout the century, imitative materials continued to evolve.
For example, ornamental window hoods were originally made of wood
or carved stone. In an effort to find a cheaper substitute for
carved stone and to speed fabrication time, cast stone, an early
form of concrete, or cast-iron hoods often replaced stone. Toward
the end of the century, even less expensive sheet metal hoods,
imitating stone, also came into widespread use. All of these
materials, stone, cast stone, cast-iron, and various pressed metals
were in production at the same time and were selected on the basis
of the availability of materials and local craftsmanship, as well
as durability and cost. The criteria for selection today are not
Many of the materials used historically to imitate other materials
are still available. These are often referred to as the
traditional materials: wood, cast stone, concrete, terra cotta and
cast metals. In the last few decades, however, and partly as a
result of the historic preservation movement, new families of
synthetic materials, such as fiberglass, acrylic polymers, and
epoxy resins, have been developed and are being used as substitute
materials in construction. In some respects these newer products
(often referred to as high tech materials) show great promise; in
others, they are less satisfactory, since they are often difficult
to integrate physically with the porous historic materials and may
be too new to have established solid performance records.
***WHEN TO CONSIDER USING SUBSTITUTE MATERIALS IN PRESERVATION
Because the overzealous use of substitute materials can greatly
impair the historic character of an historic structure, all
preservation options should be explored thoroughly before
substitute materials are used. It is important to remember that
the purpose of repairing damaged features and of replacing lost and
irreparably damaged ones is both to match visually what was there
and to cause no further deterioration. For these reasons it is not
appropriate to cover up historic materials with synthetic materials
that will alter the appearance, proportions and details of an
historic building and that will conceal future deterioration.
Some materials have been used successfully for the repair of
damaged features such as epoxies for wood infilling, cementitious
patching for sandstone repairs, or plastic stone for masonry
repairs. Repairs are preferable to replacement whether or not the
repairs are in kind or with a synthetic substitute material.
In general, four circumstances warrant the consideration of
substitute materials: 1) the unavailability of historic materials;
2) the unavailability of skilled craftsmen; 3) inherent flaws in
the original materials; and 4) code-required changes (which in many
cases can be extremely destructive of historic resources).
Cost may or may not be a determining factor in considering the use
of substitute materials. Depending on the area of the country, the
amount of material needed, and the projected life of less durable
substitute materials, it may be cheaper in the long run to use the
original material, even though it may be harder to find. Due to
many early failures of substitute materials, some preservationists
are looking abroad to find materials (especially stone) that match
the historic materials in an effort to restore historic
buildings accurately and to avoid many of the uncertainties that
come with the use of substitute materials.
1. THE UNAVAILABILITY OF THE HISTORIC MATERIAL:
The most common reason for considering substitute materials is the
difficulty in finding a good match for the historic material
(particularly a problem for masonry materials where the color and
texture are derived from the material itself). This may be due to
the actual unavailability of the material or to protracted delivery
dates. For example, the local quarry that supplied the sandstone
for a building may no longer be in operation. All efforts should
be made to locate another quarry that could supply a satisfactory
match. If this approach fails, substitute materials such as
dry-tamp cast stone or textured precast concrete may be a suitable
substitute if care is taken to ensure that the detail, color and
texture of the original stone are matched. In some cases, it may
be possible to use a sand-impregnated paint on wood as a
replacement section, achieved using readily available traditional
materials, conventional tools and work skills. Simple solutions
should not be overlooked.
2. THE UNAVAILABILITY OF HISTORIC CRAFT TECHNIQUES AND LACK OF
These two reasons complicate any preservation or rehabilitation
project. This is particularly true for intricate ornamental work,
such as carved wood, carved stone, wrought iron, cast iron, or
molded terra cotta. However, a number of stone and wood cutters
now employ sophisticated carving machines, some even computerized.
It is also possible to cast substitute replacement pieces using
aluminum, cast stone, fiberglass, polymer concretes, glass fiber
reinforced concretes and terra cotta. Mold making and casting
takes skill and craftsmen who can undertake this work are
available. Efforts should always be made, prior to replacement, to
seek out artisans who might be able to repair ornamental elements
and thereby save the historic features in place.
3. POOR ORIGINAL BUILDING MATERIALS:
Some historic building materials were of inherently poor quality or
their modern counterparts are inferior. In addition, some
materials were naturally incompatible with other materials on the
building, causing staining or galvanic corrosion. Examples of poor
quality materials were the very soft sandstones which eroded
quickly. An example of poor quality modern replacement material is
the tin-coated steel roofing which is much less durable than the
historic tin or terne iron which is no longer available. In some
cases, more durable natural stones or precast concrete might be
available as substitutes for the soft stones and modern terne-coated
stainless steel or lead-coated copper might produce a more
durable yet visually compatible replacement roofing.
