1 PRESERVATION 

BRIEFS 

Assessing Cleaning and Water-Repellent 

Treatments for Historic Masonry Buildings 

Robert C. Mack, AlA 

Anne Grimmer 

u.s. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

Inappropriate cleaning and coating treatments are a major 

cause of damage to historic masonry buildings. While 

either or both treatments may be appropriate in some cases, 

they can be very destructive to historic masonry if they are 

not selected carefully. Historic masonry, as considered 

here, includes stone, brick, architectural terra cotta, cast 

stone, concrete and concrete block. It is frequently cleaned 

because cleaning is equated with improvement. Cleaning 

may sometimes be followed by the application of a waterrepellent coating. However, unless these procedures are 

carried out under the guidance and supervision of an 

architectural conservator, they may result in irrevocable 

damage to the historic resource. 

The purpose of this Brief is to provide information on the 

variety of cleaning methods and materials that are available 

for use on the exterior of historic masonry buildings, and 

to provide guidance in selecting the most appropriate 

method or combination of methods. The difference between 

water-repellent coatings and waterproof coatings 

is explained, and the purpose of each, the suitability of 

their application to historic masonry buildings, and the 

possible consequences of their inappropriate use are 

discussed. 

The Brief is intended to help develop sensitivity to the 

qualities of historic masonry that makes it so special, and 

to assist historic building owners and property managers 

in working cooperatively with architects, architectural 

conservators and contractors (Fig. 1). Although specifically 

intended for historic buildings, the information is applicable 

to all masonry buildings. This publication updates and 

expands Preservation BriefI: The Cleaning and Waterproof 

Coating of Masonry Buildings. The Brief is not meant to be 

a cleaning manual or a guide for preparing specifications. 

Rather, it provides general information to raise awareness 

of the many factors involved in selecting cleaning and 

water-repellent treatments for historic masonry buildings. 

Figure 1. Low-to medium- pressure steam (hot-pressurized water was/ling), is being used to clean the exterior of the U.S. Tariff Commission Building, the first marble building constructed in Wa shington, D.C., in 1839. This method was selected by an architecural conservator as the "gentlest means possible" 

to clean the marble. Steam ca n soften heavy soiling deposits such as those on the cornice and column capitals, and facilitate easy removal. Note how these deposits have been removed from the right side oJ the cornice which has already been cleaned. 
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Figure 2. Biological growth as shown on this marble foundation 

can usually be removed using a low-press ure water wash, possibly witiz 

a non-ionic detergent added to it, and scrubbing with a natura l or 

syllthetic bristle brush. 

Preparing for a Cleaning Project 

Reasons for cleaning. First, it is important to determine 

whether it is appropriate to clean the masonry. The objective 

of cleaning a historic masonry building must be considered 

carefully before arriving at a decision to clean. There are 

several major reasons for cleaning a historic masonry 

building: improve the appearance of the building by 

removing unattractive dirt or soiling materials, or nonhistoric paint from the masonry; retard deterioration by 

removing soiling materials that may be damaging the 

masonry; or provide a clean surface to accurately match 

rep ointing mortars or patching compounds, or to conduct 

a condition survey of the masonry. 

Identify what is to be removed. The general nature and 

source of dirt or soiling material on a building must be 

identified to remove it in the gentlest means possible -

that is, in the most effective, yet least harmful, manner. 

Soot and smoke, for example, require a different cleaning 

agent to remove than oil stains or metallic stains. Other 

common cleaning problems include biological growth such 

as mold or mildew, and organic matter such as the tendrils 

left on masonry after removal of ivy (Fig. 2). 

Consider the historic appearance of the building. If the 

proposed cleaning is to remove paint, it is important in 

each case to learn whether or not unpainted masonry is 

historically appropriate. And, it is necessary to consider 

why the building was painted (Fig. 3). Was it to cover bad 

rep ointing or unmatched repairs? Was the building 

painted to protect soft brick or to conceal deteriorating 

stone? Or, was painted masonry simply a fashionable 

Figu re 3. Th is small test area has revealed a red brick patch that does 11 0t 

match the original beige brick. Th is may explain why the building was 

painted, and may suggest to the owner that it may be preferable to keep 

it pa inted. 

treatment in a particular historic period? Many buildings 

were painted at the time of construction or shortly thereafter; 

retention of the paint, therefore, may be more appropriate 

historically than removing it. And, if the building appears 

to have been painted for a long time, it is also important 

to think about whether the paint is part of the character of 

the historic building and if it has acquired significance over 

time. 

Consider the practicalities of cleaning or paint removal. 

Some gypsum or sulfate crusts may have become integral 

with the stone and, if cleaning could result in removing 

some of the stone surface, it may be preferable not to clean. 

Even where unpainted masonry is appropriate, the retention 

of the paint may be more practical than removal in terms 

of long range preservation of the masonry. In some cases, 

however, removal of the paint may be desirable. For 

example, the old paint layers may have built up to such 

an extent that removal is necessary to ensure a sound 

surface to which the new paint will adhere. 

Study the masonry. Although not always necessary, in 

some instances it can be beneficial to have the coating or 

paint type, color, and layering on the masonry researched 

before attempting its removal. Analysis of the nature of 

the soiling or of the paint to be removed from the masonry, 

as well as guidance on the appropriate cleaning method, 

may be provided by professional consultants, including 

architectural conservators, conservation scientists and 

preservation architects. The State Historic Preservation 

Office (SHPO), local historic district commissions, 

architectural review boards and preservation-oriented 

websites may also be able to supply useful information on 

masonry cleaning techniques. 
Understanding the Building Materials 

The construction of the building must be considered when 

developing a cleaning program because inappropriate 

cleaning can have a deleterious effect on the masonry as 

well as on other building materials. The masonry material 

or materials must be correctly identified. It is sometimes 

difficult to distinguish one type of stone from another; for 

example, certain sandstones can be easily confused with 

limestones. Or, what appears to be natural stone may not 

be stone at all, but cast stone or concrete. Historically, cast 

stone and architectural terra cotta were frequently used in 

combination with natural stone, especially for trim elements 

or on upper stories of a building where, from a distance, 

these substitute materials looked like real stone (Fig. 4). 

Other features on historic buildings that appear to be stone, 

such as decorative cornices, entablatures and window 

hoods, may not even be masonry, but metal. 

Identify prior treatments. Previous treatments of the 

building and its surroundings should be researched and 

building maintenance records should be obtained, if 

available. Sometimes if streaked or spotty areas do not 

seem to get cleaner following an initial cleaning, closer 

inspection and analysis may be warranted. The 

discoloration may turn out not to be dirt but the remnant 

of a water-repellent coating applied long ago which has 

darkened the surface of the masonry over time (Fig. 5). 

Successful removal may require testing several cleaning 

agents to find something that will dissolve and remove the 

coating. Complete removal may not always be possible. 

Repairs may have been stained to match a dirty building, 

and cleaning may make these differences apparent. Deicing salts used near the building that have dissolved can 

Figure 4. The foundation of this brick building is limestone, but the 

decorative trim above is architectural terra cotta intended to simuillte 

stone. 

Figure 5. Repeated wllter washing did rIOt remove the staining inside 

this limestone porte cochere. Upon closer examination, it was 

determined to be a water-repellent coating that had been applied many 

years earlier. An alkaline cleaner may be effective in removing it. 

migrate into the masonry. Cleaning may draw the salts to 

the surface, where they will appear as efflorescence (a 

powdery, white substance), which may require a second 

treatment to be removed. Allowances for dealing with 

such unknown factors, any of which can be a potential 

problem, should be included when investigating cleaning 

methods and materials. Just as more than one kind of 

masonry on a historic building may necessitate multiple 

cleaning approaches, unknown conditions that are 

encountered may also require additional cleaning 

treatments. 

Choose the appropriate cleaner. The importance of testing 

cleaning methods and materials cannot be over emphasized. 

Applying the wrong cleaning agents to historic masonry 

can have disastrous results. Acidic cleaners can be extremely 

damaging to acid-sensitive stones, such as marble and 

limestone, resulting in etching and dissolution of these 

stones. Other kinds of masonry can also be damaged by 

incompatible cleaning agents, or even by cleaning agents 

that are usually compatible. There are also numerous kinds 

of sandstone, each with a considerably different geological 

composition. While an acid-based cleaner may be safely 

used on some sandstones, others are acid-sensitive and 

can be severely etched or dissolved by an acid cleaner. 

Some sandstones contain water-soluble minerals and can 

be eroded by water cleaning. And, even if the stone type 

is correctly identified, stones, as well as some bricks, may 

contain unexpected impurities, such as iron particles, that 

may react negatively with a particular cleaning agent and 

result in staining. Thorough understanding of the physical 

and chemical properties of the masonry will help avoid 

the inadvertent selection of damaging cleaning agents. 

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Figure 6. Timed water soaking can be very effective for cleaning 

limestone and marble as shown here at the Marble Collegiate Church 

in New York City. In this case, a twelve-hour water soak using a 

multi-nozzle manifold was followed by a final water rinse. Photo: Diane 

S. Kaese, Wiss, Janney, Elstner Associates, lnc., N. Y. , N. Y. 

Other building materials also may be affected by the 

cleaning process. Some chemicals, for example, may have 

a corrosive effect on paint or glass. The portions of building 

elements most vulnerable to deterioration may not be 

visible, such as embedded ends of iron window bars. 

Other totally unseen items, such as iron cramps or ties 

which hold the masonry to the structural frame, also may 

be subject to corrosion from the use of chemicals or even 

from plain water. The only way to prevent problems in 

these cases is to study the building construction in detail 

and evaluate proposed cleaning methods with this 

information in mind. However, due to the very likely 

possibility of encountering unknown factors, any cleaning 

project involving historic masonry should be viewed as 

unique to that particular building. 

Cleaning Methods and Materials 

Masonry cleaning methods generally are divided into 

three major groups: water, chemical, and abrasive. Water 

methods soften the dirt or soiling material and rinse the 

deposits from the masonry surface. Chemical cleaners 

react with dirt, soiling material or paint to effect their 

removal, after which the cleaning effluent is rinsed off the 

masonry surface with water. Abrasive methods include 

blasting with grit, and the use of grinders and sanding 

discs, all of which mechanically remove the dirt, soiling 

material or paint (and, usually, some of the masonry 

surface). Abrasive cleaning is also often followed with a 

water rinse. Laser cleaning, although not discussed here 

in detail, is another technique that is used sometimes by 

conservators to clean small areas of historic masonry. It 

can be quite effective for cleaning limited areas, but it is 

expensive and generally not practical for most historic 

masonry cleaning projects. 

Although it may seem contrary to common sense, masonry 

cleaning projects should be carried out starting at the 

bottom and proceeding to the top of the building always 

keeping all surfaces wet below the area being cleaned. 

The rationale for this approach is based on the principle 

that dirty water or cleaning effluent dripping from cleaning 

in progress above will leave streaks on a dirty surface but 

will not streak a clean surface as long as it is kept wet and 

rinsed frequently. 

Water Cleaning 

Water cleaning methods are generally the gentlest means 

possible, and they can be used safely to remove dirt from 

all types of historic masonry.* There are essentially four 

kinds of water-based methods: soaking; pressure water 

washing; water washing supplemented with non-ionic 

detergent; and steam, or hot-pressurized water cleaning. 

Once water cleaning has been completed, it is often 

necessary to follow up with a water rinse to wash off the 

loosened soiling material from the masonry. 

Soaking. Prolonged spraying or misting with water is 

particularly effective for cleaning limestone and marble. 

It is also a good method for removing heavy accumulations 

of soot, sulfate crusts or gypsum crusts that tend to form 

in protected areas of a building not regularly washed by 

rain. Water is distributed to lengths of punctured hose or 

pipe with non-ferrous fittings hung from moveable 

scaffolding or a swing stage that continuously mists the 

surface of the masonry with a very fine spray (Fig. 6). A 

timed on-off spray is another approach to using this 

cleaning technique. After one area has been cleaned, the 

apparatus is moved on to another. Soaking is often used 

in combination with water washing and is also followed 

by a final water rinse. Soaking is a very slow methodit may take several days or a week-but it is a very gentle 

method to use on historic masonry. 

Water Washing. Washing with low-pressure or mediumpressure water is probably one of the most commonly 

used methods for removing dirt or other pollutant soiling 

from historic masonry buildings (Fig. 7). Starting with a 

very low pressure (100 psi or below), even using a garden 

hose, and progressing as needed to slightly higher pressure 

-generally no higher than 300-400 psi-is always the 

recommended way to begin. Scrubbing with natural bristle 

or synthetic bristle brushes-never metal which can abrade 

the surface and leave metal particles that can stain the 

masonry-can help in cleaning areas of the masonry that 

are especially dirty. 

Water Washing with Detergents. Non-ionic detergents 

-which are not the same as soaps -are synthetic organic 

compounds that are especially effective in removing oily 

soil. (Examples of some of the numerous proprietary nonionic detergents include Igepal by GAF, Tergitol by Union 

Carbide and Triton by Rohm & Haas.) Thus, the addition 

of a non-ionic detergent, or surfactant, to a low- or mediumpressure water wash can be a useful aid in the cleaning 

'Water cleaning methods may not be appropriate to use on some badly 

deteriorated masonry because water may exacerbate the deterioration, or on gypsum or alabaster which are very soluble in water. 
process. (A non-ionic detergent, unlike most household 

detergents, does not leave a solid, visible residue on the 

masonry.) Adding a non-ionic detergent and scrubbing 

with a natural bristle or synthetic bristle brush can facilitate 

cleaning textured or intricately carved masonry. This 

should be followed with a final water rinse. 

Steam/Hot-Pressurized Water Cleaning. Steam cleaning 

is actually low-pressure hot water washing because the 

steam condenses almost immediately upon leaving the 

hose. This is a gentle and effective method for cleaning 

stone and particularly for acid-sensitive stones. Steam can 

be especially useful in removing built-up soiling deposits 

and dried-up plant materials, such as ivy disks and tendrils. 

It can also be an efficient means of cleaning carved stone 

details and, because it does not generate a lot of liquid 

water, it can sometimes be appropriate to use for cleaning 

interior masonry (Figs. 8-9). 

Potential hazards of water cleaning. Despite the fact that 

water-based methods are generally the most gentle, even 

they can be damaging to historic masonry. Before beginning 

a water cleaning project, it is important to make sure that 

all mortar joints are sound and that the building is 

watertight. Otherwise water can seep through the walls 

to the interior, resulting in rusting metal anchors and 

stained and ruined plaster. 

Some water supplies may contain traces of iron and copper 

which may cause masonry to discolor. Adding a chelating 

or complexing agent to the water, such as EDTA (ethylene 

diamine tetra-acetic acid), which inactivates other metallic 

ions, as well as softens minerals and water hardness, will 

help prevent staining on light-colored masonry. 

Any cleaning method involving water should never be 

done in cold weather or if there is any likelihood of frost 

or freezing because water within the masonry can freeze, 

causing spalling and cracking. Since a masonry wall may 

take over a week to dry after cleaning, no water cleaning 

should be permitted for several days prior to the first 

average frost date, or even earlier if local forecasts predict 

cold weather. 

Most essential of all, it is important to be aware that using 

water at too high a pressure, a practice common to "power 

washing" and "water blasting", is very abrasive and can 

easily etch marble and other soft stones, as well as some 

types of brick (Figs. 10-11). In addition, the distance of the 

nozzle from the masonry surface and the type of nozzle, 

as well as gallons per minute (gpm), are also important 

variables in a water cleaning process that can have a 

significant impact on the outcome of the project. This is 

why it is imperative that the cleaning be closely monitored 

to ensure that the cleaning operators do not raise the 

pressure or bring the nozzle too close to the masonry in 

an effort to "speed up" the process. The appearance of 

grains of stone or sand in the cleaning effluent on the 

ground is an indication that the water pressure may be too 

high. 

Figure 7. Glazed architectural terra cotta often may be cleaned 

successfully with a low-pressure water wash and hand scrubbing 

supplemented, if necessary, with a non-ionic detergent. Pho to: Na tional 

Park Service Files. 

Chemical Cleaning 

Chemical cleaners, generally in the form of proprietary 

products, are another material frequently used to clean 

historic masonry. They can remove dirt, as well as paint 

and other coatings, metallic and plant stains, and graffiti. 

Chemical cleaners used to remove dirt and soiling include 

acids, alkalies and organic compounds. Acidic cleaners, 

of course, should not be used on masonry that is acid 

sensitive. Paint removers are alkaline, based on organic 

solvents or other chemicals. 

Chemical Cleaners to Remove Dirt 

Both alkaline and acidic cleaning treatments include the 

use of water. Both cleaners are also likely to contain 

surfactants (wetting agents), that facilitate the chemical 

reaction that removes the dirt. Generally, the masonry is 

wet first for both types of cleaners, then the chemical 

cleaner is sprayed on at very low pressure or brushed onto 

the surface. The cleaner is left to dwell on the masonry 

for an amount of time recommended by the product 

manufacturer or, preferably, determined by testing, and 

rinsed off with a low- or moderate-pressure cold, or 

sometimes hot, water wash. More than one application 

of the cleaner may be necessary, and it is always a 

good practice to test the product manufacturer's 

recommendations concerning dilution rates and dwell 

times. Because each cleaning situation is unique, dilution 

rates and dwell times can vary considerably. The masonry 

surface may be scrubbed lightly with natural or synthetic 

bristle brushes prior to rinsing. After rinsing, pH strips 

should be applied to the surface to ensure that the masonry 

has been neutralized completely. 

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Figure 8. (Left) Low-press ure (under 100 psi) steam cleaning 

(hot-pressurized water washing), is part of the regular maintenance 

program at the Jefferson Memorial, Washington, D.C. The white marble 

interior of this open structure is subject to constant soiling by birds, 

insects and visitors. (Right) Th is portable steam cleaner enables prompt 

clea nup when necessary. Photos: Na tional Park Service Files. 

Acidic Cleaners. Acid-based cleaning products may be 

used on non-acid sensitive masonry, which generally 

includes: granite, most sandstones, slate, unglazed brick 

and unglazed architectural terra cotta, cast stone and 

concrete (Fig. 12). Most commercial acidic cleaners are 

composed primarily of hydrofluoric acid, and often include 

some phosphoric acid to prevent rust-like stains from 

developing on the masonry after the cleaning. Acid cleaners 

are applied to the pre-wet masonry which should be kept 

wet while the acid is allowed to "work", and then removed 

with a water wash. 

Alkaline Cleaners. Alkaline cleaners should be used on 

acid-sensitive masonry, including: limestone, polished 

and unpolished marble, calcareous sandstone, glazed brick 

and glazed architectural terra cotta, and polished granite. 

(Alkaline cleaners may also be used sometimes on masonry 

materials that are not acid sensitive-after testing, of course 

- but they may not be as effective as they are on acidsensitive masonry.) Alkaline cleaning products consist 

primarily of two ingredients: a non-ionic detergent or 

surfactant; and an alkali, such as potassium hydroxide or 

ammonium hydroxide. Like acidic cleaners, alkaline 

products are usually applied to pre-wet masonry, allowed 

to dwell, and then rinsed off with water. (Longer dwell 

times may be necessary with alkaline cleaners than with 

acidic cleaners.) Two additional steps are required to 

remove alkaline cleaners after the initial rinse. First the 

masonry is given a slightly acidic wash-often with acetic 

acid-to neutralize it, and then it is rinsed again with water. 

Chemical Cleaners to Remove Paint and Other Coatings, 

Stains and Graffiti 

Removing paint and some other coatings, stains and graffiti 

can best be accomplished with alkaline paint removers, 

organic solvent paint removers, or other cleaning 

compounds. The removal of layers of paint from a masonry 

surface usually involves applying the remover either by 

brush, roller or spraying, followed by a thorough water 

wash. As with any chemical cleaning, the manufacturer's 

recommendations regarding application procedures should 

always be tested before beginning work. 

Alkaline Paint Removers. These are usually of much the 

same composition as other alkaline cleaners, containing 

potassium or ammonium hydroxide, or trisodium 

phosphate. They are used to remove oil, latex and acrylic 

paints, and are effective for removing multiple layers of 

paint. Alkaline cleaners may also remove some acrylic, 

water-repellent coatings. As with other alkaline cleaners, 

both an acidic neutralizing wash and a final water rinse 

are generally required following the use of alkaline paint 

removers. 

Organic Solvent Paint Removers. The formulation of 

organic solvent paint removers varies and may include a 

combination of solvents, including methylene chloride, 

methanol, acetone, xylene and toluene. 

Figure 9. (Left) This small steam cleaner- the size of a vacuum cleaner- offers a very controlled and gentle means of cleaning limited, or hard-to-reach 

areas or carved stone details. (Right) It is particularly useful for interiors where it is important to keep moisture to a minumum, such as inside 

the Washington Monument, Washington, D.C., where it was used to clean the commemorative stones. Photos: Audrey T. Tepper. 
Figure 10. High-pressure water washing too close to the surface has 

abraded and, consequently, marred the limestone on this early-20th 

century building. 

Other Paint Removers and Cleaners. Other cleaning 

compounds that can be used to remove paint and some 

painted graffiti from historic masonry include paint 

removers based on N-methyl-2-pyrrolidone (NMP), or on 

petroleum-based compounds. Removing stains, whether 

they are industrial (smoke, soot, grease or tar), metallic 

(iron or copper), or biological (plant and fungal) in origin, 

depends on carefully matching the type of remover to the 

type of stain (Fig. 13). Successful removal of stains from 

historic masonry often requires the application of a number 

of different removers before the right one is found. The 

removal of layers of paint from a masonry surface is usually 

accomplished by applying the remover either by brush, 

roller or spraying, followed by a thorough water wash 

(Fig. 14). 

Potential hazards of chemical cleaning. Since most 

chemical cleaning methods involve water, they have many 

of the potential problems of plain water cleaning. Like 

water methods, they should not be used in cold weather 

because of the possibility of freezing. Chemical cleaning 

should never be undertaken in temperatures below 40 

degrees F (4 degrees C), and generally not below 50 degrees 

F. In addition, many chemical cleaners simply do not work 

in cold temperatures. Both acidic and alkaline cleaners 

can be dangerous to cleaning operators and, clearly, there 

are environmental concerns associated with the use of 

chemical cleaners. 

Figure 11. Rinsing with high-pressure water following chemical 

cleaning has left a horizontal line of abrasion across the bricks on this 

late-19th century row house. 

If not carefully chosen, chemical cleaners can react adversely 

with many types of masonry. Obviously, acidic cleaners 

should not be used on acid-sensitive materials; however, 

it is not always clear exactly what the composition is of 

any stone or other masonry material. For, this reason, 

testing the cleaner on an inconspicuous spot on the building 

is always necessary. While certain acid-based cleaners 

may be appropriate if used as directed on a particular type 

of masonry, if left too long or if not adequately rinsed from 

the masonry they can have a negative effect. For example, 

hydrofluoric acid can etch masonry leaving a hazy residue 

(whitish deposits of silica or calcium fluoride salts) on the 

surface. While this efflorescence may usually be removed 

by a second cleaning-although it is likely to be expensive 

and time-consuming- hydrofluoric acid can also leave 

calcium fluoride salts or a colloidal silica deposit on 

masonry which may be impossible to remove (Fig. 15). 

Other acids, particularly hydrochloric (muriatic) acid, 

which is very powerful, should not be used on historic 

masonry, because it can dissolve lime-based mortar, 

damage brick and some stones, and leave chloride deposits 

on the masonry. 

Figure 12. A mild acidic clean ing agent is being used to clean this 

heavily soiled brick and granite building. Additional applications of the 

cleaner and hand-scrubbing, and even poulticing, may be necessary to 

remove the dark stains on the granite arches below. Photo: Sharon C. 

Park, FAlA. 

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Alkaline cleaners can stain sandstones that contain a ferrous 

compound. Before using an alkaline cleaner on sandstone 

it is always important to test it, since it may be difficult to 

know whether a particular sandstone may contain a ferrous 

compound. Some alkaline cleaners, such as sodium 

hydroxide (caustic soda or lye) and ammonium bifluoride, 

can also damage or leave disfiguring brownish-yellow 

stains and, in most cases, should not be used on historic 

masonry. Although alkaline cleaners will not etch a 

masonry surface as acids can, they are caustic and can burn 

the surface. In addition, alkaline cleaners can deposit 

potentially damaging salts in the masonry which can be 

difficult to rinse thoroughly. 

Abrasive and Mechanical Cleaning 

Generally, abrasive cleaning methods are not appropriate 

for use on historic masonry buildings. Abrasive cleaning 

methods are just that-abrasive. Grit blasters, grinders, 

and sanding discs all operate by abrading the dirt or paint 

off the surface of the masonry, rather than reacting with 

the dirt and the masonry which is how water and chemical 

methods work. Since the abrasives do not differentiate 

between the dirt and the masonry, they can also remove 

the outer surface of the masonry at the same time, and 

result in permanently damaging the masonry. Brick, 

architectural terra cotta, soft stone, detailed carvings, and 

polished surfaces are especially susceptible to physical and 

aesthetic damage by abrasive methods. Brick and 

architectural terra cotta are fired products which have a 

smooth, glazed surface which can be removed by abrasive 

blasting or grinding (Figs. 18-19). Abrasively-cleaned 

masonry is damaged aesthetically as well as physically, 

and it has a rough surface which tends to hold dirt and 

the roughness will make future cleaning more difficult. 

Abrasive cleaning processes can also increase the likelihood 

of subsurface cracking of the masonry. Abrasion of carved 

details causes a rounding of sharp corners and other loss 

of delicate features, while abrasion of polished surfaces 

removes the polished finish of stone. 

Figure 13. Sometimes it may be preferable to paint over a thick asphaltic 

coating rather than try to remove it, because it can be difficult to remove 

completely. However, in this case, many layers of asphaltic coating 

were removed through multiple applications of a heavy duty chemical 

cleaner. Each application of the cleaner was left to dwell following the 

manufacturer's reccommendations, and then rinsed thoroughly. 

(As much as possible of the asphalt was first removed with wooden 

scrapers.) Although not all the asphalt was removed, this was 

determined to be an acceptable level of cleanliness for the project. 

Figure 14. Chemical removal of paint from this brick building has 

revealed that the cornice and window hoods are metal rather than 

masonry. 

Mortar joints, especially those with lime mortar, also can 

be eroded by abrasive or mechanical cleaning. In some 

cases, the damage may be visual, such as loss of joint detail 

or increased joint shadows. As mortar joints constitute a 

significant portion of the masonry surface (up to 20 per 

cent in a brick wall), this can result in the loss of a 

considerable amount of the historic fabric. Erosion of the 

mortar joints may also permit increased water penetration, 

which will likely necessitate repainting. 

Figure 15. The whitish deposits left on the brick by a chemical paint 

remover may have resulted from inadequate rinsing or from the 

chemical being left on the surface too long and may be impossible to 

remove. 
Poulticing to Remove Stains and Graffiti 

a 

c 

d 

Figure 16. (a) The limestone base was heavily stained by runoff 

from the bronze statue above. (b) A poultice consisting of copper 

stain remover and ammonia mixed with fuller's earth was applied 

to the stone base and covered with plastic sheeting to keep it from 

drying out too quickly. (c) As the poultice dried, it pulled the stain 

out of the stone. (d) The poultice residue was removed carefully 

from the stone surface with wooden scrapers and the stone was 

rinsed with wa ter. Photos: John Dugger. 

b 

Graffiti and stains, which have penetrated into the masonry, 

often are best removed by using a poultice. A poultice 

consists of an absorbent material or clay powder (such as 

kaolin or fuller 's earth, or even shredded paper or paper 

towels), mixed with a liquid (solvent or other remover) to 

form a paste which is applied to the stain (Figs. 16-17). 

As it dries, the paste absorbs the staining material so that 

it is not redeposited on the masonry surface. Some 

commercial cleaning products and paint removers are 

specially formulated as a paste or gel that will cling to a 

vertical surface and remain moist for a longer period of 

time in order to prolong the action of the chemical on the 

stain. Pre-mixed poultices are also available as a paste or 

in powder form needing only the addition of the 

appropriate liquid. The masonry must be pre-wet before 

applying an alkaline cleaning agent, but not when using 

a solvent. Once the stain has been removed, the masonry 

must be rinsed thoroughly. 

Figure 17. A poultice is being used to remove salts from the brownstone 

statuary on the facade of this late-19th century stone church. Photo: 

National Park Service Files. 

9 
10 

Figure 18. The glazed bricks in the center of the pier were covered by a 

signboard that protected them being damaged by the sandblasting 

which removed the glaze from the surrounding bricks. 

Abrasive Blasting. Blasting with abrasive grit or another 

abrasive material is the most frequently used abrasive 

method. Sandblasting is most commonly associated with 

abrasive cleaning. Finely ground silica or glass powder, 

glass beads, ground garnet, powdered walnut and other 

ground nut shells, grain hulls, aluminum oxide, plastic 

particles and even tiny pieces of sponge, are just a few of 

the other materials that have also been used for abrasive 

cleaning. Although abrasive blasting is not an appropriate 

method of cleaning historic masonry, it can be safely used 

to clean some materials. Finely-powdered walnut shells 

are commonly used for cleaning monumental bronze 

sculpture, and skilled conservators clean delicate museum 

objects and finely detailed, carved stone features with very 

small, micro-abrasive units using aluminum oxide. 

Figure 19. A comparison of undamaged bricks surroundng the electrical 

conduit with the rest of the brick facade emphasizes the severity of the 

erosion caused by sandblasting. 

A number of current approaches to abrasive blasting rely 

on materials that are not usually thought of as abrasive, 

and not as commonly associated with traditional 

abrasive grit cleaning. Some patented abrasive cleaning 

processes - one dry, one wet -use finely-ground glass 

powder intended to "erase" or remove dirt and surface 

soiling only, but not paint or stains (Fig. 20). Cleaning with 

baking soda (sodium bicarbonate) is another patented 

process. Bakmg soda blasting is being used in some 

communities as a means of quick graffiti removal. 

However, it should not be used on historic masonry which 

it can easily abrade and can permanently "etch" the graffiti 

into the stone; it can also leave potentially damaging salts 

in the stone which cannot be removed. Most of these 

abrasive grits may be used either dry or wet, although dry 

grit tends to be used more frequently. 

Figure 20. (Left) A comparison of the limestone surface of a 1920s office building before and after "cleaning" with a proprietary abrasive process using 

fine glass powder clearly shows the effectiveness of this method. But this is an abrasive technique and it has "cleaned" by removing part of the masonry 

surface with the dirt. Because it is abrasive, it is generally not recommended for large-scale cleaning of historic masonry, although it may be suitable to 

use in certain, very limited cases under controlled circumstances. (Right) A vacum chamber where the used glass powder is collected for environmentally 

safe disposal is a unique feature of this particular process. The specially-trained operators in the chamber wear protective clothing, masks and breathing 

equipment. Photos: Tom Keohan. 
Figure 21. Low-pressure blasting with ice pellets or ice crystals (left) is 

an abrasive cleaning method that is sometimes recommended for use 

on interior masonry because it does not involve large amounts of water. 

However, like other abrasive materials, ice crystals "clean" by removing 

a portion of the masonry surface with the dirt, and may not remove 

sOllie stains that have penetrated into the l1lasollry without causing 

further abrasion (right). Photos: Audrey T. Tepper. 

Ice particles, or pelletized dry ice (carbon dioxide or C02), 

are another medium used as an abrasive cleaner (Fig. 21). 

TItis is also too abrasive to be used on most historic masonry, 

but it may have practical application for removing mastics 

or asphaltic coatings from some substrates. 

Some of these processes are promoted as being more 

environmentally safe and not damaging to historic masonry 

buildings. However, it must be remembered that they are 

abrasive and that they "clean" by removing a small portion 

of the masonry surface, even though it may be only a 

minuscule portion. The fact that they are essentially 

abrasive treatments must always be taken into consideration 

when planning a masonry cleaning project. In general, 

abrasive methods should not be used to clean historic 

masonry buildings. In some, very limited instances, highlycontrolled, gentle abrasive cleaning may be appropriate 

on selected, hard-to-clean areas of a historic masonry 

building if carried out under the watchful supervision of 

a professional conservator. But, abrasive cleaning should 

never be used on an entire building. 

Grinders and Sanding Disks. Grinding the masonry 

surface with mechanical grinders and sanding disks is 

another means of abrasive cleaning that should not be used 

on historic masonry. Like abrasive blasting, grinders and 

disks do not really clean masonry but instead grind away 

and abrasively remove and, thus, damage the masonry 

surface itself rather than remove just the soiling material. 

Planning A Cleaning Project 

Once the masonry and soiling material or paint have been 

identified, and the condition of the masonry has been 

evaluated, planning for the cleaning project can begin. 

Testing cleaning methods. In order to determine the 

gentlest means possible, several cleaning methods or 

materials may have to be tested prior to selecting the best 

one to use on the building. Testing should always begin 

with the gentlest and least invasive method proceeding 

gradually, if necessary, to more complicated methods, or 

a combination of methods. All too often simple methods, 

such as low-pressure water wash, are not even considered, 

yet they frequently are effective, safe, and not expensive. 

Water of slightly higher pressure or with a non-ionic 

detergent additive also may be effective. It is worth 

repeating that these methods should always be tested prior 

to considering harsher methods; they are safer for the 

building and the environment, often safer for the applicator, 

and relatively inexpensive. 

The level of cleanliness desired also should be determined 

prior to selection of a cleaning method. Obviously, the 

intent of cleaning is to remove most of the dirt, soiling 

material, stains, paint or other coating. A "brand new" 

appearance, however, may be inappropriate for an older 

building, and may require an overly harsh cleaning method 

to be achieved. When undertaking a cleaning project, it is 

important to be aware that some stains simply may not be 

removable. It may be wise, therefore, to agree upon a 

slightly lower level of cleanliness that will serve as the 

standard for the cleaning project. The precise amount of 

residual dirt considered acceptable may depend on the 

type of masonry, the type of soiling and difficulty of total 

removal, and local environmental conditions. 

Cleaning tests should be carried out in an area of sufficient 

size to give a true indication of their effectiveness. It is 

preferable to conduct the test in an inconspicuous location 

on the building so that it will not be obvious if the test is 

not successful. A test area may be quite small to begin, 

sometimes as small as six square inches, and gradually 

may be increased in size as the most appropriate methods 

and cleaning agents are determined. Eventually the test 

area may be expanded to a square yard or more, and it 

should include several masonry units and mortar joints 

(Fig. 22). It should be remembered that a single building 

may have several types of masonry and that even similar 

materials may have different surface finishes. Each material 

and different finish should be tested separately. Cleaning 

tests should be evaluated only after the masonry has dried 

completely. The results of the tests may indicate that 

several methods of cleaning should be used on a single 

building. 

When feasible, test areas should be allowed to weather for 

an extended period of time prior to final evaluation. A 

waiting period of a full year would be ideal in order to 

expose the test patch to a full range of seasons. If this is 

not possible, the test patch should weather for at least a 

month or two. For any building which is considered 

historically important, the delay is insignificant compared 

to the potential damage and disfigurement which may 

result from using an incompletely tested method. The 

successfully cleaned test patch should be protected as it 

will serve as a standard against which the entire cleaning 

project will be measured. 

11 
12 

Environmental considerations. The potential effect of any 

method proposed for cleaning historic masonry should be 

evaluated carefully. Chemical cleaners and paint removers 

may damage trees, shrubs, grass, and plants. A plan must 

be provided for environmentally safe removal and disposal 

of the cleaning materials and the rinsing effluent before 

beginning the cleaning project. Authorities from the local 

regulatory agency - usually under the jurisdiction of the 

federal or state Environmental Protection Agency (EPA) 

should be consulted prior to beginning a cleaning project, 

especially if it involves anything more than plain water 

washing. This advance planning will ensure that the 

cleaning effluent or run-off, which is the combination of 

the cleaning agent and the substance removed from the 

masonry, is handled and disposed of in an environmentally 

sound and legal manner. Some alkaline and acidic cleaners 

can be neutralized so that they can be safely discharged 

into storm sewers. However, most solvent-based cleaners 

cannot be neutralized and are categorized as pollutants, 

and must be disposed of by a licensed transport, storage 

and disposal facility. Thus, it is always advisable to consult 

with the appropriate agencies before starting to clean to 

ensure that the project progresses smoothly and is not 

intermpted by a stop-work order because a required permit 

was not obtained in advance. 

Vinyl guttering or polyethylene-lined troughs placed around 

the perimeter of the base of the building can serve to catch 

chemical cleaning waste as it is rinsed off the building. 

This will reduce the amount of chemicals entering and 

polluting the soil, and also will keep the cleaning waste 

contained until it can be removed safely. Some patented 

cleaning systems have developed special equipment to 

facilitate the containment and later disposal of cleaning 

waste. 

Concern over the release of volatile organic compounds 

(VOCs) into the air has resulted in the manufacture of new, 

more environmentally responsible cleaners and paint 

removers, while some materials traditionally used in 

cleaning may no longer be available for these same reasons. 

Other health and safety concerns have created additional 

cleaning challenges, such as lead paint removal, which is 

likely to require special removal and disposal techniques. 

Cleaning can also cause damage to non-masonry materials 

on a building, including glass, metal and wood. Thus, it 

is usually necessary to cover windows and doors, and 

other features that may be vulnerable to chemical cleaners. 

They should be covered with plastic or polyethylene, or a 

masking agent that is applied as a liquid which dries to 

form a thin protective film on glass, and is easily peeled 

off after the cleaning is finished. Wind drift, for example, 

can also damage other property by carrying cleaning 

chemicals onto nearby automobiles, resulting in etching 

of the glass or spotting of the paint finish. Similarly, 

airborne dust can enter surrounding buildings, and excess 

water can collect in nearby yards and basements. 

Safety considerations. Possible health dangers of each 

method selected for the cleaning project must be considered 

before selecting a cleaning method to avoid harm to the 

Figure 22. Cleaning test areas may be quite small at first and gradually 

increase in size as testing determines the "gentlest means possible". 

Photo: Frances Gale. 

cleaning applicators, and the necessary precautions must 

be taken. The precautions listed in Material Safety Data 

Sheets (MSDS) that are provided with chemical products 

should always be followed. Protective clothing, respirators, 

hearing and face shields, and gloves must be provided to 

workers to be worn at all times. Acidic and alkaline 

chemical cleaners in both liquid and vapor forms can also 

cause serious injury to passers-by (Fig. 23). It may be 

necessary to schedule cleaning at night or weekends if the 

building is located in a busy urban area to reduce the 

potential danger of chemical overspray to pedestrians. 

Cleaning during non-business hours will allow HVAC 

systems to be turned off and vents to be covered to prevent 

dangerous chemical fumes from entering the building 

which will also ensure the safety of the building's occupants. 

Abrasive and mechanical methods produce dust which 

can pose a serious health hazard, particularly if the abrasive 

or the masonry contains silica. 

Water-Repellent Coatings and Waterproof 

Coatings 

To begin with, it is important to understand that waterproof 

coatings and water-repellent coatings are not the same. 

Although these terms are frequently interchanged and 

commonly confused with one another, they are completely 

different materials. Water-repellent coatings --Dften 

referred to incorrectly as "sealers", but which do not or 

should not seal- are intended to keep liquid water from 

penetrating the surface but to allow water vapor to enter 

and leave, or pass through, the surface of the masonry (Fig. 

24). Water-repellent coatings are generally transparent, or 

clear, although once applied some may darken or discolor 

certain types of masonry while others may give it a glossy 

or shiny appearance. Waterproof coatings seal the surface 

from liquid water and from water vapor. They are usually 

opaque, or pigmented, and include bituminous coatings 

and some elastomeric paints and coatings. 
Water-Repellent Coatings 

Water-repellent coatings are formulated to be vapor 

permeable, or "breathable". They do not seal the surface 

completely to water vapor so it can enter the masonry 

wall as well as leave the wall. While the first waterrepellent coatings to be developed were primarily acrylic 

or silicone resins in organic solvents, now most waterrepellent coatings are water-based and formulated from 

modified siloxanes, silanes and other alkoxysilanes, or 

metallic stearates. While some of these products are 

shipped from the factory ready to use, other waterborne 

water repellents must be diluted at the job site. Unlike 

earlier water-repellent coatings which tended to form a 

"film" on the masonry surface, modem water-repellent 

coatings actually penetrate into the masonry substrate 

slightly and, generally, are almost invisible if properly 

applied to the masonry. They are also more vapor 

permeable than the old coatings, yet they still reduce the 

vapor permeability of the masonry. Once inside the wall, 

water vapor can condense at cold spots producing liquid 

water which, unlike water vapor, cannot escape through 

a water-repellent coating. The liquid water within the 

wall, whether from condensation, leaking gutters, or other 

sources, can cause considerable damage. 

Water-repellent coatings are not consolidants. Although 

modem water repellents may penetrate slightly beneath 

the masonry surface, instead of just "sitting" on top of it, 

they do not perform the same function as a consolidant 

which is to "consolidate" and replace lost binder to 

strengthen deteriorating masonry. Even after many years 

of laboratory study and testing few consolidants have 

proven very effective. The composition of fired products 

such as brick and architectural terra cotta, as well as many 

types of building stone, does not lend itself to consolidation. 

Some modem water-repellent coatings which contain a 

binder intended to replace the natural binders in stone 

that have been lost through weathering and natural erosion 

are described in product literature as both a water repellent 

and a consolidant. The fact that newer water-repellent 

coatings penetrate beneath the masonry surface instead 

of just forming a layer on top of the surface may indeed 

convey at least some consolidating properties to certain 

stones. However, a water-repellent coating cannot be 

considered a consolidant. In some instances, a waterrepellent or "preservative" coating, if applied to already 

damaged or spalling stone, may form a surface crust which, 

if it fails, may exacerbate the deterioration by pulling off 

even more of the stone (Fig. 25). 

Is a Water-Repellent Treatment Necessary? 

Water-repellent coatings are frequently applied to historic 

masonry buildings for the wrong reason. They also are 

often applied without an understanding of what they are 

and what they are intended to do. And these coatings can 

be very difficult, if not impossible, to remove from 

the masonry if they fail or become discolored. Most 

importantly, the application of water-repellent coatings to 

historic masonry is usually unnecessary. 

Figure 23. A tarpaulin protects and shields pedestrians from potentially 

harmful spray while chemical cleaning is underway on the granite 

exterior of the U.S. Treasury Building, Washington, D.C. 

Most historic masonry buildings, unless they are painted, 

have survived for decades without a water-repellent 

coating and, thus, probably do not need one now. Water 

penetration to the interior of a masonry building is seldom 

due to porous masonry, but results from poor or deferred 

maintenance. Leaking roofs, clogged or deteriorated 

gutters and downspouts, missing mortar, or cracks and 

open joints around door and window openings are almost 

always the cause of moisture-related problems in a historic 

masonry building. If historic masonry buildings are kept 

watertight and in good repair, water-repellent coatings 

should not be necessary. 

Rising damp (capillary moisture pulled up from the 

ground), or condensation can also be a source of excess 

moisture in masonry buildings. A water-repellent coating 

will not solve this problem either and, in fact, may be 

likely to exacerbate it. Furthermore, a water-repellent 

coating should never be applied to a damp wall. Moisture 

in the wall would reduce the ability of a coating to adhere 

to the masonry and to penetrate below the surface. But, 

if it did adhere, it would hold the moisture inside the 

masonry because, although a water-repellent coating is 

permeable to water vapor, liquid water cannot pass through 

it. In the case of rising damp, a coating may force the 

moisture to go even higher in the wall because it can slow 

down evaporation, and thereby retain the moisture in the 

wall. 

Excessive moisture in masonry walls may carry waterborne 

soluble salts from the masonry units themselves or from 

the mortar through the walls. If the water is permitted to 

come to the surface, the salts may appear on the masonry 

surface as efflorescence (a whitish powder) upon 

evaporation. However, the salts can be potentially 

dangerous if they remain in the masonry and crystallize 

13 
14 

Figure 24. Although the application of a water-repellent coating W IlS 

probably not needed on either of these buildings, the coating on the 

brick building (above), is not visible and has not changed tile character 

of the brick. But the coating on the brick colllmn (below), has a high 

gloss that is incompatible with the historic character of the masonry. 

beneath the surface as subflorescence. Subflorescence 

eventually may cause the surface of the masonry to spall, 

particularly if a water-repellent coating has been applied 

which tends to reduce the flow of moisture out from the 

subsurface of the masonry. Although many of the newer 

water-repellent products are more breathable than their 

predecessors, they can be especially damaging if applied 

to masonry that contains salts, because they limit the flow 

of moisture through masonry. 

When a Water-Repellent Coating May be Appropriate 

There are some instances when a water-repellent coating 

may be considered appropriate to use on a historic masonry 

building. Soft, incompletely fired brick from the 18th- and 

early-19th centuries may have become so porous that paint 

or some type of coating is needed to protect it from further 

deterioration or dissolution. When a masonry building 

has been neglected for a long period of time, necessary 

repairs may be required in order to make it watertight. 

If, following a reasonable period of time after the building 

has been made watertight and has dried out completely, 

moisture appears actually to be penetrating through the 

repointed and repaired masonry wails, then the application 

of a water-repellent coating may be considered in selected 

areas only. This decision should be made in consultation 

with an architectural conservator. And, if such a treatment 

is undertaken, it should not be applied to the entire exterior 

of the building. 

Anti-graffiti or barrier coatings are another type of clear 

coating-although barrier coatings can also be pigmentedthat may be applied to exterior masonry, but they are not 

formulated primarily as water repellents. The purpose of 

these coatings is to make it harder for graffiti to stick to 

a masonry surface and, thus, easier to clean. But, like 

water-repellent coatings, in most cases the application 

of anti-graffiti coatings is generally not recommended for 

historic masonry buildings. These coatings are often quite 

shiny which can greatly alter the appearance of a historic 

masonry surface, and they are not always effective (Fig. 

26). Generally, other ways of discouraging graffiti, such 

as improved lighting, can be more effective than a coating. 

However, the application of anti-graffiti coatings may be 

appropriate in some instances on vulnerable areas of 

historic masonry buildings which are frequent targets of 

graffiti that are located in out-of-the-way places where 

constant surveillance is not possible. 

Some water-repellent coatings are recommended by 

product manufacturers as a means of keeping dirt and 

pollutants or biological growth from collecting on the 

surface of masonry buildings and, thus, reducing the need 

for frequent cleaning. While this at times may be true, in 

some cases a coating may actually retain dirt more than 

uncoated masonry. Generally, the application of a waterrepellent coating is not recommended on a historic masonry 

building as a means of preventing biological growth. 

Some water-repellent coatings may actually encourage 

biological growth on a masonry wall. Biological growth 

on masonry buildings has traditionally been kept at bay 

through regularly-scheduled cleaning as part of a 

maintenance plan. Simple cleaning of the masonry with 

low-pressure water using a natural- or synthetic-bristled 

scrub brush can be very effective if done on a regular basis. 

Commercial products are also available which can 

be sprayed on masonry to remove biological growth. 

In most instances, a water-repellent coating is not 

necessary if a building is watertight. The application of 

a water-repellent coating is not a recommended treatment 

for historic masonry buildings unless there is a specific 
Figure 25. The clear coating applied to this limestone molding has 

fa iled and is taking off some of the stone surface as it peels. Photo: 

Frances Ga le. 

problem which it may help solve. If the problem 

occurs on only part of the building, it is best to treat only 

that area rather than an entire building. Extreme exposures 

such as parapets, for example, or portions of the building 

subject to driving rain can be treated more effectively and 

less expensively than the entire building. Water-repellent 

coatings are not permanent and must be reapplied 

Figure 26. The anti-graffiti or barrier coating on this column is very 

shiny and wou ld not be appropriate to use on a historic masonry 

building. The coating has discolored as it has aged and whitish streaks 

reveal areas of bare concrete where the coating was incompletely 

applied . 

periodically although, if they are truly invisible, it can be 

difficult to know when they are no longer providing the 

intended protection. 

Testing a water-repellent coating by applying it in one 

small area may not be helpful in determining its suitability 

for the building because a limited test area does not allow 

an adequate evaluation of such a treatment. Since water 

may enter and leave through the surrounding untreated 

areas, there is no way to tell if the coated test area is 

"breathable." But trying a coating in a small area may help 

to determine whether the coating is visible on the surface 

or if it will otherwise change the appearance of the masonry. 

Waterproof Coatings 

In theory, waterproof coatings usually do not cause 

problems as long as they exclude all water from the 

masomy. If water does enter the wall from the ground or 

from the inside of a building, the coating can intensify the 

damage because the water will not be able to escape. 

During cold weather this water in the wall can freeze 

causing serious mechanical disruption, such as spalling. 

In addition, the water eventually will get out by the path 

of least resistance. If this path is toward the interior, 

damage to interior finishes can result; if it is toward the 

exterior, it can lead to damage to the masomy caused by 

built-up water pressure (Fig. 27). 

In most instances, waterproof coatings should not be 

applied to historic masonry. The possible exception to 

this might be the application of a waterproof coating to 

below-grade exterior foundation walls as a last resort to 

stop water infiltration on interior basement walls. 

Generally, however, waterproof coatings, which include 

elastomeric paints, should almost never be applied above 

grade to historic masonry buildings. 

Figure 27. Instead of correcting the roof drainage problems, an 

elastomeric coating was applied to the already saturated limeston e 

cornice. An elastomeric coating holds moisture in the masonry because 

it does not "breathe" and does not allow liquid moisture to escape. If 

the water pressure builds up sufficiently it can cause the coating to 

break and pop off as shown in this example, often pulling pieces of the 

masonry with it. Photo: National Park Service Files. 

15 
16 

Summary 

A well-planned cleaning project is an essential step in 

preserving, rehabilitating or restoring a historic masonry 

building. Proper cleaning methods and coating treatments, 

when determined necessary for the preservation of the 

masonry, can enhance the aesthetic character as well as the 

structural stability of a historic building. Removing years 

of accumulated dirt, pollutant crusts, stains, graffiti or 

paint, if done with appropriate caution, can extend the life 

and longevity of the historic resource. Cleaning that is 

carelessly or insensitively prescribed or carried out by 

inexperienced workers can have the opposite of the intended 

effect. It may scar the masonry permanently, and may 

actually result in hastening deterioration by introducing 

harmful residual chemicals and salts into the masonry or 

causing surface loss. Using the wrong cleaning method or 

using the right method incorrectly, applying the wrong 

kind of coating or applying a coating that is not needed 

can result in serious damage, both physically and 

aesthetically, to a historic masonry building. Cleaning a 

historic masonry building should always be done using 

the gentlest means possible that will clean, but not damage 

the building. It should always be taken into consideration 

before applying a water-repellent coating or a waterproof 

coating to a historic masonry building whether it is really 

necessary and whether it is in the best interest of preserving 

the building. 

Selected Reading 

Architectural Ceramics: Their History, Manufacture and 

Conservation. A Joint Symposium of English Heritage and the 

United Kingdom Institute for Conservation, September 22-25, 

1994. London: English Heritage, 1996. 

Ashurst, Nicola. Cleaning Historic Buildings. Volume One: 

Substrates, Soiling & Investigation. Volume Two: Cleaning 

Materials & Processes. London: Donhead Publishing Ltd., 1994. 

Association for Preservation Technology. Special Issue: 

Preservation of Historic Masonry. Papers from the Symposium 

on Preservation Treatments for Historic Masonry: Consolidants, 

Coatings, and Water Repellents, New York, New York, November 

11-12,1994. APT Bulletin. Vol. XXVI, No.4 (1995). 

Grimmer, Anne E. Preservation Brief 6: Dangers of Abrasive 

Cleaning to Historic Buildings. Washington, D.C: Preservation 

Assistance Division, National Park Service, U.s. Department 

of the Interior, 1979. 

Grimmer, Anne E. Ke~ping it Clean: Removing Exterior Dirt, Paint, Stains and Graffiti ]rom Historic Masonry Buildings. Washington, D.C.: Preservation Assistance Division, National 

Park Service, U.S. Department of the Interior, 1988. 

Park, Sharon c., AlA. Preservation Brief39: Holding the Line: 

Controlling Unwanted Moisture in Historic Buildings. 

Washington, D.C: Heritage Preservation Services, National Park 

Service, U.s. Department of the Interior, 1996. 

Powers, Robert M. Preservation Tech Note, Masonry No.3, 

"Water Soak Cleaning of Limestone". Washington, D.C: 

Preservation Assistance Division, National Park Service, U.s. 

Department of the Interior, 1992. 

Sinvinski, Valerie. "Gentle Blasting." Old-House Journal. Vol. 

XXIV, No.4 Ouly-August 1996), pp. 46-49. 

Weaver, Martin E. Conserving Buildings: A Guide to 

Techniques and Materials. New York: John Wiley & Sons, Inc., 

1993. 

Weaver, Martin E. Preservation Brief 38: Removing Graffiti from 

Historic Masonry. Washington, D.C: Preservation Assistance 

Division, National Park Service, U.S. Department of the Interior, 

1995. 

Winkler, E.M. Stone in Architecture: Properties, Durability. 

Third, completely revised and extended edition. Berlin, 

Germany: Springer-Verlag, 1997. 

Acknowledgments 

Robert C. Mack, FAIA, is a principal in the firm of MacDonald 

& Mack Architects, Ltd., an architectural firm that specializes in 

historic buildings in Minneapolis, Minnesota. 

Anne Grimmer is a Senior Architectural Historian in the Technical 

Preservation Services Branch, Heritage Preservation Services 

Program, National Park Service, Washington, D.C 

The original version of Preservation Brief 1: The Cleaning and 

Waterproof Coating of Masonry Buildings was written by Robert C 

Mack, AlA. It inaugurated the Preservation Briefs series when 

it was published in 1975. 

The following historic preservation specialists provided technical 

review of this publication: Frances Gale, Training Director, 

National Center for Preservation Technology and Training, 

National Park Service, Natchitoches, LA; Judith M. Jacob, 

Architectural Conservator, Building Conservation Branch, 

Northeast Cultural Resources Center, National Park Service, N.Y., 

NY; Robert M. Powers, Architectural Conservator, Powers and 

Company, Inc., Philadelphia, PA; Antonio Aguilar, Kaaren Dodge, 

JoEllen Hensley, Gary Sachau, John Sandor and Audrey T. Tepper, 

Technical Preservation Services Branch, Heritage Preservation 

Services Program, National Park Service, Washington, D.C; and 

Kay D. Weeks, Heritage Preservation Services Program, National 

Park Service, Washington, D.C. 

This publication has been prepared pursuant to the National 

Historic Preservation Act of 1966, as amended, which directs the 

Secretary of the Interior to develop and make available information 

concerning historic properties. Comments on the usefulness of 

this publication may be directed to: Sharon C. Park, FAIA, Chief, 

Technical Preservation Services Branch, Heritage Preservation 

Services Program, National Park Service, 1849 C Street, N.W., 

Suite NC200, Washington, D.C 20240 (www2.cr.nps.gov/tps). 

This publication is not copyrighted and can be reproduced without 

penalty. Normal procedures for credit to the authors and the 

National Park Service are appreciated. 

Front Cover: Chemical cleaning of the brick and architectural terra 

cotta frieze on the 1880s Pellsioll Buildillg, Washington, D.C. (now the 

National Building Museulll), is showll here in progress. Photo: 

Christina Henry. 

Photographs llsed to illustrate this Brief were taken by Anne Grimmer unless 

otherwise credited. 

ISSN:0885-7016 November 2000 
1

The concept of energy conservation in buildings is not 

new. Throughout history building owners have dealt 

with changing fuel supplies and the need for effi cient 

use of these fuels. Gone are the days of the cheap and 

abundant energy of the 1950’s. Today with energy 

resources being depleted and the concern over the 

effect of greenhouse gases on climate change, owners 

of historic buildings are seeking ways to make their 

buildings more energy effi cient. These concerns are key 

components of sustainability — a term that generally 

refers to the ability to maintain the environmental, 

social, and economic needs for human existence. The 

topic of sustainable or “green” building practices is too 

broad to cover in this brief. Rather, this preservation 

brief is intended to help property owners, preservation 

professionals, and stewards of historic buildings make 

informed decisions when considering energy effi ciency 

improvements to historic buildings. 

Sound energy improvement measures must take into 

consideration not only potential energy savings, but 

also the protection of the historic property’s materials 

and features. This guidance is provided in accordance 

with the Secretary of the Interior’s Standards for 

Rehabilitation to ensure that the architectural integrity 

of the historic property is preserved. Achieving a 

successful retrofi t project must balance the goals of 

energy effi ciency with the least impact to the historic 

building. Planning must entail a holistic approach that 

considers the entire building envelope, its systems and 

components, its site and environment, and a careful 

evaluation of the effects of the measures undertaken. 

Treatments common to new construction need to be 

evaluated carefully before implementing them in historic 

buildings in order to avoid inappropriate alteration of 

important architectural features and irreparable damage 

to historic building materials. This brief targets primarily 

small-to medium-size historic buildings, both residential 

and commercial. However, the general decision-making 

principles outlined here apply to buildings of any size 

and complexity. 

Inherent Energy Effi cient Features 

of Historic Buildings

Before implementing any energy conservation measures, 

the existing energy-effi cient characteristics of a historic 

building should be assessed. Buildings are more 

than the sum of their individual components. The 

design, materials, type of construction, size, shape, site 

orientation, surrounding landscape, and climate all 

play a role in how buildings perform. Historic building 

construction methods and materials often maximized 

natural sources of heat, light and ventilation to respond 

to local climatic conditions. The key to a successful 

rehabilitation project is to understand and identify the 

existing energy-effi cient aspects of the historic building 

and how they function, as well as to understand and 

identify its character-defi ning features to ensure they 

are preserved. Whether rehabilitated for a new or 

continuing use, it is important to utilize the historic 

building’s inherent sustainable qualities as they were 

intended to ensure that they function effectively together 

with any new treatments added to further improve 

energy effi ciency.

Windows, courtyards, and light wells

Operable windows, interior courtyards, clerestories, 

skylights, rooftop ventilators, cupolas, and other features 

that provide natural ventilation and light can reduce 

energy consumption. Whenever these devices can be 

used to provide natural ventilation and light, they save 

energy by reducing the need to use mechanical systems 

and interior artifi cial lighting. 

Historically, builders dealt with the potential heat loss 

and gain from windows in a variety of ways depending 

on the climate. In cold climates where winter heat 

loss from buildings was the primary consideration 

before mechanical systems were introduced, windows 

were limited to those necessary for adequate light and 

ventilation. In historic buildings where the ratio of glass 

Improving Energy Effi ciency 

in Historic Buildings 

Jo Ellen Hensley and Antonio Aguilar

PRESERVATION 3 BRIEFS

National Park Service

U.S. Department of the Interior

Technical Preservation Services
2

to wall is less than 20%, the potential heat loss through 

the windows is likely minimal; consequently, they are 

more energy effi cient than most recent construction. In 

hot climates, numerous windows provided valuable 

ventilation, while features such as wide roof overhangs, 

awnings, interior or exterior shutters, venetian blinds, 

shades, curtains and drapes signifi cantly reduced heat 

gain through the windows. Historic windows can play 

an important role in the effi cient operation of a building 

and should be retained.

New architectural styles, beginning with the 

International Style of the 1920’s, brought about an 

increase in the percentage of glazing in the total 

building envelope. By the 1950’s, with the advent of 

the glass curtain wall, glazing constituted nearly 100% 

of a building’s exterior walls in many buildings. While 

many early modern buildings continued to use operable 

windows as a way to provide natural ventilation, greater 

reliance on mechanical heating and air conditioning 

systems eventually reduced the function of exterior 

glazing to providing light only, particularly in 

commercial, offi ce, and institutional buildings. 

Walls

Thick masonry walls typical of the late-nineteenth 

and early-twentieth centuries have inherent thermal 

characteristics that keep the buildings cooler in 

the summer and warmer in the winter. Walls with 

substantial mass have the advantage of high thermal 

inertia, which reduces the rate of heat transfer through 

the wall. For instance, a wall with high thermal inertia, 

subjected to solar radiation for an hour, will absorb the 

heat at its outside surface, but slowly transfer it to the 

interior over a period as long as six hours. Conversely, a 

wall having the equivalent thermal resistance (R-value), 

but a substantially lower thermal inertia, will transfer 

the heat in perhaps as little as two hours. Heavy 

masonry walls also reduce the need for summer cooling. 

High thermal inertia is the reason many older public and 

commercial buildings without air conditioning still feel 

cool during the summer. The heat from the midday sun 

does not penetrate the buildings until late afternoon and 

evening, when it is less likely to be occupied or when 

exterior temperatures have fallen. Heavy masonry walls 

are also effective in moderating internal temperatures 

in the winter by dampening the overall peaks of heat 

gain and loss resulting in a fl atter and more tolerable 

daily cycle. In areas that require cooling during the day 

and heating at night, masonry walls can help spread 

out excess heat gain from the day to cover some of the 

needed heating for the evening and night hours. 

Roofs 

Roof construction and design in historic buildings, 

particularly vernacular buildings, are strongly 

Fig. 1. A decorative, stained glass skylight allows natural daylight 

into the interior.

Fig. 2. Upper and lower shutters control daylight and provide privacy.

Fig. 3. Stone walls with substantial mass have high thermal inertia.
3

infl uenced by the conditions of the local climate. Wide 

overhangs that sometimes extend to create porches 

minimize the heat gain from the sun in warmer climates, 

while steep, sloping roofs with minimal or no overhang 

prevail in colder climates to allow for shedding snow 

and increasing benefi cial solar heat gain through the 

windows. Materials and color also infl uence the thermal 

performance of roofs. Metal and light colored roofs, for 

example, refl ect sunlight and thereby reduce the heat 

gain from solar radiation. 

Floor Plans 

The fl oor plan of many historic buildings, particularly 

traditional vernacular ones, was also designed to 

respond to the local climate. In cold climates, rooms 

with low ceilings were clustered around central 

chimneys to share the heat, while small windows 

with interior shutters reduced drafts and heat loss. In 

warmer climates, wide central halls with tall ceilings, 

breezeways, and large porches all maximized 

air circulation.

Landscape

Site orientation was another factor considered especially 

in the location of a historic building on its property. 

In cold climates, buildings were oriented against 

northern winds while buildings in warm climates were 

sited to take advantage of prevailing breezes. Evergreen 

trees planted on the north side of buildings shielded 

from winter winds; deciduous trees planted to the 

south provided summer shade and maximum sun in 

the winter. 

Energy Audit

Before implementing any measures to improve the 

thermal performance of a historic building, an energy 

audit should be undertaken to evaluate the current 

energy use of the building and identify defi ciencies 

in the building envelope or mechanical systems. In 

some areas, the local utility company may offer a free 

simple audit, however a more in-depth audit should 

be obtained from a professional energy auditor. The 

goal of the audit is to establish a baseline of building 

performance data to serve as a reference point 

when evaluating the effectiveness of future energy 

improvements. It is important to hire an independent 

auditor who does not have fi nancial interests in the 

results, such as a product vendor. 

An energy auditor fi rst documents the current energy 

use patterns in the building to establish an energy use 

history. This initial step includes obtaining the billing 

history from the local utility company over a one- or 

two-year period, as well as documenting the number 

of building occupants, how the building is used, and 

the type of fuel consumed. The location of any existing 

insulation is recorded and the approximate R-value of 

various components of the building envelope including 

walls, ceilings, fl oors, doors, windows and skylights 

is calculated. The building envelope is inspected to 

identify areas of air infi ltration and air loss. The type 

and age of mechanical systems and major appliances 

are also recorded. 

Tools such as a blower door test or infrared 

thermography are useful to identify specifi c areas of 

infi ltration, lack of insulation and thermal bridging. 

Mechanical depressurization along with infrared 

thermography is extremely useful in identifying 

locations of air leakage and heat loss followed by the use 

of tracer smoke to isolate specifi c air leaks. These tests 

are often challenging to perform on buildings and must 

be undertaken by experienced professionals to avoid 

Fig. 4. A typical 

New England saltbox 

features a steeply 

sloping roof to shed 

snow and a fl oor plan 

organized around a 

central chimney to 

conserve heat.

Lean-to 

Addition

Parlor Hall

Fig. 5. The side porches of this house in Charleston, SC, shade the 

large windows and provide outdoor living spaces that take advantage 

of sea breezes.
4

misleading or inaccurate results. There are professional 

standards for audits, those of the Building Performance 

Institute (BPI) being the most widely used. 

The energy auditor then produces a detailed report 

that documents the fi ndings of the audit and includes 

specifi c recommendations for upgrades such as air 

sealing, adding insulation, general repairs, lighting, and 

improvements to or replacement of mechanical systems 

or major appliances. Cost estimates are provided 

for each of the improvements including the cost of 

implementation, potential operating cost savings, and, 

importantly, the anticipated payback period. Armed 

with this information, historic building owners can 

start to make informed decisions on how to improve 

the performance of their buildings. Usually the auditor 

fi nds a few locations where there is major air leakage; 

large “holes” that are unique to a particular building 

and require equipment to fi nd them. These anomalies 

are often invisible to the people who use the building on 

a regular basis. It is important to retest the performance 

of the building following the implementation of any 

upgrades undertaken as a result of an energy audit to 

ensure that the upgrades are performing as expected.

Prioritizing Energy Upgrades 

When implementing energy upgrades, efforts should 

be concentrated on improvements that will provide the 

most payback for the money expended and the least 

compromise to the historic character of the building. 

Some upgrades recommended in energy audits may not 

be introduced into a historic building feasibly without 

damaging historic fabric or altering the appearance 

of signifi cant features. Removing historic siding and 

replacing it with new siding to introduce insulation 

into the wall cavity of a frame building or replacing 

repairable historic windows are examples of treatments 

that should not be undertaken on historic buildings. 

A common misconception is that replacing windows 

alone will result in major energy savings. This argument, 

often used to sell replacement windows, is simply not 

true. Although it varies from building to building, the 

U.S. Department of Energy (DOE) has documented that 

air loss attributable to windows in most buildings is 

only about 10% of the total air loss. Studies have shown 

that window replacement does not pay for itself in 

energy savings in a reasonable length of time. Moreover, 

there are ways to improve the performance of historic 

windows that do not require their replacement. 

In addition, historic windows can usually be repaired 

and are, thus, sustainable, while most new windows 

cannot be repaired, or even recycled, and may wind up 

in landfi lls. 

When considering energy upgrades, it is imperative to 

get a clear picture of what an improvement will cost 

initially and how long it will take to pay back the cost 

in energy savings. Therefore, the life cycle cost of the 

improvement must be considered as well as its impact 

on historic fabric. Reducing infi ltration around existing 

windows and doors, sealing penetrations in the building 

envelope, and adding insulation — particularly in the 

attic where it has little impact on historic fabric — can 

result in signifi cant improvements at relatively little 

cost. Updating mechanical systems or changing the way 

in which they are operated can also be cost-effective 

interventions. For example, installing a more effi cient 

mechanical system alone may pay for itself in ten years.

Fig. 6. (left) A blower door is used to depressurize a building by exhausting air at a rate that allows pressure gages and tracer smoke to measure the 

amount and location of air leakage. Photo: Robert Cagnetta, Heritage Restoration, Inc. 

Fig. 7. (center and right) The left thermal image shows the walls of this building before insulating. After insulation was added, the cooler and, thus 

darker exterior walls evidence how much the heat loss has been reduced. Photos: EYP Architecture & Engineering.
5

Actions to Improve Energy Effi ciency

Reducing energy demands for heating and cooling 

may be accomplished in two steps. First, implement 

operational changes and upgrades to mechanical 

systems and major appliances — measures that do not 

require making alterations or adding new materials — 

to ensure that a building functions as effi ciently 

as possible. After all these measures have been 

implemented, corrective work or treatments, such as 

weatherization, that require other alterations to the 

building may be considered. 

Establishing Realistic Goals

Energy consumption data gathered by the U.S. Energy 

Information Administration (see chart) shows that 

residential buildings built before 1950 (the largest 

percentage of historic building stock) are about 30 to 

40 percent less energy effi cient than buildings built 

after 2000. Using this as a baseline, a 30 to 40 percent 

upgrade of a historic building’s energy performance 

can be a realistic goal. A 40 percent increase in energy 

effi ciency would of course be a more achievable goal for 

buildings that have had minimal upgrades since their 

original construction, i.e., added insulation, tightening 

of the exterior envelope, or more effi cient mechanical 

equipment. On the other hand, achieving “net zero” 

energy goals as it is currently done with some new 

construction can be a much more diffi cult challenge to 

achieve in a historic retrofi t. Attempting to reach such a 

goal with a historic building would most likely result in 

signifi cant alterations and loss of historic materials. [The 

data for commercial buildings documents that buildings 

in 2003 used approximately the same energy as they did 

before 1920, after reaching their peak in the 1980’s.]

Operational Changes

One of the greatest effects on energy use is user 

behavior. Once an energy audit has established a 

baseline for the current energy use in a building, 

operational changes should be identifi ed to control 

how and when the building is used to minimize the use 

of energy-consuming equipment. These changes can 

range from simple measures such as regular cleaning 

and maintenance of mechanical equipment to installing 

sophisticated controls that cycle equipment on and off 

in specifi ed intervals for maximum performance. The 

following changes are recommended to reduce heating 

and cooling costs.

• Install programmable thermostats. 

• Close off rooms that are not in use and adjust the 

temperature in those rooms.

• Do not condition rooms that do not need to be 

conditioned, thereby reducing the thermal envelope.

• Use insulated shades and curtains to control heat gain 

and loss through windows.

• Use operable windows, shutters, awnings and 

vents as originally intended to control temperature 

and ventilation.

• Take advantage of natural light. 

• Install compact fl uorescent lights (CFL) and lightemitting diode (LED) lights. 

• Install motion sensors and timers for lighting and local 

ventilation, such as bathroom exhaust fans.

• Reduce “phantom” electricity loads by turning 

equipment off when not in use. 

• Clean and service mechanical equipment regularly. 

These measures should be undertaken fi rst to save 

energy in any existing building and are particularly 

appropriate for historic buildings because they do not 

require alterations to historic materials. 

Upgrading Equipment and Appliances

In addition to maximizing the energy effi ciency of 

existing building systems, substantial savings can be 

achieved through upgrading equipment and appliances. 

One should still weigh the operational savings against 

the initial cost of the new equipment, particularly if the 

existing equipment is not near the end of its life. 

Calculator aids that take into account the effi ciency 

of both the existing and new equipment are available 

Residential Energy Use Intensity by Age

Year Built KBtu/sq ft/yr

Prior to 1950 74.5

1950 to 1969 66.0

1970 to 1979 59.4

1980 to 1989 51.9

1990 to 1999 48.2

2000 to 2005 44.7

Source: Residential Energy Consumption Survey, 2005

Fig. 8. Where Air Escapes From a House (by percentage) – Image 

based on data from Energy Savers, U.S. Department of Energy. 

Illustration: Blank Space LLC.
6

online to assist in determining the payback. Advance 

planning will allow time to fi nd the most effi cient unit, 

as well as to investigate the availability of any state and 

federal energy credits. As energy prices continue to rise 

and technology advances, options such as the installation 

of a solar hot water heater or geothermal ground 

source or water source heat pumps are becoming more 

economically feasible. Recommendations for upgrading 

equipment and appliances include:

• Upgrade the heating system. It is important to install 

new furnaces that utilize outside combustion air to 

reduce air drawn into the building through uncontrolled 

infi ltration. [All furnaces and boilers are now measured 

by their annual fuel utilization effi ciency or AFUE.] 

Heating equipment is now more effi cient and gas 

furnaces that used to have a 60% (AFUE) rating can 

now operate at as much as 90 to 97% effi ciency.

• Upgrade the air conditioning system. 

• Replace the water heater. High-effi ciency water heaters 

use far less energy than earlier models, and high-effi ciency 

tankless water heaters heat water on demand and offer 

even greater savings. Point of use water heat can also 

reduce costs and water consumption by reducing the time 

it takes to draw hot water.

• Upgrade appliances. Energy Star appliances, 

particularly refrigerators, washing machines and 

dishwashers can all reduce electricity use and additional 

indoor heating loads. 

Upgrading Building Components 

In addition to operational and mechanical upgrades, it 

can be possible to upgrade many building components 

in a manner that will not jeopardize the historic character 

of the building and can be accomplished at a reasonable 

cost. The goal of these upgrades is to improve the 

thermal performance of the building, resulting in even 

greater energy savings. Retrofi t measures to historic 

buildings should be limited to those that achieve at least 

reasonable energy savings, at reasonable costs, with the 

least impact on the character of the building. 

The following list includes the most common measures 

proposed to improve the thermal performance of 

an existing building; some measures are highly 

recommended for historic buildings, but others are less 

benefi cial, and can even be harmful to a historic building.

Requires Minimal Alteration 

• Reduce air leakage. 

• Add attic insulation.

• Install storm windows.

• Insulate basements and crawlspaces. 

• Seal and insulate ducts and pipes.

• Weather strip doors and add storm doors.

• Add awnings and shading devices where appropriate.

Requires More Alteration 

• Add interior vestibules.

• Replace windows.

• Add insulation to wood-frame walls.

• Add insulation to masonry walls.

• Install cool roofs and green roofs.

The treatments listed fi rst have less potential to 

negatively impact the historic fabric of a building. They 

tend to be less intrusive, are often reversible, and offer 

the highest potential for energy savings. Undertaking 

any of the treatments in the second group, however, may 

pose technical problems and damage to historic building 

materials and architectural features. Their installation 

costs may also outweigh the anticipated energy savings 

and must be evaluated on a case-by-case basis with 

advice from professionals experienced in historic 

preservation and building performance.

Requires Minimal Alteration 

Reduce air leakage. Reducing air leakage (infi ltration 

and exfi ltration) should be the fi rst priority of a 

preservation retrofi t plan. Leakage of air into a building 

can account for 5 to 40 percent of space-conditioning 

costs, which can be one of the largest operational costs 

for buildings.1

 In addition, unwanted air leakage into 

and out of the building can lead to occupant comfort 

issues resulting from drafts. Air infi ltration can be 

especially problematic in historic buildings because it 

is closely linked to increased moisture movement into 

building systems. 

Air fl ow into and out of buildings is driven by three 

primary forces: wind pressure, mechanical pressure 

and the stack effect. Cold outside air that infi ltrates the 

building through big holes, as well as through loose 

windows, doors, and cracks in the outer shell of the 

building, causes the heating system to work harder and 

consume more energy. In a multi-story building, cold 

air that enters the building at lower levels, including 

the basement or crawlspace, will travel up through 

the building and exit out leaky windows, gaps around 

windows and the attic as a result of temperature and 

pressure differential. This pattern of air movement 

Fig. 9. An energy auditor tests the effi ciency of a boiler.
7

is called the “stack effect.” Not only is valuable 

conditioned air lost, but damaging moisture may also 

enter the wall cavities and attic spaces. To stop the stack 

effect, the top and bottom of the exterior walls, interfl oor bypasses, and any existing chases or shafts must 

be sealed, or “draft proofed.” The use of spray foam 

sealants in basement and attic cracks is a particularly 

useful technique for reducing air infi ltration. 

Adding weatherstripping to doors and windows, sealing 

open cracks and joints at the base of walls and around 

windows and doors, sealing off recessed lighting fi xtures 

from above, and sealing the intersection of walls and 

attic, will substantially reduce air leakage. When using 

exterior caulk to seal the intersection of siding and doors 

or windows, do not caulk the underside of clapboards 

or below windows to allow any liquid water to escape. 

When infi ltration and, consequently, exfi ltration are 

reduced, mechanical ventilation may be necessary to 

meet occupants’ requirements for fresh air.

Add attic or roof insulation. Heat loss and gain caused 

by increased interior/exterior temperature differentials 

primarily due to the stack effect and solar radiation are 

greatest at the top of a building. Therefore, reducing 

heat transfer through the roof or attic should be one of 

the highest priorities in reducing energy consumption. 

Adding insulation in unoccupied, unfi nished attics 

is not only very effective from an energy-savings 

perspective, but it is also generally simple to install and 

causes minimal disruption to historic materials. The U.S. 

Department of Energy (DOE) provides a recommended 

R-value chart based on climate zones to help determine 

the optimal amount of insulation that should be installed 

in a particular project. Local codes may also have specifi c 

insulation requirements. Insulating trap or access doors 

should not be overlooked. Even though they may be 

small, attic doors can be responsible for substantial 

heat loss and should be addressed as part of any attic 

insulation project.

In unfi nished and unheated attics, the insulation 

material is typically placed between the fl oor joists 

using blown-in, batt, or rigid foam insulation. When 

using fi berglass batts faced with a vapor retarder, 

the vapor retarder should be face down towards the 

heated interior. However, the use of a vapor retarder 

is not necessary in attic applications. If additional batt 

insulation is being added over existing insulation that 

is near or above the top of the joists, new un-faced batts 

should be placed perpendicular to the old ones to cover 

the top of the joists and reduce thermal bridging through 

the frame members. In low-pitched roofs, or where 

installing batt insulation is diffi cult, a more complete 

coverage of the attic fl oor may be achieved by using 

blown-in insulation. Unfi nished attics must be properly 

ventilated to allow excess heat to escape.

 

Fig. 10. The pattern of air movement referred to as the ”stack effect”. 

Illustration: Blank Space LLC.

Fig. 11. Air infi ltration and exfi ltration. 

Illustration: Blank Space LLC.

Fig. 12. Recommended energy improvements vary widely based on 

climate. The information contained in this document is based primarily 

on the available data for the Northeast and Mid-Atlantic regions.

DOE Climate Zone Map
8

Radiant barriers may be used in attics to reduce thermal 

radiation across the air space between the roof deck 

and the attic fl oor in order to reduce summer heat gain. 

They are most benefi cial in reducing cooling loads in 

hot climates and consist of a highly refl ective sheet or 

coating, usually aluminum, applied to one or both sides 

of a fl exible material. They are effective only when the 

foil surface faces an air space, and as long as the surface 

remains shiny – that is, free from dirt, dust, condensation 

and oxidation. Radiant barriers should not be installed 

directly over insulation on the attic fl oor, as they can act 

as vapor retarders and trap moisture in the insulation 

unless they are perforated. Their placement should be 

ventilated on both sides.

Insulating the underside of the roof rather than the attic 

fl oor increases the volume of the thermal envelope of 

the building, thus making this treatment inherently less 

energy effi cient. However, when mechanical equipment 

and/or ductwork are housed in an attic space, placing 

the insulation under the roof and treating the attic 

as a conditioned space is strongly recommended. 

This treatment allows the equipment to operate more 

effi ciently and can prevent moisture-related problems 

caused by condensation on the mechanical equipment.

When insulation is placed under the roof, all vents in 

the attic and the intersection between the walls and roof 

rafters must be sealed. Rigid foam or batt insulation 

placed between the roof rafters is a common method of 

insulating the underside of a roof. Open cell spray foam 

(.5lb/cuft) may sometimes be applied under the roof 

deck only when there are no gaps in the sheathing which 

could allow the foam to expand under slates or shingles, 

preventing the re-use of the roofi ng material. Also, if 

roof leaks do occur, they may go undetected until after 

major damage occurs. Consideration must also be given 

to the irreversibility of this procedure because the foam 

enters the pores of the wood. It may be more advisable 

to install a breathable layer of material that will allow for 

future removal without leaving a residue. 

When total roof replacement is required because of 

deterioration, installing rigid foam insulation on top 

of the roof deck before laying the new roofi ng material 

can be simple and effective, particularly on low-pitched 

or fl at roofs. However, the added thickness of the roof 

caused by installing rigid foam can alter the appearance 

of projecting eaves, dormers, and other features. If this 

application would signifi cantly alter the appearance of 

these features, consider other methods. 

Install storm windows. The addition of metal or wood 

exterior or interior storm windows may be advisable 

to increase the thermal performance of the windows 

in ways that weatherstripping and caulking cannot 

address. A single-glazed storm window may only 

increase a single-pane window’s thermal resistance to 

R2, however, that is twice as good as a single-glazed 

window alone. It will make a noticeable contribution 

to the comfort level of the building occupant, with 

the added benefi t of protecting the historic window 

from weathering. Using clear, non-tinted, low-e glass 

in the storm window can further increase the thermal 

performance of the window assembly without the loss of 

historic fabric. Studies have shown that the performance 

of a traditional wood window with the addition of a 

storm window can approach that of a double-glazed 

replacement window.2

 Some storm windows are 

available with insulated low-e glass, offering even 

higher thermal performance without the loss of the 

historic window. Furthermore, a storm window avoids 

the problem of irreparable seal failure on insulated glass 

units (IGUs) used in modern replacement windows. 

While the lifespan of the IGU depends both on the 

quality of the seal and other factors, it is unreasonable to 

expect more than 25 years. Once the seal fails, the sash 

itself will usually need to be entirely replaced.

By providing an additional insulating air space and 

adding a barrier to infi ltration, storm windows improve 

comfort and reduce the potential for condensation on the 

glass. To be effective and compatible, storm windows 

must be tight fi tting; include a sealing gasket around the 

glass; align with the meeting rail of the primary sash; 

match the color of the sash; and be caulked around the 

frame to reduce infi ltration without interfering with any 

weep holes. 

Whether a storm window or the historic window itself, 

the interior window must be the tighter of the two units 

to avoid condensation between the windows that can 

Fig. 14. Sample installation of rigid foam insulation, tapered at the 

edge to avoid altering the appearance of the roof.

Fig. 13. Sample installation of a radiant barrier.

Roof Decking

Air Space

Radiant Barrier

Insulation

Joist
9

occur in a cold climate that requires indoor heating. 

Condensation is a particular concern if it collects on 

the historic window, as can easily happen with a loosefi tting, storm window. While interior storm windows 

can be as thermally effective as exterior storm windows, 

appropriate gaskets must be used to ensure that 

damage-causing condensation does not form on the 

inside face of the historic window. Opening or removing 

the interior storm windows during non-heating months 

also helps to avoid the negative effects of moisture 

build-up. 

For large, steel industrial windows, the addition of 

interior, insulated sliding glass windows that align 

with the primary vertical mullions has proven to be a 

successful treatment that allows the primary window to 

remain operable. 

Insulate basements and crawlspaces. The fi rst step in 

addressing the insulation of basements and crawl spaces 

is to decide if they are to be part of the conditioned 

space and, therefore, within the thermal envelope of 

the building. If these areas are kept outside the thermal 

envelope of the building and treated as unconditioned 

areas, insulating between the fl oor joists on the 

underside of the subfl oor is generally recommended. 

Alternatively, rigid foam insulation installed over 

the bottom of the fl oor joists on the basement or 

crawlspace side may also be used. All gaps between the 

unconditioned and conditioned areas of the building, 

including the band joists, should be air sealed to prevent 

air infi ltration into the upper levels of the building. 

If the crawlspace contains mechanical equipment, or if 

high levels of moist air enter the crawlspace through 

vents during the summer months, it is advisable to 

include the crawlspace within the thermal boundary of 

the building. As in attics, water vapor can condense on 

ducts and other equipment located in unconditioned 

basements and crawlspaces. In the past, building codes 

routinely required that crawlspaces be treated as nonconditioned spaces and be ventilated. However, this has 

not proven to be a best practice in all cases. Ventilation 

through crawlspace vents does not keep the space dry 

during humid summers. All vents should be sealed and 

access doors weather-stripped. Rigid foam insulation 

installed on the interior face of the wall is recommended 

for basement and crawlspace foundation walls, only 

after all drainage issues have been addressed. Special 

attention should be given to ensure that all the joints 

between the insulation boards are sealed. 

A moisture barrier on exposed dirt in a crawlspace is 

strongly recommended to prevent ground moisture 

from entering the building envelope. Whenever feasible, 

pouring a concrete slab over a moisture barrier in 

crawlspaces or basements with exposed dirt fl oors 

should be considered. 

Seal and insulate ducts and pipes. A surprisingly 

enormous amount of energy is wasted when heated 

or cooled air escapes from supply ducts or when 

hot attic air leaks into air conditioning return ducts. 

Based on data collected in energy audits, as much as 

35 percent of the conditioned air in an average central 

air conditioning system may escape from the ducts.3

Care must be taken to completely seal all connections 

in the duct system and adequately insulate the ducts, 

especially in unconditioned spaces. This loss of 

energy is another reason to treat attics, basements and 

crawlspaces as conditioned spaces. Ducts located in 

unconditioned spaces should be insulated based on the 

recommendations for the appropriate climate zone. Hot 

water pipes and water heaters should be insulated in 

unconditioned spaces to retain heat, and all water pipes 

insulated to prevent freezing in cold climates.

Weather strip doors and add storm doors. Historic 

wood doors are often signifi cant features and should 

always be retained, rather than replaced. While an 

insulated replacement door may have a higher R-value, 

doors represent a small area of the total building 

envelope, and the difference in energy savings after 

replacement would be insignifi cant. The doors and 

frames should, however, have proper maintenance 

including regular painting, and the addition or renewal 

of weatherstripping. Storm doors can improve the 

thermal performance of the historic door in cold 

climates and may be especially recommended for a 

door with glazing. The design of the storm door should 

be compatible with the character of the historic door. 

A fully glazed storm door with a frame that matches 

the color of a historic door is often an appropriate 

choice because it allows for the historic door to remain 

visible. Storm doors are recommended primarily for 

residential buildings. They are not appropriate for 

commercial or industrial buildings. These buildings 

never had storm doors, because the doors were opened 

frequently or remained open for long periods. It may 

also not be appropriate to install a storm door on a 

highly signifi cant entrance door. In some instances, 

Fig. 15. Original steel windows were 

retained and made operable during 

the rehabilitation of this historic mill 

complex. Insulated sliding windows 

were added on the interior to improve 

energy effi ciency.
10

the addition of a storm door could add signifi cant heat 

gain on certain exposures or in hot climates, which could 

degrade the material or fi nish of the historic door.

Add awnings and shading devices. Awnings and other 

shading devices can provide a considerable reduction 

of heat gain through windows and storefronts. Keeping 

existing awnings, or replacing them if previously 

removed, is a relatively easy way to enhance the energy 

performance of a building. Awnings should only be 

installed when they are compatible with the building 

type and character. In building types that did not have 

awnings historically, interior shades, blinds or shutters 

should be considered instead.

A wide range of shades, blinds and shutters is available 

for use in all types of buildings to control heat gain or 

loss through windows, as well as lighting levels. When 

properly installed, shades are a simple and cost-effective 

means of saving energy. Some shade fabrics block only 

a portion of the light coming in — allowing the use of 

natural light — while others block all or most of the 

light. The light-colored or refl ective side of the shades 

should face the window to reduce heat gain. Quilted 

roller shades feature several layers of fi ber batting and 

sealed edges, and these shades act as both insulation 

and an air barrier. They control air infi ltration more 

effectively than other soft window treatments. Pleated or 

cellular shades provide dead air spaces within the cells 

to add insulation value. These shades, however, do not 

measurably control air infi ltration.

Retractable awnings and interior shades should be 

kept lowered during the summer to prevent unwanted 

heat gain, but raised in the winter to take advantage 

of the heat gain. Interior shades, especially those that 

have some insulation value, should be lowered at night 

during the winter months.

Light shelves are architectural devices designed to 

maximize daylight coming through windows by 

refl ecting it deeper into the building. These horizontal 

elements are usually mounted on the interior above 

head height in buildings with high ceilings. Although 

they can provide energy savings, they are not compatible 

with most historic buildings. In general, light shelves 

are most likely to be appropriate in some industrial or 

modernist-style buildings, or where the historic integrity 

of interior spaces has been lost and they can be installed 

without being visible from the exterior.

Requires More Alteration 

Add interior vestibules. Vestibules that create a 

secondary air space or “air lock” are effective in 

reducing air infi ltration when the exterior door is 

open. Exterior and interior vestibules are common 

architectural features of many historic buildings and 

should be retained wherever they exist. Adding an 

interior vestibule may also be appropriate in some 

historic buildings. For example, new glazed interior 

vestibules may be compatible changes to historic 

commercial and industrial buildings. New exterior 

vestibules will usually result in too great a change to the 

character of primary entrances, but may be acceptable 

in very limited instances, such as at rear entrances. Even 

in such instances, new vestibules should be compatible 

with the architectural character of the historic building.

Replace windows. Windows are character-defi ning 

features of most historic buildings. As discussed 

previously, the replacement of a historic window with 

a modern insulated unit is not usually a cost-effective 

choice. Historic wood windows have a much longer 

service life than replacement insulated windows, which 

cannot be easily repaired. Therefore, the sustainable 

choice is to repair historic windows and upgrade their 

thermal performance. However, if the historic windows 

are deteriorated beyond repair, if repair is impractical 

because of poor design or material performance, or 

if repair is economically infeasible, then replacement 

windows may be installed that match the historic 

windows in size, design, number of panes, muntin 

profi le, color, refl ective qualities of the glass, and the 

same relationship to the window opening. 

Other options should also be considered before 

undertaking complete window replacement. If only the 

sash is severely deteriorated and the frame is repairable, 

then only the sash may need to be replaced. If the 

limited lifespan of insulated glass is not a concern, the 

Fig. 16. Historic vestibules retain conditioned air in the living spaces. new sash can be made to accommodate double glazing.
11

Where the sashes are sound, but improved thermal 

performance without the use of a storm window 

is desired, some windows may be retrofi tted with 

insulated glass. If the existing sash is of suffi cient 

thickness, it may be routed to accept insulated, clear 

low-e glass without extensive loss of historic material 

or historic character. When insulated glass is added in 

a new or retrofi tted sash, any weights will have to be 

modifi ed to accommodate the signifi cant extra weight.

Wall Insulation

Adding wall insulation must be evaluated as part of 

the overall goal to improve the thermal effi ciency of 

a building and should only be considered after the 

installation of attic and basement insulation. Can this 

goal be achieved without the use of wall insulation? Can 

insulation be added without causing signifi cant loss 

of historic materials or accelerated deterioration of the 

wall assembly? Will it be cost effective? These are basic 

questions that must be answered before a decision is 

made to insulate the walls and may require professional 

evaluation.

Add insulation to wood-frame walls. Wood is 

particularly susceptible to damage from high moisture 

levels; therefore, addressing existing moisture problems 

before the addition of insulation is essential. 

Un-insulated historic wood buildings have a higher 

rate of air infi ltration than modern buildings; while 

this makes older buildings less effi cient thermally, it 

helps dissipate the unwanted moisture and thus keeps 

building assemblies dry. Climate, building geometry, 

the condition of the building materials, construction 

details, and many other factors make it diffi cult to assess 

the impact that adding insulation will have on reducing 

the air fl ow and, hence, the drying rate in a particular 

building. For this reason, predicting the impact of 

adding insulation to wood-frame walls is diffi cult. 

Insulation Installed in the Wall Cavity: When sheathing is 

part of the wall assembly, and after any moisture-related 

problems have been addressed, adding insulation to the 

interior cavity of a wood-frame wall may be considered. 

Adding insulation in a wall where there is no sheathing 

between the siding and studs is more problematic, 

however, because moisture entering the wall cavity 

through cracks and joints by wind-driven rain or 

capillary action will wet the insulation in contact with 

the back of the siding. 

Installing blown-in insulation, either dense-packed 

cellulose or fi berglass, into the wall cavity causes 

the least amount of damage to historic materials and 

fi nishes when there is access to the cavity walls, and 

it is therefore a common method of insulating woodframe walls in existing buildings. In most cases, blowing 

insulation material into the wall cavity requires access 

through the exterior or interior wall surfaces. When 

historic plaster, wood paneling, or other interior 

historic decorative elements are present, accessing the 

cavity from the exterior is recommended by removing 

individual siding boards at the top of each wall cavity. 

In this manner the boards can be reinstalled without 

unsightly drill holes on the exterior. If the plaster is 

deteriorated and will require repair, then the wall cavity 

may be accessed from the interior through holes drilled 

through non-decorative plaster.

Of the materials available, dense-packed cellulose fi ber 

is most commonly used. Its R-value, ability to absorb 

and diffuse moisture, impediment to air fl ow, relatively 

simple installation, and low cost make it a popular 

choice. Cellulose insulation from most manufacturers is 

available in at least two grades that are characterized by 

the type of fi re retardant added to the insulation. The fi re 

retardants are usually: (1) a mix of ammonium sulfate 

and boric acid or (2) boric acid only (termed “borate 

only”). The recommended type of cellulose insulation 

for historic buildings is the “borate only” grade, as 

cellulose treated with sulfates reacts with moisture in the 

air and forms sulfuric acid which corrodes many metals.

Optimum conditions for installing insulation inside 

the wall cavity occur in buildings where either the 

exterior materials or interior fi nishes have been lost, 

or where the materials are deteriorated beyond repair 

and total replacement is necessary. However, wholesale 

removal of historic materials either on the exterior or 

interior face of a historic wall to facilitate insulation is 

Fig. 17. Illustration of insulation from the 1889 trade catalog 

“The Uses of Mineral Wool in Architecture, Car Building and Steam 

Engineering”. Collection Centre Canadien d’Architecture/Canadian 

Centre for Architecture, Montreal, Canada.
12

not recommended. Even when the exterior materials, 

such as wood siding, could potentially be reinstalled, 

this method, no matter how carefully executed, usually 

results in damage to, and loss of historic materials.

If the wall cavity is open, the opportunity to properly 

install batt insulation is available. A tight fi t between 

the insulation and the adjacent building components 

is critical to the performance of the insulation. Batt 

insulation must be cut to the exact length of the cavity. 

A batt that is too short creates air spaces above and 

beneath the batt, allowing convection. A batt that is too 

long will bunch up, creating air pockets. Air pockets 

and convection currents signifi cantly reduce the thermal 

performance of insulation. Each wall cavity should be 

completely fi lled. Unfaced, friction-fi t batt insulation 

fl uffed to fi ll the entire wall cavity is recommended. Any 

air gaps between the insulation and the framing or other 

assembly components must be avoided. Batts should be 

split around wiring, pipes, ducts and other elements in 

the wall rather than be pushed or compressed around 

obstacles.

When adding insulation to the sidewalls, the band joist 

area between fl oors in multi-story, platform-framed 

buildings should be included in the sidewall insulation 

retrofi t. The R-value of the insulation installed in the 

band joist area should be at least equal to the R-value of 

the insulation in the adjacent wall cavities. In balloonframed buildings, the wall cavity is continuous between 

fl oors except where fi re stops have been inserted. 

The use of spray foam or foamed-in-place insulation

would appear to have great potential for application 

in historic wood-frame buildings due to their ability to 

fl ow into wall cavities and around irregular obstacles. 

Their high R-value and function as an air barrier make 

them a tempting choice. However, their use presents 

several problems. The injected material bonds tightly to 

historic materials making its removal diffi cult, especially 

if it is encased in an existing wall. The pressure caused 

by the expansion rate of these foams within a wall can 

also damage historic material, including breaking the 

plaster keys or cracking existing plaster fi nishes.

Insulation Installed on Either Side of the Wall: Batt, rigid 

foam board, and spray foam insulation are commonly 

added to the interior face of walls in existing buildings 

by furring-out the walls to accommodate the additional 

thickness. However, this often requires the destruction 

or alteration of important architectural features, such 

as cornices, base boards, and window trim, and the 

removal or covering of plaster or other historic wall 

fi nishes. Insulation installed in this manner is only 

recommended in buildings where interior spaces and 

features lack architectural distinction or have lost 

signifi cance due to previous alterations. 

Adding rigid foam insulation to the exterior face of 

wood-frame buildings, while common practice in 

new construction, is never an appropriate treatment 

for historic buildings. Exterior installation of the foam 

boards requires removal of the existing siding and trim 

to install one or more layers of polyisocyanurate or 

polystyrene foam panels. Depending on the amount 

of insulation added for the particular climate, the wall 

thickness may be dramatically increased by moving 

Band Joist

Fig. 18. Dense-packed cellulose insulation is being blown in through 

holes drilled in the sheathing. Once the operation has been completed, 

the shingles will be reinstalled. Photo: Edward Minch.

Fig. 20. The walls 

have been furred 

out inappropriately 

around the historic 

window trim creating 

an appearance the 

interior never had 

historically. 

Fig. 19. Platform framing (left) and Balloon framing (right).
13

the siding as much as 4 inches out from the sheathing. 

Even if the historic siding and trim could be removed 

and reapplied without signifi cant damage, the historic 

relationship of windows to walls, walls to eaves, 

and eaves to roof would be altered, which would 

compromise the architectural integrity and appearance 

of the historic building.

Solid Masonry Walls: As with frame buildings, installing 

insulation on the interior walls of a historic masonry 

structure should be avoided when it would involve 

covering or removing important architectural features 

and fi nishes, or when the added thickness would 

signifi cantly alter the historic character of the interior. 

The addition of insulation on solid masonry walls 

in cold climates results in a decreased drying rate, 

an increased frequency of freeze-thaw cycles, and 

prolonged periods of warmer and colder temperatures 

of the masonry. These changes can have a direct effect on 

the durability of materials.

Depending on the type of masonry, exterior masonry 

walls can absorb a signifi cant amount of water when it 

rains. Masonry walls dry both toward the exterior and 

the interior. When insulation is added to the interior side 

of a masonry wall, the insulation material reduces the 

drying rate of the wall toward the interior, causing the 

wall to stay wet for longer periods of time. Depending 

on the local climate, this could result in damage to 

the historic masonry, damage to interior fi nishes, and 

deterioration of wood or steel structural components 

What about moisture?

The issue of moisture in insulated assemblies is the 

subject of much debate. While there is no conclusive 

way to predict all moisture problems, especially 

in historic buildings, experts seem to agree on a 

few basic tenants. Exterior materials in insulated 

buildings become colder in the winter and stay wet 

longer following a rain event. While the wetness may 

not pose a problem for robust materials, it may speed 

the deterioration of some building materials, and 

lead to more frequent maintenance such as repainting 

of wood or repointing of masonry. Summer moisture 

problems are most commonly associated with 

excessive indoor cooling and the use of interior wall 

fi nishes that act as vapor retarders (paint buildup or 

vinyl wall coverings). Good air-sealing at the ceiling 

plane usually controls moisture in insulated attics.

Most problems are caused by poor moisture 

management, poor detailing which does not allow 

the building to shed water, or inadequate drainage. 