4. CODE-RELATED CHANGES:
Sometimes referred to as life and safety codes, building codes
often require changes to historic buildings. Many cities in
earthquake zones, for example, have laws requiring that overhanging
masonry parapets and cornices, or freestanding urns or finials be
securely reanchored to new structural frames or be removed
completely. In some cases, it may be acceptable to replace these
heavy historic elements with light replicas. In other cases, the
extent of historic fabric removed may be so great as to diminish
the integrity of the resource. This could affect the significance
of the structure and jeopardize National Register status. In
addition, removal of repairable historic materials could result in
loss of Federal tax credits for rehabilitation. Department of the
Interior regulations make clear that the Secretary of the
Interior's Standards for Rehabilitation take precedence over other
regulations and codes in determining whether a project is
consistent with the historic character of the building undergoing
Two secondary reasons for considering the use of substitute
materials are their lighter weight and, for some materials, a
reduced need of maintenance. These reasons can become important if
there is a need to keep dead loads to a minimum or if the feature
being replaced is relatively inaccessible for routine maintenance.
***CAUTIONS AND CONCERNS***
In dealing with exterior features and materials, it must be
remembered that moisture penetration, ultraviolet degradation, and
differing thermal expansion and contraction rates of dissimilar
materials make any repair or replacement problematic. To ensure
that a repair or replacement will perform well over time, it is
critical to understand fully the properties of both the original
and the substitute materials, to install replacement materials
correctly, to assess their impact on adjacent historic materials,
and to have reasonable expectations of future performance.
Many high tech materials are too new to have been tested
thoroughly. The differences in vapor permeability between some
synthetic materials and the historic materials have in some cases
caused unexpected further deterioration. It is therefore difficult
to recommend substitute materials if the historic materials are
still available. As previously mentioned, consideration should
always be given first to using traditional materials and methods of
repair or replacement before accepting unproven techniques,
materials or applications.
Substitute materials must meet three basic criteria before being
considered: they must be compatible with the historic materials in
appearance; their physical properties must be similar to those of
the historic materials, or be installed in a manner that tolerates
differences; and they must meet certain basic performance
expectations over an extended period of time.
MATCHING THE APPEARANCE OF THE HISTORIC MATERIALS:
In order to provide an appearance that is compatible with the
historic material, the new material should match the details and
craftsmanship of the original as well as the color, surface
texture, surface reflectivity and finish of the original material.
The closer an element is to the viewer, the more closely the
material and craftsmanship must match the original.
Matching the color and surface texture of the historic material
with a substitute material is normally difficult. To enhance the
chances of a good match, it is advisable to clean a portion of the
building where new materials are to be used. If pigments are to be
added to the substitute material, a specialist should determine the
formulation of the mix, the natural aggregates and the types of
pigments to be used. As all exposed material is subject to
ultra-violet degradation, if possible, samples of the new materials
made during the early planning phases should be tested or allowed
to weather over several seasons to test for color stability.
Fabricators should supply a sufficient number of samples to permit
on-site comparison of color, texture, detailing, and other critical
qualities. In situations where there are subtle variations in
color and texture within the original materials, the substitute
materials should be similarly varied so that they are not
conspicuous by their uniformity.
Substitute materials, notably the masonry ones, may be more water
absorbent than the historic material. If this is visually
distracting, it may be appropriate to apply a protective
vapor-permeable coating on the substitute material. However, these
clear coatings tend to alter the reflectivity of the material, must
be reapplied periodically, and may trap salts and moisture, which
can in turn produce spalling. For these reasons, they are not
recommended for use on historic materials.
MATCHING THE PHYSICAL PROPERTIES:
While substitute materials can closely match the appearance of
historic ones, their physical properties may differ greatly. The
chemical composition of the material (i.e., presence of acids,
alkaline, salts, or metals) should be evaluated to ensure that the
replacement materials will be compatible with the historic
resource. Special care must therefore be taken to integrate and to
anchor the new materials properly. The thermal expansion and
contraction coefficients of each adjacent material must be within
tolerable limits. The function of joints must be understood and
detailed either to eliminate moisture penetration or to allow vapor
permeability. Materials that will cause galvanic corrosion or
other chemical reactions must be isolated from one another.
To ensure proper attachment, surface preparation is critical.
Deteriorated underlying material must be cleaned out.
Non-corrosive anchoring devices or fasteners that are designed to
carry the new material and to withstand wind, snow, and other
destructive elements should be used. Properly chosen fasteners
allow attached materials to expand and contract at their own rates.