Therefore, a thorough assessment of the building’s 

ability to keep out unwanted moisture must be done 

before adding new insulation materials. Refer to 

Preservation Brief #39: Holding the Line: Controlling 

Unwanted Moisture in Historic Buildings for more 

information. Because of all the uncertainties 

associated with insulating walls, brick walls in 

particular, it may be advisable to hire a professional 

consultant who specializes in the many factors that 

affect the behavior of moisture in a building and 

can apply this expertise to the unique characteristics 

of a particular structure. Sophisticated tools such 

as computer modeling are useful to predict the 

performance of building assemblies, but they require 

interpretation by a skilled practitioner and the results 

are only as good as the data entered. It is important 

to remember, there are no reliable prescriptive 

measures to prevent moisture problems.4

Vapor Retarders (Barriers): Vapor retardants are 

commonly used in modern construction to manage 

the diffusion of moisture into wall cavities and 

attics. For vapor retardants to work properly, 

however, they must be continuous, which makes 

their installation diffi cult in existing buildings, and 

therefore generally not recommended. Even in new 

construction, installation of vapor retardants is not 

always indicated. Formerly, the recommended 

treatment was to install a vapor retardant toward 

the heated side of the wall (toward the interior 

space in cold climates and toward the exterior in hot 

climates). DOE now recommends that if moisture 

moves both to the interior and exterior of a building 

for signifi cant parts of the year, it is better not to use 

a vapor retarder at all.5

Fig. 21. The interior face of a brick masonry wall shows damage that 

resulted from the installation of a vapor retardant (foil facing) and 

thermal insulation. Photo: Simpson Gumpertz & Heger.
14

imbedded in the wall. Masonry walls of buildings that 

are heated during the winter benefi t from the transfer of 

heat from the inside to the outside face of the walls. This 

thermal transfer protects the exterior face of the wall by 

reducing the possibility of water freezing in the outer 

layers of the wall, particularly in cold and wet climates. 

The addition of insulation on the interior of the wall not 

only prolongs the drying rate of the exterior masonry 

wall, but keeps it colder as well, thereby increasing the 

potential for damage due to freeze-thaw cycles.6

Extreme swings in temperature may also have negative 

effects on a historic masonry wall. The addition of 

insulation materials to a historic masonry wall decreases 

its ability to transfer heat; thus, walls tend to stay warm 

or cold for longer periods of time. In addition, walls 

exposed to prolonged solar radiation during winter 

months can also be subject to higher swings in surface 

temperature during the day. Deleterious effects due 

to stress caused by expansion and contraction of the 

building assembly components can result. 

Buildings with masonry materials of higher porosity, 

such as those built with low-fi red brick, or certain 

soft stones, are particularly susceptible to freeze-thaw 

cycles and must be carefully evaluated prior to adding 

insulation. Inspection of the masonry in areas that are 

not heated such as parapets, exposed wing walls, or 

other parts of the building is particularly important. A 

noticeable difference in the amount of spalling or sanding 

of the masonry in these areas could predict that the same 

type of deterioration will occur throughout the building 

after the walls are insulated. Brick that was fi red at lower 

temperatures was often used on the inside face of the 

wall or on secondary elevations. Even masonry walls 

faced with more robust materials such as granite may 

have brick, rubble, mortar or other less durable materials 

as backing.

Spray foams are being used for insulation in many 

masonry buildings. Their ability to be applied over 

irregular surfaces, provide good air tightness, and 

continuity at intersections between, walls, ceilings, fl oors 

and window perimeters makes them well suited for use 

in existing buildings. However, the long-term effects 

of adding either open- or closed-cell foams to insulate 

historic masonry walls as well as performance of these 

products have not been adequately documented. 

Use of foam insulation in buildings with poor quality 

masonry or uncontrolled rising damp problems should 

be avoided. 

Periodic monitoring of the condition of insulated 

masonry walls is strongly recommended regardless of 

the insulation material added.

Install cool roofs and green roofs: Cool roofs and 

vegetated “green roofs” help to reduce the heat gain 

from the roof, thereby cooling the building and its 

environment. Cool roofs include refl ective metal roofs, 

light-colored or white roofs, and fi berglass shingles 

that have a coating of refl ective crystals. All of these 

roofi ng materials refl ect the sun’s radiation away from 

the building, which lessens heat gain, resulting in a 

reduction of the cooling load. Cool roofs are generally 

not practical in northern climates where buildings 

benefi t from the added heat gain of a dark-colored 

roof during colder months. Cool and green roofs are 

appropriate for use on historic buildings only when they 

are compatible with their architectural character, such 

as fl at roofs with no visibility. A white-colored roof that 

is readily visible is not appropriate for historic metal 

roofs that were traditionally painted a dark color, such 

as green or iron oxide red. A white refl ective roof is most 

suitable on fl at roofed historic buildings. If a historic 

building has a slate roof, for example, removing the 

slate to install a metal roof is not a compatible treatment. 

It is never appropriate to remove a historic roof if the 

material is in good or repairable condition to install 

a cool roof. However, if the roof has previously been 

changed to an asphalt shingle roof, fi berglass shingles 

with special refl ective granules may be an appropriate 

replacement. 

A green roof consists of a thin layer of vegetation 

planted over a waterproofi ng system or in trays installed 

on top of an existing fl at or slightly sloped roof. Green 

roofs are primarily benefi cial in urban contexts to reduce 

the heat island effect in cities and to control storm water 

run-off. A green roof also reduces the cooling load of 

the building and helps cool the surrounding urban 

environment, fi lters air, collects and fi lters storm water, 

and can provide urban amenities, including vegetable 

gardens, for building occupants. The impact of increased 

Fig. 22. Installation of both cool and green roofs in an urban 

environment.
15

structural loads, added moisture, and potential for 

leaks must be considered before installing a green roof. 

A green roof is compatible on a historic building only if 

the plantings are not visible above the roofl ine as seen 

from below. 

Alternative Energy Sources 

Although not the focus of this publication, alternative 

energy sources are dealt with in more detail in The 

Secretary of the Interior’s Standards for Rehabilitation & 

Illustrated Guidelines on Sustainability for Rehabilitating 

Historic Buildings and other NPS publications. 

Devices that utilize solar, geothermal, wind and other 

sources of energy to help reduce consumption of 

fossil fuel-generated energy can often be successfully 

incorporated in historic building retrofi ts. However, if 

the alterations or costs required to install these devices 

do not make their installation economically feasible, 

buying power generated off site from renewable 

sources may also be a good alternative. The use of most 

alternative energy strategies should be pursued only 

after all other upgrades have been implemented to make 

the building more energy effi cient because their initial 

installation cost is usually high. 

Solar Energy: Man has sought to harness the power 

of solar energy to heat, cool, and illuminate buildings 

throughout history. Construction techniques and design 

strategies that utilize building materials and components 

to collect, store, and release heat from the sun are 

described as “passive solar design.” As previously 

discussed, many historic buildings include passive solar 

features that should be retained and may be enhanced. 

Compatible additions to historic buildings also offer 

opportunities to incorporate passive solar features. 

Active solar devices, such as solar heat collectors and 

photovoltaic systems, can be added to historic buildings 

to decrease reliance on grid-source fossil-fuel powered 

electricity. Incorporating active solar devices in existing 

buildings is becoming more common as solar collector 

technology advances. Adding this technology to historic 

buildings, however, must be done in a manner that 

has a minimal impact on historic roofi ng materials and 

preserves their character by placing them in locations 

with limited or no visibility, i.e., on fl at roofs at a low 

angle or on a secondary roof slope. 

Solar collectors used to heat water can be relatively 

simple. More complex solar collectors heat a fl uid or 

air that is then pumped through the system to heat or 

cool interior spaces. Photovoltaic panels (PV) transform 

solar radiation into electricity. The greatest potential 

for the use of PV panels in historic buildings is on 

buildings with large fl at roofs, high parapets, or roof 

confi gurations that allow solar panels to be installed 

without being prominently visible. The feasibility 

of installing solar devices in small commercial and 

residential buildings will depend on installation costs, 

conventional energy rates, and available incentives, all of 

which will vary with time and location. The same factors 

apply to the use of solar collectors for heating water, but 

smaller installations may meet a building’s need and the 

technology has a considerable track record. 

Geothermal Energy: The use of the earth’s heat is 

another source of readily-accessible clean energy. The 

most common systems that utilize this form of energy 

are geothermal heat pumps, also known as geoexchange, earth-coupled, ground-source, or water-source 

heat pumps. Introduced in the late 1940s, geothermal 

heat pumps rely on heat from the constant temperature 

of the earth, unlike most other heat pumps which use 

the outside air temperature as the exchange medium. 

This makes geothermal heat pumps more effi cient than 

conventional heat pumps because they do not require an 

electric back-up heat source during prolonged periods of 

cold weather. 

There are many reasons that geothermal heat pumps 

are well suited for use in historic buildings. They 

can reduce the amount of energy consumption and 

emissions considerably, compared to the air exchange 

systems or electric resistance heating of conventional 

HVAC systems. They require less equipment space, 

have fewer moving parts, provide better zone space 

conditioning, and maintain better internal humidity 

levels. Geothermal heat pumps are also quieter because 

they do not require external air compressors. Despite 

higher installation costs, geothermal systems offer 

long-term operational savings and adaptability that 

may make them a worthwhile investment in some 

historic buildings. 

Fig. 23. Solar collectors installed in a compatible manner on low 

sloping sawtooth monitors. Top Photo: Neil Mishalov, Berkeley, CA.
16

Wind Energy: For historic properties in rural areas, 

where wind power has been utilized historically, 

installation of a wind mill or turbine may be suitable to 

the historic setting and cost effective. Before choosing to 

install wind-powered equipment, the potential benefi t 

and the impact on the historic character of the building, 

the site and surrounding historic district must be 

analyzed. In order for the turbines to work effectively, 

average wind speeds of 10 mph or higher are necessary. 

This technology may not be practical in more denselypopulated areas sheltered from winds or regions where 

winds are not consistent. In cities with tall buildings, 

there is potential for installing relatively small rooftop 

turbines that are not visible from the ground. However, 

because of the initial cost and size of some turbines, it is 

generally more practical to purchase wind power from 

an off-site wind farm through the local utility company. 

Summary

With careful planning, the energy effi ciency of historic 

buildings can be optimized without negatively 

impacting their historic character and integrity. 

Measuring the energy performance of buildings after 

improvements are completed must not be overlooked, 

as it is the only way to verify that the treatments 

have had the intended effect. Ongoing monitoring of 

buildings and their components after alterations to 

historic building assemblies are completed can prevent 

irreparable damage to historic materials. This, along 

with regular maintenance, can ensure the long-term 

preservation of our historic built environment and the 

sustainable use of our resources.

Acknowledgements

Jo Ellen Hensley, Senior Architectural Historian, LEED Green Associate, and Antonio Aguilar, Senior Historical Architect, Technical 

Preservation Services Branch, National Park Service, revised Preservation Brief 3: Conserving Energy in Historic Buildings, written 

by Baird M. Smith, FAIA and published in 1978. The revised Brief contains expanded and updated information on the subject of 

energy effi ciency in historic buildings. A number of individuals and organizations have contributed their time and expertise in the 

development of this Brief, beginning with the participants of the “Improving Energy Effi ciency in Historic Buildings—A Round Table 

Symposium,” Washington, DC, 2002. Special thanks go to Mike Jackson, FAIA, Illinois Historic Preservation Agency; Edward Minch,

Energy Services Group; William B. Rose, Research Architect, University of Illinois; Bradford S. Carpenter, P.E., LEED AP; and Mark 

Thaler, AIA, for their technical advice. The Advisory Council on Historic Preservation’s Sustainability Task Force, the General Services 

Administration’s Center for Historic Buildings, and our colleagues at the National Center for Preservation Technology and Training 

commented on the manuscript. In addition, the Technical Preservation Services professional staff, in particular Anne Grimmer, 

Michael J. Auer and John Sandor, provided critical and constructive review of the publication.

This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which directs the Secretary 

of the Interior to develop and make available information concerning historic properties. Additional information about the programs of 

Technical Preservation Services is available on the website at www.nps.gov/tps. Comments about this publication should be addressed 

to: Charles E. Fisher, Technical Preservation Publications Program Manager, Technical Preservation Services, National Park Service, 

1201 Eye Street, NW, 6th Floor, Washington, DC 20005. This publication is not copyrighted and can be reproduced without penalty. 

Normal procedures for credit to the authors and the National Park Service are appreciated. The photographs used in this publication 

may not be used to illustrate other publications without permission of the owners. Cover photograph: Farmhouse with energy effi cient 

historic storm windows.

ISBN: 978-0-16-089762-7

U.S. Government Printing Offi ce Stock Number: 024-005-01294-0 

December 2011

End Notes

1

 John Krigger and Chris Dorsi, “Air Leakage,” in Residential 

Energy: Cost Savings and Comfort for Existing Buildings. Helena, 

Montana: Saturn Resource Management, 2004, p. 73.

2 Measured Winter Performance of Storm Windows. A 2002 study done 

by Lawrence Berkeley National Labs.

3 Midwest Weatherization Best Practices Field Guide. Prepared for the 

U.S. Department of Energy Weatherization Assistance Program, 

May 2007, p. 157.

4

 Adapted from comments provided by William B. Rose, Research 

Architect, University of Illinois, April 2011.

5

 U.S. Department of Energy, Insulation Fact Sheet, DOE/CE-0180, 

2008, p.14.

6

 Bradford S. Carpenter, P.E., LEED AP et al., The Designer’s 

Dilemma: Modern Performance Expectations and Historic Masonry 

Walls (paper presented at the RCI 2010 Symposium on Building 

Envelope Technology, San Antonio, Texas).
4 PRESERV ATION 

BRIEFS 

Roofing for Historic Buildings 

Sarah M. Sweetser 

u.s. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

Significance of the Roof 

A weather-tight roof is basic in the preservation of a structure, regardless of its age, size, or design. In the system that 

allows a building to work as a shelter, the roof sheds the rain, 

shades from the sun, and buffers the weather. 

During some periods in the history of architecture, the roof 

imparts much of the architectural character. It defines the 

style and contributes to the building's aesthetics. The hipped 

roofs of Georgian architecture, the tllrrets of Queen Anne, the 

Mansard roofs, and the graceful slopes of the Shingle Style 

and Bungalow designs are examples of the use of roofing as a 

major design feature. 

But no matter how decorative the patterning or how compelling the form, the roof is a highly vulnerable element of a 

shelter that will inevitable fail. A poor roof will permit the 

accelerated deterioration of historic building materialsmasonry, wood, plaster, paint-and will cause general disintegration of the basic structure. Furthermore, there is an 

urgency involved in repairing a leaky roof since such repair 

costs will quickly become prohibitive. Although such action is 

desirable as soon as a failure is discovered, temporary patching methods should be carefully chosen to prevent inadvertent 

damage to sound or historic roofing materials and related 

features. Before any repair work is performed, the historic 

value of the materials used on the roof should be understood. 

Then a complete internal and external inspection of the roof 

should be planned to determine all the causes of failure and to 

identify the alternatives for repair or replacement of the 

roofing. 

Historic Roofing Materials in America 

Clay Tile: European settlers used clay tile for roofing as early 

as the mid-17th century; many pantiles (S-curved tiles), as well 

as flat roofing tiles, were used in Jamestown, Virginia. In 

some cities such as New York and Boston, clay was popularly 

used as a precaution against such fire as those that engulfed 

London in 1666 and scorched Boston in 1679. 

Tiles roofs found in the mid-18th century Moravian settlements in Pennsylvania closely resembled those found in Germany. Typically, the tiles were 14- 15" long, 6- 7" wide with a 

curved butt. A lug on the back allowed the tiles to hang on the 

lathing without nails or pegs. The tile surface was usually 

scored with finger marks to promote drainage, In the Southwest, the tile roofs of the Spanish missionaries (mission tiles) 

were first manufactured (ca. 1780) at the Mission San Antonio de Padua in California. These semicircular tiles were 

Repairs on this pantile roof were made with new tiles held in place 

with metal hangers. (Main Building, Ellis Island, New York) 

made by molding clay over sections of logs, and they were 

generally 22" long and tapered in width. 

HABS 

The plain or flat rectangular tiles most commonly used from 

the 17th through the beginning of the 19th century measured 

about 10" by 6" by W ', and had two holes at one end for a 

nail or peg fastener. Sometimes mortar was applied between 

the courses to secure the tiles in a heavy wind. 

In the mid-19th century, tile roofs were often replaced by 

sheet-metal roofs, which were lighter and easier to install and 

maintain. However, by the turn of the century, the Romanesque Revival and Mission style buildings created a new demand 

and popularity for this picturesque roofing material. 

Slate: Another practice settlers brought to the New World was 

slate roofing. Evidence of roofing slates have been found also 

among the ruins of mid-17th-century Jamestown. But because 

of the cost and the time required to obtain the material, which 

was mostly imported from Wales, the use of slate was initially 

limited. Even in Philadelphia (the second largest city in the 

English-speaking world at the time of the Revolution) slates 

were so rare that' 'The Slate Roof House" distinctly referred 

to William Penn's home built late in the 16oos. Sources of 

native slate were known to exist along the eastern seaboard 

from Maine to Virginia, but difficulties in inland transportation limited its availability to the cities, and contributed to its 

expense. Welsh slate continued to be imported until the 

development of canals and railroads in the mid-19th century 

made American slate more accessible and economical. 

Slate was popular for its durability, fireproof qualities, and 
The Victorians loved to used different colored slates to create 

decorative patterns on their roofs, an effect which cannot be easily 

duplicated by substitute materials. Before any repair work on a roof 

such as this, the slate sizes, colors, and position of the patterning 

should be carefully recorded to assure proper replacement. (Ebenezer 

Maxwell Mansion, Philadelphia, Pennsylvania. photo courtesy of 

William D. Hershey) 

aesthetic potential. Because slate was available in different 

colors (red, green, purple, and blue-gray), it was an effective 

material for decorative patterns on many 19th-century roofs 

(Gothic and Mansard styles). Slate continued to be used well 

into the 20th century, notably on many Tudor revival style 

buildings of the 1920s. 

Shingles: Wood shingles were popular throughout the country 

in all periods of building history. The size and shape of the 

shingles as well as the detailing of the shingle roof differed according to regional craft practices. People within particular 

regions developed preferences for the local species of wood 

that most suited their purposes. In New England and the Delaware Valley, white pine was frequently used: in the South, 

cypress and oak; in the far west, red cedar or redwood. Sometimes a protective coating was applied to increase the durability of the shingle such as a mixture of brick dust and fish oil, 

or a paint made of red iron oxide and linseed oil. 

Commonly in urban areas, wooden roofs were replaced 

with more fire resistant materials, but in rural areas this was 

not a major concern. On many Victorian country houses, the 

practice of wood shingling survived the technological advances of metal roofing in the 19th century, and near the turn 

of the century enjoyed a full revival in its namesake, the 

Shingle Style. Colonial revival and the Bungalow styles in the 

20th century assured wood shingles a place as one of the most 

fashionable, domestic roofing materials. 

Metal: Metal roofing in America is principally a 19thcentury phenomenon. Before then the only metals commonly 

2 

Replacement of particular historic details is important to the individual historic character of a roof, such as the treatment at the eaves 

of this rounded butt wood shingle roof Also note that the surface of 

the roof was carefully sloped to drain water away from the side of the 

dormer. In the restoration, this function was augmented with the addition of carefully concealed modern metalflashing. (Mount Vernon. 

VirJ?inial 

Galvanized sheet-metal shingles imitating the appearance of pantiles 

remained popular from the second half of the 19th century into the 

20th century. (Episcopal Church, now the Jerome Historical Society 

Building, Jerome. Arizona, 1927) 

used were lead and copper. For example, a lead roof covered 

"Rosewell," one of the grandest mansions in 18th-century 

Virginia. But more often, lead was used for protective 

flashing. Lead, as well as copper, covered roof surfaces where 

wood, tile, or slate shingles were inappropriate because of the 

roofs pitch or shape. 

Copper with standing seams covered some of the more 

notable early American roofs including that of Christ Church 

(1727-1744) in Philadelphia. Flat-seamed copper was used on 

many domes and cupolas. The copper sheets were imported 

from England until the end of the 18th century when facilities 

for rolling sheet metal were developed in America. 

Sheet iron was first known to have been manufactured here 

by the Revolutionary War financier, Robert Morris, who had 

a rolling mill near Trenton, New Jersey. At his mill Mor·ris 

produced the roof of his own Philadelphia mansion, which he 

started in 1794. The architect Benjamin H. Latrobe used sheet 

iron to replace the roof on Princeton's "Nassau Hall," which 

had been gutted by fire in 1802. 

The method for corrugating iron was originally patented in 

England in 1829. Corrugating stiffened the sheets, and 

allowed greater span over a lighter framework, as well as 

reduced installation time and labor. In 1834 the American 

architect William Strickland proposed corrugated iron to 

cover his design for the market place in Philadelphia. 

Galvanizing with zinc to protect the base metal from rust 

was developed in France in 1837. By the 1850s the material 

was used on post offices and customhouses, as well as on train 

sheds and factories. In 1857 one of the first metal roofs in the 
Repeated repair with asphalt, which cracks as it hardens, has created a 

blistered surface on this sheet-metal roof and built-in gutter, which 

will retain water. Repairs could be made by carefully heating and 

scraping the surface clean, repairing the holes in the metal with aflexible mastic compound or a metal patch, and coating the surface with a 

fibre paint. (Roane County Courthouse, Kingston, Tennessee, photo 

courtesy of Building Conservation Technology, Inc.) 

South was installed on the U.S. Mint in New Orleans. The 

Mint was thereby "fireproofed" with a 20-gauge galvanized, 

corrugated iron roof on iron trusses. 

Tin-plate iron, commonly called "tin roofing," was used 

extensively in Canada in the 18th century, but it was not as 

common in the United States until later. Thomas Jefferson 

was an early advocate of tin roofing, and he installed a 

standing-seam tin roof on "Monticello" (ca. 1770-1802). The 

Arch Street Meetinghouse (1804) in Philadelphia had tin 

shingles laid in a herringbone pattern on a "piazza" roof. 

However, once rolling mills were established in this country, 

the low cost, light weight, and low maintenance of tin plate 

made it the most common roofing material. Embossed tin 

shingles, whose surfaces created interesting patterns, were 

popular throughout the country in the late 19th century. Tin 

roofs were kept well-painted, usually red; or, as the architect 

A. J. Davis suggested, in a color to imitate the green patina of 

copper. 

Terne plate differed from tin plate in that the iron was 

dipped in an alloy of lead and tin, giving it a duller finish. 

Historic, as well as modern, documentation often confuses 

the two, so much that it is difficult to determine how often 

actual "terne" was used. 

Zinc came into use in the 1820s, at the same time tin plate 

was becoming popular. Although a less expensive substitute 

for lead, its advantages were controversial, and it was never 

widely used in this country. 

A Chicago firm's catalog dated 1896 illustrates a method of unrolling, 

turning the edges, andfinishing the standing seam on a metal roof 

Tin shingles, commonly embossed to imitate wood or tile, or with a 

decorative design, were popular as an inexpensive, textured roofing 

material. These shingles 8% inch by 12'/2 inch on the exposed surface) 

were designed with interlocking edges, but they have been repaired by 

surface nailing, which may cause future leakage. (Ballard House, 

Yorktown, Virgina, photo by Gordie Whittington, National Park 

Service) 

Other Materials: Asphalt shingles and roll roofing were used 

in the 1890s. Many roofs of asbestos, aluminum, stainless 

steel, galvinized steel, and lead-coated copper may soon have 

historic values as well. Awareness- of these and other traditions of roofing materials and their detailing will contribute to 

more sensitive preservation treatments. 

Locating the Problem 

Failures of Surface Materials 

When trouble occurs, it is important to contact a professional, either an architect, a reputable roofing contractor, or a 

craftsman familiar with the inherent characteristics of the 

particular historic roofing system involved. These professionals may be able to advise on immediate patching procedures and help plan more permanent repairs. A thorough 

examination of the roof should start with an appraisal of the 

existing condition and quality of the roofing material itself. 

Particular attention should be given to any southern slope 

because year-round exposure to direct sun may cause it to 

break down first. 

Wood: Some historic roofing materials have limited life 

expectancies because of normal organic decay and "wear." 

For example, the flat surfaces of wood shingles erode from 

exposure to rain and ultraviolet rays. Some species are more 

hardy than others, and heartwood, for example, is stronger 

and more durable than sapwood. 

Ideally, shingles are split with the grain perpendicular to 

3 
the surface. This is because if shingles are sawn across the 

grain, moisture may enter the grain and cause the wood to 

deteriorate. Prolonged moisture on or in the wood allows 

moss or fungi to grow, which will further hold the moisture 

and cause rot. 

Metal: Of the inorganic roofing materials used on historic 

buildings, the most common are perhaps the sheet metals: 

lead, copper, zinc, tin plate, terne plate, and galvanized iron. 

In varying degrees each of these sheet metals are likely to 

deteriorate from chemical action by pitting or streaking. This 

can be caused by airborn pollutants; acid rainwater; acids from 

lichen or moss; alkalis found in lime mortars or portland 

cement, which might be on adjoining features and washes 

down on the roof surface; or tannic acids from adjacent wood 

sheathings or shingles made of red cedar or oak. 

Corrosion from "galvanic action" occurs when dissimilar 

metals, such as copper and iron, are used in direct contact. 

Corrosion may also occur even though the metals are physically separated; one of the metals will react chemically 

against the other in the presence of an electrolyte such as rainwater. In roofing, this situation might occur when either a 

copper roof is decorated with iron cresting, or when steel nails 

are used in copper sheets. In some instances the corrosion can 

be prevented by inserting a plastic insulator between the 

dissimilar materials. Ideally, the fasteners should be a metal 

sympathetic to those involved. 

Iron rusts unless it is well-painted or plated. Historically 

this problem was avoided by use of tin plating or galvinizing. 

But this method is durable only as long as the coating remains 

intact. Once the plating is worn or damaged, the exposed iron 

will rust. Therefore, any iron-based roofing material needs to 

be undercoated, and its surface needs to be kept well-painted 

to prevent corrosion. 

One cause of sheet metal deterioration is fatigue . Depending 

upon the size and the gauge of the metal sheets, wear and 

metal failure can occur at the joints or at any protrusions in 

the sheathing as a result from the metal's alternating movement to thermal changes. Lead will tear because of" creep, " 

or the gravitational stress that causes the material to move 

down the roof slope. 

Slate: Perhaps the most durable roofing materials are slate 

and tile. Seemingly indestructable, both vary in quality. Some 

slates are hard and tough without being brittle. Soft slates are 

more subject to erosion and to attack by airborne and rainThis detail shows slate delamination caused by a combination of 

weathering and pol/ution. In addition, the slates have eroded around 

the repair nails, incorrectly placed in the exposed surface of the slates. 

(Lower Pontalba Building, New Orleans, photo courtesy of Building 

Conservation Technology, Inc.) 

4 

water chemicals, which cause the slates to wear at nail holes, 

to delaminate, or to break. In winter, slate is very susceptible 

to breakage by ice, or ice dams. 

Tile: Tiles will weather well, but tend to crack or break if hit, 

as by tree branches, or if they are walked on improperly. Like 

slates, tiles cannot support much weight. Low quality tiles 

that have been insufficiently fired during manufacture, will 

craze and spall under the effects of freeze and thaw cycles on 

their porous surfaces. 

Failures of Support Systems 

Once the condition of the roofing material has been determined, the related features and support systems should be 

examined on the exterior and on the interior of the roof. 

The gutters and downspouts need periodic cleaning and 

maintenance since a variety of debris fill them, causing water 

to back up and seep under roofing units. Water will eventually 

cause fasteners, sheathing, and roofing structure to deteriorate. During winter, the daily freeze-thaw cycles can cause 

ice floes to develop under the roof surface. The pressure from 

these ice floes will dislodge the roofing material, especially 

slates, shingles, or tiles. Moreover, the buildup of ice dams 

above the gutters can trap enough moisture to rot the 

sheathing or the structural members. 

Many large public buildings have built-in gutters set within 

the perimeter of the roof. The downspouts for these gutters 

may run within the walls of the building, or drainage may be 

through the roof surface or through a parapet to exterior 

downspouts. These systems can be effective if properly maintained; however, if the roof slope is inadequate for good 

runoff, or if the traps are allowed to clog, rainwater will form 

pools on the roof surface. Interior downspouts can collect 

debris and thus back up, perhaps leaking water into the surrounding walls. Exterior downspouts may fill with water, 

which in cold weather may freeze and crack the pipes. Conduits from the built-in gutter to the exterior downspout may 

also leak water into the surrounding roof structure or walls. 

Failure of the flashing system is usually a major cause of 

roof deterioration. Flashing should be carefully inspected for 

failure caused by either poor workmanship, thermal stress, or 

metal deterioration (both of flashing material itself and of the 

fasteners). With many roofing materials, the replacement of 

flashing on an existing roof is a major operation, which may 

require taking up large sections of the roof surface. 

Therefore, the installation of top quality flashing material on 

Temporary stabilization or " mothballing" with materials such as 

plywood and building paper can protect the roof of a project until it 

can be properly repaired or replaced. (Narbonne House, Salem, 

Massachusetts) 
These two views of the same house demonstrate how the use of a substitute material can drastically affect the overall character of a structure. The 

textural interest of the original tile roof was lost with the use of asphalt shingles. Recent preservation efforts are replacing the tile roof (Frank 

House, Kearney, Nebraska, photo courtesy of the Nebraska State Historical Society, Lincoln, Nebraska) 

a new or replaced roof should be a primary consideration. 

Remember, some roofing andflashing materials are not 

compatible. 

Roof fasteners and clips should also be made of a material 

compatible with all other materials used, or coated to prevent 

rust. For example, the tannic acid in oak will corrode iron 

nails. Some roofs such as slate and sheet metals may fail if 

nailed too rigidly. 

If the roof structure appears sound and nothing indicates 

recent movement, the area to be examined most closely is the 

roof substrate-the sheathing or the battens. The danger spots 

would be near the roof plates, under any exterior patches, at 

the intersections of the roof planes, or at vertical surfaces 

such as dormers. Water penetration, indicating a breach in the 

roofing surface or flashing, should be readily apparent, usually as a damp spot or stain. Probing with a small pen knife may 

reveal any rot which may indicate previously undetected 

damage to the roofing membrane. Insect infestation evident 

by small exit holes and frass (a sawdust-like debris) should 

also be noted. Condensation on the underside of the roofing is 

undesirable and indicates improper ventilation. Moisture will 

have an adverse effect on any roofing material; a good roof 

stays dry inside and out. 

Repair or Replace 

Understanding potential weaknesses of roofing material also 

requires knowledge of repair difficulties. Individual slates can 

be replaced normally without major disruption to the rest of 

the roof, but replacing flashing on a slate roof can require 

substantial removal of surrounding slates. If it is the substrate 

or a support material that has deteriorated, many surface 

materials such as slate or tile can be reused if handled carefully during the repair. Such problems should be evaluated at 

the outset of any project to determine if the roof can be effectively patched, or if it should be completely replaced. 

Will the repairs be effective? Maintenance costs tend to 

multiply once trouble starts. As the cost of labor escalates, 

repeated repairs could soon equal the cost of a new roof. 

The more durable the surface is initially, the easier it will be 

to maintain. Some roofing materials such as slate are expensive to install, but if top quality slate and flashing are used, it 

will last 40-60 years with minimal maintenance. Although the 

installation cost of the roof will be high, low maintenance 

needs will make the lifetime cost of the roof less expensive. 

Historical Research 

In a restoration project, research of documents and physical 

investigation of the building usually will establish the roofs 

history. Documentary research should include any original 

plans or building specifications, early insurance surveys, 

newspaper descriptions, or the personal papers and files of 

people who owned or were involved in the history of the 

building. Old photographs of the building might provide 

evidence of missing details. 

Along with a thorough understanding of any written history 

of the building, a physical investigation of the roofing and its 

structure may reveal information about the roofs construction history. Starting with an overall impression of the structure, are there any changes in the roof slope, its configuration, or roofing materials? Perhaps there are obvious patches 

or changes in patterning of exterior brickwork where a gable 

roof was changed to a gambrel, or where a whole upper story 

was added. Perhaps there are obvious stylistic changes in the 

roof line, dormers, or ornamentation. These observations 

could help one understand any important alteration, and 

could help establish the direction of further investigation. 

Because most roofs are physically out of the range of 

careful scrutiny, the" principle of least effort" has probably 

limited the extent and quality of previous patching or replacing, and usually considerable evidence of an earlier roof surface remains. Sometimes the older roof will be found as an 

underlayment of the current exposed roof. Original roofing 

may still be intact in awkward places under later features on a 

roof. Often if there is any unfinished attic space, remnants of 

roofing may have been dropped and left when the roof was 

being built or repaired. If the configuration of the roof has 

been changed, some of the original material might still be in 

place under the existing roof. Sometimes whole sections of the 

roof and roof framing will have been left intact under the 

higher roof. The profile and/ or flashing of the earlier roof 

may be apparent on the interior of the walls at the level of the 

alteration. If the sheathing or lathing appears to have survived 

changes in the roofing surface, they may contain evidence of 

the roofing systems. These may appear either as dirt marks, 

which provide "shadows" of a roofing material, or as nails 

broken or driven down into the wood, rather than pulled out 

during previous alterations or repairs. Wooden headers in the 

roof framing may indicate that earlier chimneys or skylights 

have been removed. Any metal ornamentation that might 

have existed may be indicated by anchors or unusual markings 

along the ridge or at other edges of the roof. This primary 

5 
evidence is essential for a full understanding of the 

roofs history. 

Caution should be taken in dating early" fabric" on the 

evidence of a single item, as recycling of materials is not a 

mid-20th-century innovation. Carpenters have been reusing 

materials, sheathing, and framing members in the interest of 

economy for centuries. Therefore, any analysis of the materials found, such as nails or sawmarks on the wood, requires 

an accurate knowledge of the history of local building practices before any final conclusion can be accurately reached. It 

is helpful to establish a sequence of construction history for 

the roof and roofing materials; any historic fabric or pertinent 

evidence in the roof should be photographed, measured, and 

recorded for future reference. 

During the repair work, useful evidence might unexpectedly 

appear. It is essential that records be kept of any type of work 

on a historic building, before, during, and after the project. 

Photographs are generally the easiest and fastest method, and 

should include overall views and details at the gutters, flashing, dormers, chimneys, valleys, ridges, and eaves. All 

photographs should be immediately labeled to insure accurate 

identification at a later date. Any patterning or design on the 

roofing deserves particular attention. For example, slate roofs 

are often decorative and have subtle changes in size, color, 

and texture, such as a gradually decreasing coursing length 

from the eave to the peak. If not carefully noted before a 

project begins, there may be problems in replacing the surface. The standard reference for this phase of the work is 

Recording Historic Buildings, compiled by Harley J. McKee 

for the Historic American Buildings Survey, National Park 

Service, Washington, D.C., 1970. 

Replacing the Historic Roofing Material 

Professional advice will be needed to assess the various 

aspects of replacing a historic roof. With some exceptions, 

most historic roofing materials are available today. If not, an 

architect or preservation group who has previously worked 

with the same type material may be able to recommend suppliers. Special roofing materials, such as tile or embossed 

metal shingles, can be produced by manufacturers of related 

products that are commonly used elsewhere, either on the exterior or interior of a structure. With some creative thinking 

and research, the historic materials usually can be found. 

Because of the roof's visibility, the slate detailing around the dormers 

is important to the character of this structure. Note how the slates 

swirlfrom a horizontal pattern on the main roof to a diamond pattern 

on the dormer roofs and side walls. (18th and Que Streets, NW, 

Washington, D.C.) 

6 

Craft Practices: Determining the craft practices used in the installation of a historic roof is another major concern in roof 

restoration. Early builders took great pride in their work, and 

experience has shown that the" rustic" or irregular designs 

commercially labled "Early American" are a 20th-century invention. For example, historically, wood shingles underwent 

several distinct operations in their manufacture including 

splitting by hand, and smoothing the surface with a draw 

knife. In modern nomenclature, the same item would be a 

"tapersplit" shingle which has been dressed. Unfortunately, 

the rustic appearance of today's commercially available 

•• handsplit" and re-sawn shingle bears no resemblance to the 

hand-made roofing materials used on early American 

buildings. 

Good design and quality materials for the roof surface, fastenings, 

andf/ashing minimize roofing failures. This is essential on roofs such 

as on the National Cathedral where a thorough maintenance inspection and minor repairs cannot be done easily without special scaffolding. However, the success of the roof on any structure depends on 

frequent cleaning and repair of the gutter system. (Washington, D. c., 

photo courtesy of John Burns, A.I.A.) 

Early craftsmen worked with a great deal of common sense; 

they understood their materials. For example they knew that 

wood shingles should be relatively narrow; shingles much 

wider than about 6" would split when walked on, or they may 

curl or crack from varying temperature and moisture. It is important to understand these aspects of craftsmanship, remembering that people wanted their roofs to be weather-tight 

and to last a long time. The recent use of •• mother-goose" 

shingles on historic structures is a gross underestimation of 

the early craftsman's skills. 

Supervision: Finding a modern craftsman to reproduce historic details may take some effort. It may even involve 

some special instruction to raise his understanding of certain historic craft practices. At the same time, it may be 

pointless (and expensive) to follow historic craft practices 

in any construction that will not be visible on the finished 

product. But if the roofing details are readily visible, their 

appearance should be based on architectural evidence or 

on historic prototypes. For instance, the spacing of the 

seams on a standing-seam metal roof will affect the 

building's overall scale and should therefore match the 

original dimensions of the seams. 
Many older roofing practices are no longer performed 

because of modern improvements. Research and review of 

specific detailing in the roof with the contractor before beginning the project is highly recommended. For example, one 

early craft practice was to finish the ridge of a wood shingle 

roof with a roof "comb"-that is, the top course of one slope 

of the roof was extended uniformly beyond the peak to shield 

the ridge, and to provide some weather protection for the raw 

horizontal edges of the shingles on the other slope. If the 

" comb" is known to have been the correct detail, it should be 

used. Though this method leaves the top course vulnerable to 

the weather, a disguised strip of flashing will strengthen this 

weak point. 

Detail drawings or a sample mock-up will help ensure that 

the contractor or craftsman understands the scope and special 

requirements of the project. It should never be assumed that 

the modern carpenter, slater, sheet metal worker, or roofer 

will know all the historic details. Supervision is as important 

as any other stage of the process. 

Special problems inherent in the design of an elaborate historic roof 

can be controlled through the use of good materials and regular 

maintenance. The shape and detailing are essential elements of the 

building's historic character, and should not be modified, despite the 

use of alternative surface materials. (Gam well House, Bellingham, 

Washington) 

Alternative Materials 

The use of the historic roofing material on a structure may be 

restricted by building codes or by the availability of the 

materials, in which case an appropriate alternative will have 

to be found. 

Some municipal building codes allow variances for roofing 

materials in historic districts. In other instances, individual 

variances may be obtained. Most modern heating and cooking 

is fueled by gas, electricity, or oil-none of which emit the hot 

embers that historically have been the cause of roof fires. 

Where wood burning fireplaces or stoves are used, spark arrestor screens at the top of the chimneys help to prevent flaming material from escaping, thus reducing the number of fires 

that start at the roof. In most states, insurance rates have been 

equalized to reflect revised considerations for the risks involved with various roofing materials. 

In a rehabilitation project, there may be valid reasons for 

replacing the roof with a material other than the original. The 

historic roofing may no longer be available, or the cost of obtaining specially fabricated materials may be prohibitive. But 

the decision to use an alternative material should be weighed 

carefully against the primary concern to keep the historic 

character of the building. If the roof is flat and is not visible 

from any elevation of the building, and if there are advantages to substituting a modern built-up composition roof for 

what might have been a flat metal roof, then it may make better economic and construction sense to use a modern roofing 

method. But if the roof is readily visible, the alternative 

material should match as closely as possible the scale, texture, 

and coloration of the historic roofing material. 

Asphalt shingles or ceramic tiles are common substitute materials intended to duplicate the appearance of wood shingles, 

slates, or tiles. Fire-retardant, treated wood shingles are currently available. The treated wood tends, however, to be brittle, and may require extra care (and expense) to install. In 

some instances, shingles laid with an interlay of fire-retardent 

building paper may be an acceptable alternative. 

Lead-coated copper, terne-coated steel, and aluminum/ 

zinc-coated steel can successfully replace tin, terne plate, zinc, 

or lead. Copper-coated steel is a less expensive (and less 

durable) substitute for sheet copper. 

The search for alt~rnative roofing materials is not new. As 

early as the 18th century, fear of fire cause many wood shingle 

or board roofs to be replaced by sheet metal or clay tile. Some 

historic roofs were failures from the start, based on overambitious and naive use of materials as they were first developed. Research on a structure may reveal that an inadequately 

designed or a highly combustible roof was replaced early in its 

history, and therefore restoration of a later roof material 

would have a valid precedent. In some cities, the substitution 

of sheet metal on early row houses occurred as soon as the 

rolled material became available. 

Cost and ease of maintenance may dictate the substitution 

of a material wholly different in appearance from the 

original. The practical problems (wind, weather, and roof 

pitch) should be weighed against the historical consideration 

of scale, texture, and color. Sometimes the effect of the alternative material will be minimal. But on roofs with a high 

degree of visibility and patterning or texture, the substitution 

may seriously alter the architectural character of the building. 

Temporary Stabilization 

It may be necessary to carry out an immediate and temporary 

stabilization to prevent further deterioration until research 

can determine how the roof should be restored or rehabilitated, or until funding can be provided to do a proper job. A 

simple covering of exterior plywood or roll roofing might provide adequate protection, but any temporary covering should 

be applied with caution. One should be careful not to 

overload the roof structure, or to damage or destroy historic 

evidence or fabric that might be incorporated into a new roof 

at a later date. In this sense, repairs with caulking or 

bituminous patching compounds should be recognized as potentially harmful, since they are difficult to remove, and at 

their best, are very temporary. 

Precautions 

The architect or contractor should warn the owner of any 

precautions to be taken against the specific hazards in installing the roofing material. Soldering of sheet metals, for instance, can be a fire hazard, either from the open flame or 

from overheating and undected smoldering of the wooden 

substrate materials. 

Thought should be given to the design and placement of any 

modern roof appurtenances such as plumbing stacks, air 

vents, or TV antennas. Consideration should begin with the 

placement of modern plumbing on the interior of the building, otherwise a series of vent stacks may pierce the roof membrane at various spots creating maintenance problems as well 

as aesthetic ones. Air handling units placed in the attic space 

will require vents which, in turn, require sensitive design. Incorporating these in unused chimneys has been very successful 

7 
in the past. 

Whenever gutters and downspouts are needed that were not 

on the building historically, the addj.tions should be made as 

unobtrusively as possible, perhaps by painting them out with 

a color compatible with the nearby wall or trim. 

Maintenance 

Although a new roof can be an object of beauty, it will not be 

protective for long without proper maintenance. At least 

twice a year, the roof should be inspected against a checklist. 

All changes should be recorded and reported. Guidelines 

should be established for any foot traffic that may be required 

for the maintenance of the roof. Many roofing materials 

should not be walked on at all. For some-slate, asbestos, and 

clay tile-a self-supporting ladder might be hung over the 

ridge of the roof, or planks might be spanned across the roof 

surface. Such items should be specifically designed and kept 

in a storage space accessible to the roof. If exterior work ever 

requires hanging scaffolding, use caution to insure that the 

anchors do not penetrate, break, or wear the roofing surface, 

gutters, or flashing. 

Any roofing system should be recognized as a membrane 

that is designed to be self-sustaining, but that can be easily 

damaged by intrusions such as pedestrian traffic or fallen tree 

branches. Certain items should be checked at specific times. 

For example, gutters tend to accumulate leaves and debris 

during the spring and fall and after heavy rain. Hidden gutter 

screening both at downspouts and over the full length of the 

gutter could help keep them clean. The surface material would 

require checking after a storm as well. Periodic checking of 

the underside of the roof from the attic after a storm or winter 

freezing may give early warning of any leaks. Generally, 

damage from water or ice is less likely on a roof that has good 

flashing on the outside and is well ventilated and insulated on 

the inside. Specific instructions for the maintenance of the 

different roof materials should be available from the architect 

or contractor. 

Summary 

The essential ingredients for replacing and maintaIning a 

historic roof are: 

• Understanding the historic character of the building and 

being sympathetic to it. 

• Careful examination and recording of the existing roof 

and any evidence of earlier roofs. 

• Consideration of the historic craftsmanship and detailing and implementing them in the renewal wherever 

visible. 

• Supervision of the roofers or maintenance personnel to 

assure preservation of historic fabric and proper understanding of the scope and detailing of the project. 

• Consideration of alternative materials where the original cannot be used. 

• Cyclical maintenance program to assure that the staff 

understands how to take care of the roof and of the particular trouble spots to safeguard. 

With these points in mind, it will be possible to preserve the 

architectural character and maintain the physical integrity of 

the roofing on a historic building. 

This Preservation Brief was written by Sarah M. Sweetser, Architectural Historian, Technical Preservation Services Division. Much of 

the technical information was based upon an unpublished report prepared under cont.ract for this office by John G. and Diana S. Waite. 

Some of the historical information was from Charles E. Peterson, 

FAIA, "American Notes," Journal of the Society of Architectural 

Historians. 

The illustrations for this brief not specifically credited are from the 

files of the Technical Preservation Services Division. 

This publication was prepared pursuant to Executive Order 11593, "Protection 

and Enhancement of the Cultural Environment," which directs the Secretary 

of the Interior to "develop and make available to Federal agencies and State 

and local governments information concerning professional methods and tech8 

Decorative features such as cupolas require extra maintenance. The 

flashing is carefully detailed to promote run-off, and the wooden ribbing must be kept well-painted. This roof surface, which was originally 

tin plate, has been replaced with lead-coated copper for maintenance 

purposes. (Lyndhurst, Tarrytown, New York, photo courtesy of the 

National Trust for Historic Preservation) 

niques lor preserving, improving, restoring and maintaining historic properties." The Brief has been developed under the technical editorship of Lee H. 

Nelson, AlA, Chief, Preservation Assistance Division, National Park Service. 

U.S. Department of the Interior, Washington. D.C. 20240. Comments on the 

usefulness of this information are welcome and can be sent to Mr. Nelson at 

the above address. This publication is not copyrighted and can be reproduced 

without penalty. Normal procedures for credit to the author and the National 

Park Service are appreciated. February 1978. 

Additional readings on the subject of roofing are listed below. 

Boaz, Joseph N., ed. Architectural Graphic Standards. New York: 

John Wiley and Sons, Inc., 1970. (Modern roofing types and detailing) 

Briggs, Martin S. A Short History of the Building Crafts. London: 

Oxford University Press, 1925. (Descriptions of historic roofing 

materials) 

Bulletin of the Association for Preservation Technology. Vol. 2 (nos. 

1-2) 1970. (Entirely on roofing) 

Holstrom, Ingmar; and Sandstrom, Christina. Maintenance of Old 

Buildings: Preservation from the Technical and Antiquarian Standpoint. Stockholm: National Swedish Building Research, 1972. 

(Contains a section on roof maintenance problems) 

Insall, Donald. The Care of Old Buildings Today. London: The 

Architectural Press, 1972. (Excellent guide to some problems and 

solutions for historic roofs) 

Labine, R.A. Clem. "Repairing Slate Roofs. " The Old House Journal3 (no. 12, Dec. 1975): 6- 7. 

Lefer, Henry. " A Birds-eye View." Progressive Architecture. (Mar. 

1977), pp. 88-92. (Article on contemporary sheet metal) 

National Slate Association. Slate Roofs. Reprint of 1926 edition, now 

available from the Vermont Structural Slate Co., Inc., Fairhaven, 

VT 05743 . (An excellent reference for the many designs and details 

of slate roofs) 

Peterson, Charles E. " Iron in Early American Roofs. " The Smithsonian Journal of History 3 (no. 3). Edited by Peter C. Welsh. 

Washington, D.C. : Smithsonian Institution, 1968, pp. 41-76. 

Waite, Diana S. Nineteenth Century Tin Roofing and its Use at Hyde 

Hall. Albany: New York State Historic Trust, 1971. 

- -. "Roofing for Early America." Building Early America. Edited 

by Charles E. Peterson. Radnor, Penn.: Chilton Book Co. , 1976. 
PRESERV ATION 

BRIEFS 

The Repair and 

Thermal Upgrading of 

Historic Steel Windows 

Sharon C. Park, AlA 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

The Secretary of the Interior's "Standards for Rehabilitation" require that where historic windows are individually significant features, or where 

they contribute to the character of significant facades, their distinguishing visual qualities must not be destroyed. Further, the rehabilitation 

guidelines recommend against changing the historic appearance of windows through the use of inappropriate designs, materials, finishes, or colors 

which radically change the sash, depth of reveal, and muntin configuration; the reflectivity and color of the glazing; or the appearance of the 

frame. 

Windows are among the most vulnerable features of 

historic buildings undergoing rehabilitation. This is 

especially the case with rolled steel windows, which are 

often mistakenly not deemed worthy of preservation in the 

conversion of old buildings to new uses. The ease with 

which they can be replaced and the mistaken assumption 

that they cannot be made energy efficient except at great 

expense are factors that typically lead to the decision to 

remove them. In many cases, however, repair and retrofit 

of the historic windows are more economical than wholesale replacement, and all too often, replacement units are 

unlike the originals in design and appearance. If the windows are important in establishing the historic character of 

the building (see fig. 1), insensitively designed replacement 

windows may diminish-or destroy-the building's historic 

character. 

This Brief identifies various types of historic steel 

windows that dominated the metal window market from 

1890-1950. It then gives criteria for evaluating deterioration 

and for determining appropriate treatment, ranging from 

routine maintenance and weatherization to extensive 

repairs, so that replacement may be avoided where possible. I This information applies to do-it-yourself jobs and to 

large rehabilitations where the volume of work warrants the 

removal of all window units for complete overhaul by professional contractors. 

This Brief is not intended to promote the repair of ferrous metal windows in every case, but rather to insure 

that preservation is always the first consideration in a 

rehabilitation project. Some windows are not important 

elements in defining a building's historic character; others 

are highly significant, but so deteriorated that repair is infeasible. In such cases, the Brief offers guidance in 

evaluating appropriate replacement windows. 

Fig. 1 Often highly distinctive in design and craftsmanship, rolled steel 

windows play an important role in defining the architectural character of 

many later nineteenth and early twentieth century buildings. Art Deco, 

Art Moderne, the International Style, and Post World War II Modernism depended on the slim profiles and streamlined appearance of metal 

windows for much of their impact. Photo: William G. Johnson. 

'The technical information given in this brief is intended for most ferrous (or 

magnetic) metals, particularly rolled steel. While stainless steel is a ferrous metal, 

the cleaning and repair techniques outlined here must not be used on it as the finish 

will be damaged. For information on cleaning stainless steel and non-ferrous 

metals, such as bronze, Monel, or aluminum, refer to Metals in America's Historic 

Buildings (see bibliography). 
HISTORICAL DEVELOPMENT 

Although metal windows were available as early as 1860 

from catalogues published by architectural supply firms, 

they did not become popular until after 1890. Two factors 

combined to account for the shift from wooden to metal 

windows about that time. Technology borrowed from the 

rolling industry permitted the mass production of rolled 

steel windows. This technology made metal windows cost 

competitive with conventional wooden windows. In addition, a series of devastating urban fires in Boston, 

Baltimore, Philadelphia, and San Francisco led to the 

enactment of strict fire codes for industrial and multistory commercial and office buildings. 

As in the process of making rails for railroads, rolled 

steel windows were made by passing hot bars of steel 

through progressively smaller, shaped rollers until the appropriate angled configuration was achieved (see fig. 2). 

The rolled steel sections, generally 118" thick and 1" -

1 112" wide, were used for all the components of the windows: sash, frame,and subframe (see fig. 3). With the addition of wire glass, a fire-resistant window resulted. 

These rolled steel windows are almost exclusively found in 

masonry or concrete buildings. 

A byproduct of the fire-resistant window was the 

strong metal frame that permitted the installation of 

larger windows and windows in series. The ability to have 

expansive amounts of glass and increased ventilation 

dramatically changed the designs of late 19th and early 

20th century industrial and commercial buildings. 

The newly available, reasonably priced steel windows 

soon became popular for more than just their fireresistant qualities. They were standardized, extremely 

durable, and easily transported. These qualities led to the 

use of steel windows in every type of construction, from 

simple industrial and institutional buildings to luxury 

commercial and apartment buildings. Casement, doublehung, pivot, projecting, austral, and continuous windows 

differed in operating and ventilating capacities. Figure 4 

outlines the kinds and properties of metal windows 

available then and now. In addition, the thin profiles of 

metal windows contributed to the streamlined appearance 

of the Art Deco, Art Moderne, and International Styles, 

among others. 

The extensive use of rolled steel metal windows continued until after World War II when cheaper, noncorroding aluminum windows became increasingly 

popular. While aluminum windows dominate the market 

today, steel windows are still fabricated. Should replacement of original windows become necessary, replacement 

windows may be available from the manufacturers of 

some of the earliest steel windows. Before an informed 

decision can be made whether to repair or replace metal 

windows, however, the significance of the windows must 

be determined and their physical condition assessed. 

Cover illustration: from Hope's Metal Windows and Casements: 

1818-1926, currently Hope's Architectural Products, Inc. Used with permission. 

2 

ROLLING SECTION FROM BAR 

1ST PASS 2ND PASS 3RD PASS 4TH PASS 

5TH PASS 6TH PASS 7TH PASS 8TH PASS 

FULL SIZE 

Fig. 2. The process of rolling a steel bar into an angled section is illustrated above. The shape and size of the rolled section will vary slightly depending on the overall strength needed for the window opening and 

the location of the section in the assembly: subframe, frame, or sash. 

The 1/ 8 " thickness of the metal section is generally standard. Drawing: 

A Metal Window Dictionary. Used with permission. 

?A?H· 

Fig. 3 A typical section through the top and bottom of a metal window 

shows the three component parts of the window assembly: subframe, 

frame, and sash. Drawings: Catalogue No. 15, January 1931; International Casement Co, Inc., presently Hope's Architectural Products, Inc., 

Jamestown, NY. Used with permission. 
EVALUATION 

Historic and Architectural Considerations 

An assessment of the significance of the windows should 

begin with a consideration of their function in relation to 

the building's historic use and its historic character. Windows that help define the building's historic character 

should be preserved even if the building is being converted 

to a new use. For example, projecting steel windows used 

to introduce light and an effect of spaciousness to a 

warehouse or industrial plant can be retained in the conversion of such a building to offices or residences. 

Other elements in assessing the relative importance of 

the historic windows include the design of the windows 

and their relationship to the scale, proportion, detailing 

and architectural style of the building. While it may be 

easy to determine the aesthetic value of highly ornamented 

windows, or to recognize the importance of streamlined 

windows as an element of a style, less elaborate windows 

can also provide strong visual interest by their small panes 

or projecting planes when open, particularly in simple, 

unadorned industrial buildings (see fig. 5). 

One test of the importance of windows to a building is 

to ask if the overall appearance of the building would be 

changed noticeably if the windows were to be removed or 

radically altered. If so, the windows are important in 

defining the building's historic character, and should be 

repaired if their physical condition permits. 

Physical Evaluation 

Steel window repair should begin with a careful evaluation 

of the physical condition of each unit. Either drawings or 

photographs, liberally annotated, may be used to record 

the location of each window, the type of operability, the 

condition of all three parts-sash, frame and subframe-and the repairs essential to its continued use. 

Specifically, the evaluation should include: presence and 

degree of corrosion; condition of paint; deterioration of 

the metal sections, including bowing, misalignment of the 

sash, or bent sections; condition of the glass and glazing 

compound; presence and condition of all hardware, 

screws, bolts, and hinges; and condition of the masonry 

or concrete surrounds, including need for caulking or 

resetting of improperly sloped sills. 

Corrosion, principally rusting in the case of steel windows, is the controlling factor in window repair; 

therefore, the evaluator should first test for its presence. 

Corrosion can be light, medium, or heavy, depending on 

how much the rust has penetrated the metal sections. If 

the rusting is merely a surface accumulation or flaking, 

then the corrosion is light. If the rusting has penetrated 

the metal (indicated by a bubbling texture), but has not 

caused any structural damage, then the corrosion is 

medium. If the rust has penetrated deep into the metal, 

the corrosion is heavy. Heavy corrosion generally results 

in some form of structural damage,through delamination, 

to the metal section, which must then be patched or spliced. A sharp probe or tool, such as an ice pick, can be used to determine the extent of corrosion in the metal. If 

the probe can penetrate the surface of the metal and brittle strands can be dug out, then a high degree of corrosive 

deterioration is present. 

In addition to corrosion, the condition of the paint, the 

presence of bowing or misalignment of metal sections, the 

amount of glass needing replacement, and the condition 

of the masonry or concrete surrounds must be assessed in 

the evaluation process. These are key factors in determining whether or not the windows can be repaired in place. 

The more complete the inventory of existing conditions, 

the easier it will be to determine whether repair is feasible 

or whether replacement is warranted. 

Rehabilitation Work Plan 

Following inspection and analysis, a plan for the 

rehabilitation can be formulated. The actions necessary to 

return windows to an efficient and effective working condition will fall into one or more of the following 

categories: routine maintenance, repair, and weatherization. The routine maintenance and weatherization 

measures described here are generally within the range of 

do-it-yourselfers. Other repairs, both moderate and major, require a professional contractor. Major repairs normally require the removal of the window units to a 

workshop, but even in the case of moderate repairs, the 

number of windows involved might warrant the removal 

of all the deteriorated units to a workshop in order to 

realize a more economical repair price. Replacement of 

windows should be considered only as a last resort. 

Since moisture is the primary cause of corrosion in steel 

windows, it is essential that excess moisture be eliminated 

and that the building be made as weathertight as possible 

before any other work is undertaken. Moisture can accumulate from cracks in the masonry, from spalling mortar, from leaking gutters, from air conditioning condensation runoff, and from poorly ventilated interior spaces. 

Finally, before beginning any work, it is important to 

be aware of health and safety risks involved. Steel windows have historically been coated with lead paint. The 

removal of such paint by abrasive methods will produce 

toxic dust. Therefore, safety goggles, a toxic dust 

respirator, and protective clothing should be worn. 

Similar protective measures should be taken when acid 

compounds are used. Local codes may govern the 

methods of removing lead paints and proper disposal of 

toxic residue. 

ROUTINE MAINTENANCE 

A preliminary step in the routine maintenance of steel 

windows is to remove surface dirt and grease in order to 

ascertain the degree of deterioration, if any. Such minor 

cleaning can be accomplished using a brush or vacuum 

followed by wiping with a cloth dampened with mineral 

spirits or denatured alcohol. 

3 
Double-hung industrial windows 

duplicated the look of traditional wooden 

windows. Metal double-hung windows were 

early examples of a building product adapted to meet stringent new fire code requirements for manufacturing and high-rise 

buildings in urban areas. Soon supplanted 

in industrial buildings by less expensive 

pivot windows, double-hung metal windows regained popularity in the 1940s for 

use in speculative suburban housing. 

Pivot windows were an early type of industrial window that combined inexpensive first cost and low maintenance. Pivot 

windows became standard for warehouses 

and power plants where the lack of screens 

was not a problem. The window shown 

here is a horizontal pivot. Windows that 

turned about a vertical axis were also 

manufactured (often of iron). Such vertical pivots are rare today. 

Projecting windows, sometimes called 

awning or hopper windows, were perfected 

in the 1920s for industrial and institutional 

buildings. They were often used in "combination" windows, in which upper panels 

opened out and lower panels opened in. 

Since each movable panel projected to 

one side of the frame only, unlike pivot 

windows, for example, screens could be 

introduced. 

Austral windows were also a product of 

the 1920s. They combined the appearance 

of the double-hung window with the increased ventilation and ease of operation 

of the projected window. (When fully 

opened, they provided 70070 ventilation as 

compared to 50% ventilation for doublehung windows.) Austral windows were 

often used in schools, libraries and other 

public buildings. 

Casement windows adapted the English 

tradition of using wrought iron casements 

with leaded carnes for residential use. 

Rolled steel casements (either single, as 

shown, or paired) were popular in the 

1920s for cottage style residences and 

Gothic style campus architecture. More 

streamlined casements were popular in the 

1930s for institutional and small industrial 

buildings. 

Continuous windows were almost exclusively used for industrial buildings requiring high overhead lighting. Long 

runs of clerestory windows. operated by mechanical 

tension rod gears were typical. Long banks 

of continuous windows were possible 

because the frames for such 

windows were often 

structural elements 

of the building. 

Fig. 4 Typical rolled steel windows available jrom 1890 to the present. The various operating and ventilating capacities in combination 

with the aesthetics oj the window style were important considerations in the selection oj one window type over another. Drawings: 

Sharon C. Park; AlA. 

If it is determined that the windows are in basically 

sound condition, the following steps can be taken: 1) 

removal of light rust, flaking and excessive paint; 2) priming of exposed metal with a rust-inhibiting primer; 3) 

replacement of cracked or broken glass and glazing compound; 4) replacement of missing screws or fasteners; 5) 

cleaning and lubrication of hinges; 6) repainting of all 

steel sections with two coats of finish paint compatible 

with the primer; and 7) caulking the masonry surrounds 

with a high quality elastomeric caulk. 

Recommended methods for removing light rust include 

manual and mechanical abrasion or the application of 

chemicals. Burning off rust with an oxy-acetylene or propane torch, or an inert gas welding gun, should never be 

attempted because the heat can distort the metal. In addition, such intense heat (often as high as 3800 0 F) 

vaporizes the lead in old paint, resulting in highly toxic 

fumes. Furthermore, such heat will likely result in broken 

glass. Rust can best be removed using a wire brush, an 

aluminum oxide sandpaper, or a variety of power tools 

4 

Fig. 5 Windows ojten provide a strong visual element to relatively simple or unadorned industrial or commercial buildings. This 

design element should be taken into consideration when evaluating the significance oj the windows. Photo: Michael Auer. 
adapted for abrasive cleaning such as an electric drill with 

a wire brush or a rotary whip attachment. Adjacent sills 

and window jambs may need protective shielding. 

Rust can also be removed from ferrous metals by using 

a number of commercially prepared anti-corrosive acid 

compounds. Effective on light and medium corrosion, 

these compounds can be purchased either as liquids or 

gels. Several bases are available, including phosphoric 

acid, ammonium citrate, oxalic acid and hydrochloric 

acid. ¥ydrochloric acid is generally not recommended; it 

can leave chloride deposits, which cause future corrosion. 

Phosphoric acid-based compounds do not leave such 

deposits, and are therefore safer for steel windows. 

However, any chemical residue should be wiped off with 

damp cloths, then dried immediately. Industrial blowdryers work well for thorough drying. The use of running 

water to remove chemical residue is never recommended 

because the water may spread the chemicals to adjacent 

surfaces, and drying of thes~ surfaces may be more difficult. Acid cleaning compounds will stain masonry; 

therefore plastic sheets should be taped to the edge of the 

metal sections to protect the masonry surrounds. The 

same measure should be followed to protect the glazing 

from etching because of acid cop.tact. 

Measures that remove rust will ordinarily remove flaking paint as well. Remaining loose or flaking paint can be 

removed with a chemical paint remover or with a 

pneumatic needle scaler or gun, which comes with a series 

of chisel blades and has proven effective in removing flaking paint from metal windows. Well-bonded paint may 

serve to protect the metal further from corrosion, and 

need not be removed unless paint build-up prevents the 

window from closing tightly. The edges should be feathered by sanding to give a good surface for repainting. 

Next, any bare metal should be wiped with a cleaning 

solvent such as denatured alcohol, and dried immediately 

in preparation for the application of an anti-corrosive 

primer. Since corrosion can recur very soon after metal 

has been exposed to the air, the metal should be primed 

immediately after cleaning. Spot priming may be required 

periodically as other repairs are undertaken. Anticorrosive primers generally consist of oil-alkyd based 

paints rich in zinc or zinc chromate.

2 Red lead is no 

longer available because of its toxicity. All metal primers, 

however, are toxic to some degree and should be handled 

carefully. Two coats of primer are recommended. Manufacturer's recommendations should be followed concerning application of primers. 

REPAIR 

Repair in Place 

The maintenance procedures described above will be insufficient when corrosion is extensive, or when metal window sections are misaligned. Medium to heavy corrosion 

that has not done any structural damage to the metal sections can be removed either by using the chemical cleaning 

process described under "Routine Maintenance" or by 

sandblasting. Since sandblasting can damage the masonry 

surrounds and crack or cloud the glass, metal or plywood 

shields should be used to protect these materials. The 

sandblasting pressure should be low, 80-100 pounds per 

square inch, and the grit size should be in the range of 

#10-#45. Glass peening beads (glass pellets) have also been 

successfully used in cleaning steel sections. While sandblasting equipment comes with various nozzle sizes, 

pencil-point blasters are most useful because they give the 

operator more effective control over the direction of the 

spray. The small aperture of the pencil-point blaster is 

also useful in removing dried putty from the metal sections that hold the glass. As with any cleaning technique, 

once the bare metal is exposed to air, it should be primed 

as soon as possible. This includes the inside rabbeted section of sash where glazing putty has been removed. To reduce the dust, some local codes allow only wet blasting. 

In this case, the metal must be dried immediately, generally with a blow-drier (a step that the owner should consider 

when calculating the time and expense involved). Either 

form of sandblasting metal covered with lead paints produces toxic dust. Proper precautionary measures should 

be taken against toxic dust and silica particles. 

Bent or bowed metal sections may be the result of 

damage to the window through an impact or corrosive expansion. If the distortion is not too great, it is possible to 

realign the metal sections without removing the window to 

a metal fabricator's shop. The glazing is generally removed and pressure is applied to the bent or bowed section. 

In the case of a muntin, a protective 2 x 4 wooden bracing can be placed behind the bent portion and a wire 

cable with a winch can apply progressively more pressure 

over several days until the section is realigned. The 2 x 4 

bracing is necessary to distribute the pressure evenly over 

the damaged section. Sometimes· a section, such as the 

bottom of the frame, will bow out as a result of pressure 

exerted by corrosion and it is often necessary to cut the 

metal section to relieve this pressure prior to pressing the 

section back into shape and making a welded repair. 

Once the metal sections have been cleaned of all corrosion and straightened, small holes and uneven areas 

resulting from rusting should be filled with a patching 

material and sanded smooth to eliminate pockets where 

water can accumulate. A patching material of steel fibers 

and an epoxy binder may be the easiest to apply. This 

steel-based epoxy is available for industrial steel repair; it 

can also be found in auto body patching compounds or in 

plumber's epoxy. As with any product, it is important to 

follow the manufacturer's instructions for proper use and 

best results. The traditional patching technique-melting 

steel welding rods to fill holes in the metal sections-may 

be difficult to apply in some situations; moreover, the 

window glass must be removed during the repair process, 

or it will crack from the expansion of the heated metal 

sections. After these repairs, glass replacement, hinge 

lubrication, painting, and other cosmetic repairs can be 

undertaken as necessary. 

'Refer to Table IV, Types of Paint Used for Painting Metal in Metals in America's 

Historic Buildings, p, 139. (See bibliography). 

5 
To complete the checklist for routine maintenance, 

cracked glass, deteriorated glazing compound, missing 

screws, and broken fasteners will have to be replaced; 

hinges cleaned and lubricated; the metal windows painted, 

and the masonry surrounds caulked. If the glazing must 

be replaced, all clips, glazing beads, and other fasteners 

that hold the glass to the sash should be retained, if possible, although replacements for these parts are still being 

fabricated. When bedding glass, use only glazing compound formulated for metal windows. To clean the hinges 

(generally brass or bronze), a cleaning solvent and fine 

bronze wool should be used. The hinges should then be 

lubricated with a non-greasy lubricant specially formulated for metals and with an anti-corrosive agent. 

These lubricants are available in a spray form and should 

be used periodically on frequently opened windows. 

Final painting of the windows with a paint compatible 

with the anti-corrosive primer should proceed on a dry 

day. (Paint and primer from the same manufacturer 

should be used.) Two coats of finish paint are recommended if the sections have been cleaned to bare metal. 

The paint should overlap the glass slightly to insure 

weathertightness at that connection. Once the paint dries 

thoroughly, a flexible exterior caulk can be applied to 

eliminate air and moisture infiltration where the window 

and the surrounding masonry meet. 

Caulking is generally undertaken after the windows 

have received at least one coat of finish paint. The 

perimeter of the masonry surround should be caulked 

with a flexible elastomeric compound that will adhere well 

to both metal and masonry. The caulking used should be 

a type intended for exterior application, have a high 

tolerance for material movement, be resistant to 

ultraviolet light, and have a minimum durability of 10 

years. Three effective compounds (taking price and other 

factors into consideration) are polyurethane, vinyl acrylic, 

and butyl rubber. In selecting a caulking material for a 

window retrofit, it is important to remember that the 

caulking compound may be covering other materials in a 

substrate. In this case, some compounds, such as silicone, 

may not adhere well. Almost all modern caulking compounds can be painted after curing completely. Many 

come in a range of colors, which eliminates the need to 

paint. If colored caulking is used, the windows should 

have been given two coats of finish paint prior to caulking. 

Repair in Workshop 

Damage to windows may be so severe that the window 

sash and sometimes the frame must be removed for cleaning and extensive rust removal, straightening of bent sections, welding or splicing in of new sections, and reglazing. These major and expensive repairs are reserved for 

highly significant windows that cannot be replaced; the 

procedures involved should be carried out only by skilled 

workmen. (see fig. 6a-6f.) 

6 

As part of the orderly removal of windows, each window should be numbered and the parts labelled. The 

operable metal sash should be dismantled by removing the 

hinges; the fixed sash and, if necessary, the frame can 

then be unbolted or unscrewed. (The subframe is usually 

left in place. Built into the masonry surrounds, it can only 

be cut out with a torch.) Hardware and hinges should be 

labelled and stored together. 

The two major choices for removing flaking paint and 

corrosion from severely deteriorated windows are dipping 

in a chemical bath or sandblasting. Both treatments require removal of the glass. If the windows are to be dipped, a phosphoric acid solution is preferred, as mentioned 

earlier. While the dip tank method is good for fairly evenly distributed rust, deep set rust may remain after dipping. 

For that reason, sandblasting is more effective for heavy 

and uneven corrosion. Both methods leave the metal sections clean of residual paint. As already noted, after 

cleaning has exposed the metal to the air, it should be 

primed immediately after drying with an anti-corrosive 

primer to prevent rust from recurring. 

Sections that are seriously bent or bowed must be 

straightened with heat and applied pressure in a 

workshop. Structurally weakened sections must be cut 

out, generally with an oxy-acetylene torch, and replaced 

with sections welded in place and the welds ground 

smooth. Finding replacement metal sections, however, 

may be difficult. While most rolling mills are producing 

modern sections suitable for total replacement, it may be 

difficult to find an exact profile match for a splicing 

repair. The best source of rolled metal sections is from 

salvaged windows, preferably from the same building. If 

no salvaged windows are available, two options remain. 

Either an ornamental metal fabricator can weld flat plates 

into a built-up section, or a steel plant can mill bar steel 

into the desired profile. 

While the sash and frame are removed for repair, the 

subframe and masonry surrounds should be inspected. 

This is also the time to reset sills or to remove corrosion 

from the subframe, taking care to protect the masonry 

surrounds from damage. 

Missing or broken hardware and hinges should be 

replaced on all windows that will be operable. Salvaged 

windows, again, are the best source of replacement parts. 

If matching parts cannot be found, it may be possible to 

adapt ready-made items. Such a substitution may require 

filling existing holes with steel epoxy or with plug welds 

and tapping in new screw holes. However, if the hardware 

is a highly significant element of the historic window, it 

may be worth having reproductions made. 

Following are illustrations of the repair and thermal 

upgrading of the rolled steel windows in a National 

Historic Landmark (fig. 6). Many of the techniques 

described above were used during this extensive rehabilitation. The complete range of repair techniques is then summarized in the chart titled Steps jor Cleaning and Repairing Historic Steel Windows (see fig. 7). 
• 

Fig. 6 a. View of the wing of the State Capitol where the 

rolled steel casement windows are being removed for repair. 

Fig. 6 c. View of the rusted frame which was unscrewed from the 

sub frame and removed from the window opening and taken to a 

workshop for sandblasting. In some cases, severely deteriorated sections of the frame were replaced with new sections of milled bar 

steel. 

Fig. 6 e. View looking down towards the sill. The cleaned frame 

was reset in the window opening. The frame was screwed to the 

refurbished subframe at the jamb and the head only. The screw 

holes at the sill, which had been the cause of much of the earlier 

rusting, were infilled. Vinyl weatherstripping was added to the 

frame. 

Fig. 6 d. View looking down towards the sill. The sub frames appeared very rusted, but were in good condition once debris was 

vacuumed and surface rust was removed, in place, with chemical 

compounds. Where necessary, epoxy and steel filler was used to 

patch depressions in order to make the sub frame serviceable again. 

Fig. 6 f View from 

the outside of the 

completely 

refurbished window. 

In addition to the 

steel repair and the 

installation of vinyl 

weatherstripping, 

the exterior was 

caulked with 

polyurethane and 

the single glass was 

replaced with 

individual lights of 

thermal glass. The 

repaired and 

upgraded windows 

have comparable 

energy efficiency 

ratings to new 

replacement units 

while retaining the 

historic steel sash, 

frames and 

subframes. 

Fig. 6. The repair and thermal upgrading oj the historic steel windows at the State Capitol, Lincoln, Nebraska. This early twentieth 

century building, designed by Bertram Goodhue, is a National Historic Landmark. Photos: All photos in this series were provided by 

the State Building Division. 

7 
STEPS FOR CLEANING AND REPAIRING HISTORIC STEEL WINDOWS ________ _ 

Recommended Tools, Products and 

Work Item Techniques Procedures Notes 

*(Must be done in a 

workshop) 

1. Removing General maintenance Vacuum and bristle brushes to Solvents can cause eye and skin irdirt and and chemical cleaning remove dust and dirt; solvents ritation. Operator should wear progrease-from (denatured alcohol, mineral tective gear and work in ventilated 

metal spirits), and clean cloths to area. Solvents should not contact 

remove grease. masonry. Do not flush with water. 

2. Removing 

Rust! 

Corrosion 

Light Manual and mechanical Wire brushes, steel wool, Handsanding will probably be 

abrasion rotary attachments to electric necessary for corners. Safety goggles 

drill, sanding blocks and and masks should be worn. 

disks. 

Chemical cleaning Anti-corrosive jellies and li- Protect glass and metal with plastic 

quids (phosphoric acid prefer- sheets attached with tape. Do not 

red); clean damp cloths. flush with water. Work in ventilated 

area. 

Medium Sandblasting! abrasive Low pressure (80-100 psi) and Removes both paint and rust. Codes 

cleaning small grit (#10-#45); glass should be checked for environ menpeening beads. Pencil blaster tal compliance. Prime exposed 

gives good control. metal promptly. Shield glass and 

masonry. Operator should wear 

safety gear. 

Heavy *Chemical dip tank Metal sections dipped into Glass and hardware should be 

chemical tank (phosphoric removed. Protect operator. Deepset 

acid preferred) from several rust may remain, but paint will be 

hours to 24 hours. removed. 

*Sandblasting! Low pressure (80-100 psi) and Excellent for heavy rust. Remove or 

abrasive cleaning small grit (#10-#45). protect glass. Prime exposed metal 

promptly. Check codes for environmental compliance. Operator 

should wear safety gear. 

3. Removing Chemical method Chemical paint strippers Protect glass and masonry. Do not 

flaking suitable for ferrous metals. flush with water. Have good venpaint. Clean cloths. tilation and protection for operator. 

Mechanical abrasion Pneumatic needle gun chisels, Protect operator; have good ventilasanding disks. tion. Well-bonded paint need not be 

removed if window closes properly. 

4. Aligning Applied pressure Wooden frame as a brace for Remove glass in affected area. 

bent, bowed cables and winch mechanism. Realignment may take several days. 

metal 

sections *Heat and pressure Remove to a workshop. Apply Care should be taken that heat does 

heat and pressure to bend not deform slender sections. 

back. 

S 
Recommended Tools, Products and 

Work Item Techniques Procedures Notes 

*(Must be done in a 

workshop) 

5. Patching Epoxy and steel filler Epoxy fillers with high con- Epoxy patches generally are easy to 

depressions tent of steel fibers; plumber's apply, and can be sanded smooth. 

epoxy or autobody patching Patches should be primed. 

compound. 

Welded patches Weld in patches using steel Prime welded sections after grinding 

rods and oxy-acetylene torch connections smooth. 

or arc welder. 

6. Splicing in *Cut out decayed sec- Torch to cut out bad sections Prime welded sections after grinding 

new metal tions and weld in new back to 45 0 joint. Weld in connection smooth. 

sections or salvaged sections new pieces and grind smooth. 

7. Priming Brush or spray At least one coat of anti-cor- Metal should be primed as soon as 

metal application rosive primer on bare metal. it is exposed. If cleaned metal will 

sections Zinc-rich primers are general- be repaired another day, spot prime 

ly recommended. to protect exposed metal. 

8. Replacing Routine maintenance Pliers to pull out or shear off If new holes have to be tapped into 

missing rusted heads. Replace screws the metal sections, the rusted holes 

screws and and bolts with similar ones, should be cleaned, filled and primed 

bolts readily available. prior to redrilling. 

9. Cleaning, Routine maintenance, Most hinges and closure hard- Replacement hinges and fasteners 

lubricating solvent cleaning ware are bronze. Use solvents may not match the original exactly. 

or rep lac- (mineral spirits), bronze wool If new holes are necessary, old ones 

ing hinges and clean cloths. Spray with should be filled. 

and other non-greasy lubricant containhardware ing anti-corrosive agent. 

10. Replacing Standard method for Pliers and chisels to remove Heavy gloves and other protective 

glass and application old glass, scrape putty out of gear needed for the operator. All 

glazing glazing rabbet, save all clips parts saved should be cleaned prior 

compound and beads for reuse. Use only to reinstallation. 

glazing compound formulated 

for metal windows. 

11. Caulking Standard method for Good quality (10 year or bet- The gap between the metal frame 

masonry application ter) elastomeric caulking com- and the masonry opening should be 

surrounds pound suitable for metal. caulked; keep weepholes in metal 

for condensation run-off clear of 

caulk. 

12. Repainting Spray or brush At least 2 coats of paint com- The final coats of paint and the 

metal patible with the anti-corrosive primer should be from the same 

windows primer. Paint should lap the manufacturer to ensure compatibiliglass about 1/8" to form a ty. If spraying is used, the glass and 

seal over the glazing masonry should be protected. 

compound. 

Fig. 7. STEPS FOR CLEANING AND REPAIRING HISTORIC STEEL WINDOWS. Compiled by Sharon C. Park, AlA. 

9 
WEATHERIZATION 

Historic metal windows are generally not energy efficient; 

this has often led to their wholesale replacement. Metal 

windows can, however, be made more energy efficient in 

several ways, varying in complexity and cost. Caulking 

around the masonry openings and adding weatherstripping, for example, can be do-it-yourself projects and are 

important first steps in reducing air infiltration around the 

windows. They usually have a rapid payback period. 

Other treatments include applying fixed layers of glazing 

over the historic windows, adding operable storm windows, or installing thermal glass in place of the existing 

glass. In combination with caulking and weatherstripping, 

these treatments can produce energy ratings rivaling those 

achieved by new units. 3 

Weatherstripping 

The first step in any weatherization program, caulking, 

has been discussed above under "Routine Maintenance." 

The second step is the installation of weatherstripping 

where the operable portion of the sash, often called the 

ventilator, and the fixed frame come together to reduce 

perimeter air infiltration (see fig. 8). Four types of 

weatherstripping appropriate for metal windows are 

spring-metal, vinyl strips, compressible foam tapes, and 

sealant beads. The spring-metal, with an integral friction 

fit mounting clip, is recommended for steel windows in 

good condition. The clip eliminates the need for an applied glue; the thinness of the material insures a tight 

closure. The weatherstripping is clipped to the inside 

channel of the rolled metal section of the fixed frame. To 

insure against galvanic corrosion between the weatherstripping (often bronze or brass), and the steel window, 

the window must be painted prior to the installation of 

the weatherstripping. This weatherstripping is usually applied to the entire perimeter of the window opening, but 

in some cases, such as casement windows, it may be best 

to avoid weatherstripping the hinge side. The natural 

wedging action of the weatherstripping on the three sides 

of the window often creates an adequate seal. 

Vinyl weatherstripping can alSO be applied to metal windows. Folded into a "V" configuration, the material 

forms a barrier against the wind. Vinyl weatherstripping is 

usually glued to the frame, although some brands have an 

adhesive backing. As the vinyl material and the applied 

glue are relatively thick, this form of weatherstripping 

may not be appropriate for all situations. 

Compressible foam tape weatherstripping is often best 

for large windows where there is a slight bending or 

distortion of the sash. In some very tall windows having 

closure hardware at the sash mid-point, the thin sections 

'One measure of energy efficiency is the U·value (the number of BTUs per hour 

transferred through a square foot of material). The lower the U·value. the better 

the performance. According to ASHRAE HANDBOOK·1977 Fundamentals, the 

U·value of historic rolled steel sash with single glazing is 1.3. Adding storm win· 

dows to the existing units or reglazing with 5/ S" insulating glass produces a 

U·value of .69. These methods of weatherizing historic steel windows compare 

favorably with rolled steel replacement alternatives: with factory installed I" in· 

sulating glass (.67 U·value); with added thermal·break construction and factory 

finish coatings (.62 U·value). 

10 

of the metal window will bow away from the frame near 

the top. If the gap is not more than 114", foam 

weatherstripping can normally fill the space. If the gap exceeds this, the window may need to be realigned to close 

more tightly. The foam weatherstripping comes either 

with an adhesive or plain back; the latter variety requires 

application with glue. Compressible foam requires more 

frequent replacement than either spring-metal or vinyl 

wea therstri pping. 

A fourth type of successful weatherstripping involves 

the use of a caulking or sealant bead and a polyethylene 

bond breaker tape. After the window frame has been 

thoroughly cleaned with solvent, permitted to dry, and 

primed, a neat bead of low modulus (firm setting) caulk, 

such as silicone, is applied. A bond breaker tape is then 

applied to the operable sash covering the metal section 

where contact will occur. The window is then closed until 

the sealant has set (2-7 days, depending on temperature 

and humidity). When the window is opened, the bead will 

have taken the shape of the air infiltration gap and the 

bond breaker tape can be removed. This weatherstripping 

method appears to be successful for all types of metal 

windows with varying degrees of air infiltration. 

Since the several types of weatherstripping are appropriate for different circumstances, it may be necessary 

to use more than one type on any given building. Successful weatherstripping depends upon using the thinnest 

material adequate to fill the space through which air 

enters. Weatherstripping that is too thick can spring the 

hinges, thereby resulting in more gaps. 

Sprlng·metal 

Vinyl Strips 

/ 

Spring·metal com~s in bronze, brass or 

stainless steel with an integral friction·fit 

clip. The weatherstripping is applied after 

the repaired windows are painted to avoid 

galvanic corrosion. This type of thin 

weatherstripping is intended for windows 

in good condition. 

Vinyl strips are scored and fold into a "V" 

configuration. Applied adhesive is necessary 

.. 7A7H which will increase the thickness of the . ? weatherstripping, making it inappropriate 

Fffl1 -z..-.r .',' ' for some situations. The weatherstripping • is generally applied to the window after 

'C.WEATHE'f'>;rI\If" painting. 

Foam Tape <=n~I'JIOI'? 

Sealant Bead EXlel"lloJ<? 

Closed cell foam tape comes either with 

or without an adhesive backing, It is 

effective for windows with a gap of 

approximately 1,4" and is easy to install . 

However, this type of weatherstripping 

will need frequent replacement on 

windows in regular use. The metal section 

should be cleaned of all di rt and grease 

prior to its application. 

This very effective type of weatherstripping 

involves the application of a clean bead of 

firm setting caulk on the primed frame 

with a polyethelene bond breaker tape on 

the operable sash. The window is then 

closed until the bead has set and takes the 

form of the gap. The sash is then opened 

and the tape is removed leaving the set 

caulk as the weatherstripping. 

Fig. 8 APPROPRIA TE TYPES OF WEA THERSTRIPPING 

FOR METAL WINDOWS. Weatherstripping is an important 

part of upgrading the thermal efficiency of historic steel windows. 

The chart above shows the jamb section of the window with the 

weatherstripping in place. Drawings: Sharon C. Park, AlA. 
Thermal Glazing 

The third weatherization treatment is to install an additional layer of glazing to improve the thermal efficiency 

of the existing window. The decision to pursue this treatment should proceed from careful analysis. Each of the 

most common techniques for adding a layer of glazing 

will effect approximately the same energy savings (approximately double the original insulating value of the windows); therefore, cost and aesthetic considerations usually 

determine the choice of method. Methods of adding a 

layer of glazing to improve thermal efficiency include adding a new layer of transparent material to the window; 

adding a separate storm window; and replacing the single 

layer of glass in the window with thermal glass. 

The least expensive of these options is to install a clear 

material (usually rigid sheets of acrylic or glass) over the 

original window. The choice between acrylic and glass is 

generally based on cost, ability of the window to support 

the material, and long-term maintenance outlook. If the 

material is placed over the entire window and secured to 

the frame, the sash will be inoperable. If the continued 

use of the window is important (for ventilation or for fire 

exits), separate panels should be affixed to the sash 

without obstructing operability (see fig. 9). Glass or 

acrylic panels set in frames can be attached using magnetized gaskets, interlocking material strips, screws or 

adhesives. Acrylic panels can be screwed directly to 

the metal windows, but the holes in the acrylic panels 

should allow for the expansion and contraction of this 

material. A compressible gasket between the prime sash 

and the storm panel can be very effective in establishing a 

thermal cavity between glazing layers. To avoid condensation, 1/S" cuts in a cop corner and diagonally opposite 

bottom corner of the gasket will provide a vapor bleed, 

through which moisture can evaporate. (Such cuts, however, reduce thermal performance slightly.) If condensation does occur, however, the panels should be easily removable in order to wipe away moisture before it causes 

corrosion. 

The second method of adding a layer of glazing is to 

have independent storm windows fabricated. (Pivot and 

austral windows, however, which project on either side of 

the window frame when open, cannot easily be fitted with 

storm windows and remain operational.) The storm window should be compatible with the original sash configuration. For example, in paired casement windows, 

either specially fabricated storm casement windows or 

sliding units in which the vertical meeting rail of the slider 

reflects the configuration of the original window should 

be installed. The decision to place storm windows on the 

inside or outside of the window depends on whether the 

historic window opens in or out, and on the visual impact 

the addition of storm windows will have on the building. 

Exterior storm windows, however, can serve another purpose besides saving energy: they add a layer of protection 

against air pollutants and vandals, although they will partially obscure the prime window. For highly ornamental 

windows this protection can determine the choice of exterior rather then interior storm windows. 

The third method of installing an added layer of glazing 

is to replace the original single glazing with thermal glass. 

Except in rare instances in which the original glass is of 

special interest (as with stained or figured glass), the glass 

can be replaced if the hinges can tolerate the weight of the 

additional glass. The rolled metal sections for steel windows are generally from 1" - 1 1/2" thick. Sash of this 

thickness can normally tolerate thermal glass, which 

ranges from 3/S" - 5/S". (Metal glazing beads, readily 

available, are used to reinforce the muntins, which hold 

the glass.) This treatment leaves the window fully operational while preserving the historic appearance. It is, 

however, the most expensive of the treatments discussed 

here. (See fig. 6f). 

A 1\F'P'LlEi7 

ACFlYL I C 

EXlmIOf\. 

EXTEPlIOFI 

I""ANEL? I 

~'" ? 1~",,?A~~ ~~~~:r .... _1-"'-

Fig. 9 Two examples of adding a second layer of glazing in order to impro.ve the thermal performance of historic steel windows. Scheme A 

(showing jamb detail) is of a "acrylic panel with a closed cell foam 

gasket attached with self-tapping stainless steel screws directly to the exterior of the outwardly opening sash. Scheme B (showing jamb detail) is 

of a glass panel in a magnetized frame affixed directly to the interior of 

the historic steel sash. The choice of using glass or acrylic mounted on 

the inside or outside will depend on the ability of the window to tolerate 

additional weight, the location and size of the window, the cost, and the 

long-term maintenance outlook. Drawing: Sharon C. Park, AlA. 

WINDOW REPLACEMENT 

Repair of historic windows is always preferred within a 

rehabilitation project. Replacement should be considered 

only as a last resort. However, when the extent of 

deterioration or the unavailability of replacement sections 

renders repair impossible, replacement of the entire window may be justified. In the case of significant windows, 

replacement in kind is essential in order to maintain the 

historic character of the building. However, for less 

significant windows, replacement with compatible new 

windows may be acceptable. In selecting compatible 

replacement windows, the material, configuration, color, 

operability, number and size of panes, profile and proportion of metal sections, and reflective quality of the 

original glass should be duplicated as closely as possible. 

A number of metal window manufacturing companies 

produce rolled steel windows. While stock modern window designs do not share the multi-pane configuration of 

11 
historic windows, most of these manufacturers can 

reproduce the historic configuration if requested, and the 

cost is not excessive for large orders (see figs. lOa and 

lOb). Some manufacturers still carry the standard preWorld War II multi-light windows using the traditional 

12" x 18" or 14" x 20" glass sizes in industrial, commercial, security, and residential configurations. In addition, 

many of the modern steel windows have integral 

weatherstripping, thermal break construction, durable 

vinyl coatings, insulating glass, and other desirable 

features. 

Fig. 10 a. A six-story concrete manufacturing building prior to 

the replacement of the steel pivot windows. Photo: Charles 

Parrott. 

Fig. 10 b. Close-up view of the new replacement steel windows 

which matched the multi-lighted originals exactly. Photo: Charles 

Parrott. 

Windows manufactured from other materials generally 

cannot mat~the thin profiles of the rolled steel sections. 

AluminulIl, fpr example, is three times weaker than steel 

and must be extruded into a box-like configuration that 

does not reflect the thin historic profiles of most steel 

windows. Wooden and vinyl replacement windows 

generally are not fabricated in the industrial style, nor can 

they reproduce the thin profiles of the rolled steel sections, and consequently are generally not acceptable 

replacements. 

For product information on replacement windows, the 

owner, architect, or contractor should consult manufacturers' catalogues, building trade journals, or the Steel 

Window Institute, 1230 Keith Building, Cleveland, Ohio 

44115. 

SUMMARY 

The National Park Service recommends the retention of 

significant historic metal windows whenever possible. 

Such windows, which can be a character-defining feature 

of a historic building, are too often replaced with inappropriate units that impair rather than complement the 

overall historic appearance. The repair and thermal 

upgrading of historic steel windows is more practicable 

than most people realize. Repaired and properly maintained metal windows have greatly extended service lives. 

They can be made energy efficient while maintaining their 

contribution to the historic character of the building. 

BIBLIOGRAPHY 

ASHRAE Handbook - 1977 Fundamentals. New York: American 

Society of Heating, Refrigerating and Airconditioning Engineers, 

1978. 

Crittal, W. F. A Metal Window Dictionary. London:Curwen Press, 

1926. Reprinted by B.T. Batsford, Ltd., 1953. 

Gayle, Margot; David W. Look, AlA; John G. Waite.Metals in 

America's Historic Buildings: Uses and Preservation Treatments. 

Technical Preservation Services, U.S. Department of the Interior. 

Washington,D.C.: U.S.Government Printing Office, 1980. 

Gillet, William. "Steel Windows." Windows and Glass in the Exterior 

of Buildings. National Academy of Sciences Publication 478. 

Washington, D.C.: 1957,75-78. 

Sarton, R. H. "Selecting and Specifying an Appropriate Type of Steel 

Window." Metalcraft. Vol 6, No. 1 (January, 1931): 43-48, 64-65. 

Sweet's Architectural Catalogue. 13th Edition, New York, Sweets 

Catalogue Service, Inc., 1918. 

The author gratefully acknowledges the invaluable assistance of coworker Michael Auer in preparing this brief for publication. This 

publication is an extension of research initiated by Frederec E. Kleyle. 

Special thanks are given to Hope's Architectural Products, Inc., 

Jamestown, NY, for their generous contribution of historic metal window catalogues which were an invaluable source of information. The 

following individuals are also to be thanked for reviewing the manuscript 

and making suggestions: Hugh Miller, Chief, Park Historic Architecture 

Division, National Park Service; Barclay L. Rogers, Museum Services, 

National Park Service; Susan M. Young, Steel Window Institute, and 

Danny Schlichenmaier, State Building Division, Lincoln, Nebraska. 

Finally, thanks go to Technical Preservation Services Branch staff and to 

cultural resources staff of the National Park Service Regional Offices, 

whose valua~e comments were incorporated into the final text and who 

contributed t the publication of this brief. 

This public tion has been prepared pursuant to the Economic 

Recovery Tax Act of 1981, which directs the Secretary of the Interior to 

certify rehabilitations of historic buildings that are consistent with their 

historic character; the guidance provided in this brief will assist property 

owners in complying with the requirements of this law. 

Preservation Briefs: 13 has been developed under the technical editorship of Lee H. Nelson, AlA, Chief, Preservation Assistance Division, 

National Park Service, U.S. Department of the Interior, Washington, 

D.C. 20240. Comments on the usefulness of this information are 

welcomed and can be sent to Mr. Nelson at the above address. 
PRESERVATION 

BRIEFS 

New Exterior Additions to Historic 

Buildings: Preservation Concerns 

Anne E. Grimmer and Kay D. Weeks 

National Park Service 

U.S. Department of the Interior 

Technical Preservation Services 

A new exterior addition to a historic building should 

be considered in a rehabilitation project only after 

determining that requirements for the new or adaptive 

use cannot be successfully met by altering nonsignificant interior spaces. If the new use cannot be 

accommodated in this way, then an exterior addition 

may be an acceptable alternative. Rehabilitation as a 

treatment "is defined as the act or process of making 

possible a compatible use for a property through repair, 

alterations, and additions while preserving those portions 

or features which convey its historical, cultural, or 

architectural values." 

The topic of new additions, including rooftop additions, 

to historic buildings comes up frequently, especially as it 

relates to rehabilitation projects. It is often discussed and 

it is the subject of concern, consternation, considerable 

disagreement and confusion. Can, in certain instances, 

a historic building be enlarged for a new use without 

destroying its historic character? And, just what is 

significant about each particular historic building 

that should be preserved? Finally, what kind of new 

construction is appropriate to the historic building? 

The vast amount of literature on the subject of additions 

to historic buildings reflects widespread interest as well 

as divergence of opinion. New additions have been 

discussed by historians within a social and political 

framework; by architects and architectural historians 

in terms of construction technology and style; and 

by urban planners as successful or 

unsuccessful contextual design. However, 

within the historic preservation and 

rehabilitation programs of the National 

Park Service, the focus on new additions 

is to ensure that they preserve the 

character of historic buildings. 

Most historic districts or neighborhoods 

are listed in the National Register of 

Historic Places for their significance within 

a particular time frame. This period of 

significance of historic districts as well 

Figure 1. The addition to the right with its connecting hyphen is compatible with the 

Collegiate Gothic-style library. The addition is set back from the front of the library and 

uses the same materials and a simplified design that references, but does not copy, the 

historic building. Photo: David Wakely Photography. 

as individually-listed properties may 

sometimes lead to a misunderstanding 

that inclusion in the National Register may 

prohibit any physical change outside of a 

certain historical period - particularly in 

the form of exterior additions. National 

Register listing does not mean that a 

building or district is frozen in time and 

that no change can be made without 

compromising the historical significance. 

It does mean, however, that a new 

addition to a historic building should 

preserve its historic character. 

1 
2 

Figure 2. The new section on the right is appropriately scaled and 

reflects the design of the historic Art Deco-style hotel. The apparent 

separation created by the recessed connector also enables the addition 

to be viewed as an individual building. 

Guidance on New Additions 

To meet Standard 1 of the Secretary of the Interior's 

Standards for Rehabilitation, which states that "a 

property shall be used for its historic purpose or be 

placed in a new use that requires minimal change to 

the defining characteristics of the building and its site 

and environment," it must be determined whether a 

historic building can accommodate a new addition. 

Before expanding the building's footprint, consideration 

should first be given to incorporating changes-such as 

code upgrades or spatial needs for a new use-within 

secondary areas of the historic building. However, this 

is not always possible and, after such an evaluation, 

the conclusion may be that an addition is required, 

particularly if it is needed to avoid modifications to 

character-defining interior spaces. An addition should 

be designed to be compatible with the historic character 

of the building and, thus, meet the Standards for 

Rehabilitation. Standards 9 and 10 apply specifically to 

new additions: 

(9) "New additions, exterior alterations, or related 

new construction shall not destroy historic 

materials that characterize the property. The new 

work shall be differentiated from the old and 

shall be compatible with the massing, size, scale, 

and architectural features to protect the historic 

integrity of the property and its environment." 

(10) "New additions and adjacent or related new 

construction shall be undertaken in such a manner 

that if removed in the future, the essential form 

and integrity of the historic property and its 

environment would be unimpaired." 

The subject of new additions is important because a 

new addition to a historic building has the potential to 

change its historic character as well as to damage and 

destroy significant historic materials and features. A new 

addition also has the potential to confuse the public and 

to make it difficult or impossible to differentiate the old 

from the new or to recognize what part of the historic 

building is genuinely historic. 

The intent of this Preservation Brief is to provide 

guidance to owners, architects and developers on 

how to design a compatible new addition, including a 

rooftop addition, to a historic building. A new addition 

to a historic building should preserve the building's 

historic character. To accomplish this and meet the 

Secretary of the Interior's Standards for Rehabilitation, a 

new addition should: 

• Preserve significant historic materials, 

features and form; 

• Be compatible; and 

• Be differentiated from the historic building. 

Every historic building is different and each 

rehabilitation project is unique. Therefore, the guidance 

offered here is not specific, but general, so that it can 

be applied to a wide variety of building types and 

situations. To assist in interpreting this guidance, 

illustrations of a variety of new additions are provided. 

Good examples, as well as some that do not meet the 

Standards, are included to further help explain and 

clarify what is a compatible new addition that preserves 

the character of the historic building. 

Figure 3. The red and buff-colored parking addition with a rooftop 

playground is compatible with the early-20th century school as 

well as with the neighborhood in which it also serves as infill in the 

urban setting. 
Preserve Significant Historic 

Materials, Features and Form 

Attaching a new exterior addition usually 

involves some degree of material loss to 

an external wall of a historic building, 

but it should be minimized. Damaging 

or destroying significant materials and 

craftsmanship should be avoided, as 

much as possible. 

Generally speaking, preservation of 

historic buildings inherently implies 

minimal change to primary or "public" 

elevations and, of course, interior 

features as well. Exterior features that 

distinguish one historic building or 

a row of buildings and which can be 

seen from a public right of way, such 

as a street or sidewalk, are most likely 

to be the most significant. These can 

include many different elements, such 

as: window patterns, window hoods 

or shutters; porticoes, entrances and 

doorways; roof shapes, cornices and 

decorative moldings; or commercial 

storefronts with their special detailing, 

signs and glazing patterns. Beyond a 

single building, entire blocks of urban 

or residential structures are often closely 

related architecturally by their materials, 

detailing, form and alignment. Because 

significant materials and features should 

be preserved, not damaged or hidden, 

the first place to consider placing a 

new addition is in a location where 

the least amount of historic material 

and character-defining features will 

be lost. In most cases, this will be on a 

secondary side or rear elevation. 