Caulking, flexible sealants or expansion joints between the
historic material and the substitute material can absorb slight
differences of movement. Since physical failures often result from
poor anchorage or improper installation techniques, a structural
engineer should be a member of any team undertaking major repairs.
Some of the new high tech materials such as epoxies and polymers
are much stronger than historic materials and generally impermeable
to moisture. These differences can cause serious problems unless
the new materials are modified to match the expansion and
contraction properties of adjacent historic materials more closely,
or unless the new materials are isolated from the historic ones
altogether. When stronger or vapor impermeable new materials are
used alongside historic ones, stresses from trapped moisture or
differing expansion and contraction rates generally hasten
deterioration of the weaker historic material. For this reason, a
conservative approach to repair or replacement is recommended, one
that uses more pliant materials rather than high-strength ones.
Since it is almost impossible for substitute materials to match the
properties of historic materials perfectly, the new system
incorporating new and historic materials should be designed so that
if material failures occur, they occur within the new material
rather than the historic material.
While a substitute material may appear to be acceptable at the time
of installation, both its appearance and its performance may
deteriorate rapidly. Some materials are so new that industry
standards are not available, thus making it difficult to specify
quality control in fabrication, or to predict maintenance
requirements and long term performance. Where possible, projects
involving substitute materials in similar circumstances should be
examined. Material specifications outlining stability of color and
texture; compressive or tensile strengths if appropriate; the
acceptable range of thermal coefficients, and the durability of
coatings and finishes should be included in the contract documents.
Without these written documents, the owner may be left with little
recourse if failure occurs.
The tight controls necessary to ensure long-term performance extend
beyond having written performance standards and selecting materials
that have a successful track record. It is important to select
qualified fabricators and installers who know what they are doing
and who can follow up if repairs are necessary. Installers and
contractors unfamiliar with specific substitute materials and how
they function in your local environmental conditions should be
The surfaces of substitute materials may need special care once
installed. For example, chemical residues or mold release agents
should be removed completely prior to installation, since they
attract pollutants and cause the replacement materials to appear
dirtier than the adjacent historic materials. Furthermore,
substitute materials may require more frequent cleaning, special
cleaning products and protection from impact by hanging
window-cleaning scaffolding. Finally, it is critical that the
substitute materials be identified as part of the historical record
of the building so that proper care and maintenance of all the
building materials continue to ensure the life of the historic
***CHOOSING AN APPROPRIATE SUBSTITUTE MATERIAL***
Once all reasonable options for repair or replacement in kind have
been exhausted, the choice among a wide variety of substitute
materials currently on the market must be made. The listings at
the end of this Brief describe a number of such materials, many of
them in the family of modified concretes which are gaining greater
use. The listings do not include wood, stamped metal, mineral
fiber cement shingles and some other traditional imitative
materials, since their properties and performance are better known.
Nor do they include vinyls or molded urethanes which are sometimes
used as cosmetic claddings or as substitutes for wooden millwork.
Since millwork is still readily available, it should be replaced
The listings describe the properties and uses of several materials
finding greater use in historic preservation projects, and outline
advantages and disadvantages of each. It should not be read as an
endorsement of any of these materials, but serves as a reminder
that numerous materials must be studied carefully before selecting
the appropriate treatment. Included are three predominantly
masonry materials (cast stone, precast concrete, and glass fiber
reinforced concrete); two predominantly resinous materials (epoxy
and glass fiber reinforced polymers also known as fiberglass), and
cast aluminum which has been used as a substitute for various
metals and woods.
Substitute materials--those products used to imitate historic
materials--should be used only after all other options for repair
and replacement in kind have been ruled out. Because there are so
many unknowns regarding the long-term performance of substitute
materials, their use should not be considered without a thorough
investigation into the proposed materials, the fabricator, the
installer, the availability of specifications, and the use of that
material in a similar situation in a similar environment.
Substitute materials are normally used when the historic materials
or craftsmanship are no longer available, if the original materials
are of a poor quality or are causing damage to adjacent materials,
or if there are specific code requirements that preclude the use of
historic materials. Use of these materials should be limited,
since replacement of historic materials on a large scale may
jeopardize the integrity of an historic resource. Every means of
repairing deteriorating historic materials or replacing them with
identical materials should be examined before turning to substitute
The importance of matching the appearance and physical properties
of historic materials and, thus, of finding a successful long-term
solution cannot be overstated. The successful solutions
illustrated in this Brief were from historic preservation projects
involving professional teams of architects, engineers, fabricators,
and other specialists. Cost was not necessarily a factor, and all
agreed that whenever possible, the historic materials should be
used. When substitute materials were selected, the solutions were
often expensive and were reached only after careful consideration
of all options, and with the assistance of expert professionals.