One way to reduce overall material 

loss when constructing a new addition 

is simply to keep the addition smaller 

Figure 4. This glass and brick structure is a harmonious addition set back and connected 

to the rear of the Colonial Revival-style brick house. Cunningham/Quill Architects. 

Photos: © Maxwell MacKenzie. 

in proportion to the size of the historic 

building. Limiting the size and number of openings 

between old and new by utilizing existing doors or 

enlarging windows also helps to minimize loss. An 

often successful way to accomplish this is to link the 

addition to the historic building by means of a hyphen 

or connector. A connector provides a physical link 

while visually separating the old and new, and the 

connecting passageway penetrates and removes only a 

small portion of the historic wall. A new addition that 

will abut the historic building along an entire elevation 

or wrap around a side and rear elevation, will likely 

integrate the historic and the new interiors, and thus 

result in a high degree of loss of form and exterior walls, 

as well as significant alteration of interior spaces and 

features, and will not meet the Standards. 

Compatible but Differentiated Design 

In accordance with the Standards, a new addition must 

preserve the building's historic character and, in order 

to do that, it must be differentiated, but compatible, 

with the historic building. A new addition must retain 

the essential form and integrity of the historic property. 

Keeping the addition smaller, limiting the removal 

of historic materials by linking the addition with a 

hyphen, and locating the new addition at the rear or on 

an inconspicuous side elevation of a historic building 

are techniques discussed previously that can help to 

accomplish this. 

Rather than differentiating between old and new, it 

might seem more in keeping with the historic character 

3 
4 

simply to repeat the historic form, material, features and 

detailing in a new addition. However, when the new 

work is highly replicative and indistinguishable from 

the old in appearance, it may no longer be possible to 

identify the "real" historic building. Conversely, the 

treatment of the addition should not be so different that 

it becomes the primary focus. The difference may be 

subtle, but it must be clear. A new addition to a historic 

building should protect those visual qualities that make 

the building eligible for listing in the National Register 

of Historic Places. 

The National Park Service policy concerning new 

additions to historic buildings, which was adopted in 

1967, is not unique. It is an outgrowth and continuation 

of a general philosophical approach to change first 

expressed by John Ruskin in England in the 1850s, 

formalized by William Morris in the founding of the 

Society for the Protection of Ancient Buildings in 

1877, expanded by the Society in 1924 and, finally, 

reiterated in the 1964 Venice Charter-a document that 

continues to be followed by the national committees 

of the International Council on Monuments and 

Sites (lCOMOS). The 1967 Administrative Policies for 

Historical Areas of the National Park System direct that 

" ... a modern addition should be readily distinguishable 

from the older work; however, the new work should be 

harmonious with the old in scale, proportion, materials, 

and color. Such additions should be as inconspicuous as 

Figure 5. This addition (a) is constructed of matching brick 

and attached by a recessed connector (b) to the 1914 apartment 

building (c). The design is compatible and the addition is 

smaller and subordinate to the historic building (d). 

possible from the public view." As a logical evolution 

from these Policies specifically for National Park 

Service-owned historic structures, the 1977 Secretary 

of the Interior's Standards for Rehabilitation, which may 

be applied to all historic buildings listed in, or eligible 

for listing in the National Register, also state that "the 

new work shall be differentiated from the old and 

shall be compatible with the massing, size, scale, and 

architectural features to protect the historic integrity of 

the property and its environment." 

Preserve Historic Character 

The goal, of course, is a new addition that preserves the 

building's historic character. The historic character of 

each building may be different, but the methodology of 

establishing it remains the same. Knowing the uses and 

functions a building has served over time will assist in 

making what is essentially a physical evaluation. But, 

while written and pictorial documentation can provide 

a framework for establishing the building's history, 

to a large extent the historic character is embodied in 

the physical aspects of the historic building itselfshape, materials, features, craftsmanship, window 

arrangements, colors, setting and interiors. Thus, it 

is important to identify the historic character before 

making decisions about the extent-or limitations-of 

change that can be made. 
Figure 6. A new addition (left) is connected to the garage which separates it from the main block of the c. 1910 former florist shop (right). The 

addition is traditional in style, yet sufficiently restrained in design to distinguish it from the historic building. 

A new addition should always be subordinate to the 

historic building; it should not compete in size, scale 

or design with the historic building. An addition that 

bears no relationship to the proportions and massing 

of the historic building-in other words, one that 

overpowers the historic form and changes the scalewill usually compromise the historic character as 

well. The appropriate size for a new addition varies 

from building to building; it could never be stated 

in a square or cubic footage ratio, but the historic 

building's existing proportions, site and setting can 

help set some general parameters for enlargement. 

Although even a small addition that is poorly 

designed can have an adverse impact, to some extent, 

there is a predictable relationship between the size of 

the historic resource and what is an appropriate size 

for a compatible new addition. 

Generally, constructing the new 

addition on a secondary side or rear 

elevation-in addition to material 

preservation-will also preserve the 

historic character. Not only will the 

addition be less visible, but because 

a secondary elevation is usually 

simpler and less distinctive, the 

addition will have less of a physical 

and visual impact on the historic 

building. Such placement will help to 

preserve the building's historic form 

and relationship to its site and setting. 

Historic landscape features, including 

distinctive grade variations, also 

property should not be covered with large paved 

areas for parking which would drastically change the 

character of the site. 

Despite the fact that in most cases it is recommended 

that the new addition be attached to a secondary 

elevation, sometimes this is not possible. There simply 

may not be a secondary elevation-some important 

freestanding buildings have significant materials and 

features on all sides. A structure or group of structures 

together with its setting (for example, a college campus) 

may be of such significance that any new addition 

would not only damage materials, but alter the 

buildings' relationship to each other and the setting. 

An addition attached to a highly-visible elevation of a 

historic building can radically alter the historic form 

or obscure features such as a decorative cornice or 

window ornamentation. Similarly, an addition that fills 

need to be respected. Any new 

landscape features, including plants 

and trees, should be kept at a scale 

and density that will not interfere with 

understanding of the historic resource 

itself. A traditionally landscaped 

Figure 7. A vacant side lot was the only place a new stair tower could be built when this 

1903 theater was rehabilitated as a performing arts center. Constructed with matching 

materials, the stair tower is set back with a recessed connector and, despite its prominent 

location, it is clearly subordinate and differentiated from the historic theater. 

5 
6 

Figure 8. The rehabilitation of this large, early-20th century warehouse (left) into affordable artists' lofts included the addition of a compatible glass 

and brick elevator/stair tower at the back (right). 

Figure 9. A simple, brick stair tower replaced two non-historic additions 

at the rear of this 1879 school building when it was rehabilitated as a 

women's and children's shelter. The addition is set back and it is not visibLe 

from the front of the school. 

Figure 10. The small size and the use of matching materials ensures that 

the new addition on the left is compatible with the historic Romanesque 

Revival-style building. 

in a planned void on a highly-visible elevation 

(such as a U-shaped plan or a feature such as a 

porch) will also alter the historic form and, as a 

result, change the historic character. Under these 

circumstances, an addition would have too much 

of a negative impact on the historic building and 

it would not meet the Standards. Such situations 

may best be handled by constructing a separate 

building in a location where it will not adversely 

affect the historic structure and its setting. 

In other instances, particularly in urban areas, 

there may be no other place but adjacent to the 

primary fa<;:ade to locate an addition needed for 

the new use. It may be possible to design a lateral 

addition attached on the side that is compatible 

with the historic building, even though it is a 

highly-visible new element. Certain types of 

historic structures, such as government buildings, 

metropolitan museums, churches or libraries, 

may be so massive in size that a relatively largescale addition may not compromise the historic 

character, provided, of course, the addition is 

smaller than the historic building. Occasionally, 

the visible size of an addition can be reduced by 

placing some of the spaces or support systems in 

a part of the structure that is underground. Large 

new additions may sometimes be successful if 

they read as a separate volume, rather than as an 

extension of the historic structure, although the 

scale, massing and proportions of the addition 

still need to be compatible with the historic 

building. However, similar expansion of smaller 

buildings would be dramatically out of scale. In 

summary, where any new addition is proposed, 

correctly assessing the relationship between 

actual size and relative scale will be a key to 

preserving the character of the historic building. 
Design Guidance for Compatible 

New Additions to Historic Buildings 

There is no formula or prescription for 

designing a new addition that meets the 

Standards. A new addition to a historic 

building that meets the Standards can be any 

architectural style-traditional, contemporary 

or a simplified version of the historic 

building. However, there must be a balance 

between differentiation and compatibility in 

order to maintain the historic character and 

the identity of the building being enlarged. 

New additions that too closely resemble the 

historic building or are in extreme contrast to 

it fall short of this balance. Inherent in all of the 

guidance is the concept that an addition needs to 

be subordinate to the historic building. 

A new addition must preserve significant 

historic materials, features and form, and it 

must be compatible but differentiated from 

the historic building. To achieve this, it is 

necessary to carefully consider the placement 

or location of the new addition, and its size, 

scale and massing when planning a new 

addition. To preserve a property's historic 

character, a new addition must be visually 

distinguishable from the historic building. 

This does not mean that the addition and the 

historic building should be glaringly different 

in terms of design, materials and other visual 

qualities. Instead, the new addition should 

take its design cues from, but not copy, the 

historic building. 

Figure 11. The addition to this early-20th 

century Gothic Revival-style church provides 

space for offices, a great hall for gatherings 

and an accessible entrance (left). The stucco 

finish, metal roof, narrow gables and the 

Gothic-arched entrance complement the 

architecture of the historic church. Placing the 

addition in back where the ground slopes away 

ensures that it is subordinate and minimizes 

its impact on the church (below). 

A variety of design techniques can be effective ways to 

differentiate the new construction from the old, while 

respecting the architectural qualities and vocabulary of the 

historic building, including the following: 

• Incorporate a simple, recessed, small-scale hyphen 

to physically separate the old and the new volumes 

or set the addition back from the wall plane(s) of the 

historic building. 

• Avoid designs that unify the two volumes into 

a single architectural whole. The new addition 

may include simplified architectural features that 

reflect, but do not duplicate, similar features on the 

historic building. This approach will not impair 

the existing building'S historic character as long 

as the new structure is subordinate in size and 

clearly differentiated and distinguishable so that the 

identity of the historic structure is not lost in a new 

and larger composition. The historic building must 

be clearly identifiable and its physical integrity must 

not be compromised by the new addition. 

7 
8 

Figure 12. This 1954 synagogue (left) is accessed through a monumental entrance to the right. The new education wing (far right) added to it features 

the same vertical elements and color and, even though it is quite large, its smaller scale and height ensure that it is secondary to the historic resource. 

Figure 13. A glass and metal structure was constructed in the 

courtyard as a restaurant when this 1839 building was converted 

to a hotel. Although such an addition might not be appropriate in 

a more public location, it is compatible here in the courtyard of this 

historic building. 

Figure 14. This glass addition was erected at the back of an 1895 

former brewery during rehabilitation to provide another entrance. 

The addition is compatible with the plain character of this 

secondary elevation. 

• Use building materials in the same color range 

or value as those of the historic building. 

The materials need not be the same as those 

on the historic building, but they should be 

harmonious; they should not be so different 

that they stand out or distract from the 

historic building. (Even clear glass can be 

as prominent as a less transparent material. 

Generally, glass may be most appropriate for 

small-scale additions, such as an entrance on a 

secondary elevation or a connector between an 

addition and the historic building.) 

• Base the size, rhythm and alignment of the 

new addition's window and door openings on 

those of the historic building. 

• Respect the architectural expression of the 

historic building type. For example, an 

addition to an institutional building should 

maintain the architectural character associated 

with this building type rather than using 

details and elements typical of residential or 

other building types. 

These techniques are merely examples of ways to 

differentiate a new addition from the historic building 

while ensuring that the addition is compatible with 

it. Other ways of differentiating a new addition from 

the historic building may be used as long as they 

maintain the primacy of the historic building. Working 

within these basic principles still allows for a broad 

range of architectural expression that can range from 

stylistic similarity to contemporary distinction. The 

recommended design approach for an addition is one 

that neither copies the historic building exactly nor 

stands in stark contrast to it. 
Revising an Incompatible Design for aNew Addition to Meet the Standards 

Figure 15. The rehabilitation of a c. 1930 high school auditorium for a clinic and offices proposed two additions: a one-story entrance and 

reception area on this elevation (a); and a four-story elevator and stair tower on another side (b). The gabled entrance (c) first proposed was not 

compatible with the flat-roofed auditorium and the design of the proposed stair tower (d) was also incompatible and overwhelmed the historic 

building. The designs were revised (e-fJ resulting in new additions that meet the Standards (g-h). 

9 
10 

Incompatible New Additions to Historic Buildings 

New Addition 

Figure 16. The proposal to add three row houses to the rear ell of this early-19th century 

residential property doubles its size and does not meet the Standards .. 

Figure 17. The small addition on the left is 

starkly different and it is not compatible with 

the eclectic, late-19th century house. 

----

Figure 19. The upper two floors of this early-20th century 

office building were part of the original design, but were 

not built. During rehabilitation, the two stories were finally 

constructed. This treatment does not meet the Standards 

because the addition has given the building an appearance it 

never had historically. 

New Addition 

Figure 20. The height, as 

well as the design, of these 

two-story rooftop additions 

overwhelms the two-story 

and the one-story, low-rise 

historic buildings. 

Figure 18. The expansion 

of a one- and one-half story 

historic bungalow (left) 

with a large two-story rear 

addition (right) has greatly 

altered and obscured its 

distinctive shape and form. 
New Additions in Densely-Built 

Environments 

In built-up urban areas, locating a new 

addition on a less visible side or rear 

elevation may not be possible simply 

because there is no available space. In this 

instance, there may be alternative ways to 

help preserve the historic character. One 

approach when connecting a new addition 

to a historic building on a primary elevation 

is to use a hyphen to separate them. A 

subtle variation in material, detailing 

and color may also provide the degree of 

differentiation necessary to avoid changing 

the essential proportions and character of 

the historic building. 

A densely-built neighborhood such as 

a downtown commercial core offers a 

particular opportunity to design an addition 

that will have a minimal impact on the 

historic building. Often the site for such 

an addition is a vacant lot where another 

building formerly stood. Treating the 

addition as a separate or infill building 

may be the best approach when designing 

an addition that will have the least impact 

on the historic building and the district. In 

these instances there may be no need for a 

direct visual link to the historic building. 

Height and setback from the street should 

generally be consistent with those of the 

historic building and other surrounding 

buildings in the district. Thus, in most 

urban commercial areas the addition 

should not be set back from the fa<;:ade of 

the historic building. A tight urban setting 

may sometimes even accommodate a larger 

addition if the primary elevation is designed 

to give the appearance of being several 

buildings by breaking up the facade into 

elements that are consistent with the scale of 

the historic building and adjacent buildings. 

New Addition 

Figure 21. Both wings of this historic L-shaped building (top), which 

fronts on two city streets, adjoined vacant lots. A two-story addition was 

constructed on one lot (above, left) and a six-story addition was built on 

the other (above, right). Like the historic building, which has two different 

facades, the compatible new additions are also different and appear to be 

separate structures rather than part of the historic building. 

Figure 22. The proposed new addition is compatible with the historic buildings that remain on the block. 

Its design with multiple storefronts helps break up the mass. 

11 
12 

Rooftop Additions 

The guidance provided on designing a compatible new 

addition to a historic building applies equally to new 

rooftop additions. A rooftop addition should preserve 

the character of a historic building by preserving historic 

materials, features and form; and it should be compatible 

but differentiated from the historic building. 

However, there are several other design principles that 

apply specifically to rooftop additions. Generally, a 

rooftop addition should not be more than one story in 

height to minimize its visibility and its impact on the 

proportion and profile of the historic building. A rooftop 

addition should almost always be set back at least one full 

bay from the primary elevation of the building, as well as 

from the other elevations if the building is free-standing or 

highly visible. 

It is difficult, if not impossible, to minimize the impact 

of adding an entire new floor to relatively low buildings, 

such as small-scale residential or commercial structures, 

even if the new addition is set back from the plane of 

the fac;ade. Constructing another floor on top of a small, 

one, two or three-story building is seldom appropriate 

for buildings of this size as it would measurably alter 

the building's proportions and profile, and negatively 

impact its historic character. On the other hand, a rooftop 

addition on an eight-story building, for example, in a 

historic district consisting primarily of tall buildings 

might not affect the historic character because the new 

construction may blend in with the surrounding buildings 

and be only minimally visible within the district. A 

rooftop addition in a densely-built urban area is more 

likely to be compatible on a building that is adjacent to 

similarly-sized or taller buildings. 

A number of methods may be used to help evaluate the 

effect of a proposed rooftop addition on a historic building 

and district, including pedestrian sight lines, threedimensional schematics and computer-generated design. 

However, drawings generally do not provide a true 

"picture" of the appearance and visibility of a proposed 

rooftop addition. For this reason, it is often necessary to 

construct a rough, temporary, full-size or skeletal mock up 

of a portion of the proposed addition, which can then be 

photographed and evaluated from critical vantage points 

on surrounding streets. 

Figure 23. Colored flags marking the location of a proposed penthouse 

addition (a) were placed on the roof to help evaluate the impact and 

visibility of an addition planned for this historic furniture store (b). 

Based on this evaluation, the addition was constructed as proposed. 

It is minimally visible and compatible with the 1912 structure (c). 

The tall parapet wall conceals the addition from the street below (d). 
Figure 24. How to Evaluate a Proposed Rooftop Addition. 

A sight-line study (above) only factors in views from directly across the 

street, which can be very restrictive and does not illustrate the full effect 

of an addition from other public rights of way. A mock up (above, right) 

or a mock up enhanced by a computer-generated rendering (below, 

right) is essential to evaluate the impact of a proposed rooftop addition 

on the historic building. 

Figure 25. It was possible to add a compatible, three-story, 

penthouse addition to the roof of this five-story, historic bank 

building because the addition is set far back, it is surrounded 

by taller buildings and a deep parapet conceals almost all of the 

addition from be/ow. 

Figure 26. A rooftop addition 

would have negatively 

impacted the character of the 

primary facade (right) of this 

mid-19th century, four-story 

structure and the low-rise 

historic district. However, a 

third floor was successfully 

added on the two-story rear 

portion (be/ow) of the same 

building with little impact to 

the building or the district 

because it blends in with the 

height of the adjacent building. 

13 
14 

Figure 27. Although the new brick stair/elevator tower (left) is not visible from the front (right), it is on a prominent side elevation of this 1890 stone 

bank. The compatible addition is set back and does not compete with the historic building. Photos: Chadd Gossmann, Aurora Photography, LLC. 

Designing a New Exterior Addition to a Historic Building 

This guidance should be applied to help in designing 

a compatible new addition that that will meet the 

Secretary of the Interior's Standards for Rehabilitation: 

• A new addition should be simple and 

unobtrusive in design, and should be 

distinguished from the historic building-a 

recessed connector can help to differentiate the 

new from the old. 

• A new addition should not be highly visible from 

the public right of way; a rear or other secondary 

elevation is usually the best location for a new 

addition. 

• The construction materials and the color of the 

new addition should be harmonious with the 

historic building materials. 

• The new addition should be smaller than the 

historic building-it should be subordinate in 

both size and design to the historic building. 

The same guidance should be applied when 

designing a compatible rooftop addition, plus 

the following: 

• A rooftop addition is generally not appropriate 

for a one, two or three-story building-and 

often is not appropriate for taller buildings. 

• A rooftop addition should be minimally visible. 

• Generally, a rooftop addition must be set back 

at least one full bay from the primary elevation 

of the building, as well as from the other 

elevations if the building is freestanding or 

highly visible. 

• Generally, a rooftop addition should not be 

more than one story in height. 

• Generally, a rooftop addition is more likely to 

be compatible on a building that is adjacent to 

similarly-sized or taller buildings. 

Figure 28. A small addition 

(left) was constructed when 

this 1880s train station was 

converted for office use. The 

paired doors with transoms 

and arched windows on the 

compatible addition reflect, but 

do not replicate, the historic 

building (right). 
Summary 

Figure 29. This simple 

glass and brick entrance 

(left) added to a secondary 

elevation of a 1920s 

school building (right) 

is compatible with the 

original structure. 

Because a new exterior addition to a historic building can damage or destroy significant materials and can change the 

building's character, an addition should be considered only after it has been determined that the new use cannot be 

met by altering non-significant, or secondary, interior spaces. If the new use cannot be met in this way, then an attached 

addition may be an acceptable alternative if carefully planned and designed. A new addition to a historic building should 

be constructed in a manner that preserves significant materials, features and form, and preserves the building's historic 

character. Finally, an addition should be differentiated from the historic building so that the new work is compatible 

with - and does not detract from - the historic building, and cannot itself be confused as historic. 

Additional Reading 

Byard, Paul Spencer. The Architecture of New Additions: Design 

and ReguLation. New York, NY: W.W. Norton & Company, 1998. 

Day, Steven, AlA. "Modernism Meets History: New 

Additions to Historic Structures." Preservation Seattle [Historic 

Seattle's online monthly preservation magazine.] May 2003. 

www.historicseattle.orglpreservationseattle/publicpolicy/ 

defaultmay2.htm. 

Incentives! A Guide to the Federal Historic Preservation Tax 

Incentives Program for Income-Producing Properties. "Avoiding 

Incompatible Treatments: New Additions & Rooftop 

Additions." Technical Preservation Services Branch, National 

Park Service. Online at www.nps.gov/history/hps/tps/. 

Interpreting the Standards Bulletins (ITS). Technical Preservation 

Services Branch, National Park Service. Online at 

www.nps.gov/history/hps/tps/. 

New Additions to Historic Buildings. Technical Preservation 

Services Branch, National Park Service. Online at www.nps. 

gov /history/hps/tps/. 

O'Connell, Kim A. "Making Connections." Traditional Building. 

March/April 2004. (Vol. 17, No.2), pp. 12-15. 

The Secretary of the Interior's Standards for Rehabilitation and 

Guidelines for Rehabilitating Historic Buildings. Washington, 

D.C.: U.S. Department of the Interior, National Park Service, 

Preservation Assistance Division, rev. 1990. 

The Secretary of the Interior's Standards for Rehabilitation & 

Illustrated Guidelines for Rehabilitating Historic Buildings. 

(Authors: W. Brown Morton, III, Gary L. Hume, Kay D. Weeks, 

and H. Ward Jandl. Project Directors: Anne E. Grimmer 

and Kay D. Weeks.) Washington, D.C.: U.S. Department of 

the Interior, National Park Service, Preservation Assistance 

Division, 1992. Online at www.nps.gov/history/hps/tps/. 

Semes, Steven W. "Differentiated and Compatible: The 

Secretary's Standards revisited." Traditional Building. February 

2009. (Vol. 22, No.1), pp. 20-23. 

Semes, Steven W. The Future of the Past: A Conservation Ethic for 

Architecture, Urbanism, and Historic Preservation. (In association 

with The Institute of Classical Architecture and Classical 

America.) New York, NY: W.w. Norton & Company, 2009. 

Figure 30. The small addition on the right of this late-19th century 

commercial structure is clearly secondary and compatible in size, 

materials and design with the historic building. 

15 
16 

Acknowledgements 

Figure 31. An elevator/stair tower 

was added at the back of this 

Richardsonian Romanesque-style 

theater when it was rehabilitated. 

Rough-cut stone and simple 

cut-out openings ensure that 

the addition is compatible and 

subordinate to the historic building. 

Photo: Chuck Liddy, AlA. 

Anne E. Grimmer, Senior Architectural Historian, Technical Preservation Services Branch, National Park Service, 

revised Preservation Brief 14, written by Kay D. Weeks and first published in 1986. The revised Brief features all new 

illustrations and contains expanded and updated design guidance on the subject of new additions that has been 

developed by the Technical Preservation Services Branch since the original publication of the Brief. Several individuals 

generously contributed their time and expertise to review the revision of this Preservation Brief including: Sharon 

C. Park, FAIA, Chief, Architectural History and Historic Preservation, Smithsonian Institution; Elizabeth Tune and 

Karen Brandt, Department of Historic Resources, Commonwealth of Virginia; and Phillip Wisley and David Ferro, 

Division of Historical Resources, Florida Department of State. The Technical Preservation Services professional staff, in 

particular Michael J. Auer, Jo Ellen Hensley, Gary Sachau and Rebecca Shiffer, also provided important guidance in the 

development of this publication. All illustrations are from National Park Service files unless otherwise credited. Front 

cover image: Detail of new addition shown in Figure 4. Photo: © Maxwell MacKenzie. 

This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which 

directs the Secretary of the Interior to develop and make available information concerning historic properties. The 

Technical Preservation Services Branch, National Park Service, prepares standards, guidelines and other educational 

materials on responsible historic preservation treatments for a broad public audience. Additional information 

about the programs of Technical Preservation Services is available on the website at www.nps.govlhistorylhps/tps. 

Comments about this publication should be addressed to: Charles E. Fisher, Technical Preservation Publications 

Program Manager, Technical Preservation Services-2255, National Park Service, 1849 C Street, NW, Washington, DC 

20240. This publication is not copyrighted and can be reproduced without penalty. Normal procedures for credit to the 

author and the National Park Service are appreciated. 

ISBN: 978-0-16-085869-7 

U.S. Government Printing Office Stock Number: 024-005-01280-0 August 2010 
15 PRESERVATION 

BRIEFS 

Preservation of Historic Concrete 

Paul Gaudette and Deborah Slaton 

National Park Service 

U.S. Department of the Interior 

Introduction to Historic Concrete 

Concrete is an extraordinarily versatile building material 

used for utilitarian, ornamental, and monumental 

structures since ancient times. Composed of a mixture 

of sand, gravel, crushed stone, or other coarse material, 

bound together with lime or cement, concrete undergoes 

a chemical reaction and hardens when water is added. 

Inserting reinforcement adds tensile strength to 

structural concrete elements. The use of reinforcement 

contributes significantly to the range and size of 

building and structure types that can be constructed 

with concrete. 

While early twentieth century proponents of modern 

concrete often considered it to be permanent, it is, 

like all materials, subject to deterioration. This Brief 

provides an overview of the history of concrete and 

its popularization in the United States, surveys the 

principal causes and modes of concrete deterioration, 

and outlines approaches to repair and protection that 

are appropriate to historic concrete. In the context of this 

Brief, historic concrete is considered to be concrete used 

in construction of structures of historical, architectural, 

or engineering interest, whether those structures are old 

or relatively new. 

Brief History of Use and Manufacture 

The ancient Romans found that a mixture of lime putty 

and pozzolana, a fine volcanic ash, would harden 

under water. The resulting hydraulic cement became 

a major feature of Roman building practice, and was 

used in many buildings and engineering projects 

such as bridges and aqueducts. Concrete technology 

was kept alive during the Middle Ages in Spain and 

Africa. The Spanish introduced a form of concrete to 

the New World in the first decades of the sixteenth 

century, referred to as "tapia" or "tabby." This material, 

a mixture of lime, sand, and shell or stone aggregate 

mixed with water, was placed between wooden forms, 

tamped, and allowed to dry in successive layers. Tabby 

was later used by the English settlers in the coastal 

southeastern United States. 

The early history of concrete was fragmented, 

with developments in materials and construction 

techniques occurring on different continents and in 

various countries. In the United States, concrete was 

slow in achieving widespread acceptance in building 

construction and did not begin to gain popularity until 

the late nineteenth century. It was more readily accepted 

for use in transportation and infrastructure systems. 

The Erie Canal in New York is an example of the 

early use of concrete in transportation in the United 

States. The natural hydraulic cement used in the canal 

construction was processed from a deposit of limestone 

found in 1818 near Chittenango, southeast of Syracuse. 

The use of concrete in residential construction was 

Figure 1. The Sebastopol House in Seguin, Texas, is an 1856 Greek 

Revival-style house constructed of lime concrete. Lime concrete 

or "limecrete" was a popular construction material, as it could be 

made inexpensively from local materials. By 1900, the town had 

approximately ninety limecrete structures, twenty of which remain. 

Photo: Texas Parks and Wildlife Department. 

1 
2 

Figure 2. Chatterton House was the home of the post trader at Fort 

Fred Steel in Wyoming, one of several forts established in the 1860s 

to protect the Union Pacific Railroad. The walls of the post trader's 

house were built using stone aggregate and lime, without cement. 

The use of this material presents special preservation challenges. 

publicized in the second edition of Orson S. Fowler's A 

Home for All (1853) which described the advantages of 

"gravel wall" construction to a wide audience. The town 

of Seguin, Texas, thirty-five miles east of San Antonio, 

already had a number of concrete buildings by the 1850s 

and came to be called "The Mother of Concrete Cities," 

with approximately ninety concrete buildings made 

from local "lime water" and gravel (Fig. 1). 

Impressed by the economic advantages of poured gravel 

wall or "lime-grout" construction, the Quartermaster 

General's Office of the War Department embarked on a 

campaign to improve the quality of building for frontier 

military posts. As a result, lime-grout structures were 

constructed at several western posts soon after the Civil 

War, including Fort Fred Steele and Fort Laramie, both 

in Wyoming (Fig. 2). By the 1880s, sufficient experience 

had been gained with unreinforced concrete to permit 

construction of much larger buildings. A notable 

example from this period is the Ponce de Leon Hotel in 

St. Augustine, Florida. 

Figure 3. The Lincoln Highway Association promoted construction of 

a high quality continuous hard surface roadway across the country. 

The Boys Scouts of America installed concrete road markers along the 

Lincoln Highway in 1928. 

Extensive construction in concrete also occurred through 

the system of coastal fortifications commissioned by the 

federal government in the 1890s for the Atlantic, Pacific, 

and Gulf coasts. Unlike most concrete construction 

to that time, the special requirements of coastal 

fortifications called for concrete walls as much as 20 feet 

thick, often at sites that were difficult to access. Major 

structures in the coastal defenses of the 1890s were built 

of mass concrete with no internal reinforcing, a practice 

that was replaced by the use of reinforcing bars in 

fortifications constructed after about 1905. 

The use of reinforced concrete in the United States dates 

from 1860, when S.T. Fowler obtained a patent for a 

reinforced concrete wall. In the early 1870s, William E. 

Ward built his own house in Port Chester, New York, 

using concrete reinforced with iron rods for all structural 

elements. Despite these developments, such construction 

remained a novelty until after 1880, when innovations 

introduced by Ernest L. Ransome made the use of 

reinforced concrete more practicable. Ransome made 

many contributions to the development of concrete 

construction technology, including the use of twisted 

reinforcing bars to improve bond between the concrete 

and the steel, which he patented in 1884. Two years later, 

Ransome introduced the rotary kiln to United States 

cement production. The new kiln had greater capacity 

and burned more thoroughly and uniformly, allowing 

development of a less expensive, more uniform, and 

more reliable manufactured cement. Improvements in 

concrete production initiated by Ransom led to a much 

greater acceptance of concrete after 1900. 

The Lincoln Highway Association, incorporated in 

1913, promoted the use of concrete in construction of a 

coast-to-coast roadway system. The goal of the Lincoln 

Highway Association and highway advocate Henry 

B. Joy was to educate the country in the need for good 

roads made of concrete, with an improved Lincoln 

Figure 4. The highly ornamental concrete panels on the exterior 

facade of the Baha'i House of Worship in Wilmette, Illinois, illustrate 

the work of fabricator John J. Earley, known as "the man who made 

concrete beautiful. " 
Figure 5. Following World War II, architects and engineers took 

advantage of improvements in concrete production, quality control, 

and advances in precast concrete to design structures such as the Police 

Headquarters building in Philadelphia, Pennsylvania, constructed in 

1961. Photo: Courtesy of the Philadelphia Police Department. 

Highway as an example. Concrete "seedling miles" 

were constructed in remote areas to emphasize the 

superiority of concrete over unimproved dirt. The 

Association believed that as people learned about 

concrete, they would press the government to construct 

good roads throughout their states. Americans' 

enthusiasm for good roads led to the involvement 

of the federal government in road-building and the 

creation of numbered U.S. routes in the 1920s (Fig. 3). 

During the early twentieth century, Ernest Ransome 

in Beverly, Massachusetts, Albert Kahn in Detroit, and 

Richard E. Schmidt in Chicago, promoted concrete 

for use in "Factory Style" utilitarian buildings with 

an exposed concrete frame infilled with expanses 

of glass. Thomas Edison's cast-in-place reinforced 

concrete homes in Union Township, New Jersey 

(1908), proclaimed a similarly functional emphasis 

in residential construction. From the 1920s onward, 

concrete began to be used with spectacular design 

results: examples include John J. Earley's Meridian 

Hill Park in Washington, D.C.; Louis Bourgeois' 

exuberant, graceful Baha'i Temple in Wilmette, Illinois 

(1920-1953), for which Earley fabricated the concrete 

(Fig. 4); and Frank Lloyd Wright's Fallingwater 

near Bear Run, Pennsylvania (1934). Continuing 

improvements in quality control and development 

of innovative fabrication processes, such as the 

Shockbeton method for precast concrete, provided 

increasing opportunities for architects and engineers. 

Wright's Guggenheim Museum in New York City 

(1959); Geddes Brecher Qualls & Cunningham'S Police 

Headquarters building in Philadelphia, Pennsylvania 

(1961); and Eero Saarinen's soaring terminal building at 

Dulles International Airport outside Washington, D.C., 

and the TWA terminal at Kennedy Airport in New 

York (1962), exemplify the masterful use of concrete 

achieved in the modern era (Fig. 5). 

Figure 6. The Bailey Magnet School in Jackson, Mississippi, was 

designed as the Jackson Junior High School by the firm of N. W. 

Overstreet & Town in 1936. The streamlined building exemplifies the 

applicability of concrete to creating a modern architectural aesthetic. 

Photo: Bill Burris, Burris/Wagnon Architects, P.A. 

Figure 7. Detailed bas reliefs as well as sculptures, such as this lion at 

the Bailey Magnet School, could be used as ornamentation on concrete 

buildings. Sculptural concrete elements were typically cast in molds. 

Throughout the twentieth century, a wide range of 

architectural and engineering structures were built using 

concrete as a practical and cost-effective choice-and 

concrete also became valued for its aesthetic qualities. 

Cast in place and precast concrete were readily adapted 

to the Streamlined Moderne style, as exemplified by the 

Bailey Magnet School in Jackson, Mississippi, designed 

as the Jackson Junior High School by N.W. Overstreet 

& Town in 1936 (Figs. 6 and 7). The school is one of 

many concrete buildings designed and constructed 

under the auspices of the Public Works Administration. 

Recreational structures and landscape features also 

utilized the structural range and unique character of 

exposed concrete to advantage, as seen in Chicago'S 

Lincoln Park Chess Pavilion, designed by Morris 

Webster in 1956 (Fig. 8), and the Ira C. Keller Fountain 

in Portland Oregon, designed by Lawrence Halprin in 

1969 (Fig. 9). Concrete was also popular for building 

interiors, with ornamental features and exposed 

structural elements recognized as part of the design 

aesthetic (See Figs. 10 and 11 in sidebar). 

3 
4 

Historic Interiors 

The expanded use of concrete provided new opportunities to 

create dramatic spaces and ornate architectural detail on the 

interiors of buildings, at a significant cost savings over traditional 

construction practices. The architectural design of the Berkeley 

City Club in Berkeley, California, expressed Moorish and Gothic 

elements in concrete on the interior of the building (Fig. 10). Used 

as a woman's social club, the building was designed by noted 

California architect Julia Morgan and constructed in 1929. The 

vaulted ceilings, columns, and ornamental capitals of the lobby 

and the ornamental arches and beamed ceiling of the "plunge" are 

all constructed of concrete. 

Figure 10. The Berkeley City Club has significant interior spaces alld features of 

concrete construction, including the lobby and pool. Photos: Una Gilmartin (left) 

and Brian Kehoe (right), Wiss, Janney, Elstner Associates, Illc. 

The historic character of a building's interior can also be conveyed 

in a more utilitarian manner in terms of concrete features and 

finishes (Fig. 11). The exposed concrete structure-columns, 

capitals, and drop panels-is an integral part of the character 

of this old commercial building in Minneapolis. In concrete 

warehouse and factory buildings of the early twentieth century, 

exposed concrete columns and formboard finish concrete slab 

ceilings are common features as seen in this warehouse, now 

converted for use as a parking garage and shops. 

Figure 11. Whether in a circa 1925 office (left) or in a parking garage 

and retail facility (right), exposed concrete structures help characterize 

these building interiors. Photo: Minnesota Historical Society (left). 

Concrete Characteristics 

Concrete is composed of fine (sand) and 

coarse (crushed stone or gravel) aggregates 

and paste made of portland cement and water. 

The predominant material in terms of bulk is 

the aggregate. Portland cement is the binder 

most commonly used in modern concrete. It 

is commercially manufactured by blending 

limestone or chalk with clays that contain 

alumina, silica, lime, iron oxide and magnesia, 

and heating the compounds together to high 

temperatures. The hydration process that 

occurs between the portland cement and water 

results in formation of an alkali paste that 

surrounds and binds the aggregate together as 

a solid mass. 

The quality of the concrete is dependent on 

the ratio of water to the binder; binder content; 

sound, durable, and well-graded aggregates; 

compaction during placement; and proper 

curing. The amount of water used in the mix 

affects the concrete permeability and strength. 

The use of excess water beyond that required 

in the hydration process results in more 

permeable concrete, which is more susceptible 

to weathering and deterioration. Admixtures 

are commonly added to concrete to adjust 

concrete properties such as setting or hardening 

time, requirements for water, workability, and 

other characteristics. For example, the advent 

of air entraining agents in the 1930s provided 

enhanced durability for concrete. 

During the twentieth century, there was 

a steady rise in the strength of ordinary 

concrete as chemical processes became better 

understood and quality control measures 

improved. In addition, the need to protect 

embedded reinforcement against corrosion 

was acknowledged. Requirements for concrete 

cover over reinforcing steel, increased cement 

content, decreased water-cement ratio, and air 

entrainment all contributed to greater concrete 

strength and improved durability. 

Mechanisms and Modes of 

Deterioration 

Causes of Deterioration 

Concrete deterioration occurs primarily because 

of corrosion of the embedded steel, degradation 

of the concrete itself, use of improper techniques 

or materials in construction, or structural 

problems. The causes of concrete deterioration 

must be understood in order to select an 

appropriate repair and protection system. 
While reinforcing steel has played a pivotal role in 

expanding the applications of concrete in twentieth 

century architecture, corrosion of this steel has also 

caused deterioration in many historic structures. 

Reinforcing steel embedded in the concrete is normally 

surrounded by a passivating 

oxide layer that, when present, 

protects the steel from corrosion 

and aids in bonding the 

steel and concrete. When the 

concrete's normal alkaline 

environment (above a pH 

of 10) is compromised and 

the steel is exposed to water, 

water vapor, or high relative 

humidity, corrosion of the 

Lack of proper maintenance of building elements 

such as roofs and drainage systems can contribute to 

water-related deterioration of the adjacent concrete, 

particularly when concrete is saturated with water 

and then exposed to freezing temperatures. As water 

within the concrete freezes, it 

expands and exerts forces on 

the adjacent concrete. Repeated 

freezing and thawing can result 

in the concrete cracking and 

delaminating. Such damage 

appears as surface degradation, 

including severe scaling and 

micro-cracking that extends 

into the concrete. The condition 

is most often observed near 

steel reinforcing takes place. A 

reduction in alkalinity results 

from carbonation, a process that 

occurs when the carbon dioxide 

in the atmosphere reacts with 

calcium hydroxide and moisture 

in the concrete. Carbonation 

starts at the concrete's exposed 

surface but may extend to the 

reinforcing steel over time. 

When carbonation reaches 

Figure 8. The Chess Pavilion in Chicago'S Lincoln Park 

was designed by architect Morris Webster and constructed 

in 1956. The pavilion is a distinctive landscape feature, 

with its reinforced concrete cantilevered slab that provides 

cover for chess players. 

the surface of the concrete but 

can also eventually occur deep 

within the concrete. This type 

of deterioration is usually most 

severe at joints, architectural 

details, and other areas with 

more surface exposure to 

weather. In the second half of 

the twentieth century, concrete 

has utilized entrained air (the 

incorporation of microscopic 

air bubbles) to provide 

enhanced protection against 

damage due to cyclic freezing 

of saturated concrete. 

the metal reinforcement, the 

concrete no longer protects the 

steel from corrosion. 

Corrosion of embedded 

reinforcing steel may be 

initiated and accelerated if 

calcium chloride was added to 

the concrete as a set accelerator 

during original construction 

to promote more rapid curing. 

It may also take place if the 

concrete is later exposed to 

deicing salts, as may occur 

during the winter in northern 

climates. Seawater or other 

marine environments can 

Figure 9. The Ira C. Keller Fountain in Portland, Oregon, 

was designed by Lawrence Halprin and constructed in 

1969. The fountain is constructed primarily of concrete 

pillars with formboard textures and surrounding elements, 

patterned with geometric lines, which fa cilitate the path 

The use of certain aggregates 

can also result in deterioration 

of the concrete. Alkaliaggregate reactions-in some 

cases alkali-silica reaction 

(ASR)-occur when alkalis 

normally present in cement 

react with certain aggregates, 

leading to the development of 

an expansive crystalline gel. 

When this gel is exposed to 

moisture, it expands and causes of water. Photo: Anita Washko, Wiss, Janney, Elstner 

Associates, Inc. cracking of the aggregate and 

concrete matrix. Deleterious also provide large amounts 

of chloride, either from 

inadequately washed original aggregate or from 

exposure of the concrete to seawater. 

Corrosion-related damage to reinforced concrete is 

the result of rust, a product of the corrosion process of 

steel, which expands and thus requires more space in 

the concrete than the steel did at the time of installation. 

This change in volume of the steel results in expansive 

forces, which cause cracking and spalling of the 

adjacent concrete (Fig. 12). Other signs of corrosion of 

embedded steel include delamination of the concrete 

(planar separations parallel to the surface) and rust 

staining (often a precursor to spalling) on the concrete 

near the steel. 

aggregates are typically found only in certain areas of 

the country and can be detected through analysis by an 

experienced petrographer. Low-alkali cements as well 

as fly ash are used today in new construction to prevent 

such reactions where this problem may occur. 

Problems Specifically Encountered with 

Historic Concrete 

Materials and workmanship used in the construction 

of historic concrete structures, particularly those built 

before the First World War, sometimes present potential 

sources of problems. For example, where the aggregate 

consisted of cinder from burned coal or crushed brick, 

5 
6 

Figure 12. The concrete lighthouse at the Kilauea Point 

Light Station, Kilauea, Kauai, Hawaii, was constructed 

circa 1913. The concrete, which was a good quality, high 

strength mix for its day, is in good condition after almost 

one hundred years in service. Deterioration in the form of 

spalling related to corrosion of embedded reinforcing steel 

has occurred primarily in areas of higher ornamentation 

such as projecting bands and brackets (see close-up photo). 

the concrete tends to be weak and porous 

because these aggregates absorb water. Some of 

these aggregates can be extremely susceptible 

to deterioration when exposed to moisture 

and cyclic freezing and thawing. Concrete 

was sometimes compromised by inclusion of 

seawater or beach sand that was not thoroughly 

washed with fresh water, a condition more 

common with coastal fortifications built prior to 

1900. The sodium chloride present in seawater 

and beach sand accelerates the rate of corrosion 

of the reinforced concrete. 

Another problem encountered with historic 

concrete is related to poor consolidation of the 

concrete during its placement in forms, or in molds in the case 

of precasting. This problem is especially prevalent in highly 

ornamental units. Early twentieth century concrete was often 

tamped or rodded into place, similar to techniques used in 

forming cast stone. Poorly consolidated concrete often contains 

voids (lfbugholes" or "honeycombs"), which can reduce the 

protective concrete cover over the embedded reinforcing 

bars, entrap water, and, if sufficiently large and strategically 

numerous, reduce localized concrete strength. Vibration 

technology has improved over time and flowability agents are 

also used today to address this problem. 

A common type of deterioration observed in concrete is the 

effect of weathering from exposure to wind, rain, snow, and 

salt water or spray. Weathering appears as erosion of the 

cement paste, a condition more prevalent in northern regions 

where precipitation can be highly acidic. This results in the 

exposure of the aggregate particles on the exposed concrete 

surface. Variations may occur in the aggregate exposure due 

to differential erosion or dissolution of exposed cement paste. 

Erosion can also be caused by the mechanical action of water 

channeled over concrete, such as by the lack of drip grooves in 

belt courses and sills, and by inadequate drainage. In addition, 

high-pressure water when used for cleaning can also erode the 

concrete surface. 

In concrete structures built prior to the First World War, 

concrete was often placed into forms in relatively short 

vertical lifts due to limitations in lifting and pouring 

techniques available at the time. Joints between different 

concrete placements (often termed cold joints or lift lines) may 

sometimes be considered an important part of the character of 

a concrete element (Fig. 13). However, wide joints may permit 

water to infiltrate the concrete, resulting in more rapid paste 

erosion or freeze-thaw deterioration of adjacent concrete in 

cold climates. 

In the early twentieth century, concrete was sometimes placed 

in several layers parallel to the exterior surface. A base concrete 

was first created with form work and then a more cement rich 

mortar layer was applied to the exposed vertical face of the 

Figure 13. Fort Casey on Admiralty Head, Fort Casey, Washington, was 

constructed in 1898. The lift lines from placement of concrete are clearly 

visible on the exterior walls and characterize the finished appearance. 
base concrete. The higher cement content in the facing 

concrete provided a more water-resistant outer layer 

and finished surface. The application of a cement-rich 

top layer, referred to in some early concrete publications 

as "waterproofing," was also used on top surfaces of 

concrete walls, or as the top layer in sidewalks. With this 

type of concrete construction, deterioration can occur 

over time as a result of debonding between layers, and 

can proceed very rapidly once the protective cement-rich 

layer begins to break down. 

It is common for historic concrete to have a highly 

variable appearance, including color and finish texture. 

Different levels of aggregate exposure due to paste 

erosion are often found in exposed aggregate concrete. 

This variability in the appearance of historic concrete 

increases the level of difficulty in assessing and repairing 

weathered concrete. 

Signs of Distress and Deterioration 

Characteristic signs of failure in concrete include 

cracking, spalling, staining, and deflection. Cracking 

occurs in most concrete but will vary in depth, width, 

direction, pattern, and location, and can be either active 

or dormant (inactive). Active cracks can widen, deepen, 

or migrate through the concrete, while dormant cracks 

remain relatively unchanged in size. Some dormant 

cracks, such as those caused by early age shrinkage of 

the concrete during curing, are not a structural concern 

but when left unrepaired, can provide convenient 

channels for moisture penetration and subsequent 

damage. Random surface cracks, also called map cracks 

due to their resemblance to lines on a map, are usually 

related to early-age shrinkage but may also indicate 

other types of deterioration such as alkali-silica reaction. 

Structural cracks can be caused by temporary or 

continued overloads, uneven foundation settling, seismic 

forces, or original design inadequacies. Structural cracks 

are active if excessive loads are applied to a structure, if 

the overload is continuing, or if settlement is ongoing. 

These cracks are dormant if the temporary overloads 

have been removed or if differential settlement has 

stabilized. Thermally-induced cracks result from 

stresses produced by the expansion and contraction 

of the concrete during temperature changes. These 

cracks frequently occur at the ends or re-entrant corners 

of older concrete structures that were built without 

expansion joints to relieve such stress. 

Spalling (the loss of surface material) is often associated 

with freezing and thawing as well as cracking and 

delamination of the concrete cover over embedded 

reinforcing steel. Spalling occurs when reinforcing 

bars corrode and the corrosion by-products expand, 

creating high stresses on the adjacent concrete, which 

cracks and is displaced. Spalling can also occur when 

water absorbed by the concrete freezes and thaws (Fig. 

14). In addition, surface spalling or scaling may result 

from the improper finishing, forming, or other surface 

Figures 14. Layers of architectural concrete that have debonded 

(spaUed) from the surface were removed from a historic water tank 

during the investigation performed to assess existing conditions. 

Photos: Anita Washko, Wiss, Janney, Elstner Associates, Inc. 

phenomena when water-rich cement paste (laitance) 

rises to the surface. The resulting weak material is 

vulnerable to spalling of thin layers, or scaling. In some 

cases, spalling of the concrete can diminish the loadcarrying capacity of the structure. 

Deflection is the bending or sagging of structural beams, 

joists, or slabs, and can be an indication of deficiencies in 

the strength and structural soundness of concrete. This 

condition can be produced by overloading, corrosion 

of embedded reinforcing, or inadequate design or 

construction, such as use of low-strength concrete or 

undersized reinforcing bars. 

Staining of the concrete surface can be related to soiling 

from atmospheric pollutants or other contaminants, 

dirt accumulation, and the presence of organic growth. 

However, stains can also indicate more serious 

underlying problems, such as corrosion of embedded 

reinforcing steel, improper previous surface treatments, 

alkali-aggregate reaction, or efflorescence, the deposition 

of soluble salts on the surface of the concrete as a result 

of water migration (Fig. 15). 

7 
8 

Planning for Concrete Preservation 

The significance of a historic concrete building or 

structure-including whether it is important for its 

architectural or engineering design, for its materials 

and construction techniques, or both-guides decision 

making about repair and, if needed, replacement 

methods. Determining the causes of deterioration is also 

central to the development of a conservation and repair 

plan. With historic concrete buildings, one of the more 

difficult challenges is allowing for sufficient time during 

the planning phase to analyze the concrete, develop 

mixes, and provide time for adequate aging of mock-ups 

for matching to the original concrete. 

An understanding of the original construction 

techniques (cement characteristics, mix design, original 

intent of assembly, type of placement, precast versus 

cast in place, etc.) and previous repair work performed 

on the concrete is important in determining causes 

of existing deterioration and the susceptibility of the 

structure to potential other types of deterioration. 

For example, concrete placed in short lifts (individual 

concrete placements) or constructed in precast segments 

will have numerous joints that can provide entry points 

for water infiltration. Inappropriate prior repairs, such 

as installation of patches using an incompatible material, 

can affect the future performance of the concrete. Such 

prior repairs may require corrective work. 

As with other preservation projects, three primary 

approaches are usually considered for historic concrete 

structures: maintenance, repair, or replacement. 

Maintenance and repair best achieve the preservation 

goal of minimal intervention and the greatest retention 

of existing historic fabric. However, where elements of 

the building are severely deteriorated or where inherent 

problems with the material lead to ongoing failures, 

replacement may be necessary. 

During planning, information is gathered through 

research, visual survey, inspection openings, and 

laboratory studies. The material should then be 

reviewed by professionals experienced in concrete 

deterioration to help evaluate the nature and causes of 

the concrete problems, to assess both the short-term and 

long-term effects of the deterioration, and to formulate 

proper repair approaches. 

Condition Assessment 

A condition assessment of a concrete building or 

structure should begin with a review of all available 

documents related to original construction and prior 

repairs. While plans and specifications for older 

concrete buildings are not always available, they can 

be an invaluable resource and every attempt should be 

made to find them. They may provide information on 

the composition of the concrete mix or on the type and 

location of reinforcing bars. If available, documents 

related to past repairs should also be reviewed to 

Figure 15. Evidence of 

moisture movement through 

concrete is apparent 

in the form of mineral 

deposits on the concrete 

surface. Cyclic freezing 

and thawing of entrapped 

moisture, and corrosion of 

embedded reinforcement, 

have also contributed to 

deterioration of the concrete 

column on this fence at 

Crocker Field in Fitchburg, 

Massachusetts, designed by 

the Olmsted Brothers. 

understand how the repairs were made and to help 

evaluate their anticipated performance and service life. 

Archival photographs can also provide a valuable source 

of information about original construction. 

A visual condition survey will help identify and 

evaluate the extent, types, and patterns of distress 

and deterioration. The American Concrete Institute 

offers several useful guides on how to perform a visual 

condition survey of concrete. Generally, the condition 

assessment begins with an overall visual survey, 

followed by a close-up investigation of representative 

areas to obtain more detailed information about modes 

of deterioration. 

A number of nondestructive testing methods can be 

used in the field to evaluate concealed conditions. Basic 

techniques include sounding with a hand-held hammer 

(or for horizontal surfaces, a chain) to help identify areas 

of delamination. More sophisticated techniques include 

impact-echo testing (Fig. 16), ground penetrating radar, 

pulse velocity, and other methods that characterize 

concrete thickness and locate voids or delaminations. 

Magnetic detection instruments are used to locate 

embedded reinforcing steel and can be calibrated to 

identify the size and depth of reinforcement. Corrosion 

measurements can be taken using copper-copper 

sulfate half-cell tests or linear polarization techniques to 

determine the probability or rate of active corrosion of 

the reinforcing steel. 

To further evaluate the condition of the concrete, 

samples may be removed for laboratory study to 

determine material components and composition, 

and causes of deterioration. Samples need to be 

representative of existing conditions but should be taken 

from unobtrusive locations. Laboratory studies of the 

concrete may include petrographic evaluation following 

ASTM C856, Practice for Petrographic Examination 

of Hardened Concrete. Petrographic examination, 

consisting of microscopical studies performed by a 

geologist specializing in the evaluation of construction 

materials, is performed to determine air content, watercement ratio, cement content, and general aggregate 

characteristics. Laboratory studies can also include 
chemical analyses to determine chloride content, sulfate 

content, and alkali levels of the concrete; identification 

of deleterious aggregates; and determination of depth 

of carbonation. Compressive strength studies can 

be conducted to evaluate the strength of the existing 

concrete and provide information for repair work. The 

laboratory studies provide a general identification of 

the original concrete's components and aggregates, 

and evidence of damage due to various mechanisms 

including cyclic freezing and thawing, alkali-aggregate 

reactivity, or sulfate attack. Information gathered 

through laboratory studies can also be used to help 

develop a mix design for the repair concrete. 

Cleaning 

As with other historic structures, concrete structures are 

cleaned for several reasons: to improve the appearance 

of the concrete, as a cyclical maintenance measure, or 

in preparation for repairs. Consideration should first be 

given to whether the historic concrete structure needs to 

be cleaned at all. If cleaning is required, then the gentlest 

system that will be effective should be selected. 

Three primary methods are used for cleaning concrete: 

water methods, abrasive surface treatments, and 

chemical surface treatments. Low-pressure water (less 

than 200 psi) or steam cleaning can effectively remove 

surface soiling from sound concrete; however, care is 

required on fragile or deteriorated surfaces. In addition, 

water and steam methods are typically not effective in 

removing staining or severe soiling. Power washing 

with high-pressure water is sometimes used to clean or 

remove coatings from sound, high-strength concrete, but 

high-pressure water washing is generally damaging to 

and not appropriate for concrete on historic structures. 

When used with proper controls and at very low 

pressures (typically 35 to 75 psi), microabrasive 

Figure 16. Impact echo testing is performed on a concrete structural 

slab to help determine depth of deterioration. In this method, a short 

pulse of energy is introduced into the structure and a transducer 

mounted on the impacted surface of the structure receives the 

reflected input waves or echoes. These waves are analyzed to help 

identify flaws and deterioration within the concrete. 

surface treatments using very fine particulates, such 

as dolomitic limestone powder, can sometimes clean 

effectively. However, micro abrasive cleaning may alter 

the texture and surface reflectivity of concrete. Some 

concrete can be damaged even by fine particulates 

applied at very low pressures. 

Chemical surface treatments can clean effectively 

but may also alter the appearance of the concrete by 

bleaching the concrete, removing the paste, etching 

the aggregate, or otherwise altering the surface. 

Detergent cleaners or mild, diluted acid cleaners may 

be appropriate for removal of staining or severe soiling. 

Cleaning products that contain strong acids such as 

hydrochloric (muriatic) or hydrofluoric acid, which will 

damage concrete and are harmful to persons, animals, 

site features, and the environment, should not be used. 

For any cleaning process, trial samples should be 

performed prior to full-scale implementation. The 

intent of the cleaning program should not be to return 

the structure to a like new appearance. Concrete can 

age gracefully, and as long as soiling is not severe or 

deleterious, many structures can still be appreciated 

without extensive cleaning. 

Methods of Maintenance and Repair 

The maintenance of historic concrete often is thought of 

in terms of appropriate cleaning to remove unattractive 

dirt or soiling materials. However, the implementation 

of an overall maintenance plan for a historic structure is 

the most effective way to help protect historic concrete. 

For examples, the lack of maintenance to roofs and 

drainage systems can promote water related damage 

to adjacent concrete features. The repeated use of 

deicing salts in winter climates can pit the surface of old 

concrete and also may promote decay in embedded steel 

reinforcements. Inadequate protection of concrete walls 

adjacent to driveways and parking areas can result in the 

need for repair work later on. 

The maintenance of historic concrete involves the regular 

inspection of concrete to establish baseline conditions 

and identify needed repairs. Inspection tasks involve 

monitoring protection systems, including sealant joints, 

expansion joints, and protective coatings; reviewing 

existing conditions for development of distress such as 

cracking and delaminations; documenting conditions 

observed; and developing and implementing a cyclical 

repair program. 

Sealants are an important part of maintenance of historic 

concrete structures. Elastomeric sealants, which have 

replaced traditional oil-resin based caulks for many 

applications, are used to seal cracks and joints to keep 

out moisture and reduce air infiltration. Sealants are 

commonly used at windows and door perimeters, 

at interfaces between concrete and other materials, 

and at attachments to or through walls or roofs, such 

as with lamps, signs, or exterior plumbing fixtures. 

9 
10 

Figure 17. (a) The 63rd Street Beach House was constructed on the shoreline of Chicago in 1919. The highly exposed aggregate concrete of the 

exterior walls of the beach house was used for many buildings in the Chicago parks as an alternative to more expensive stone construction. Photo: 

Leslie Schwartz Photography. (b) Concrete deterioration included cracking, spalling, and delamination caused by corrosion of embedded reinforcing 

steel and concrete damage due to cyclic freezing and thawing. (c) Various sizes and types of aggregates were reviewed for matching to the original 

concrete materials. (d) Mock-ups of the concrete repair mix were prepared for comparison to the original concrete. Considerations included aggregate 

type and size, cement color, proportions, aggregate exposure, and surface finish. (e) The craftsman finished the surface to replicate the original 

appearance in a mock-up on the structure. Here, he used a nylon bristle brush to remove loose paste and expose the aggregate, creating a variable 

surface to match the adjacent original concrete. 

Where used for crack repairs on historic facades, the 

finished appearance of the sealant application must 

be considered, as it may be visually intrusive. In some 

cases, sand can be broadcast onto the surface of the 

sealant to help conceal the repair. 

Urethane and polyurethane sealants are often used to 

seal joints and cracks in concrete structures, paving, 

and walkways; these sealants provide a service life of 

up to ten years. High-performance silicone sealants 

also are often used with concrete, as they provide a 

range of movement capabilities and a service life of 

twenty years or more. Some silicone sealants may stain 

adjacent materials, which may be a problem with more 

porous concrete, and may also tend to accumulate 

dust and dirt. The effectiveness of sealants for sealing 

joints and cracks depends on numerous factors 

including proper surface preparation and application. 

Sealants should be examined as part of routine 

maintenance inspections, as these materials deteriorate 

faster than their substrates and must be replaced 

periodically as a part of cyclical maintenance. 

Repair of historic concrete may be required to 

address deterioration because the original design and 

construction did not provide for long-term durability, 

or to facilitate a change in use of the structure. 

Examples include increasing concrete cover to protect 

reinforcing steel and reducing water infiltration into the 

structure by repair of joints. Any such improvements 

must be thoroughly evaluated for compatibility with 

the original design and appearance. Care is required in 

all aspects of historic concrete repair, including surface 

preparation; installation of form work; development 

of the concrete mix design; and concrete placement, 

consolidation, and curing. 

An appropriate repair program addresses existing 

distress and reduces the rate of future deterioration, 

which in many cases involves moisture-related issues. 

The repair program should incorporate materials and 

methods that are sympathetic to the existing materials 

in character and appearance, and which provide good 

long-term performance. In addition, repair materials 

should age and weather similarly to the original 

materials. In order to best achieve these goals, concrete 

repair projects should be divided into three phases: 

development of trial repair procedures, trial repairs and 

evaluation, and production repair work. 
For any concrete repair project, the process of investigation, 

laboratory analysis, trial samples, mock-ups, and full-scale 

repairs allows ongoing refinement of the repair work as 

well as implementation of quality-control measures. The 

trial repair process provides an opportunity for the owner, 

architect, engineer, and contractor to evaluate the concrete 

mix design and the installation and finishing techniques for 

the repairs from both technical and aesthetic standpoints. 

The final repair materials and procedures should match 

the original concrete in appearance while meeting the 

established criteria for durability. Information gathered 

through trial repairs and mock-ups is invaluable in refining 

the construction documents prior to the start of the overall 

repair project (Fig. 17). 

Surface Preparation 

In undertaking surface preparation for historic concrete 

repair, care must be taken to limit removal of existing 

material while still providing an appropriate substrate for 

repairs. This is particularly important where ornamentation 

and fine details are involved. Preparation for localized 

repairs usually begins with removal of the loose concrete 

to determine the general extent of the repair, followed by 

saw-cutting the perimeter of the repair area. The repair area 

should extend beyond the area of concrete deterioration 

to a sufficient extent to provide a sound substrate. When 

repairing concrete with an exposed aggregate or other 

special surface texture, a sawcut edge may be too visually 

evident. To hide the repair edge, techniques such as lightly 

hand-chipping the edge of the patch may be used to 

conceal the joint between the original concrete and the new 

repair material. The depth to which the concrete needs to 

be removed may be difficult to determine without invasive 

probing in the repair area. Removal of concrete should 

typically extend beyond the level of the reinforcing steel, if 

present, so that the patch encapsulates the reinforcing steel, 

which provides mechanical attachment for the repair. 

If the concrete was originally of lower strength and quality, 

the assessment of present soundness is more difficult. 

Deteriorated and unsound concrete is typically removed 

using pneumatic chipping hammers. Removal of concrete 

in historic structures is better controlled by using smaller 

chipping hammers or hand tools. The area of the concrete 

to be repaired and the exposed reinforcing steel are 

then cleaned, usually by careful sandblast and air blast 

procedures applied only within the repair area. Adjacent 

original concrete surfaces should be protected during this 

work. In some cases, project constraints such as dust control 

may limit the ability to thoroughly clean the concrete and 

steel. For example, it may be necessary to use needle scaling 

(a small pneumatic impact device) and wire brushing 

instead of sandblasting. 

Supplemental steel may be needed when existing 

reinforcing steel is severely deteriorated, or if reinforcing 

steel is not present in repair areas. Exposed existing 

reinforcing and other embedded steel elements can be 

cleaned, primed, and painted with a corrosion-inhibiting 

coating. The patching material should be reinforced 

and mechanically attached to the existing concrete. 

Reinforcement materials used in repairs most often 

include mild steel, epoxy-coated steel, or stainless steel, 

depending on existing conditions. 

Formwork and Molds 

Special formwork is needed to recreate ornamental 

concrete features - which may be complex, in high 

relief, or architecturally detailed-and to provide special 

surface finishes such as wood form board textures. 

Construction of the formwork itself requires particular 

skill and craftsmanship. Reusable forms can be used for 

concrete ornamentation that is repeated across a building 

facade, or precast concrete elements may be used to 

replace missing or unrepairable architectural features. 

Formwork for ornamental concrete is often created using 

a four-step process: a casting of the original concrete is 

taken; a plaster replica of the unit is prepared; a mold or 

form is made from the plaster replica; and a new concrete 

unit is cast. Custom formwork and molds are often the 

work of specialty companies, such as precasters and cast 

stone fabricators. 

The process of forming architectural features or special 

surface textures is particularly challenging if early age 

stripping (removal of formwork early in the concrete 

curing process) is needed to perform surface treatment 

on the concrete. Timing for formwork removal is related 

to strength gain, which in turn is partly dependent on 

temperature and weather conditions. Early age removal of 

formwork in highly detailed concrete can lead to damage 

of the new concrete that has not yet gained sufficient 

strength through curing. 

Selection of Repair Materials and Mix Design 

Selection and design of proper repair materials is a 

critical component of the repair project. This process 

requires evaluation of the performance, characteristics, 

and limitations of the repair materials, and may involve 

laboratory testing of proposed materials and trial repairs. 

The materials should be selected to address the specific 

type of repair required and to be compatible with special 

characteristics of the original concrete. Some modern 

repair materials are designed to have a high compressive 

strength and to be impermeable. Even though inherently 

durable, these newer materials may not be appropriate for 

use in repairing a low strength historic concrete. 

The concrete's durability, or resistance to deterioration, 

and the materials and methods selected for repair 

depend on its composition, design, and quality of 

workmanship. In most cases, a mix design for durable 

replacement concrete should use materials similar to 

those of the original concrete mix. Prepackaged materials 

are often not appropriate for repair of historic concrete. 

The concrete patching material can be air entrained or 

polymer-modified if subject to exterior exposure, and 

should incorporate an appropriate selection of aggregate 

and cement type, and proper water content and water 

11 
12 

• • I '{\ I ., • 

• , -II t, . \ ',~,' j ~ ;.\ '\ -.: " \: .... ·~·~I"I I "'".",'. , .. . ..r.,,. ,T, . " I.... • •. ~ •. .. ~ • -: • \ I·, ... ~ t' I'!, ' ;. ' "w --. I '.' 

. I, .',: "i '" 1 ~ ) .' '. j I I - • , , 

b .• I 

Figure 18. (a) Exposed aggregate precast concrete is 

sounded with a hammer to detect areas of deterioration. 

Corrosion of the exposed reinforcing steel bar has led to 

spalling of the adjacent concrete. (b) Samples of aggregate 

considered for use in repair concrete are compared to the 

original concrete materials in terms of size, color, texture, 

and reflectance. (c) Various sample panels are made using 

the selected concrete repair mix design for comparison to 

the original concrete on the building, and the mix design is 

adjusted based on review of the samples. (d) After removal 

of the spall, the concrete surface is prepared for installation 

of a formed patch. (e) Prior to placement of the concrete, 

a retarding agent is brush-applied to the inside face of the 

formwork to slow curing at the surface. After the concrete 

is partially cured, the forms are removed and the surface 

of the concrete is rubbed to remove some of the paste and 

expose the aggregate to match the original concrete. 

to cement ratio. Some admixtures, including polymer modifiers, 

may change the appearance of the concrete mix. Design of the 

concrete patching material should address characteristics required 

for durability, workability, strength gain, compressive strength, 

and other performance attributes. During installation of the 

repair, skilled workmanship is required to ensure proper mixing 

procedures, placement, consolidation, and curing. 

Matching and Repair Techniques for Historic Concrete 

Repair measures should be selected that retain as much of the 

original material as possible, while providing for removal of an 

adequate amount of deteriorated concrete to provide a sound 

substrate for a durable repair. The installed repair must visually 

match the existing concrete as closely as possible and should be 

similar in other aspects such as compressive strength, permeability, 

and other characteristics important in the mix design of the 

concrete (Fig. 18). 

Understanding the original construction techniques often provides 

opportunities in the design of repairs. For example, joints between 

the new and old concrete can be hidden in changes in surface 

profile and cold joints. The required patching mix for the concrete 

to be used in the repair will likely need to be specially designed 

to replicate the appearance of the adjacent historic concrete. A 

high level of craftsmanship is required for finishing of historic 

concrete, in particular to create the sometimes inconsistent finish 

and variation in the original concrete in contrast to the more even 

appearance required for most non-historic repairs. 

To match the various characteristics of the original concrete, trial 

mixes should be developed. These mixes need to take into account 

the types and colors of aggregates and paste present in the original 

concrete. Different mixes may be needed because of variations 

in the appearance and composition of the historic concrete. The 

trials should utilize different forming and finishing techniques 

to achieve the best possible match to the original concrete. Initial 

trials should first take place on site but off the structure. The mix 

designs providing the best match are then installed as trial repairs 

on the structure, and assessed after they have cured. 

Achieving compatibility between repair work and original 

concrete may be difficult, especially given the variability often 

present in historic concrete materials and finishes. Formed rather 

than trowel-applied patch repairs are recommended for durability, 

as forming permits better ranges of mix ingredients (such as coarse 

aggregates) and improved consolidation as compared to trowelapplied repairs. Parge coatings usually are not recommended 

as they do not provide as durable repair as formed concrete. 

However, in some cases parge coatings may be appropriate to 

match an original parged surface treatment Proper placement 

and finishing of the repair are important to obtain a match with 

the original concrete. To minimize problems associated with rapid 

curing of concrete, such as surface cracking, it is important to use 

proper curing methods and to allow for sufficient time. 

Hairline cracks that show no sign of increasing in size may often 

be left unrepaired. The width of the crack and the amount of 

movement usually limits the selection of crack repair techniques 

that are available. Although it is difficult to determine whether 

cracks are moving or non-moving, and therefore most cracks 
should be assumed to be moving, it is possible to repair 

non-moving cracks by installation of a cementitious 

repair mortar matching the adjacent concrete. It is 

generally desirable not to widen cracks prior to the mortar 

application. Repair mortar containing sand in the mix may 

be used for wider cracks; unsanded repair mortar may be 

used for narrower cracks. 

When it is desirable to re-establish the structural integrity 

of a concrete structure involving dormant cracks, epoxy 

injection repair has proven to be an effective procedure. 

Such a repair is made by first sealing the crack on both sides 

of a wall or structural member with epoxy, polyester, wax, 

tape, or cement slurry, and then injecting epoxy through 

small holes or ports drilled in the concrete. Once the epoxy 

in the crack has hardened, the surface sealing material 

may be removed; however, this type of repair is usually 

quite apparent. Although it may be possible to inject epoxy 

without leaving noticeable residue, this process is difficult 

and, in general, the use of epoxy repairs in visible areas of 

concrete on historic structures is not recommended. 

Active structural cracks (which move as loads are 

added or removed) and thermal cracks (which move as 

temperatures fluctuate) must be repaired in a manner that 

will accommodate the anticipated movement. In some more 

extreme cases, expansion joints may have to be introduced 

before crack repairs are undertaken. Active cracks may 

be filled with sealants that will adhere to the sides of 

the cracks and will compress or expand during crack 

movement. The design, detailing, and execution of sealant 

repairs require considerable attention, or they will detract 

from the appearance of the historic building. The routing 

and cleaning of a crack, and installation of an elastomeric 

sealant to prevent water penetration, is used to address 

cracks where movement is anticipated. However, unless 

located in a concealed area of the concrete, this technique 

is often not acceptable for historic structures because the 

repair will be visually intrusive (Fig. 19). Other approaches, 

such as installation of a cementitious crack repair, may need 

to be considered even though this type of repair may be less 

effective or have a shorter service life than a sealant repair. 

Replacement 

If specific components of historic concrete structures are 

beyond repair, replacement components can be cast to 

match historic ones. Replacement of original concrete 

should be carefully considered and viewed as a method of 

last resort. In some cases, such as for repeated ornamental 

units, it may be more cost-effective to fabricate precast 

concrete units to replace missing elements. The forms 

created for precast or cast-in-place units can then be used 

again during future repair projects. 

Careful mix formulation, placement, and finishing are 

required to ensure that replacement concrete units will 

match the historic concrete. There is often a tendency to 

make replacement concrete more consistent in appearance 

than the original concrete. The consistency can be in 

stark contrast with the variability of the original concrete 

Figure 19. A high-speed grinder ia used to widen a crack in 

preparation for installation of a sealant. This process is called 

"routing. " After the crack is prepared, the sealant is installed to 

prevent moisture infiltration through the crack. Although sealant 

repairs can provide a durable, watertight repair for moving cracks, 

they tend to be very visible. 

due to original construction techniques, architectural 

design, or differential exposure to weather. Trial repairs 

and mock-ups are used to evaluate the proposed 

replacement concrete work and to refine construction 

techniques (Fig 20). 

Protection Systems 

Coatings and Penetrating Sealers. Protection systems 

such as a penetrating sealers or film forming coating 

are often used with non-historic structures to protect 

the concrete and increase the length of the service life 

of concrete repairs. However, film-forming coatings 

are often inappropriate for use on a historic structure, 

unless the structure was coated historically. Filmforming coatings will often change the color and 

appearance of a surface, and higher build coatings can 

also mask architectural finishes and ornamental details. 

For example, the application of a coating on concrete 

having a formboard finish may hide the wood texture 

of the surface. Pigmented film-forming coatings are 

also typically not appropriate for use over exposed 

aggregate concrete, where the uncoated exposed surface 

contributes significantly to the historic character of 

13 
14 

Figure 20. (a) The Jefferson Davis Memorial in Fairview, Kentucky, 

constructed from 1917-1924, is 351 feet tall and constructed of 

unreinforced concrete. The walls of the memorial are 8 feet thick at the 

base and 2 feet thick at the top of the wall. Access to the monument 

for investigation was provided by rappelling techniques, while ground 

supported and suspended scaffolding was used to access the exterior during 

repairs. (b) The concrete was severely deteriorated at isolated locations, with 

spalling and damage from cyclic freezing and thawing of entrapped water. 

In addition, previous repairs were at the end of their service life and removal 

of deteriorated concrete and failed previous repairs was required. Light 

duty chipping hammers were used to avoid damage to adjacent material 

when removing deteriorated concrete to the level of sound concrete. (c) 

Field samples were performed to match the color, finish, and texture of the 

original concrete. A challenge in matching of historic concrete is achieving 

variability of appearance. (d) The completed surface after repairs exhibits 

intentional variability of the concrete surface to match the appearance of 

the original concrete. Some formwork imperfections that would normally be 

removed by finishing were intentionally left in place, to replicate the highly 

variable finish of the original concrete. (e) The Jefferson Davis Memorial 

after completion of repairs in 2004. Photo e: Joseph Lenzi, Senler, Campbell 

& Associates, Inc. 
concrete. In cases where the color of a substrate needs to 

be changed, such as to modify the appearance of existing 

repairs, an alternative to pigmented film-forming coatings is 

the use of pigmented stains. 

Many proprietary clear, penetrating sealers are currently 

available to protect concrete substrates. These products 

render fine cracks and pores within the concrete 

hydrophobic; however, they do not bridge or fill cracks. 

Clear sealers may change the appearance of the concrete in 

that treated areas become more visible after rain in contrast 

to the more absorptive areas of original concrete. Once 

applied, penetrating sealers cannot be effectively removed 

and are therefore considered irreversible. They should 

not be used on historic concrete without thorough prior 

consideration. However, clear penetrating sealers provide 

an important means of protection for historic concrete that 

is not of good quality and can help to avoid more extensive 

future repairs or replacement. Thus they are sometimes 

appropriate for use on historic concrete. Once applied, these 

sealers will require periodic re-application. 

Waterproofing membranes are systems used to protect 

concrete surfaces such as roofs, terraces, plazas, or balconies, 

as well as surfaces below grade. Systems range from coal 

tar pitch membranes used on older buildings, to asphalt or 

urethane-based systems. On historic buildings, membrane 

systems are typically used only on surfaces that were 

originally protected by a similar system and surfaces that are 

not visible from grade. Waterproofing membranes may be 

covered by roofing, paving, or other architectural finishes. 

Laboratory and field testing is recommended prior to 

application of a protection system or treatment on any 

concrete structure; testing is even more critical for historic 

structures because many such treatments are not reversible. 

As with other repairs, trial samples are important to 

evaluate the effectiveness of the treatment and to determine 

whether it will harm the concrete or affect its appearance. 

Cathodic Protection. Corrosion is an electrochemical 

process in which electrons flow between cathodic (positively 

charged) and anodic (negatively charged) areas on a 

metal surface; corrosion occurs at the anodes. Cathodic 

protection is a technique used to control the corrosion of 

metal by making the whole metal surface the cathode of 

an electrochemical cell. This technique is used to protect 

metal structures from corrosion and is also sometimes 

used to protect steel reinforcement embedded in concrete. 

For reiniorced concrete, cathodic protection is typically 

accomplished by connecting an auxiliary anode to the 

reiniorcing so that the entire reiniorcing bar becomes a 

cathode. In sacrificial anode (passive) systems, current flows 

naturally by galvanic action between the less noble anode 

(such as zinc) and the cathode. In impressed-current (active) 

systems, current is impressed between an inert anode 

(such as titanium) and the cathode. Cathodic protection is 

intended to reduce the rate of corrosion of embedded steel 

in concrete, which in turn reduces overall deterioration. 

Protecting embedded steel from corrosion helps to prevent 

concrete cracking and spalling. 

Impressed-current cathodic protection is the most 

effective means of mitigating steel corrosion and has 

been used in practical structural applications since the 

1970s. However, impressed-current cathodic protection 

systems are typically the most costly to install and 

require substantial ongoing monitoring, adjustment, 

and maintenance to ensure a proper voltage output 

(protection current) over time. Sacrificial anode cathodic 

protection dates back to the 1800s, when the hulls of 

ships were protected using this technology. Today 

many industries utilize the concept of sacrificial anode 

cathodic protection for the protection of steel exposed 

to corrosive environments. It is less costly than an 

impressed-current system, but is somewhat less effective 

and requires reapplication of the anode when it becomes 

depleted. 

Re-alkalization. Another technique currently available 

to protect concrete is realkalization, which is a process 

to restore the alkalinity of carbonated concrete. The 

treatment involves soaking the concrete with an alkaline 

solution, in some cases forcing it into the concrete to 

the level of the reiniorcing steel by passage of direct 

current. These actions increase the alkalinity of the 

concrete around the reiniorcement, thus restoring the 

protective alkaline environment for the reiniorcement. 

Like impressed-current cathodic protection methods, it 

is costly. Other corrosion methods are also available but 

have a somewhat shorter history of use. 

Careful evaluation of existing conditions, the causes and 

nature of distress, and environmental factors is essential 

before a protection method is selected and implemented. 

Not every protection system will be effective on each 

structure. In addition, the level of intrusion caused by 

the protection system must be carefully evaluated before 

it is used on a historic concrete structure. 

Summary 

In the United States, concrete has been a popular 

construction material since the late nineteenth century 

and recently has gained greater recognition as a historic 

material. Preservation of historic concrete requires a 

thorough understanding of the causes and types of 

deterioration, as well as of repair and replacement 

materials and methods. It is important that adequate 

time is allotted during the planning phase of a project 

to provide for trial repairs and mock-ups in order 

to evaluate the effectiveness and aesthetics of the 

repairs. Careful design is essential and, as with other 

preservation efforts, the skill of those performing 

the work is critical to the success of the repairs. The 

successful repair of many historic concrete structures 

in recent years demonstrates that the techniques and 

materials now available can extend the life of such 

structures and help ensure their preservation. 

15 
16 

Selected Reading 

American Concrete Institute. Guide for Making a Condition Survey of Concrete in Service. ACI Committee 201, ACI 201.1R-92. 

American Concrete Institute. Guide to Evaluation of Concrete Structures before Rehabilitation. ACI Committee 364, ACI 364.1R-07. 

American Concrete Institute. Concrete Repair Guide. ACI Committee 546, ACI 546R-04. 

American Concrete Institute. Guide for Evaluation of Existing Concrete Buildings. ACI Committee 437, ACI 437R-03. 

Childe, H.L. Manufacture and Uses of Concrete Products and Cast Stone. London: Concrete Publications Limited, 1930. 

Collins, Peter. Concrete: The Vision of a New Architecture. New York, New York: Faber and Faber, 1959. 

Cowden, Adrienne B., compo Historic Concrete: An Annotated Bibliography. Washington, D.C: National Park Service, 1993. 

Komandant, August E. Contemporary Concrete Structures. New York, New York: McGraw Hill, 1972. 

Erlemann, Gustav G. "Steel Reinforcing Bar Specification in Old Structures." Concrete International, April 1999, 49-50. 

Federal Highway Administration. Guide to Nondestructive Testing of Concrete. FHWA Publication Number FHWA-SA-97-105. 

Gaudette, Paul E. "Special Considerations in Repair of Historic Concrete." Concrete Repair Bulletin, January/February 2000,12-13. 

Jester, Thomas C, ed. Twentieth CentunJ Building Materials. New York, New York: McGraw-Hill, 1995. 

Johnson, Arne P., and Seung Kyoung Lee. "Protection Methods for Historic Concrete at Soldier Field." Preserve and Play: Preserving 

Historic Recreation and Entertainment Sites. Washington, D.C: Historic Preservation Education Foundation, National Council for 

Preservation Education, and National Park Service, 2006. 

Macdonald, Susan, ed. Concrete: Building Pathology. Osney Mead, Oxford, u.K.: Blackwell Science, 2003. 

McGovern, Martin S. "A Clear View of Sealers." Concrete Construction, January 2000, 53-58. 

Morton, W. Brown III, Gary L. Hume, Kay D. Weeks, H. Ward Jandl, and Anne E. Grimmer. The Secretary of the Interior's Standards for 

Rehabilitation & Illustrated Guidelines for Rehabilitating Historic Buildings. Washington, D.C: National Park Service, 1983, reprinted 1997. 

"Repairing Cracks." Concrete Repair Digest, August/September 1992, 160-164. Condensed from ACI document 224.1R-93. 

Slaton, Deborah. "Cleaning Historic Concrete." Concrete Repair Bulletin, January/February 2000,14-15. 

Acknowledgements 

Paul Gaudette is an engineer with Wiss, Janney, Elstner Associates, Inc., in Chicago, Illinois. Deborah Slaton is an architectural 

conservator with Wiss, Janney, Elstner Associates, Inc., in Northbrook, Illinois. All photographs by Paul Gaudette unless otherwise 

stated. Front cover image: Kyle Normandin, Wiss, Janney, Elstner Associates, Inc. 

The authors wish to thank William Bing Coney; author of the first edition of this preservation brief, who served as a peer reviewer 

for the current edition. In addition, the authors gratefully acknowledge the assistance of the following individuals as peer reviewers 

of this brief: Arne Johnson and Una Gilmartin, Wiss, Janney, Elstner Associates, Inc.; Robert Joyce, Quality Restorations, Inc.; Susan 

Macdonald, New South Wales Heritage Office; Miles T. Murray; Restruction Corporation; and Jack Pyburn, OJP/Architect, Inc. Anne 

E. Grimmer, Chad Randl, and former staff Sharon C Park, FAIA, of the Technical Preservation Services, National Park Service, offered 

valuable comments during development of the brief. Charles E. Fisher of the National Park Service was the technical editor for this 

publication project. 

This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which directs the Secretary 

of the Interior to develop and make available information concerning historic properties. Comments about this publication should 

be addressed to: Charles E. Fisher, Technical Preservation Publications Program Manager, Technical Preservation Services-2255, 

National Park Service, 1849 C Street, NW, Washington, DC 20240. This publication is not copyrighted and can be reproduced without 

penalty. Normal procedures for credit to the authors and the National Park Service should be provided. The photographs used in 

this publication may not be used to illustrate other publications without permission of the owners. For more information about the 

programs of the National Park Service's Technical Preservation Services see our website at http://www.nps.govlhistorylhps/tps.htm 

ISBN: 978-0-16-078946-5 

U.S. Government Printing Office Stock Number: 024-005-01253-2 2007 
PRESERVATION 

BRIEFS 

The Use of Substitute Materials 

on Historic Building Exteriors 

Sharon C. Park, AlA 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

·",; 

• 

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 and general properties of the historic material and will not damage 

the historic resource . 

Introduction 

When deteriorated, damaged, or lost features of a 

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 historic resources. 

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 sandimpregnated 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 (see illus. 1, 2). 

Molded or cast masonry substitutes, such as drytamp cast stone and poured concrete, became popular 

in place of quarried stone during the 19th century. 

These masonry units were fabricated locally, avoiding 
Illus. 1. An early 18th-century technique for imitating caroed or 

quarried stone was the use of sand-impregnated paint applied to 

wood. The facade stones and quoins are of wood. The Lindens 

(1754), Washington, D.C. Photo: Sharon C. Park, AlA. 

Illus. 3. Casting concrete to represent quarried stone was a popular late 19th-century technique seen in this circa 1910 mailorder house. While most components were delivered by rail, the 

foundations and exterior masonry were completed by local craftsmen. Photo: Sharon C. Park, AlA. 

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 (see illus. 3). 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 (see illus. no. 4). 

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 ap2 

Illus. 2. Stucco has for many centuries represented a number of 

building materials. Seen here is the ground floor of a Beaux Arts 

mansion, circa 1900, which represents a finely laid stone foundation wall executed in scored stucco. Photo: Sharon C. Park, AlA. 

Illus. 4. The 19th-century also produced a variety of metal products used in imitation of other materials. In this case, the entire 

exterior of the Long Island Safety Deposit Company is cast-iron 

representing stone. Photo: Becket Logan, Friends of Cast Iron 

Architecture. 

pearance 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 polychrome effect. 

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 
a b 

c 

Illus. 5. The four historic examples of various window hoods 

shown are: (a) stone; (b) cast stone; (c) cast-iron; and (d) sheet 

metal. The criteria for selecting substitute materials today 

(availability, quality, delivery dates, cost) are not much different 

from the past. Photo: Sharon C. Park, AlA. 

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 (see illus. 5). 

The criteria for selection today are not much 

different. 

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 Projects 

Because the overzealous use of substitute materials 

can greatly impair the historic character of a 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 a 

historic building and that will conceal future 

deterioration (see illus. 6). 

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 (see 

illus. 7). 

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 mayor 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 preservationist are looking 

abroad to find materials (especially stone) that match 

the historic materials in an effort to restore historic 

Illus. 6. Substitute materials should never be considered as a 

cosmetic cover-up for they can cause great physical damage and 

can alter the appearance of historic buildings. For example, a 

fiberglass coating was used at Ranchos de Taos, NM, in place of 

the historic adobe coating which had deteriorated. The waterproof 

coating sealed moisture in the walls and caused the sfXllIing 

shown. It was subsequently removed and the walls were properly 

repaired with adobe. Photo: Lee H. Nelson, FAlA. 

3 
Illus. 7. Whenever possible, historic materials should be repaired 

rather than replaced. Epoxy, a synthetic resin, has been used to 

repair the wood window frame and sill at the Auditors Building 

(1878) Washington, DC. The cured resin is white in this photo 

and will be primed and painted. Photo: Lee H. Nelson, FAIA. 

Illus. 9. Simple solutions should not be overlooked when materials 

are no longer available. In the case of the Morse-Libby Mansion 

(1859), Portland, ME, the deteriorated brownstone porch beam 

was replaced with a carved wooden beam painted with sand impregnated paint. Photo: Stephen Sewall. 

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 (see illus. 8). 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 

4 

Illus. 8. Even when materials are not locally available, it may be 

possible and cost effective to find sources elsewhere. For example, 

the local sandstone was no longer available for the restoration of 

the New York Shakespeare Festival Public Theater. The 

deteriorated sandstone window hoods, were replaced with stone 

from Germany that closely matched the color and texture of the 

historic sandstone. Photo: John G. Waite. 

Illus. 10. The use of substitute materials is not necessarily cheaper 

or easier than using the original materials. The complex process of 

fabricating the polyester bronze reproduction pieces of the gilded 

wood molding for the clockcase at Independence Hall required 

talented artisans and substantial mold-making time. From left to 

right is the final molded polyester bronze detail; the plaster 

casting mold; the positive and negative interim neoprene rubber 

molds; and the expertly carved wooden master. Photo: Courtesy of 

Independence National Historical Park. 

as a replacement section, achieved using readily 

available traditional materials, conventional tools and 

work skills. (see illus. 9). Simple solutions should not 

be overlooked. 

2. The unavailability of historic craft techniques 

and lack of skilled artisans. 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 
Illus. 11. The unavailability of historic craft techniques is another 

reason to consider substitute materials. The original first floor cast 

iron front of the Grand Opera House, Wilmington, DE, was 

missing; the expeditious reproduction in cast aluminum was possible because artisans working in this medium were available. 

Photo: John G. Waite. 

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. (see illus. 10, 11). 

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 ternecoated stainless steel or lead-coated copper might 

produce a more durable yet visually compatible 

replacement roofing (see illus. 12). 

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 (see illus. 

13). 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 

Illus. 12. Substitute materials may be considered when the 

original materials have not performed well. For example, early 

sheet metals used for roofing, such as tinplate, were reasonably 

durable, but the modem equivalent, terne-coated steel, is subject 

to corrosion once the thin tin plating is damaged. Terne-coated 

stainless steel or lead-coated copper (shown here) are now used as 

substitutes. Photo: John G. Waite. 

Illus. 13. Code-related changes are of concern in historic preservation projects because the integrity of the historic resource may be 

irretrievably affected. In the case of the Old San Francisco Mint, 

the fiberglass cornice was used to bring the building into seismic 

conformance. The original cornice was deteriorated, and the 

replacement (1982) was limited to the projecting pediment. The 

historic stone fascia was retained as were the stone columns. The 

limited replacement of deteriorated material did not jeopardize the 

integrity of the building. Photo: Walter M. Sontheimer. 

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 rehabilitation. 

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 

5 
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 perfonn 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 (see illus. 14). 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 (see illus. 

15, 16). In situations where there are subtle variations 

in color and texture within the original materials, the 

6 

Illus. 14. The visual qualities of the historic feature must be 

matched when using substitute materials. In this illustration, the 

lighter weight mineral fiber cement shingles used to replace the 

deteriorated historic slate roof were detailed to match the color, 

size, shape and pattern of the original roofing and the historic 

snow birds were reattached. Photo: Sharon C. Park, AlA. 

Illus. 15. Poor quality workmanship can be avoided. In this 

example, the crudely cast concrete entrance pier (shown) did not 

match the visual qualities of the remaining historic sandstone (not 

shown). The aggregate is too large and exposed; the casting is not 

crisp; the banded tooling edges are not articulated; and the color 

is too pale. Photo: Sharon C. Park, AIA. 
Illus. 16. The good quality substitute materials shown here do 

match the historic sandstone in color, texture, tooling and surface 

details. Dry-tamp cast stone was used to match the red sandstone 

that was no longer available. The reconstructed first floor incorporated both historic and substitute materials. Sufficient molds 

were made to avoid the problem of detecting the substitutes by 

their uniformity. Photo: Sharon C. Park, AlA. 

Illus. 17. Care must be taken to ensure that the replacement 

materials will work within a predesigned system. At the Norris 

Museum, Yellowstone National Park, the 12-inch diameter log 

rafters, part of an intricate truss system, had rotted at the inner 

core from the exposed ends back to a depth of 48 inches. The exterior wooden shells remained intact. Fiberglass rods (left photo) 

and specially formulated structural epoxy were used to fill the 

cleaned out cores and a cast epoxy wafer end with all the detail of 

the original wood graining was laminated onto the log end (right 

photo). This treatment preserved the original feature with a combination of repair and replacement using substitute materials as part of a well thought out system. Photos: Courtesy of Harrison 

Goodall. 

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. 

Illus. 18. Substitute materials must be properly installed to allow 

for expansion, contraction, and structural security. The new 

balustrade (a polymer concrete modified with glass fibers) at 

Carnegie Hall, New York City, was installed with steel structural 

supports to allow window-washing equipment to be suspended 

securely. In addition, the formulation of this predominantly epoxy 

material allowed for the natural expansion and contradion within 

the predesigned joints. Photo: Courtesy of MJM Studios. 

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, alkalines, 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 

(see illus. 17). 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 (see illus. 18). 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 

7 
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 

(see illus. 19). 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. 

Performance Expectations 

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 (see illus. 20, 21). 

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 

avoided. 

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 resource. 

8 

Illus. 19. When the physical properties are not matched, particularly thermal expansion and contraction properties, great 

damage can occur. In this case, an extremely rigid epoxy replacement unit was installed in a historic masonry wall. Because the 

epoxy was not modified with fillers, it did not expand or contract 

systematically with the natural stones in the wall surrounding it. 

Pressure built up resulting in a vertical crack at the center of the 

unit, and spalled edges to every historic stone that was adjacent 

to the rigid unit. Photo: Walter M. Sontheimer. 

Illus. 20. Long-term performance can be affected by where the 

substitute material is located. In this case, fiberglass was used as part of a storefront at street level. Due to the brittle nature of the 

material and the frequency of impact likely to occur at this location, an unsightly chip has resulted. Photo: Sharon C. Park, AIA. 
Illus. 21. Change of color over time is one of the greatest problems of synthetic substitute materials used outdoors. Ultra-violet 

light can cause materials to change color over time; some will 

lighten and others will darken. In this photograph, the synthetic 

patching material to the sandstone banding to the left of the window has aged to a darker color. Photos: Sharon C. Park, AlA. 

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 (see illus. 22). The charts 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 charts 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 the charts include vinyls or 

molded urethanes which are sometimes used as 

cosmetic claddings or as substitutes for wooden 

millwork. Because millwork is still readily available, it 

should be replaced in kind. 

The charts 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. 

Illus. 22. A fiber reinforced polymer (fiberglass) cornice and 

precast concrete elements replaced deteriorated features on the 

19th-century exterior. Photo: Sharon C. Park, AlA. 

Summary 

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 a historic resource. Every means of 

repairing deteriorating historic materials or replacing 

them with identical materials should be examined 

before turning to substitute materials. 

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. 

FOLLOWING ARE DESCRIPTIONS OF VARIOUS SUBSTITUTE MATERIALS 

9 
PROs and CONs of VARIOUS SUBSTITUTE MATERIALS 

Cast Aluminum 

Material: 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. 

Application: Cast aluminum can be a substitute for castiron or other decorative elements. This would include 

grillwork, roof crestings, 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. 

Close-up detail showing the crisp casting in aluminum of this 

19th-century replica column and capital for a storefront. Photo: 

Advantages: 

• light weight (112 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 

Sharon C. Park, AlA. 

Disadvantages: 

• lower structural strength than 

cast-iron 

• difficult to prevent galvanic corrosion 

with other metals 

• greater expansion and contraction 

than cast-iron; requires gaskets or 

caulked joints 

• difficult to keep paint on aluminum 

Checklist: 

• 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 cast? 

• How are expansion joints detailed? • Will there be a galvanic corrosion 

problem? 

• Have factory finishes been protected 

during installation? 

• Are fabricators/installers experienced? 

The new cast aluminum storefront replaced the lost 19th-century cast-iron original. Photo: Sharon C. Park, AlA. 

10 
PROs and CONs of VARIOUS SUBSTITUTE MATERIALS 

Cast Stone (dry-tamped): 

Material: 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 ressembling 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 . 

Application: Cast stone is often the most visually similar 

material as a replacement for unveined 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. 

Dry-tamped cast stone can reproduce the sandy texture of some natural stones. Photo: Sharon C. Park, AlA. 

Advantages: 

• 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 

Disadvantages: 

• 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 made 

Glass Fiber Reinforced Concretes (GFRC) 

Material: 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. 

Checklist: 

• Are the original or similar materials 

available? 

• How are units to be installed and 

anchored? 

• Have performance standards been 

developed to ensure color stability? 

• 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? 

Application: 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 vapor permeable. 

This glass fiber reinforced concrete sculptural wall panel will 

replace the seriously damaged resin and plaster original. A finely 

textured surface was achieved by spraying the GFRC mix into 

molds that were created from the historic panel and resculpted 

based on historic photographs. Photo: Courtesy of MJM Studios. 

Advantages: 

• 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 

Disadvantages: 

• non-Ioadbearing 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 material 

Checklist: 

• Are the original materials and craftsmanship still available? 

• Have samples been inspected on the 

site to ensure detail/texture match? 

• Has anchorage system been properly 

designed? 

• Have performance standards been 

developed? 

• Are fabricators/installers experienced? 

11 
PROs and CONs of VARIOUS SUBSTITUTE MATERIALS 

Cast Stone (dry-tamped): 

Material: 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 ressembling 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 . 

Application: Cast stone is often the most visually similar 

material as a replacement for unveined 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. 

Dry-tamped cast stone can reproduce the sandy texture of some natural stones. Photo: Sharon C. Park, AlA. 

Advantages: 

• 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 

Disadvantages: 

• 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 made 

Glass Fiber Reinforced Concretes (GFRC) 

Material: 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. 

Checklist: 

• Are the original or similar materials 

available? 

• How are units to be installed and 

anchored? 

• Have performance standards been 

developed to ensure color stability? 

• 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? 

Application: 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 vapor permeable. 

This glass fiber reinforced concrete sculptural wall panel will 

replace the seriously damaged resin and plaster original. A finely 

textured surface was achieved by spraying the GFRC mix into 

molds that were created from the historic panel and resculpted 

based on historic photographs. Photo: Courtesy of MJM Studios. 

Advantages: 

• 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 

Disadvantages: 

• non-Ioadbearing 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 material 

Checklist: 

• Are the original materials and craftsmanship still available? 

• Have samples been inspected on the 

site to ensure detail/texture match? 

• Has anchorage system been properly 

designed? 

• Have performance standards been 

developed? 

• Are fabricators/installers experienced? 

11 
PROs and CONs of VARIOUS SUBSTITUTE MATERIALS 

Epoxies (Epoxy Concretes, Polymer Concretes): 

Material: Epoxy is a resinous two-part thermo-setting 

material used as a consolidant, an adhesive, a patching 

compound, and as a molding resin. It can repair damaged 

material or recreate lost features. The resins which are 

poured into molds are usually mixed with fillers such as 

sand, or glass spheres, to lighten the mix and modify their 

expansion/contraction properties. When mixed with aggregates, such as sand or stone chips, they are often called 

epoxy concrete or polymer concrete, which is a misnomer 

as there are no cementitious materials contained within the 

mix. Epoxies are vapor impermeable, which makes detailing 

of the new elements extremely important so as to avoid 

trapping moisture behind the replacement material. It can 

be used with wood, stone, terra cotta, and various metals. 

Application: Epoxy is one of the most versatile of the new 

materials. It can be used to bind together broken fragments 

of terra cotta; to build up or infill missing sections of ornamental metal; or to cast missing elements of wooden ornaments. Small cast elements can be attached to existing 

materials or entire new features can be cast. The resins are 

poured into molds and due to the rapid setting of the 

material and the need to avoid cracking, the molded units 

are generally small or hollow inside. Multiple molds can be 

combined for larger elements. With special rods, the epoxies can be structurally reinforced. Examples of epoxy 

replacement pieces include: finials, sculptural details, small 

column capitals, and medallions. 

This replica column capital was made using epoxy resins poured 

into a mold taken from the building. The historic wooden column 

shaft was repaired during the restoration. Photo: Courtesy Dell 

Corporation. 

Advantages: 

• can be used for repair/replacement 

• lightweight, easily installed 

• good casting ability; molds can be 

taken from building 

• material can be sanded and carved. 

• color and ultra-violet screening can 

be added; takes paint well 

• durable, rot and fungus resistant 

Disadvantages: 

• materials are flammable and generate 

heat as they cure and may be toxic 

when burned 

• toxic materials require special protection for operator and adequate ventilation while curing 

• material may be subject to ultra-violet 

deterioration unless coated or filters 

added 

• rigidity of material often must be 

modified with fillers to match expansion coefficients 

• vapor impermeable 

Checklist: 

• Are historic materials available for 

molds, or for splicing-in as a repair 

option? 