***PROS AND CONS OF VARIOUS SUBSTITUTE MATERIALS***
Cast aluminum is a molten aluminum alloy cast in permanent (metal)
molds or one-time sand molds which must be adjusted for shrinkage
during the curing process. Color is from paint applied to primed
aluminum or from a factory finished coating. Small sections can be
bolted together to achieve intricate or sculptural details. Unit
castings are also available for items such as column plinth blocks.
Cast aluminum can be a substitute for cast iron or other decorative
elements. This would include grillwork, roof cresting, cornices,
ornamental spandrels, storefront elements, columns, capitals, and
column bases and plinth blocks. If not self-supporting, elements
are generally screwed or bolted to a structural frame. As a result
of galvanic corrosion problems with dissimilar metals, joint
details are very important.
- light weight (=AB of cast-iron)
- corrosion-resistant, non-combustible
- intricate castings possible
- easily assembled, good delivery time
- can be prepared for a variety of colors
- long life, durable, less brittle than cast iron
- lower structural strength than cast iron
- difficult to prevent galvanic corrosion with other metals<= br> - greater expansion and contraction than cast-iron; requires
gaskets or caulked joints
- difficult to keep paint on aluminum
- Can existing be repaired or replaced in-kind?
- How is cast aluminum to be attached?
- Have full-size details been developed for each piece to be
- How are expansion joints detailed?
- Will there be a galvanic corrosion problem?
- Have factory finishes been protected during installation?<= br> - Are fabricator/installers experienced?
CAST STONE (DRY-TAMPED):
Cast stone is an almost-dry cement, lime and aggregate mixture
which is dry-tamped into a mold to produce a dense stone-like unit.
Confusion arises in the building industry as many refer to high
quality precast concrete as cast stone. In fact, while it is a
form of precast concrete, the dry-tamp fabrication method produces
an outer surface resembling a stone surface. The inner core can be
either dry-tamped or poured full of concrete. Reinforcing bars and
anchorage devices can be installed during fabrication.
Cast stone is often the most visually similar material as a
replacement for unveiled deteriorated stone, such as brownstone or
sandstone, or terra cotta in imitation of stone. It is used both
for surface wall stones and for ornamental features such as window
and door surrounds, voussoirs, brackets and hoods. Rubber-like
molds can be taken of good stones on site or made up at the factory
from shop drawings.
- replicates stone texture with good molds (which can come
from extant stone) and fabrication
- expansion/contraction similar to stone
- minimal shrinkage of material
- anchors and reinforcing bars can be built in
- material is fire-rated
- range of color available
- vapor permeable
- heavy units may require additional anchorage
- color can fade in sunlight
- may be more absorbent than natural stone
- replacement stones are obvious if too few models and molds
- Are the original or similar materials available?
- How are units to be installed and anchored?
- Have performance standards been developed to ensure color
- Have large samples been delivered to site for color, finish
and absorption testing?
- Has mortar been matched to adjacent historic mortar to
achieve a good color/tooling match?
- Are fabricators/installers experienced?
GLASS FIBER REINFORCED CONCRETES (GFRC):
Glass fiber reinforced concretes are lightweight concrete compounds
modified with additives and reinforced with glass fibers. They are
generally fabricated as thin shelled panels and applied to a
separate structural frame or anchorage system. The GFRC is most
commonly sprayed into forms although it can be poured. The glass
must be alkaline resistant to avoid deteriorating effects caused by
the cement mix. The color is derived from the natural aggregates
and if necessary a small percentage of added pigments.
Glass fiber reinforced concretes are used in place of features
originally made of stone, terra cotta, metal or wood, such as
cornices, projecting window and door trims, brackets, finials, or
wall murals. As a molded product it can be produced in long
sections of repetitive designs or as sculptural elements. Because
of its low shrinkage, it can be produced from molds taken directly
from the building. It is installed with a separate noncorrosive
anchorage system. As a predominantly cementitious material, it is
- lightweight, easily installed
- good molding ability, crisp detail possible
- weather resistant
- can be left uncoated or else painted
- little shrinkage during fabrication
- molds made directly from historic features
- cements generally breathable
- material is fire-rated
- non-loadbearing use only
- generally requires separate anchorage system
- large panels must be reinforced
- color additives may fade with sunlight
- joints must be properly detailed
- may have different absorption rate than adjacent historic
- Are the original materials and craftsmanship still
- Have samples been inspected on the site to ensure
- Has anchorage system been properly designed?
- Have performance standards been developed?
- Are fabricators/installers experienced?
Precast concrete is a wet mix of cement and aggregate poured into
molds to create masonry units. Molds can be made from existing
good surfaces on the building. Color is generally integral to the
mix as a natural coloration of the sand or aggregate, or as a small
not thin-shell panels.