• Has the epoxy resin been formulated 

within the expansion/contraction coefficients of adjacent materials? 

• Have samples been matched for 

color/finish? 

• Are fabricators/installers experienced? 

• Is there a sound sub-strate of material 

to avoid deterioration behind new 

material? 

• Are there performance standards? 

Columns were repaired and a capital was replaced in epoxy on this 19th-century 2-story porch. Photo: Dell Corporation 

13 
PROs and CONs of VARIOUS SUBSTITUTE MATERIALS 

Epoxies (Epoxy Concretes, Polymer Concretes): 

Material: Epoxy is a resinous two-part thermo-setting 

material used as a consolidant, an adhesive, a patching 

compound, and as a molding resin. It can repair damaged 

material or recreate lost features. The resins which are 

poured into molds are usually mixed with fillers such as 

sand, or glass spheres, to lighten the mix and modify their 

expansion/contraction properties. When mixed with aggregates, such as sand or stone chips, they are often called 

epoxy concrete or polymer concrete, which is a misnomer 

as there are no cementitious materials contained within the 

mix. Epoxies are vapor impermeable, which makes detailing 

of the new elements extremely important so as to avoid 

trapping moisture behind the replacement material. It can 

be used with wood, stone, terra cotta, and various metals. 

Application: Epoxy is one of the most versatile of the new 

materials. It can be used to bind together broken fragments 

of terra cotta; to build up or infill missing sections of ornamental metal; or to cast missing elements of wooden ornaments. Small cast elements can be attached to existing 

materials or entire new features can be cast. The resins are 

poured into molds and due to the rapid setting of the 

material and the need to avoid cracking, the molded units 

are generally small or hollow inside. Multiple molds can be 

combined for larger elements. With special rods, the epoxies can be structurally reinforced. Examples of epoxy 

replacement pieces include: finials, sculptural details, small 

column capitals, and medallions. 

This replica column capital was made using epoxy resins poured 

into a mold taken from the building. The historic wooden column 

shaft was repaired during the restoration. Photo: Courtesy Dell 

Corporation. 

Advantages: 

• can be used for repair/replacement 

• lightweight, easily installed 

• good casting ability; molds can be 

taken from building 

• material can be sanded and carved. 

• color and ultra-violet screening can 

be added; takes paint well 

• durable, rot and fungus resistant 

Disadvantages: 

• materials are flammable and generate 

heat as they cure and may be toxic 

when burned 

• toxic materials require special protection for operator and adequate ventilation while curing 

• material may be subject to ultra-violet 

deterioration unless coated or filters 

added 

• rigidity of material often must be 

modified with fillers to match expansion coefficients 

• vapor impermeable 

Checklist: 

• Are historic materials available for 

molds, or for splicing-in as a repair 

option? 

• Has the epoxy resin been formulated 

within the expansion/contraction coefficients of adjacent materials? 

• Have samples been matched for 

color/finish? 

• Are fabricators/installers experienced? 

• Is there a sound sub-strate of material 

to avoid deterioration behind new 

material? 

• Are there performance standards? 

Columns were repaired and a capital was replaced in epoxy on this 19th-century 2-story porch. Photo: Dell Corporation 

13 
PRESERVATION 

BRIEFS 

Rehabilitating Interiors 

in Historic Buildings 

Identifying and Preserving Character-defining 

Elements 

H. Ward Jandl 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

A floor plan, the arrangement of spaces, and features and applied finishes may be individually or collectively important in 

defining the historic character of the building and the purpose for which it was constructed. Thus, their identification, retention, protection, and repair should be given prime consideration in every preservation project. Caution should be exercised in 

developing plans that would radically change character-defining spaces or that would obscure, damage or destroy interior 

features or finishes. 

While the exterior of a building may be its most 

prominent visible aspect, or its " public face," its interior can be even more important in conveying the 

building's history and development over time. 

Rehabilitation within the context of the Secretary of 

the Interior's Standards for Rehabilitation calls for the 

preservation of exterior and interior portions or 

features of the building that are significant to its 

historic, architectural and cultural values. 

Interior components worthy of preservation may include the building'S plan (sequence of spaces and circulation patterns), the building'S spaces (rooms and 

volumes), individual architectural features, and the 

various finishes and materials that make up the 

walls, floors, and ceilings. A theater auditorium or 

sequences of rooms such as double parlors or a lobby 

leading to a stairway that ascends to a mezzanine 

may comprise a building'S most important spaces. Individual rooms may contain notable features such as 

plaster cornices, millwork, parquet wood floors, and 

hardware . Paints, wall coverings, and finishing 

techniques such as graining, may provide color, texture, and patterns which add to abuilding's unique 

character. 

Virtually all rehabilitations of historic buildings involve some degree of interior alteration, even if the 

buildings are to be used for their original purpose. 

Interior rehabilitation proposals may range from 

preservation of existing features and spaces to total 

reconfigurations. In some cases, depending on the 

building, restoration may be warranted to preserve 

historic character adequately; in other cases, extensive 

alterations may be perfectly acceptable . 

This Preservation Brief has been developed to assist 

building owners and architects in identifying and 

evaluating those elements of a building'S interior that 

contribute to its historic character and in planning for 

the preservation of those elements in the process of 

rehabilitation . The guidance applies to all building 

types and styles, from 18th century churches to 20th 

century office buildings. The Brief does not attempt 

to provide specific advice on preservation techniques 

and treatments, given the vast range of buildings, but 

rather suggests general preservation approaches to 

guide construction work. 

Identifying and Evaluating the Importance 

of Interior Elements Prior to Rehabilitation 

Before determining what uses might be appropriate 

and before drawing up plans, a thorough professional 

assessment should be undertaken to identify those 

tangible architectural components that, prior to 

rehabilitation, convey the building's sense of time 

and place-that is , its "historic character." Such an 

assessment, accomplished by walking through and 

taking account of each element that makes up the interior, can help ensure that a truly compatible use for 

the building, one that requires minimal alteration to 

the building, is selected. 

Researching The Building's History 

A review of the building'S history will reveal why 

and when the building achieved significance or how 

it contributes to the significance of the district. This 

information helps to evaluate whether a particular 

rehabilitation treatment will be appropriate .to the 

building and whether it will preserve those tangible 

components of the building that convey its 

significance for association with specific events or 

persons along with its architectural importance. In 

this regard, National Register files may prove useful 

in explaining why and for what period of time the 
building is significant. In some cases research may 

show that later alterations are significant to the 

building; in other cases, the alterations may be 

without historical or architectural merit, and may be 

removed in the rehabilitation. 

Identifying Interior Elements 

Interiors of buildings can be seen as a series of 

primary and secondary spaces. The goal of the 

assessment is to identify which elements contribute to 

the building's character and which do not. 

Sometimes it will be the sequence and flow of spaces, 

and not just the individual rooms themselves, that 

contribute to the building's character. This is particularly evident in buildings that have strong central 

axes or those that are consciously asymmetrical in 

design. In other cases, it may be the size or shape of 

the space that is distinctive. The importance of some 

interiors may not be readily apparent based on a 

visual inspection; sometimes rooms that do not appear to be architecturally distinguished are associated 

with important persons and events that occurred 

within the building. 

Primary spaces, are found in all buildings, both 

monumental and modest. Examples may include 

foyers, corridors, elevator lobbies, assembly rooms, 

stairhalls, and parlors. Often they are the places in 

the building that the public uses and sees; sometimes 

they are the most architecturally detailed spaces in 

the building, earefully proportioned and finished with 

costly materials. They may be functionally and architecturally related to the building's external appearance. In a simpler building, a primary space may 

be distinguishable only by its location, size, proportions, or use. Primary spaces are always important to 

the character of the building and should be 

preserved. 

Secondary spaces are generally more utilitarian in 

appearance and size than primary spaces. They may 

include areas and rooms that service the building, 

such as bathrooms, and kitchens. Examples of 

secondary spaces in a commercial or office structure 

may include storerooms, service corridors, and in 

some cases, the offices themselves. Secondary spaces 

tend to be of less importance to the building and may 

accept greater change in the course of work without 

compromising the building's historic character. 

Spaces are often designed to interrelate both visually and functionally. The sequence of spaces, such 

as vestibule-haIl-parlor or foyer-Iobby-stair-auditorium 

or stairhall-corridor-classroom, can define and express 

the building's historic function and unique character. 

Important sequences of spaces should be identified 

and retained in the rehabilitation project. 

Floor plans may also be distinctive and 

characteristic of a style of architecture or a region. Examples include Greek Revival and shotgun houses. 

Floor plans may also reflect social, educational, and 

medical theories of the period. Many 19th century 

psychiatric institutions, for example, had plans based 

on the ideas of Thomas Kirkbride, a Philadelphia doctor who authored a book on asylum design. 

In addition to evaluating the relative importance of 

the various spaces, the assessment should identify architectural features and finishes that are part of the 

2 

Figure 1. This architect-designed interior reflects early 20th' century American taste: the checkerboard tile floor, wood wainscot, 

coffered ceiling, and open staircase are richly detailed and crafted 

by hand. Not only are the individual architectural features worthy 

of preservation, but the planned sequence of spaces-entry hall, stairs, stair landings, and loggia-imparts a grandeur that is 

characteristic of high style residences of this period. This interior 

is of Greystone, Los Angeles, California. Photography for HABS 

by Jack E. Boucher 

Figure 2. The interiors of mills and industrial buildings frequently 

are open, unadorned spaces with exposed structural elements. 

While the new uses to which this space could be put are manyretail, residential, or office-the generous floor-to-ceiling height 

and exposed truss system are important character-defining features 

and should be retained in the process of rehabilitation. 
DINING 

ROOM 

PARLOR 

KITCHEN 

SITTING 

ROOM 

APT. 2 

APT. ~ 

Figure 3. The floor plan at left is characteristic of many 19th century Greek Revival houses, with large rooms flanking a central hall. 

In the process of rehabilitation, the plan (at right) was drastically altered to accommodate two duplex apartments. The open stair was 

replaced with one that is enclosed, two fireplaces were eliminated, and Greek Revival trim around windows and doors was removed. 

The symmetry of the rooms themselves was destroyed with the insertion of bathrooms and kitchens. Few vestiges of the 19th century 

interior survived the rehabilitation. Drawing by Neal A. Vogel 

Figure 4. Many institutional buildings possess distinctive spaces or floor plans that are important in conveying the significance of 

the property. Finding new compatible uses for these buildings and 

preserving the buildings' historic character can be a difficult, if 

not impossible, task. One such case is Mechanics Hall in 

Worcester, Massachusetts, constructed between 1855 and 1857. 

This grand hall, which occupies the entire third floor of the 

building, could not be subdivided without destroying the integrity 

of the space. 

interior's history and character. Marble or wood 

wainscoting in corridors, elevator cabs, crown 

molding, baseboards, mantels, ceiling medallions, 

window and door trim, tile and parquet floors, and 

staircases are among those features that can be found 

in historic buildings. Architectural finishes of note 

may include grained woodwork, marbleized columns, 

and plastered walls. Those features that are 

characteristic of the building's style and period of 

construction should, again, be retained in the 

rehabilitation. 

Figure 5. The interior of a simply detailed worker's house of the 

19th century may be as important historically as the richly ornamented interior seen in figure 1. Although the interior of this 

house has not been properly maintained, the wide baseboards, flat 

window trim, and four-panel door are characteristic of workers' 

housing during this period and deserve retention during 

rehabilitation. 

Features and finishes, even if machine-made and 

not exhibiting particularly fine craftsmanship, may be 

character-defining; these would include pressed metal 

ceilings and millwork around windows and doors. 

The interior of a plain, simple detailed worker's 

house of the 19th century may be as important 

historically as a richly ornamented, high-style 

townhouse of the same period. Both resources, if 

equally intact, convey important information about 

the early inhabitants and deserve the same careful attention to detail in the preservation process. 

3 
The location and condition of the building's existing 

heating, plumbing, and electrical systems also need 

to be noted in the assessment. The visible features of 

historic systems-radiators, grilles, light fixtures, 

switchplates, bathtubs, etc.-can contribute to the 

overall character of the building, even if the systems 

themselves need upgrading. 

Assessing Alterations and Deterioration 

In assesessing a building's interior, it is important 

to ascertain the extent of alteration and deterioration 

that may have taken place over the years; these factors help determine what degree of change is appropriate in the project. Close examination of existing 

fabric and original floorplans, where available, can 

reveal which alterations have been additive, such as 

new partitions inserted for functional or structural 

reasons and historic features covered up rather than 

destroyed. It can also reveal which have been subtractive, such as key walls removed and architectural 

features destroyed. If an interior has been modified 

by additive changes and if these changes have not acquired significance, it may be relatively easy to 

remove the alterations and return the interior to its 

historic appearance. If an interior has been greatly 

altered through subtractive changes, there may be 

more latitude in making further alterations in the 

process of rehabilitation because the integrity of the 

interior has been compromised. At the same time, if 

the interior had been exceptionally significant, and 

solid documentation on its historic condition is 

available, reconstruction of the missing features may 

be the preferred option. 

It is always a recommended practice to photograph 

interior spaces and features thoroughly prior to 

rehabilitation. Measured floor plans showing the existing conditions are extremely useful. This documentation is invaluable in drawing up rehabilitation plans 

and specifications and in assessing the impact of 

changes to the property for historic preservation certification purposes. 

Drawing Up Plans and Executing Work 

If the historic building is to be rehabilitated, it is 

critical that the new use not require substantial alteration of distinctive spaces or removal of characterdefining architectural features or finishes. If an interior loses the physical vestiges of its past as well as 

its historic function, the sense of time and place 

associated both with the building and the district in 

which it is located is lost. 

The recommended approaches that follow address 

common problems associated with the rehabilitation 

of historic interiors and have been adapted from the 

Secretary of the Interior's Standards for Rehabilitation 

and Guidelines for Rehabilitating Historic Buildings. 

Adherence to these suggestions can help ensure that 

character-defining interior elements are preserved in 

the process of rehabilitation. The checklist covers a 

range of situations and is not intended to be allinclusive. Readers are strongly encouraged to review 

the full set of guidelines before undertaking any 

rehabilitation project. 

4 

Figure 6. This corridor, located in the historic Monadnock 

Building in Chicago, has glazed walls, oak trim, and marble 

wainscotting, and is typical of those found in late 19th and early 

20th century office buildings. Despite the simplicity of the 

features, a careful attention to detail can be noted in the patterned 

tile floor, bronze mail chute, and door hardware. The retention of 

corridors like this one should be a priority in rehabilitation projects involving commercial buildings. 

Figure 7. When the Monadnock Building was rehabilitated, architects retained the basic floor plan on the upper floors consisting 

of a double-loaded corridor with offices opelling onto it. The 

original floor-to-ceiling height in the corridors and outside 

offices-the most important spaces-was maintained by installing 

needed air conditioning ductwork in the less important anterooms. 

In this way, the most significant interior spaces were preserved 

intact. Drawing by Neal A. Vogel 
Recommended Approaches for Rehabilitating Historic Interiors 

1. Retain and preserve floor plans and interior 

spaces that are important in defining the overall 

historic character of the building. This includes the 

size, configuration, proportion, and relationship of 

rooms and corridors; the relationship of features to 

spaces; and the spaces themselves such as lobbies, 

reception halls, entrance halls, double parlors, 

theaters, auditoriums, and important industrial or 

commercial use spaces. Put service functions 

required by the building's new use, such as 

bathrooms, mechanical equipment, and office 

machines, in secondary spaces. 

2. Avoid subdividing spaces that are characteristic 

of a building type or style or that are directly 

associated with specific persons or patterns of 

events. Space may be subdivided both vertically 

through the insertion of new partitions or horizontally through insertion of new floors or mezzanines. The insertion of new additional floors 

should be considered only when they will not 

damage or destroy the structural system or 

obscure, damage, or destroy character-defining 

spaces, features, or finishes. If rooms have already 

been subdivided through an earlier insensitive 

renovation, consider removing the partitions and 

restoring the room to its original proportions and 

size. 

3. Avoid making new cuts in floors and ceilings 

where such cuts would change character-defining 

spaces and the historic configuration of such 

spaces. Inserting of a new atrium or a lightwell is 

appropriate only in very limited situations where 

the existing interiors are not historically or architecturally distinguished. 

4. A void installing dropped ceilings below ornamental ceilings or in rooms where high ceilings 

are part of the building's character. In addition to 

obscuring or destroying significant details, such 

treatments will also change the space's proportions. If dropped ceilings are installed in buildings 

that lack character-defining spaces, such as mills 

and factories, they should be well set back from 

the windows so they are not visible from the 

exterior. 

5. Retain and preserve interior features and 

finishes that are important in defining the overall 

historic character of the building. This might include columns, doors, cornices, baseboards, 

fireplaces and mantels, paneling, light fixtures, 

elevator cabs, hardware, and flooring; and 

wallpaper, plaster, paint, and finishes such as stenciling, marbleizing, and graining; and other 

decorative materials that accent interior features 

and provide color, texture, and patterning to walls, 

floors, and ceilings. 

6. Retain stairs in their historic configuration and 

location. If a second means of egress is required, 

consider constructing new stairs in secondary 

spaces. (For guidance on designing compatible new 

additions, see Preservation Brief 14, "New Exterior 

Additions to Historic Buildings.") The application 

of fire-retardant coatings, such as intumescent 

paints; the installation of fire suppression systems, 

such as sprinklers; and the construction of glass 

enclosures can in many cases permit retention of 

stairs and other character-defining features. 

7. Retain and preserve visible features of early 

mechanical systems that are important in defining 

the overall historic character of the building, such 

as radiators, vents, fans, grilles, plumbing fixtures, 

switchplates, and lights. If new heating, air conditioning, lighting and plumbing systems are installed, they should be done in a way that does 

not destroy character-defining spaces, features and 

finishes. Ducts, pipes, and wiring should be installed as inconspicuously as possible: in secondary 

spaces, in the attic or basement if possible, or in 

closets. 

8. Avoid "furring out" perimeter walls for insulation purposes. This requires unnecessary removal 

of window trim and can change a room's proportions. Consider alternative means of improving 

thermal performance, such as installing insulation 

in attics and basements and adding storm 

windows. 

9. Avoid removing paint and plaster from traditionally finished surfaces, to expose masonry and 

wood. Conversely, avoid painting previously unpainted millwork. Repairing deteriorated plasterwork is encouraged. If the plaster is too 

deteriorated to save, and the walls and ceilings are 

not highly ornamented, gypsum board may be an 

acceptable replacement material. The use of paint 

colors appropriate to the period of the building's 

construction is encouraged. 

10. Avoid using destructive methods-propane and 

butane torches or sandblasting-to remove paint or 

other coatings from historic features . Avoid harsh 

cleaning agents that can change the appearance of 

wood. (For more information regarding appropriate 

cleaning methods, consult Preservation Brief 6, 

" Dangers of Abrasive Cleaning to Historic 

Buildings.' ') 

5 
Figure 8. Furring out exterior walls to add insulation and suspending new ceilings to hide ductwork and wiring can change a room 's 

proportions and can cause interior features to appear fragmented. In this case, a school was converted into apartments, and individual 

classrooms became living rooms, bedrooms, and kitchens. On the left is an illustration of a classroom prior to rehabilitation; note the 

generous floor-to-ceiling height, wood wainscotting, molded baseboard, picture molding, and Eastlake Style door and window trim. After 

rehabilitation, on the right, only fragments of the historic detailing survive: the ceiling has been dropped be/ow the picture molding, the 

remaining wainscotting appears to be randomly placed, and some of the window trim has been obscured. Together with the subdivision 

of the classrooms, these rehabilitation treatments prevent a clear understanding of the original classroom 's design and space. If thermal 

performance must be improved, alternatives to furring out walls and suspending new ceilings, such as installing insulation in attics and 

basements, should be considered. Drawings by Neal A. Vogel 

Figure 9. The tangible reminders of early mechanical systems can 

be worth saving. In this example, in the Old Post Office in 

Washington, D. c., radiators encircle Corinthian columns in a 

decorative manner. Note, too, the period light fixtures. These 

features were retained when the building was rehabilitated as 

retail and office space. Photo: Historic American Buildings Survey 

6 

Figure 10. In this case plaster has been removed from perimeter 

walls, leaving brick exposed. In removing finishes from historic 

masonry walls, not only is there a loss of historic finish, but raw, 

unfinished walls are exposed, giving the interior an appearance it 

never had. Here, the exposed brick is of poor quality and the 

mortar joints are wide and badly struck. Plaster should have been 

retained and repaired, as necessary. 
Figure 11. These dramatic "before" and "after" photographs show a severely deteriorated space restored to its original elegance: plaster 

has been repaired and painted, the scagliola columns have been restored to match marble using traditional craft techniques, and missing 

decorative metalwork has been re-installed in front of the windows. Although some reorganization of the space took place, notably the 

relocation of the front desk, the overall historic character of the space has been preserved. These views are of the lobby in the Willard 

Hatel, Washington, D.C. Credit: Commercial Photographers (left); Carol M. Highsmith (right) 

Meeting Building, Life Safety and Fire 

Codes 

Buildings undergoing rehabilitation must comply 

with existing building, life safety and fire codes. The 

application of codes to specific projects varies from 

building to building, and town to town. Code requirements may make some reuse proposals impractical; in other cases, only minor changes may be 

needed to bring the project into compliance. In some 

situations, it may be possible to obtain a code 

variance to preserve distinctive interior features. (It 

should be noted that the Secretary's Standards for 

Rehabilitation take precedence over other regulations 

and codes in determining whether a rehabilitation 

project qualifies for Federal tax benefits.) A thorough 

understanding of the applicable regulations and close 

coordination with code officials, building inspectors, 

and fire marshals can prevent the alteration of significant historic interiors. 

Sources of Assistance 

Rehabilitation and restoration work should be 

undertaken by professionals who have an established 

reputation in the field. 

Given the wide range of interior work items, from 

ornamental plaster repair to marble cleaning and the 

application of graining, it is possible that a number of 

specialists and subcontractors will need to be brought 

in to bring the project to completion. State Historic 

Preservation Officers and local preservation organizations may be a useful source of information in this 

regard. Good sources of information on appropriate 

preservation techniques for specific interior features 

and finishes include the Bulletin of the Association for 

Preservation Technology and The Old-House Journal; 

other useful publications are listed in the 

bibliography. 

Protecting Interior Elements During 

Rehabilitation 

Architectural features and finishes to be preserved 

in the process of rehabilitation should be clearly 

marked on plans and at the site. This step, along with 

careful supervision of the interior demolition work 

and protection against arson and vandalism, can prevent the unintended destruction of architectural 

elements that contribute to the building's historic 

character. 

Protective coverings should be installed around 

architectural features and finishes to avoid damage in 

the course of construction work and to protect 

workers. Staircases and floors, in particular, are subjected to dirt and heavy wear, and the risk exists of 

incurring costly or irreparable damage. In most cases, 

the best, and least costly, preservation approach is to 

design and construct a protective system that enables 

stairs and floors to be used yet protects them from 

damage. Other architectural features such as mantels, 

doors, wainscotting, and decorative finishes may be 

protected by using heavy canvas or plastic sheets. 

Summary 

In many cases, the interior of a historic building is 

as important as its exterior. The careful identification 

and evaluation of interior architectural elements, after 

undertaking research on the building's history and 

use, is critically important before changes to the 

building are contemplated. Only after this evaluation 

should new uses be decided and plans be drawn up. 

The best rehabilitation is one that preserves and protects those rooms, sequences of spaces, features and 

finishes that define and shape the overall historic 

character of the building. 
This Preservation Brief is based on a discussion paper 

prepared by the author for a National Park Service 

regional workshop held in March, 1987, and on a 

paper written by Gary Hume, "Interior Spaces in 

Historic Buildings," October, 1987. Appreciation is 

extended to the staff of Technical Preservation Services Branch and to the staff of NPS regional offices 

who reviewed the manuscript and provided many 

useful suggestions. Special thanks are given to Neal 

A. Vogel, a summer intern with the NPS, for many 

of the illustrations in this Brief. 

This publication has been prepared pursuant to the 

National Historic Preservation Act of 1966, as 

amended. Preservation Briefs 18 was developed 

under the editorship of Lee H. Nelson, FAIA, Chief, 

Preservation Assistance Division, National Park Service, U.S. Department of the Interior, P.O. Box 37127, 

Washington, D.C. 20013-7127. Comments on the 

usefulness of this information are welcomed and may 

be sent to Mr. Nelson at the above address. This 

publication is not copyrighted and can be reproduced 

without penalty. Normal procedures for credit to the 

author and the National Park Service are appreciated. 

Selected Reading List 

There are few books written exclusively on preserving historic interiors, and most of these tend to focus 

on residential interiors. Articles on the subject appear 

regularly in The Old-House Journal, the Bulletin of the 

Association for Preservation Technology, and Historic 

Preservation Magazine. 

Ferro, Maximilian 1., and Melissa 1. Cook. Electric 

Wiring and Lighting in Historic American Buildings. 

New Bedford, Massachusetts: AFC/A Nortek 

Company, 1984. 

Fisher, Charles E. Temporary Protection of Historic Stairways During Rehabilitation Work. Preservation Tech 

Note. Washington, D. c.: Preservation Assistance 

Division, National Park Service, U.S. Department 

of the Interior, 1985. 

Jennings, Jan, and Herbert Gottfried. American Vernacular Interior Architecture 1870-1940. New York: 

Van Nostrand Reinhold Company, 1988. 

Johnson, Ed. Old House Woodwork Restoration: How to 

Restore Doors, Windows, Walls, Stairs and Decorative 

Trim to Their Original Beauty. Englewood Cliffs, 

New Jersey: Prentice-Hall, Inc., 1983. 

Labine, Clem, and Carolyn Flaherty (editors). The 

Old-House Journal Compendium. Woodstock, New 

York: The Overlook Press, 1980. 

The Secretary of the Interior's Standards for Rehabilitation 

and Guidelines for Rehabilitating Historic Buildings. 

Washington, D. c.: Preservation Assistance Division, National Park Service, U.s. Department of 

the Interior, rev. 1983. 

U.S. Department of Housing and Urban Development. Rehabilitation Guidelines, volumes 1-11. 

Washington, D.C.: U.S. Department of Housing 

and Urban Development, 1980-84. 

Winkler, Gail Caskey, and Roger W. Moss. Victorian 

Interior Decoration: American Interiors 1830-1900. 

New York: Henry Holt and Company, 1986. 

October 1988 

Cover: Detail of carving on interior shutter. Hammond-Harwood 

House, Annapolis, Maryland. 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
PRESERVATION 

BRIEFS 

Heating, Ventilating, and Cooling 

Historic Buildings: 

Problems and Recommended Approaches 

Sharon C. Park, AlA 

u.s. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

The need for modern mechanical systems is one of the 

most common reasons to undertake work on historic 

buildings. Such work includes upgrading older mechanical systems, improving the energy efficiency of 

existing buildings, installing new heating, ventilation or 

air conditioning (HVAC) systems, or-particularly for 

museums-installing a climate control system with 

humidification and dehumidification capabilities. Decisions to install new HVAC or climate control systems 

often result from concern for occupant health and comfort, the desire to make older buildings marketable, or 

the need to provide specialized environments for operating computers, storing artifacts, or displaying museum collections. Unfortunately, occupant comfort and 

concerns for the objects within the building are sometimes given greater consideration than the building 

itself. In too many cases, applying modern standards 

of interior climate comfort to historic buildings has 

proven detrimental to historic materials and decorative 

finishes. 

This Preservation Brief underscores the importance of 

careful planning in order to balance the preservation 

objectives with interior climate needs of the building. It 

is not intended as a technical guide to calculate tonnage or to size piping or ductwork. Rather, this Brief 

identifies some of the problems associated with installing mechanical systems in historic buildings and recommends approaches to minimizing the physical and 

visual damage associated with installing and maintaining these new or upgraded systems. 

Historic buildings are not easily adapted to house 

modern precision mechanical systems. Careful planning must be provided early on to ensure that decisions made during the design and installation phases 

of a new system are appropriate. Since new mechanical 

and other related systems, such as electrical and fire 

suppression, can use up to 10% of a building'S square 

footage and 30%-40% of an overall rehabilitation 

budget, decisions must be made in a systematic and 

coordinated manner. The installation of inappropriate 

mechanical systems may result in any or all of the 

following: 

• large sections of historic materials are removed to 

install or house new systems. 

• historic structural systems are weakened by carrying 

the weight of, and sustaining vibrations from, large 

equipment. 

• moisture introduced into the building as part of a 

new system migrates into historic materials and 

causes damage, including biodegradation, freeze/ 

thaw action, and surface staining. 

• exterior cladding or interior finishes are stripped to 

install new vapor barriers and insulation. 

• historic finishes, features, and spaces are altered by 

dropped ceilings and boxedchases or by poorly located grilles, registers, and equipment. 

• systems that are too large or too small are installed 

before there is a clearly planned use or a new tenant. 

For historic properties it is critical to understand what 

spaces, features, and finishes are historic in the building, what should be retained, and what the realistic 

heating, ventilating, and cooling needs are for the 

building, its occupants, and its contents. A systematic 

approach, involving preservation planning, preservation design, and a follow-up program of monitoring 

and maintenance, can ensure that new systems are 

successfully added-or existing systems are suitably 

upgraded-while preserving the historic integrity of tl}e 

building. 

No set formula exists for determining what type of 

mechanical system is best for a specific building. Each 

building and its needs must be evaluated separately. 

Some buildings will be so significant that every effort 

must be made to protect the historic materials and systems in place with minimal intrusion from new systems. Some buildings will have museum collections 

that need special climate control. In such cases, curatorial needs must be considered-but not to the ultimate detriment of the historic building resource. Other 
buildings will be rehabilitated for commercial use. For 

them, a variety of systems might be acceptable, as long 

as significant spaces, features, and finishes are retained. 

Most mechanical systems require upgrading or replacement within 15-30 years due to wear and tear or 

the availability of improved technology. Therefore, historic buildings should not be greatly altered or otherwise sacrificed in an effort to meet short-term systems 

objectives. 

History of Mechanical Systems 

The history of mechanical systems in buildings involves 

a study of inventions and ingenuity as building owners, architects, and engineers devised ways to improve 

the interior climate of their buildings. Following are 

highlights in the evolution of heating, ventilating, and 

cooling systems in historic buildings. 

Eighteenth Century. Early heating and ventilation in 

America relied upon common sense methods of managing the environment (see figure 1). Builders purposely 

sited houses to capture winter sun and prevailing summer cross breezes; they chose materials that could help 

protect the inhabitants from the elements, and took 

precautions against precipitation and damaging drainage patterns. The location and sizes of windows, 

doors, porches, and the floor plan itself often evolved 

to maximize ventilation. Heating was primarily from 

fireplaces or stoves and, therefore, was at the source of 

delivery. In 1744, Benjamin Franklin designed his 

"Pennsylvania stove" with a fresh air intake in order to 

maximize the heat radiated into the room and to minimize annoying smoke. 

Thermal insulation was rudimentary-often wattle 

and daub, brick and wood nogging. The comfort level 

for occupants was low, but the relatively small difference between internal and external temperatures and 

relative humidity allowed building materials to expand 

and contract with the seasons. 

Regional styles and architectural features reflected 

regional climates. In warm, dry and sunny climates, 

thick adobe walls offered shelter from the sun and kept 

the inside temperatures cool. Verandas, courtyards, 

porches, and high ceilings also reduced the impact of 

the sun. Hot and humid climates called for elevated 

living floors, louvered grilles and shutters, balconies, 

and interior courtyards to help circulate air. 

Nineteenth Century. The industrial revolution provided the technological means for controlling the environment for the first time (see figure 2). The dual 

developments of steam energy from coal and industrial 

mass production made possible early central heating 

systems with distribution of heated air or steam using 

metal ducts or pipes. Improvements were made to 

early wrought iron boilers and by late century, steam 

and low pressure hot water radiator systems were in 

common use, both in offices and residences. Some 

large institutional buildings heated air in furnaces and 

distributed it throughout the building in brick flues 

with a network of metal pipes delivering heated air to 

individual rooms. Residential designs of the period 

often used gravity hot air systems utilizing decorative 

floor and ceiling grilles. 

Ventilation became more scientific and the introduc2 

1. Eighteenth century and later vernacular architecture depended on the 

siting of the building, deciduous trees, cross ventilation, and the placement 

of windows and chimneys to maximize winter heating and natural summer 

cooling. Regional details, as seen in this Virginia house, include external 

chimneys and a separate summer kitchen to reduce fire risk and isolate heat 

in the summer. Photo: NPS Files. 

2. Nineteenth century buildings continued to use architectural features such 

as porches, cupolas, and awnings to make the buildings more comfortable in 

summer, but heating was greatly improved by hot water or steam radiators. 

Photo: NPS Files 
tion of fresh air into buildings became an important 

component of heating and cooling. Improved forced air 

ventilation became possible in mid-century with the 

introduction of power-driven fans. Architectural features such as porches, awnings, window and door 

transoms, large open-work iron roof trusses, roof monitors, cupolas, skylights and clerestory windows helped 

to dissipate heat and provide healthy ventilation. 

Cavity wall construction, popular in masonry structures, improved the insulating qualities of a building 

and also provided a natural cavity for the dissipation of 

moisture produced on the interior of the building. In 

some buildings, cinder chips and broken masonry filler 

between structural iron beams and jack arch floor 

vaults provided thermal insulation as well as fireproofing. Mineral wool and cork were new sources of 

lightweight insulation and were forerunners of contemporary batt and blanket insulation. 

The technology of the age, however, was not sufficient to produce "tight" buildings. There was still only a 

moderate difference between internal and external temperatures. This was due, in part, to the limitations of 

early insulation, the almost exclusive use of single 

glazed windows, and the absence of air-tight construction. The presence of ventilating fans and the reliance 

on architectural features, such as operable windows, 

cupolas and transoms, allowed sufficient air movement 

to keep buildings well ventilated. Building materials 

could behave in a fairly traditional way, expanding and 

contracting with the seasons. 

Twentieth Century. The twentieth century saw intensive development of new technologies and the notion 

of fully integrating mechanical systems (see figure 3). Oil 

and gas furnaces developed in the nineteenth century 

were improved and made more efficient, with electricity becoming the critical source of power for building 

systems in the latter half of the century. Forced air heating systems with ducts and registers became popular 

for all types of buildings and allowed architects to experiment with architectural forms free from mechanical 

encumbrances. In the 1920s large-scale theaters and 

auditoriums introduced central air conditioning, and by 

mid-century forced air systems which combined heating and air conditioning in the same ductwork set a 

new standard for comfort and convenience. The combination and coordination of a variety of systems came 

together in the post-World War II highrise buildings; 

complex heating and air conditioning plants, electric 

elevators, mechanical towers, ventilation fans, and full 

service electric lighting were integrated into the building's design. 

The insulating qualities of building materials improved. Synthetic materials, such as spun fiberglass 

batt insulation, were fully developed by mid-century. 

Prototypes of insulated thermal glazing and integral 

storm window systems were promoted in construction 

journals. Caulking to seal out perimeter air around 

window and door openings became a standard construction detail. 

The last quarter of the twentieth century has seen 

making HVAC systems more energy efficient and better 

integrated. The use of vapor barriers to control moisture migration, thermally efficient windows, caulking 

and gaskets, compressed thin wall insulation, has become standard practice. New integrated systems now 

combine interior climate control with fire suppression, 

lighting, air filtration, temperature and humidity control, and security detection. Computers regulate the 

performance of these integrated systems based on the 

time of day, day of the week, occupancy, and outside 

ambient temperature. 

3. The circa 1928 Fox Theater in Detroit, designed by C. Howard Crane, 

was one of the earliest twentieth century buildings to provide air conditioning to its patrons. The early water-cooled system was recently restored. 

Commercial and highrise buildings of the twentieth century were able, 

mostly through electrical power, to provide sophisticated systems that integrated many building services. Photo: William Kessler and Associates, 

Arch i tects. 

Climate Control and Preservation 

Although twentieth century mechanical systems technology has had a tremendous impact on making historic buildings comfortable, the introduction of these 

new systems in older buildings is not without problems. The attempt to meet and maintain modern climate control standards may in fact be damaging to historic 

resources. Modern systems are often over-designed to 

compensate for inherent inefficiencies of some historic 

buildings materials and plan layouts. Energy retrofit 

measures, such as installing exterior wall insulation 

and vapor barriers or the sealing of operable window 

and vents, ultimately affect the performance and can 

reduce the life of aging historic materials. 

3 
In general, the greater the differential between the 

interior and exterior temperature and humidity levels, 

the greater the potential for damage. As natural vapor 

pressure moves moisture from a warm area to a colder, 

dryer area, condensation will occur on or in building 

materials in the colder area (see figure 4). Too little 

humidity in winter, for example, can dry and crack 

historic wooden or painted surfaces. Too much humidity in winter causes moisture to collect on cold surfaces, such as windows, or to migrate into walls. As a 

result, this condensation deteriorates wooden or metal 

windows and causes rotting of walls and wooden 

structural elements, dampening insulation and holding 

moisture against exterior surfaces. Moisture migration 

through walls can cause the corrosion of metal anchors, angles, nails or wire lath, can blister and peel 

exterior paint, or can leave efflorescence and salt deposits on exterior masonry. In cold climates, freezethaw damage can result from excessive moisture in 

external walls. 

To avoid these types of damage to a historic building, 

it is important to understand how building components 

work together as a system. Methods for controlling 

interior temperature and humidity and improving ventilation must be considered in any new or upgraded 

HVAC or climate control system. While certain energy 

retrofit measures will have a positive effect on the overall building, installing effective vapor barriers in historic walls is difficult and often results in destruction of 

significant historic materials (see figure 5). 

u.: 

o 

70 

60 

~ 50 :::J 

'§ 

~40 E 

~ 

30 

20 

outside inside 

~ C) 

lC1 

1Iu.: o O>.~ 

:4::: l.() 

"0 'E II :::J_ 

..c: c Q)"O 

> c. '~ 3= 

~~ ~ C) 

0> 

1Iu.: o O>.C'") 

.~ co "0 'E II :::J~ 

..c: c w"o 

> c. '~ 3= 

-~"O '" 

outside 

5. The installation of vapor retarders in walls of historic buildings in an 

effort to contain interior moisture can cause serious damage to historic 

finishes as shown here. In this example, all the wall plaster and lath have 

been stripped in preparation for a vapor barrier prior to replastering, Controlling interior temperature and relative humidity can be more effective than 

adding insulation and vapor barriers to historic perimeter walls. Photo: 

Ernest A. Conrad, P.E. 

inside 

95 

90 

85 'f 

~ outside 80.'3 ~ sealant c. '" 

inside 

75 E 

~ electrical 

70 outlet -

65 condensation 

b. Hot, Humid Climate Condition air leak~~:+;i;) 

vapor retarder *,"~~'ir"'+~:-:::~~tTC. Moisture Condensation in Walls 

4. Mechanical heating and cooling systems change the interior climate of a building, Moisture in the air will dissipate from the warmer area of a building to 

the colder area and can cause serious deterioration of historic materials. Condensation can form if the dew point occurs within the building wall, particularly 

one that has been insulated (see a and b), Even when vapor retarders are installed (c), any non-continuous areas will provide spaces for moisture to pass. Wall 

Section Drawings: NPS files 

4 
Planning the New System 

Climate control systems are generally classified according to the medium used to condition the tem~erature: 

air, water, or a combination of both (see overvIew on 

page 6). The complexity of choices facinp a buildinp 

owner or manager means that a systematic approach IS 

critical in determining the most suitable system for a 

building, its contents, and its occup~nts. ~o m~tter . 

which system is installed, a change m the mtenor chmate will result. This physical change will in turn affect 

how the building materials perform. New registers, 

grilles, cabinets, or other accessories associated with 

the new mechanical system will also visually change 

the interior (and sometimes the exterior) appearance of 

the building. Regardless of the type or extent of a mechanical system, the owner of a historic buildi~g . 

should know before a system is installed what It will 

look like and what problems can be anticipated during 

the life of that system. The potential harm to a build~g 

and costs to an owner of selecting the wrong mechamcal system are very great. . 

The use of a building and its contents will largely 

determine the best type of mechanical system. The 

historic building materials and construction technology 

as well as the size and availability of secondary spaces 

within the historic structure will affect the choice of a 

system. It may be necessary to investigate a combination of systems. In each case, the needs of the u~er, the 

needs of the building, and the needs of a collectIon or 

equipment must be considered. It may not be neces- . 

sary to have a comprehensive climate control sy'stem if 

climate-sensitive objects can be accommodated m special areas or climate-controlled display cases. It may not 

be necessary to have central air conditioning. in a mild 

climate if natural ventilation systems can be Improved 

through the use of operable windows, awnings, exhaust fans, and other "low-tech" means. Modern 

standards for climate control developed for new construction may not be achievable or desir~ble for ?istoric 

buildings. In each case, the lowest level of mterventlOn 

needed to successfully accomplish the job should be selected. 

Before a system is chosen, the following planning 

steps are recommended: 

1. Determine the use of the building. The proposed 

use of the building (museum, commercial, residential, 

retail) will influence the type of system that should be 

installed. The number of people and functions to be 

housed in a building will establish the level of comfort 

and service that must be provided. Avoid uses that 

require major modifications to significant arc~it~ctural 

spaces. What is the intensity of u~e of the buildmg: 

intermittent or constant use, speCIal events or seasonal 

events? Will the use of the building require major new 

services such as restaurants, laundries, kitchens, locker 

rooms, or other areas that generate moisture that may 

exacerbate climate control within the historic space? In 

the context of historic preservation, uses that require 

radical reconfigurations of historic spaces are inappropriate for the building. 

2. Assemble a qualified team. This team ideally 

should consist of a preservation architect, mechanical 

engineer, electrical engineer, structural engine~r, and 

preservation consultants, each knowledgeable m codes 

and local requirements. If a special use (church, museum, art studio) or a collection is involved, a specialist 

familiar with the mechanical requirements of that 

building type or collection should also be hired. 

Team members should be familiar with the needs of 

historic buildings and be able to balance complex factors: the preservation of the historic architecture (aesthetics and conservation), requirements imposed by 

mechanical systems (quantified heating and cooling 

loads), building codes (health and safety), ten.ant requirements (quality of comfort, ease of operatIon), access (maintenance and future replacement), and the 

overall cost to the owner. 

3. Undertake a condition assessment of the existing 

building and its systems. What are the existing construction materials and mechanical systems? What condition are they in and are they reusable (see figure 6)? 

Where are existing chillers, boilers, air handlers, or 

cooling towers located? Look at the condition of all 

other services that may benefit from being integrated 

into a new system, such as electrical and fire suppression systems. Where can energy efficiency be improved 

to help downsize any new equipment added, an? 

which of the historic features, e.g. shutters, awnmgs, 

skylights, can be reused (see. figure 7)? Evaluat~ air 

infiltration through the extenor envelope; momtor the 

interior for temperature and humidity levels with hygrothermographs for at least a year. Identify building, 

site, or equipment deficiencies or the presence of .asbestos that must be corrected prior to the installation or 

upgrading of mechanical systems. 

6. A condition assessment during the planning stage would identify this 

round radiator in a small oval-shaped vestibule as a significant element of the 

historic heating system. In upgrading the mechanical system, the radiator 

should be retained. Photo: Michael C. Henry, P.E. , AlA. 

5 
Overview of HVAC Systems 

WATER SYSTEMS: Hydronic radiators, Fan coil, or 

radiant pipes 

Water systems are generally called hydronic and use a network 

of pipes to deliver water to hot water radiators, radiant pipes set 

in floors or fan coil cabinets which can give both heating and 

cooling. Boilers produce hot water or steam; chillers produce 

chilled water for use with fan coil units. Thermostats control the 

temperature by zone for radiators and radiant floors. Fan coil 

units have individual controls. Radiant floors provide quiet, 

even heat, but are not common. 

Advantages: Piped systems are generally easier to install in 

historic buildings because the pipes are smaller than ductwork. 

Disadvantages: There is the risk, however, of hidden leaks in 

the wall or burst pipes in winter if boilers fail. Fan coil condensate pans can overflow if not properly maintained. Fan coils 

may be noisy. 

Hydronic Radiators: Radiators or baseboard radiators are 

looped together and are usually set under windows or along 

perimeter walls. New boilers and circulating pumps can upgrade older systems. Most piping was cast iron although copper systems can be used if separately zoned. Modern cast iron 

baseboards and copper fin-tubes are available. Historic radiators can be reconditioned. 

cast iron baseboard 

cast iron radiator 

Fan Coil Units: Fan coil systems use terminal cabinets in each 

room serviced by 2, 3, or 4 pipes approximately 1-1/2" each in 

diameter. A fan blows air over the coils which are serviced by 

hot or chilled water. Each fan coil cabinet can be individually 

controlled. Four-pipe fan coils can provide both heating and 

cooling all year long. Most piping is steel. Non-cabinet units 

may be concealed in closets or custom cabinetry, such as benches, can be built. 

return piping Fan coil unit 

6 

CENTRAL AIR SYSTEMS 

The basic heating, ventilation and air conditioning (HVAC) system is all-air, single zone fan driven designed for low, medium 

or high pressure distribution. The system is composed of compressor drives, chillers, condensers, and furnace depending 

on whether the air is heated, chilled or both. Condensers, generally air cooled, are located outside. The ducts are sheet metal 

or flexible plastic and can be insulated. Fresh air can be circulated. Registers can be designed for ceilings, floors and walls. 

The system is controlled by thermostats; one per zone. 

Advantages: Ducted systems offer a high level of control of 

interior temperature, humidity, and filtration. Zoned units can 

be relatively small and well concealed. 

Disadvantages: The damage from installing a ducted system 

without adequate space can be serious for a historic building. 

Systems need constant balancing and can be noisy. 

Basic HVAC: Most residential or small commercial systems will 

consist of a basic furnace with a cooling coil set in the unit and 

a refrigerant compressor or condenser located outside the 

building. Heating and cooling ductwork is usually shared. If 

sophisticated humidification and dehumidification is added to 

the basic HVAC system, a full climate control system results. 

This can often double the size of the equipment. 

supply 

Heating furnace with cooling coil 

Basic Heat Pump/Air System: The heat pump is a basic HVAC 

system as described above except for the method of generating 

hot and cold air. The system operates on the basic refrigeration 

cycle where latent heat is extracted from the ambient air and is 

used to evaporate refrigerant vapor under pressure. Functions 

of the condenser and evaporator switch when heating is 

needed. Heat pumps, somewhat less efficient in cold climates, 

can be fitted with electric resistance coil. 

Cooling cycle 

compressor refrigerant flow 

he.li" ~ ,',"po .. lo. 

expansion valve 

cycle reverses for heating 
COMBINED AIR AND WATER SYSTEMS 

These systems are popular for restoration work because they 

combine the ease of installation for the piped system with the 

performance and control of the ducted system. Smaller air handling units, not unlike fan coils, may be located throughout a 

building with service from a central boiler and chiller. In many 

cases the water is delivered from a central plant which services I a complex of buildings. 

This system overcomes the disadvantages of a central ducted ; 

system where there is not adequate horizontal or vertical runs I 

for the ductwork. The equipment, being smaller, may also be 

I quieter and cause less vibration. If only one air handler is being 

utilized for the building, it is possible to house all the equipment in a vault outside the building and send only conditioned 

air into the structure. 

Advantages: flexibility for installation using greater piping runs 

with shorter ducted runs; Air handlers can fit into small spaces. 

Disadvantages: piping areas may have undetected leaks; air 

handlers may be noisy. 

Water-serviced Air Handlers: 

a supply air 

t) return air 

o 

fresh air intake 

Typical Systems Layout: 

IFI~~~~ ~~~~ heating coil supply air 

~~~~~~ cooling coil 

~ humidifier foi1 I Conditioned 

, ~ Air handler space 

n 

(J return air 

Chiller In 

{} 

Fresh air intake 

OTHER SYSTEM COMPONENTS 

Non-systems components should not be overlooked if they can 

make a building more comfortable without causing damage to 

the historic resource or its collection. 

Advantages: components may provide acceptable levels of 

comfort without the need for an entire system. 

Disadvantages: Spot heating, cooling and fluxuations in humidity may harm sensitive collections or furnishings. If an integrated system is desirable, components may provide only a 

temporary solution. 

Portable Air Conditioning: 

Most individual air conditioners are set in windows or through 

exterior walls which can be visually as well as physically damaging to historic buildings. Newer portable air conditioners are 

available which sit in a room and exhaust directly to the exterior 

through a small slot created by a raised window sash. 

~ :!~~~:t air 

(Air conditioner) 

Fans: Fans should be considered in most properties to improve 

ventilation. Fans can be located in attics, at the top of stairs, or 

in individual rooms. In moderate climates, fans may eliminate 

the need to install central air systems. 

exhaust fan 

Dehumidifiers: For houses without central air handling systems, a dehumidifier can resolve problems in humid climates. 

Seasonal use of dehumidifiers can remove moisture from damp 

basements and reduce fungal growth. 

(dehom;difie,) 

portable unit lA ~ 

Heaters: Portable radiant heaters, such as those with water 

and glycol, may provide temporary heat in buildings used infrequently or during systems breakdowns. Care should be taken 

not to create a fire hazard with improperly wired units. 

portable heater 

electric baseboard 

Compiled by Sharon C. Park. Sketches adapted from Architectural Graphic Standards with permission from John Wiley and Sons. 

7 
4. Prioritize architecturally significant spaces, finishes, and features to be preserved. Significant architectural spaces, finishes and features should be 

identified and evaluated at the outset to ensure their 

preservation. This includes significant existing mechanical systems or elements such as hot water radiators, 

decorative grilles, elaborate switchplates, and nonmechanical architectural features such as cupolas, transoms, or porches. Identify non-significant spaces 

where mechanical equipment can be placed and secondary spaces where equipment and distribution runs 

on both a horizontal and vertical basis can be located. 

Appropriate secondary spaces for housing equipment 

might include attics, basements, penthouses, mezzanines, false ceiling or floor cavities, vertical chases, 

stair towers, closets, or exterior below-grade vaults (see 

figure 8). 

5. Become familiar with local building and fire 

codes. Owners or their representatives should meet 

early and often with local officials. Legal requirements 

should be checked; for example, can existing ductwork 

be reused or modified with dampers? Is asbestos abatement required? What are the energy, fire, and safety 

codes and standards in place, and how can they be met 

while maintaining the historic character of the building? How are fire separation walls and rated mechanical systems to be handled between multiple tenants? Is 

there a requirement for fresh air intake for stair towers 

that will affect the exterior appearance of the building? 

Many of the health, energy, and safety code requirements will influence decisions made for mechanical 

equipment for climate control. It is importance to know 

what they are before the design phase begins. 

6. Evaluate options for the type and size of systems. 

A matrix or feasibility studies should be developed to 

balance the benefits and drawbacks of various systems. 

Factors to consider include heating and/or cooling, fuel 

type, distribution system, control devices, generating 

equipment and accessories such as filtration, and humidification. What are the initial installation costs, projected fuel costs, long-term maintenance, and life-cycle 

7. Operable skylights and grilles that can be adapted for return air should be 

identified as part of the planning phase for new or upgraded mechanical 

systems. Photo: Dianne Pierce, NPS files. 

8 

costs of these components and systems? Are parts of 

an existing system being reused and upgraded? The 

benefits of added ventilation should not be overlooked 

(see figure 9). What are the trade-offs between one 

large central system and multiple smaller systems? 

Should there be a forced air ducted system, a 2-pipe 

fan coil system, or a combined water and air system? 

What space is available for the equipment and distribution system? Assess the fire-risk levels of various fuels. 

Understand the advantages and disadvantages of the 

various types of mechanical systems available. Then 

evaluate each of these systems in light of the preservation 

objectives established during the design phase of planning. 

8. In considering options for new systems, existing spaces should be evaluated for their ability to house new equipment. This sketch shows several 

areas where new mechanical equipment could be located to avoid damaging 

significant spaces. Sketch: NPS files 

9. Improving ventilation through traditional means should not be overlooked 

in planning new or upgraded HVAC systems. In mild climates, good exhaust 

fans can often eliminate the need for air conditioning or can reduce equipment size by reducing cooling loads. Photo: Ernest A. Conrad, P.E. 
Designing the new system 

In designing a system, it is important to anticipate how 

it will be installed, how damage to historic materials 

can be minimized, and how visible the new mechanical 

system will be within the restored or rehabilitated 

spaces (see figure 10 a-f). Mechanical equipment space 

needs are often overwhelming; in some cases, it may 

be advantageous to look for locations outside of the 

building, including ground vaults, to house some of 

the equipment but only if it there is no adverse impact 

to the historic landscape or adjacent archeological resources. Various means for reducing the heating and 

cooling loads (and thereby the size of the equipment) 

should be investigated. This might mean reducing 

slightly the comfort levels of the interior, increasing the 

number of climate control zones, or improving the 

energy efficiency of the building. 

The following activities are suggested during the 

design phase of the new system: 

1. Establish specific criteria for the new or upgraded 

mechanical system. New systems should be installed 

with a minimum of damage to the resource and should be 

visually compatible with the architecture of the building. 

They should be installed in a way that is easy to service, 

maintain, and upgrade in the future. There should be 

safety and back-up monitors in place if buildings have collections, computer rooms, storage vaults or special 

conditions that need monitoring. The new systems 

should work within the structural limits of the historic 

building. They should produce no undue vibration, no 

undue noise, no dust or mold, and no excess moisture that 

could damage the historic building materials. If any 

equipment is to be located outside of the building, 

there should be no impact to the historic appearance of 

building or site, and there should be no impact on archeological resources. 

2. Prioritize the requirements for the new climate 

control system. The use of the building will determine 

the level of interior comfort and climate control. Sometimes, various temperature zones may safely be created 

within a historic building. This zoned approach may be 

appropriate for buildings with specialized collections 

storage, for buildings with mixed uses, or for large 

buildings with different external exposures, occupancy 

patterns, and delivery schedules for controlled air. Special archives, storage vaults or computer rooms may 

need a completely different climate control from the 

rest of the building. Determine temperature and humidity levels for occupants and collections and ventilation requirements between differing zones. Establish if 

the system is to run 24 hours a day or only during operating or business hours. Determine what controls are 

optimum (manual, computer, preset automatic, or 

other). The size and location of the equipment to handle these different situations will ultimately affect the 

design of the overall system as well. 

3. Minimize the impact of the new HVAC on the 

existing architecture. Design criteria for the new system should be based on the type of architecture of the 

historic resource. Consideration should be given as to 

whether or not the delivery system is visible or hidden. 

Utilitarian and industrial spaces may be capable of 

accepting a more visible and functional system. More 

formal, ornate spaces which may be part of an interpretive program may require a less visible or disguised 

system. A ducted system should be installed without 

ripping into or boxing out large sections of floors, 

walls, or ceilings. A wet pipe system should be installed so that hidden leaks will not damage important 

decorative finishes. In each case, not only the type of 

system (air, water, combination), but its distribution 

(duct, pipe) and delivery appearance (grilles, cabinets, 

or registers) must be evaluated. It may be necessary to 

use a combination of different systems in order to preserve the historic building. Existing chases should be 

reused whenever possible. 

4. Balance quantitative requirements and preservation objectives. The ideal system may not be achievable 

for each historic resource due to cost, space limitations, 

code requirements, or other factors beyond the owner's 

control. However, significant historic spaces, finishes, 

and features can be preserved in almost every case, 

even given these limitations. For example, if some ceiling areas must be slightly lowered to accommodate 

ductwork or piping, these should be in secondary areas away from decorative ceilings or tall windows. If 

modern fan coil terminal units are to be visible in historic spaces, consideration should be given to custom 

designing the cabinets or to using smaller units in more 

locations to diminish their impact. If grilles and registers are to be located in significant spaces, they should 

be designed to work within the geometry or placement 

of decorative elements. All new elements, such as 

ducts, registers, pipe-runs, and mechanical equipment 

should be installed in a reversible manner to be removed in the future without further damage to the 

building (see fig 11). 

Systems Performance and Maintenance 

Once the system is installed, it will require routine 

maintenance and balancing to ensure that the · proper 

performance levels are achieved. In some cases, extremely sophisticated, computerized systems have been 

developed to control interior climates, but these still 

need monitoring by trained staff. If collection exhibits 

and archival storage are important to the resource, the 

climate control system will require constant monitoring 

and tuning. Back-up systems are also needed to prevent damage when the main system is not working: 

The owner, manager, or chief of maintenance should 

be aware of all aspects of the new climate control system and have a plan of action before it is installed. 

Regular training sessions on operating, monitoring, 

and maintaining the new system should be held for 

both curatorial and building maintenance staff. If there 

are curatorial reasons to maintain constant temperature 

or humidity levels, only individuals thoroughly trained 

in how the HVAC systems operates should be able to 

adjust thermostats. Ill-informed and haphazard attempts to adjust comfort levels, or to save energy over 

weekends and holidays, can cause great damage. 

9 
10. The following photographs illustrate recent preservation projects where careful planning and design retained the historic character of the resources. 

before after 

a. Before and after of a circa 1900 school entrance. The radiators have been replaced with a two-pipe fan coil system built into bench seats. The 

ceiling was preserved and no exposed elements were required to add air conditioning. Piping runs are under the benches and there was no 

damage to the masonry walls. Photos: Notter Finegold + Alexander Inc. and Lautman Photography, Washington. 

historic after 

d. Auditors Buildings, Washington, D.C. This upper floor workspace had been modified over the years with dropped ceilings and partitions. In 

the recent restoration, the open plan workspace was restored, the false ceiling was removed, and the fireproof construction was exposed. A variable air volume (VA V) system using round double shell exposed ductwork is in keeping with the industrial character of the architectural space. 

Photo: Kenneth Wyner Photography, courtesy of Notter Finegold + Alexander Inc. Before view provided by Notter Finegold + Alexander/Mariani. 

10 
b. Central air conditioning was installed in the corridors of this circa 1900 

school building by adding an air handler over the entrance from a vestibule. 

The custom-designed slot registers provide linear diffusers without detracting 

from the architecture of the space. Photo: Lautman Photography courtesy of 

Notter Finegold + Alexander Inc. 

e. Town Hall, Andover, MA. The upstairs auditorium was restored and new 

mechanical systems were installed. Perimeter baseboard radiation provides 

heat and air handlers, located in the attic space provide air conditioning. The 

cast iron ceiling grille was adapted for return air and the supply registers 

were installed in a symmetrical and regular manner to minimize impact on 

the historic ceiling. Photo: David Hewitt/Anne Garrison for Ann Beha 

Associates. 

c. Conference room, Auditors Building, Washington, D.C. The historic 

steam radiators were retained for heating. The cast iron ceiling register was 

retained as a decorative element, but made inoperable to meet fire codes. 

Photo: Kenneth Wyner Photography courtesy of Notter Finegold + Alexander Inc. 

f. Homewood, Baltimore, MD. This elegant circa 1806 residence is now a 

house museum. The registers for the forced air ducted system seen behind the 

table legs, are grained to blend with the historic baseboards. The HVAC 

system uses a water/air system where chilled water and steam heat are 

converted to conditioned air. Photo: Courtesy Homewood Museum, Johns 

Hopkins University. 

11 
HVAC Do's and Don'ts 

DO's: 

• Use shutters, operable windows, porches, curtains, 

awnings, shade trees and other historically appropriate non-mechanical features of historic buildings 

to reduce the heating and cooling loads. Consider 

adding sensitively designed storm windows to 

existing historic windows. 

• Retain or upgrade existing mechanical systems 

whenever possible: for example, reuse radiator 

systems with new boilers, upgrade ventilation 

within the building, install proper thermostats or 

humidistats. 

• Improve energy efficiency of existing buildings by 

installing insulation in attics and basements. Add 

insulation and vapor barriers to exterior walls only 

when it can be done without further damage to the 

resource. 

• In major spaces, retain decorative elements of the 

historic system whenever possible. This includes 

switchplates, grilles and radiators. Be creative in 

adapting these features to work within the new or 

upgraded system. 

• Use space in existing chases, closets or shafts for 

new distribution systems. 

• Design climate control systems that are compatible 

with the architecture of the building: hidden system for formal spaces, more exposed systems possible in industrial or secondary spaces. In formal 

areas, avoid standard commercial registers and use 

custom slot registers or other less intrusive grilles. 

12 

• Size the system to work within the physical constraints of the building. Use multi-zoned smaller 

units in conjunction with existing vertical shafts, 

such as stacked closets, or consider locating equipment in vaults underground, if possible. 

• Provide adequate ventilation to the mechanical 

rooms as well as to the entire building. Selectively 

install air intake grilles in less visible basement, 

attic, or rear areas . 

• Maintain appropriate temperature and humidity 

levels to meet requirements without accelerating 

the deterioration of the historic building materials. 

Set up regular monitoring schedules. 

• Design the system for maintenance access and for 

future systems replacement. 

• For highly significant buildings, install safety monitors and backup features, such as double pans, 

moisture detectors, lined chases, and battery packs 

to avoid or detect leaks and other damage from 

system failures. 

• Have a regular maintenance program to extend 

equipment life and to ensure proper performance. 

• Train staff to monitor the operation of equipment and 

to act knowledgeably in emergencies or breakdowns. 

• Have an emergency plan for both the building and 

any curatorial collections in case of serious malfunctions or breakdowns. 

DON'TS: 

• Don't install a new system if you don't need it. 

• Don't switch to a new type of system (e.g. forced 

air) unless there is sufficient space for the new system or an appropriate place to put it. 

• Don't over-design a new system. Don't add air conditioning or climate control if they are not absolutely necessary. 

• Don't cut exterior historic building walls to add 

through-wall heating and air conditioning units. 

These are visually disfiguring, they destroy historic 

fabric, and condensation runoff from such units 

can further damage historic materials. 

• Don't damage historic finishes, mask historic features, or alter historic spaces when installing new 

systems. 

• Don't drop ceilings or bulkheads across window 

openings. 

• Don't remove repairable historic windows or replace them with inappropriately designed thermal 

windows. 

• Don't seal operable windows, unless part of a museum where air pollutants and dust are being 

controlled. 

• Don't place condensers, solar panels, chimney 

stacks, vents or other equipment on visible portions of roofs or at significant locations on the site. 

• Don't overload the building structure with the 

weight of new equipment, particularly in the attic. 

• Don't place stress on historic building materials 

through the vibrations of the new equipment. 

• Don't allow condensation on windows or within 

walls to rot or spall adjacent historic building 

materials. 
. Maintenance staff should learn how to operate, monItor, and maintain the mechanical equipment. They 

must know where the maintenance manuals are kept. 

Routine maintenance schedules must be developed for 

changing and cleaning filters, vents, and condensate 

pans to control fungus, mold, and other organisms that 

are dangerous to health. Such growths can harm both 

inhabitants and equipment. (In piped systems, for ex-

~mple, molds in condensate pans can block drainage 

lmes and cause an overflow to leak onto finished surfaces). Maintenance staff should also be able to monitor 

the appropriate gauges, dials, and thermographs. Staff 

must be trained to intervene in emergencies, to know 

where the master controls are, and whom to call in an 

emergency. As new personnel are hired, they will also 

require maintenance training. 

. In ad~ition to regular cyclical maintenance, thorough 

mspectIons should be undertaken from time to time to 

evaluate the continued performance of the climate control s):,stem. As the system ages, parts are likely to fail, 

and SIgns of trouble may appear. Inadequately ventilat~d. areas may smell musty. Wall surfaces may show 

stammg, wet patches, bubbling or other signs of moisture damage. Routine tests for air quality, humidity, 

and temperature should indicate if the system is performing properly. If there is damage as a result of the 

new system, it should be repaired immediately and 

then c~osely monitored to ensure complete repair. 

EqUIpment must be accessible for maintenance and 

should be visible for easy inspection. Moreover, since 

~echanical systems last only 15-30 years, the system 

~tself mu~t be "reversible." That is, the system must be 

mstalled m such a way that later removal will not damage t~e building. In addition to servicing, the back-up 

momto~s that signal malfunctioning equipment must 

be routmely checked, adjusted, and maintained. 

Checklists should be developed to ensure that all aspects. of routine maintenance are completed and that 

data IS reported to the building manager. 

a 

Conclusion 

The successful integration of new systems in historic 

buil~ings can be challenging. Meeting modern HVAC 

requrrem.ents for human comfort or installing contr~lled clImates for museum collections or for the operatIon of complex computer equipment can result in 

both visual and physical damage to historic resources. 

Owners of historic buildings must be aware that the 

final result will involve balancing multiple needs; no 

perfect. heating, ventila~ing, and air conditioning system eXIsts. In undertakmg changes to historic buildings, it is best to have the advice and input of trained 

professionals who can: 

assess the condition of the historic building, 

evaluate the significant elements th<if should be 

preserved or reused, 

prioritize the preservation objectives, 

understand the impact of new interior climate conditions on historic materials, 

integrate preservation with mechanical and code 

requirements, 

maximize the advantages of various new or upgraded 

mechanical systems, 

understand the visual and physical impact of various 

installations, 

identify maintenance and monitoring requirements 

for new or upgraded systems, and 

plan for the future removal or replacement of the 

system. 

Too often the presumed climate needs of the occupants or collections can be detrimental to the long-term 

preservation of the building. With a careful balance 

?etw:en the preservation needs of the building and the 

mtenor temperature and humidity needs of the occupants, a successful project can result. 

11. Durin$ the restoration of this 1806 National Historic Landmark (photo a), a new climate control system was installed. The architects removed all the earli-

~; m~c7anlca~~1ulp7ent fr0

b

m

l 

the house and Installed new equipment in a 30' x 40' concrete vault located underground 150 feet from the house itself (photo . n y con I lOne air IS own Into the house reusing much of the circa 1930s ductwork. Photos: Thomas C. Jester. 

13 
Bibliography 

Banham, Reyner. The Architecture of the Well-Tempered Environment. 

London: The Architectural Press, 1969. 

Burns, John A., AlA. Energy Conserving Features Inherent in Older 

Homes. Washington: US. Department of Housing and Urban Development and US. Department of the Interior, 1982. 

Cowan, Henry J. Science and Building; Structural and Environmental 

Design in the Nineteenth and Twentieth Centuries. New York: John Wiley 

& Sons, 1978. 

Ferguson, Eugene S. "An Historical Sketch of Central Heating: 1800-

1860;' in Building Early America (Charles Peterson, editor) Philadelphia: Chilton Book Co., 1976. 

Fitch, James Marston. American Building; The Environmental Forces That 

Shape It. Boston: Houghton Mifflin Co., 1972. 

Giedion, Siegfried. Mechanization Takes Command; a Contribution to 

Anonymous History. New York: Oxford University Press, 1948. 

Merritt, Frederick S. Building Engineering and Systems Design. New 

York: Van Nostrand Reinhold Co, 1979. 

Smith, Baird M. Preservation Briefs 3: Conserving Energy in Historic 

Buildings. Washington, DC: US. Department of the Interior, 1978. 

Turberg, Edward. A History of American Building Technology. Durham: 

Durham Technical Institute, 1981. 

cover photo: This historic coal boiler continues in use after its conversion to 

an oil-fired boiler. Photo: NPS files 

14 

Acknowledgements 

The author gratefully acknowledges the invaluable assistance of 

Michael C. Henry, P.E., AlA, in the development and technical 

editing of this Preservation Brief. Technical review was also provided 

by Ernest A. Conrad, P.E. Thanks is also given to staff members of 

the National Park Service Cultural Resources Programs, induding 

Tom Keohan and Catherine Colby, Rocky Mountain Region; Michael 

Crowe, Western Region; Mark Chavez, Midwest Region; Randall J. 

Biallas, AlA, Chief, Park Historic Architecture Division, and George 

A. Thorsen, Historical Architect, Denver Service Center. SpeCial 

thanks is also given to Michael J. Auer of Technical Preservation 

Services tor his editorial assistance in preparing this paper and Tim 

Buehner for his assistance with the illustrations. 

This publication has been prepared pursuant to the National Historic 

Preservation Act of 1966, as amended, which directs the Secretary of 

the Interior to develop and make available information concerning 

historic properties. Preservation Brief 24 was developed under the 

editorship of H. Ward Jand!, Chief, Technical Preservation Services. 

Comments on the usefulness of this publication may be directed to 

Chief, Technical Preservation Services Branch, Preservation Assistance Division, National Park Service, P.O. Box 37127, Washington, 

D.C. 20013-7127. This publication is not copyrighted and can be 

reproduced without penalty. Normal procedures for credit to the 

author and the National Park Service are appreciated. 

Washington, DC 1991 
1

Earthquakes result from sudden movements of the 

geological plates that form the earth’s crust, generally 

along cracks or fractures known as “faults.” When 

buildings are not designed and constructed to withstand 

these unpredictable and often violent ground motions, 

major structural damage, or outright collapse, can result, 

with grave risk to human life. Historic buildings are 

especially vulnerable to seismic events, particularly 

those built before seismic codes were adopted. Also, 

more and more communities continue to adopt higher 

standards for seismic retrofi t of existing buildings. And, 

despite popular misconceptions, the risks of earthquakes 

are not limited to the West Coast (Figures 1 and 7), but 

exist across much of the United States.

Although historic and other older buildings can be 

retrofi tted to survive earthquakes, the process of doing 

so may damage or destroy the very features that make 

such buildings signifi cant. While life-safety issues 

remain foremost concerns, fortunately, there are various 

approaches which can help protect historic buildings 

from both the devastation caused by earthquakes 

and from the damage infl icted by well-intentioned, 

but insensitive, retrofi t procedures. Building owners, 

managers, consultants, and communities need to 

be actively involved in planning for and readying 

irreplaceable historic resources from these threats.

This Preservation Brief provides information on how 

earthquakes affect historic buildings, how a historic 

preservation ethic can guide responsible retrofi t 

decisions, and how various methods of seismic 

rehabilitation can protect human lives and historic 

structures. The Brief provides a description of the most 

common vulnerabilities of various building construction 

types and the seismic strengthening methods most often 

needed to remedy them. A glossary of technical terms is 

also provided at the end of the Brief. 

The Seismic Rehabilitation 

of Historic Buildings 

Antonio Aguilar

PRESERVATION 

BRIEFS

National Park Service

U.S. Department of the Interior

Technical Preservation Services

41

Undertaking the seismic rehabilitation of a historic 

building is a process that requires careful planning 

and execution, and the coordinated work of architects, 

engineers, code offi cials, contractors, and agency 

administrators. Project personnel working together 

can ensure that the architectural, structural, fi nancial, 

programmatic, cultural, and social values of historic 

buildings are preserved, while rendering them safe for 

continued use.

Achieving Seismic Retrofi t as well 

as Preservation

Major repairs 

and alterations, 

additions, change 

in occupancy, and 

local ordinances 

can trigger 

compliance with 

current building 

code requirements, 

including seismic 

strengthening 

requirements that 

specify a minimum 

level of protection 

from earthquake 

hazards to building 

occupants. They 

also specify the 

process for the 

seismic evaluation 

of buildings, outline 

the methods for 

rehabilitation, 

specify limitations 

Figure 1. In 2011, a 5.6 magnitude 

earthquake substantially damaged all four 

turrets atop St. Gregory’s University 

Benedictine Hall in Shawnee, Oklahoma. 

Photo: St. Gregory University.
2

on selecting structural analysis procedures, identify 

acceptable rehabilitation strategies, and specify when 

alternative compliance methods may be used.

Building codes are primarily intended to guide the 

design and construction of new buildings and often 

require the application of certain design and construction 

methods that are critical for good seismic performance. 

However, determining the seismic adequacy of historic 

buildings by comparing them to the requirements 

for new construction can be diffi cult, and sometimes 

impossible, because some archaic building materials 

and construction methods are not included in the new 

building codes. While many jurisdictions have adopted 

prescriptive standards, primarily for certain building 

types such as unreinforced masonry load bearing walls, 

more sophisticated, performance-based evaluation 

methods allowed by some codes offer more fl exibility. 

Figures 2-3. Standard approaches to seismic rehabilitation, such as adding diagonal bracing to reinforce window and storefront openings as seen in 

these photos, can be visually intrusive. Figure 2 (left). Figure 3 (right). Photo: Steade Craigo.

Many prescriptive code complying standards can result 

in the destruction of much of a historic building’s 

appearance and integrity. This is because the most 

expedient way to reinforce a building is to introduce a 

completely new complying structural system, to add 

new structural members, and to fi ll in irregularities or 

large openings without regard to how the new structural 

elements or modifi cations affect its architectural design. 

The results of these approaches can be quite intrusive 

(Figures 2-3). However, structural reinforcement can 

be introduced sensitively while still meeting code 

requirements. In such cases, its design, placement, 

patterning, and detailing will respect the historic 

character of the building, even when the reinforcement 

itself is visible (Figures 4-5). 

Successful seismic rehabilitations of historic buildings 

require both skillful use of the best available technology 

Figures 4-5. Using moment frames (horizontal and vertical 

steel members identifi ed by the arrows) set back behind 

storefront openings as seen in Figure 4 (left), or placing brace 

frames (in red) away from the windows as illustrated in 

Figure 5 (right) are solutions that meet historic preservation 

goals. Figure 4. Photo: Elizabeth Hilton.
3

as well a proper understanding of historic preservation, 

and the inherent strength of archaic materials and 

structural systems. The seismic retrofi t of historic 

buildings is as much an art as it is a science; it is, 

therefore, extremely important to select a professional 

who is not only experienced with seismic rehabilitation 

of existing buildings, but is also closely familiar with 

The Secretary of the Interior’s Standards for the Treatment 

of Historic Properties. While some degree of change or 

alteration to a historic building may be inevitable and 

acceptable in a seismic rehabilitation, the Standards 

can provide critical decision-making guidance in 

the process of planning and designing a successful 

seismic rehabilitation. The goal of a successful 

seismic rehabilitation should be to reduce the seismic 

vulnerabilities of a building while retaining its historic 

materials and features to the greatest extent possible and 

avoiding or minimizing alterations to signifi cant historic 

features and spaces.

Four important preservation principles should be kept 

in mind when undertaking seismic retrofi t projects:

• Historic features and materials, both structural and 

nonstructural, should be preserved and retained, not 

as museum artifacts, but to continue to fulfi ll their 

historic function to the greatest extent possible, and 

not be replaced wholesale in the process of seismic 

strengthening.

• If historic features and materials are damaged beyond 

repair, or must be removed during the retrofi t, they 

should be replaced in kind or with compatible 

substitute materials. If they must be removed during 

the retrofi t, they should be removed carefully and 

thoroughly documented to ensure they can be properly 

re-installed in their original location.

• New seismic retrofi t systems should work in concert 

with the inherent strengths of the historic structural 

system, and, whether hidden or exposed, should 

respect the character and integrity of the historic 

building, be visually unobtrusive and compatible 

in design, and be selected and designed with due 

consideration to limiting the damage to historic features 

and materials during installation.

• Seismic work should be reversible whenever feasible 

to allow its removal for future installation of improved 

systems as well as repair of historic features and 

materials.

Earthquakes and Historic Buildings: 

Assessing Principal Risk Factors

The Federal Emergency Management Agency (FEMA) 

defi nes seismic risk as a function of earthquake hazard 

and vulnerability. Assessing the seismic risk of a historic 

property is the fi rst step to avoid the potential loss of life 

and injuries, damage and loss of property, or disruption 

of services. Seismic evaluations of historic buildings 

within areas of earthquake hazard should be conducted 

Putting a Team Together

A team that is experienced with both seismic retrofi t 

requirements and historic preservation, and can 

adopt an inter-disciplinary approach, is important 

for achieving a seismic rehabilitation that is sensitive 

to the building’s historic character, features, and 

materials. Team members should be selected for their 

experience with similar projects, and may include 

architects, engineers, code specialists, contractors, 

and preservation consultants. Because the typical 

seismic codes are written for new construction, it 

is important that both the architect and structural 

engineer be knowledgeable about historic buildings 

and about meeting building code equivalencies and 

fi nding other options. 

Local and state building offi cials can identify regulatory 

requirements, alternative approaches to meeting 

these requirements, and a historic preservation or 

building conservation code if one has been adopted 

by the jurisdiction. Even on small projects that 

cannot support a full professional team, consultants 

should be familiar with historic preservation goals. 

The State Historic Preservation Offi ce (SHPO) and 

the local historic preservation offi ce or commission 

may be able to identify consultants with experience 

in seismic rehabilitation of historic buildings, or be 

able to provide initial technical assistance on how to 

approach a seismic retrofi t.

if they have not been previously performed. This 

evaluation should identify both the potential structural 

defi ciencies of the building (any structural component 

such as columns, beams, fl oors, etc., required to resist 

seismic forces), as well as the potential vulnerabilities 

of the nonstructural components of the building (all 

components that are not part of the structural system, 

which include exterior cladding, glazing, chimneys, 

interior partitions, ceilings, and other architectural 

features, as well as building systems, and equipment). 

Nonstructural failures generally account for the majority 

of earthquake damage repair costs during earthquakes. 

Thus, it is critical to consider the risk and consequences 

of potential nonstructural failures. This is particularly 

important for historic buildings located in areas of low 

or moderate earthquake hazard, where the danger of 

collapse may be relatively small, but nonstructural 

elements such as unanchored stone veneers, cornices, 

parapets, chimneys, and gable ends may dislodge and 

fall to the ground during a moderate earthquake and 

pose severe life-safety hazards (Figure 6).
4

Other important nonstructural hazards to consider are 

the possibility that gas and water lines may rupture 

during an earthquake, which can cause fi re and water 

damage. Many of these vulnerabilities can be mitigated 

by understanding how the forces unleashed in an 

earthquake affect a building, and then planning and 

implementing appropriate remedial treatments 

(Figure 7).

Factors that infl uence how and why historic buildings 

are damaged in an earthquake:

1. Depth of the earthquake and subsequent strength of 

earthquake waves reaching the surface

2. Duration of the earthquake, including aftershock 

tremors

3. Proximity of the building to the earthquake epicenter, 

although distance is not necessarily a direct 

relationship

4. Building construction type, including structural 

systems and materials

5. Building design, including plan and elevation 

confi guration, overall massing, arrangement of 

interior spaces, and detailing of nonstructural 

elements

6. Existing building condition, including maintenance 

level

7. Site and soil conditions

In the process of assessing the potential seismic risk, 

these are crucial factors that should be considered:

1. Type of construction and condition of the building 

2. Site seismic hazards

3. Occupancy and use 

Type of Building Construction. To a great extent, a 

historic building’s construction and materials determine 

its behavior during an earthquake. Some buildings, 

such as a broad class of wood-frame structures, are 

able to absorb substantial movements with little risk 

of collapse. Others, such as unreinforced masonry 

or adobe buildings, tend to be more susceptible to 

damage from shaking (Figure 8). If an earthquake is 

strong, or continues for a long time, building elements 

that are poorly attached or unreinforced may collapse 

or dislodge. Buildings of more rigid or stronger 

construction methods such as reinforced concrete or 

steel-frame buildings may also have seismic defi ciencies 

depending on when they were constructed and whether 

or not they have been well-maintained over time. 

A thorough assessment of the building’s existing 

conditions is the basis for any seismic rehabilitation. 

This begins by gathering any available information 

about the building’s original construction. Many historic 

buildings in earthquake zones have survived episodes of 

ground shaking and may even have undergone previous 

seismic reinforcement work. Compiling any available 

documentation that quantifi es their proven seismic 

resistance or describes seismic reinforcement work 

or any other changes that have occurred over time is 

extremely useful. Some of these records may have been 

already compiled in previous documentation assembled 

to nominate the structure to the National Register of 

Historic Places or for a Historic Structure Report. (If 

not previously done, for many buildings preparing a 

Historic Structure Report is highly recommended; see 

Preservation Brief 43: The Preparation and Use of Historic 

Structure Reports). Early real estate or insurance maps, 

such as Sanborn Maps, and assessor’s records may also 

note building changes over time.

Original construction documents, plans and 

specifi cations, when available, and engineering 

drawings, in particular, which include structural layout 

and connection details are especially useful. When 

drawings documenting improvements or alterations 

over time are not available, building permits can also 

provide useful information. Historic photographs of 

the building under construction or before and after 

previous earthquakes are also invaluable. The compiled 

information, along with a thorough evaluation of the 

condition and strength of the existing building materials, 

will provide a sound basis for calculating the potential 

seismic hazards of the building and preparing a seismic 

retrofi t plan.

Building Confi guration. The geometry and shape of 

a building also play a role in how a building behaves 

during an earthquake. Buildings with regular plans, 

whether they are round, square, or rectangular, have 

a greater resistance to damage during an earthquake 

because their geometry allows for equal resistance of 

lateral forces in all directions. 

Buildings with complex and irregular plans, however, 

may be more prone to damage during an earthquake 

because of uneven strength and stiffness. For example, 

structures with an L,T, H, or other plan confi gurations 

with inward-facing, or re-entrant corners, have unequal 

resistance to stress concentrated at those corners and 

intersections (Figure 9). This is of particular concern if 

the buildings have fl exible structural systems and/or 

have an irregular layout of shear walls, which may cause 

portions of the building to pull apart. 

Similarly, the more complex and irregular buildings are 

in elevation, the more susceptible they are to damage, 

especially tall structures. Other building features such 

as large ground-level storefront or garage openings, 

or fl oors with columns and walls running in only one 

direction, are commonly known as “soft” or “weak” 

stories, which increase the seismic vulnerability of 

historic buildings (Figures 10 and 11).
5

Figure 7. A simplifi ed 2014 United States Geological Survey (USGS) seismic hazard map. Owners of certain classes of high-risk buildings in 

regions of high seismic activity are advised, and often required by local ordinances, to take immediate action in undertaking a comprehensive 

vulnerability assessment and make any necessary seismic rehabilitation measures. Owners of buildings in moderate seismic zones are advised to do 

further investigation of their building’s exposure to earthquake risk, identify seismic rehabilitation needs, and consider mitigation of risks primarily 

due to nonstructural hazards. Owners of buildings in low seismic areas are advised to consider low-cost rehabilitation measures that protect against 

casualties and property loss, if such measures are found to be necessary, even though the potential occurrence of an earthquake might be low.

Figure 6. 

Failing 

nonstructural 

elements 

such as the 

stone-veneer 

window and 

door surrounds 

shown in this 

image can 

become a lifesafety hazard 

when they 

block or impede 

an exit path. 

Photo: Wiss, 

Janney, Elstner 

and Associates, 

Inc.

Figure 8. Unreinforced masonry buildings, such as this 1875 

stone building damaged during the 2014 South Napa, California 

earthquake, can be particularly vulnerable to earthquake damage. 

Photo: Architectural Resources Group.
6

Building Condition. Damaged and deteriorated 

building materials increase the risk of serious damage 

during an earthquake. This condition can be the result 

of poor quality workmanship and materials from when 

the building was built, or lack of proper maintenance. 

Material damage and degradation due to moisture, 

erosion, mold, or insect infestation are typical problems 

resulting from poor maintenance. Well-maintained 

buildings, even without added reinforcement, survive 

better than similar buildings that have not been 

maintained. In unreinforced masonry buildings, 

deteriorated mortar joints can weaken entire walls. 

Regular cyclical maintenance is therefore essential. 

The capacity of the structural system to resist 

earthquakes may also be severely reduced if previous 

alterations or earthquakes have weakened structural 

connections. Unrepaired cracks or damage from 

previous earthquakes can progressively weaken a 

building, increasing the potential for greater damage 

during the next earthquake. Cumulative earthquake 

damage can be signifi cant; therefore, it is important to 

analyze the structural capacity of the building. 

Over time, structural members 

can become loose and pose a 

major liability. Unreinforced 

masonry buildings typically have 

a friction-fi t connection between 

horizontal and vertical structural 

members, and the shaking 

caused by an earthquake pulls 

them apart. Insuffi cient bearing 

surfaces for beams, joists, and 

rafters against the load-bearing 

walls or support columns is 

another important factor to 

consider. The resulting structural 

inadequacy can cause a partial 

or complete building collapse, 

Basic Maintenance/Earthquake 

Preparedness

Regular maintenance ensures that existing historic 

materials remain in good condition and are not 

weakened by rot, rust, decay, or other moisture 

problems. Without exception, historic buildings 

should be well maintained. An evacuation plan 

should also be developed. With the knowledge that 

an earthquake may occur at any time in the future, 

building owners should have emergency information 

and supplies on hand.

• Check roofs, gutters, and foundations for 

moisture problems, and check for corrosion 

of metal ties at parapets and chimneys. Make 

repairs and keep metal painted and in good 

condition.

• Inspect and keep termite and wood-boring 

insects away from wooden structural members.

• Check exit steps and porches to ensure that 

they are tightly connected and will not collapse 

during an emergency exit.

• Check masonry for deteriorating mortar, and 

never defer repairs. Repoint, matching the 

historic mortar in composition and detailing.

• Contact utility companies for information on 

fl exible connectors for gas and water lines and 

earthquake-activated gas shut-off valves. Strap 

oil tanks down and anchor water heaters to wall 

framing.

• Collect local emergency material for reference 

and implement simple household or offi ce 

mitigation measures, such as installing latches 

to keep cabinets from fl ying open or braces to 

attach tall bookcases to walls. Keep drinking 

water, tarpaulins, and other emergency supplies 

on hand.

Figure 9. This image shows structural deformation due to stress 

concentration in structures with re-entrant corners, the inside corner 

where the two perpendicular exterior walls meet.

Figure 10. (left) Open fi rst fl oor. Figure 11. (right) Double height second fl oor. These renderings show 

examples of “weak” or ”soft” story irregularities.
7

depending on the severity of the earthquake and the 

internal wall confi guration. 

Evaluation of the general physical condition of the 

building’s interior and exterior, and identifi cation of 

areas vulnerable to seismic damage, often requires 

testing and analysis to determine the durability 

and strength of materials and structure. This 

should be performed by a qualifi ed engineer who is 

knowledgeable of historic materials and construction 

methods. In order to evaluate the actual strength and 

condition of the historic materials, selective destructive 

testing may be required.

Site Seismic Hazards. In addition to the shaking 

motion of the ground during an earthquake, there is 

risk of damage due to site-specifi c hazards, such as fault 

rupture; liquefaction and other soil failures; landslides; 

hazards from adjacent buildings, including pounding; 

or potential inundation from nearby dam failure or a 

tsunami. If such hazards exist, they should be addressed 

along with any needed seismic rehabilitation of the 

building. 

Occupancy and Use. A building’s occupancy and use 

have a direct relationship to its seismic risk, as well as 

the social, economic, or environmental consequences 

that an earthquake may pose. From a life-safety 

perspective, warehouses, barns, and certain industrial 

buildings and structures with low human occupancy 

may present a lower risk compared to high-rise offi ce 

buildings, theaters, and other high-occupancy buildings. 

Specifi c uses such as medical facilities, housing for 

persons of limited mobility, or buildings that support 

vital community services or utilities fall within use 

categories where the risk of damage or collapse during 

an earthquake requires special consideration. Owners of 

historic buildings that are being repurposed for a new 

use should be aware that, depending on the change, the 

new use may pose a higher risk to life safety and may 

require signifi cant seismic reinforcement to mitigate its 

seismic risk. Inversely, if the change in use lowers the 

risk to life safety, the need for extensive seismic retrofi t 

work may not be necessary. 

Evaluating Signifi cant Historic Features 

and Spaces

Just as important as the assessment of the material 

and structural condition of a building is the careful 

identifi cation of the interior and exterior features and 

components that help defi ne its historic character. 

Establishing a protection and preservation plan that 

identifi es signifi cant interior spaces, features, and 

fi nishes is essential. Signifi cant architectural elements 

include domes and atriums and important or 

highly-decorative features such as staircases, ornate 

ceilings, mosaics, murals, and other historic treatments. 

Conversely, it is also important to identify secondary or 

tertiary areas of the building, or any spaces, features, or 

fi nishes that have been changed over time and no longer 

have historic signifi cance. Less important spaces may 

provide areas for additional structural reinforcement 

to be installed during a seismic rehabilitation without 

having an adverse impact on the overall historic 

character of the building. 

The placement of additional structural reinforcement 

should be carefully considered to avoid or appreciably 

minimize any impact on the building’s signifi cant or 

primary exterior and interior spaces (Figure 12). New 

structural elements should be located within interstitial 

or utilitarian spaces whenever possible. Alterations 

within secondary spaces are preferable to alterations of 

primary spaces, but care should be taken to preserve 

historic materials and character to the greatest extent 

feasible in these areas as well. When new structural 

elements must be added within signifi cant interior 

spaces, the placement and location should avoid 

major alterations to the overall volume, distinctive 

architectural features, or fi nishes within the space, as 

well as to its character (Figures 13-15). (See Preservation 

Brief 17: Architectural Character—Identifying the Visual 

Aspects of Historic Buildings as an Aid to Preserving their 

Character, and Preservation Brief 18: Rehabilitating Interiors 

in Historic Buildings: Identifying and Preserving CharacterDefi ning Elements.)

Developing a Seismic Rehabilitation Plan

Seismic Vulnerability. Seismic vulnerability is 

represented as a sliding scale of potential damage based 

on the probability of hazard by locale (site-specifi c data) 

and building use. This helps the owner understand 

the building’s vulnerability to damage, both structural 

and nonstructural, in the event of an earthquake. 

Consequences of earthquake vulnerability may be 

characterized as:

Figure 12. A mezzanine was added to the original banking hall of 

this 1921 former bank building as a seismic strengthening measure. 

Subdividing such an important character-defi ning space can result in 

a severe loss to a building’s historic integrity.
8

• Deaths and injuries to building occupants and related 

liability

• Building collapse or damage to building components, 

and related cost for repairs

• Damage to building contents and related cost or 

liabilities

• Disruption of building operations and related cost or 

liabilities

Performance Objectives. Once the risks and 

vulnerabilities of a building have been assessed, the 

next step in the process is usually setting goals for 

reducing the seismic risks. Knowing what rehabilitation 

requirements are mandated by the local jurisdiction 

is critical, as well as knowing whether a locality has 

separate codes for historic structures. There may be 

other triggers, such as a lender requiring retrofi t or 

earthquake insurance when refi nancing a loan, or a 

change of occupancy. The owner should also be aware 

Figure 15. After completion of the 

seismic reinforcement, the replicated 

plaster ornamentation was installed 

and the decorative painted fi nishes 

restored, returning the waiting room 

of King Street Station to its historic 

appearance. Photo: Doug Scott.

of the different levels of seismic risk-reduction measures 

that can be chosen and their associated cost. Alternative 

ways of reducing seismic risk to life and property, such 

as reducing the occupancy of a building or providing 

alternative facilities in case of an earthquake, should be 

studied. 

After all alternatives have been considered, modifi cation 

options for reducing the risk of damage to a historic 

building should be evaluated. Before undertaking a 

seismic rehabilitation, objectives that defi ne the level 

of acceptable damage or loss for a building during 

a potential earthquake must be determined. These 

are referred to as “rehabilitation objectives.” The 

rehabilitation objectives are usually set by the local 

code, ordinance, or code offi cial in mandatory seismic 

strengthening programs, or chosen by the owner and 

engineer in a voluntary seismic rehabilitation (see ASCE 

Standard 41-13).

Figures 13-14. Vertical steel 

reinforcement members were inserted 

and grouted into the walls of the main 

waiting room in Seattle’s historic 

1906 King Street Railroad Station to 

strengthen them (top left). To replace 

the original plaster ornamentation, 

molds were made to cast new 

replacement plaster elements as shown 

on this work-in-progress photo (top 

right). Photos: John Stamets.
9

The rehabilitation objectives are based on target 

“building performance levels” which provide a limiting 

range to the amount of predicted damage that a 

building should sustain during an earthquake. Building 

performance levels fall into four general classifi cations 

that rank from higher performance (lower risk) to lower 

performance (more risk): 

Figure 16. Computer modelling is a useful analytic tool that helps 

assess the strengths and weaknesses of a building and evaluate 

different seismic strengthening options. Three-dimensional dynamic 

analysis is required for a number of different structures located in 

certain seismic zones. 

Photo: Wiss, Janney, Elstner and Associates, Inc.

1. Operational. Backup utility services maintain 

function; the building sustains very little damage. 

This approach is intended for critical facilities, such as 

hospitals and emergency management centers, which 

must remain open and operational after a major 

earthquake. For some historic buildings, achieving 

this level of performance may be diffi cult or next 

to impossible without a great deal of modifi cation. 

However, as more buildings that were engineered to 

sustain ground-shaking become historic, more may 

achieve this level of performance.

2. Immediate Occupancy. The building remains safe to 

occupy. Damage and expected repairs are minor. 

3. Life Safety. The building remains stable and has 

substantial structural reserve capacity; hazardous 

nonstructural damage is controlled. 

4. Collapse Prevention. This addresses the most serious 

life-safety concerns by correcting those defi ciencies 

that could lead to serious human injury or total 

building collapse. The building remains standing in 

order for occupants to exit the building; any other 

damage or loss is acceptable. It is expected that if an 

earthquake were to occur, the building should not 

collapse but would be seriously damaged, could not 

be occupied, and would require major repairs or need 

to be demolished.

From a design perspective, the vast majority of historic 

buildings can tolerate a well-planned and placed system 

of seismic reinforcement. Utilitarian structures, such 

as warehouses, may be able to receive fairly visible 

reinforcement systems without undue damage to 

their historic character (Figures 17-18). Other, more 

architecturally detailed buildings or those with more 

fi nished or decorative interior surfaces will benefi t 

from more hidden systems. However, installation of 

such systems may require the temporary removal and 

reinstallation of signifi cant features as part of the seismic 

rehabilitation work. Most buildings can incorporate 

Figures 17 and 18. Careful placement of the new structural reinforcement, painted to match the surrounding building elements, if left exposed, 

minimizes its visual impact. New added structural elements may be left exposed when their visibility does not impair the historic character of the 

building. Figure 17 (left). Photo: Jason Hagin. Figure 18 (right). Photo: Architectural Resources Group.
10

Hazard Mitigation of Nonstructural Components

Nonstructural components are all those elements other 

than the columns, bearing walls, fl oors, beams or trusses, 

bracing elements, foundation, and other elements that 

make up a building’s structure (Figures 19-20). In the 

past, seismic rehabilitation of existing buildings has 

focused on mitigating structural defi ciencies. However, 

nonstructural components represent a substantial 

portion of a building’s initial capital investment; 

and, based on post-earthquake surveys, losses from 

nonstructural components also represent a high 

percentage of losses during an earthquake (see FEMA 

E-74: Reducing the Risk of Nonstructural Earthquake 

Damage—A Practical Guide).

Similar to structural components, a risk assessment of 

nonstructural components should be completed based 

on the hazards they present. The potential consequences 

of earthquake damage to nonstructural components 

depend on the type of risk they pose:

Life Safety. Could anyone be hurt by this component in 

an earthquake? 

Property Loss. Could a large property loss result? 

Functional Loss. Could the loss of this component cause 

an outage or interruption in operations? 

seismic hazard mitigation during other construction 

work in a way that ensures a high degree of in-place 

retention of historic materials.

Building performance levels are established for both 

structural and nonstructural damage. While reducing 

life-safety risks and ensuring safe post-earthquake 

occupancy of a building might be the primary concerns 

for many owners, potential damage to contents may be 

extremely important in certain circumstances. Important 

archives and records, invaluable art objects, and book 

and other collections are often housed in historic 

buildings. Risk-reduction measures associated with 

hazardous materials or valuable equipment must also be 

considered. 

Figure 19. Chimneys are 

common nonstructural 

features susceptible to 

earthquake damage, 

even in zones of low-to 

medium-seismic activity 

as shown on this building 

in Virginia damaged in a 

2011 earthquake. 

Photo: Suzanne Tripp.

Figure 20. Bracing parapet walls, as illustrated in the typical detail drawing 

on the left, and as installed in the image on the right, helps avoid hazards 

from falling debris.
11

Hazardous Nonstructural Components

The following list represents typical components 

and is not exhaustive:

Architectural

• Exterior walls (including masonry veneers, 

prefabricated panels, glazing, glass block, and 

curtain wall and storefront window systems) 

• Partitions (hollow- clay tile, unreinforced 

masonry, or similar)

• Ceilings (plaster or other heavy material, 

particularly if suspended)

• Parapets, cornices, turrets, and other projecting 

decorative elements (unreinforced masonry) 

• Balconies, canopies, marquees, and signs 

• Chimneys and exhaust/ vent stacks (unreinforced 

masonry)

• Stairways 

Mechanical, Electrical, and Plumbing Systems

• Boilers, furnaces, pumps, and chillers

• Water and fuel storage tanks

• Plumbing and piping, both nonhazardous and 

hazardous materials, including sprinkler systems

• Conduits

• Ductwork

• Light fi xtures

• Electrical and communication equipment

Furnishings and Interior Equipment

• Storage racks

• Bookcases

• Filing cabinets

• Hazardous material storage units

• Elevators

• Conveyors

• Other contents in museums and historic houses

Cost and Implementation

Designing a successful seismic rehabilitation takes 

into consideration not only seismic performance and 

historic preservation concerns, but fi nancial ones as well. 

The rehabilitation of historic buildings often triggers 

mandated seismic strengthening work to comply with 

local codes. This often occurs when the amount of work 

crosses a certain threshold or when the rehabilitation 

involves a change in use, such as the conversion of an 

industrial building into a residential use. In these cases, 

the rehabilitation includes all the work necessary to 

meet the new programmatic requirements as well as 

the seismic performance objectives in a “single-stage” 

project. All the costs and occupancy disruptions are also 

incurred at one time, and a signifi cant upfront capital 

investment is required. 

When budgetary constraints and/or occupancy 

restrictions do not make a single-stage project 

feasible, a “multi-stage” project that spreads the cost 

and occupancy disruptions over a set period of time 

should be considered. The term used for a planned 

rehabilitation, implemented over a period of time to 

meet a predetermined seismic performance objective, 

is an “incremental rehabilitation.” In an incremental 

rehabilitation, the work is integrated into ongoing 

facility maintenance and capital improvement 

operations. An incremental approach has been shown to 

have many advantages from a benefi t/cost analysis (see 

FEMA 227, 395, and 399).

Preparation of Construction Documents and Execution 

of the Work

Whether the seismic rehabilitation will be done as a 

single project or incrementally, construction documents 

must be prepared. The documents should clearly 

defi ne the overall scope of the work. In the case of an 

incremental rehabilitation, the scope and sequence 

of each phase must also be clearly detailed. A good 

set of construction documents along with a quality 

assurance program to ensure that the design is carried 

out as specifi ed is critical for any successful seismic 

rehabilitation. It is also critical that the construction 

documents include details and specifi cations for the 

treatment of historic features and materials. Selecting 

a contractor with a proven record of successful historic 

rehabilitation work is extremely important. It is also 

recommended that the project team schedule a meeting 

with the local building code offi cial to discuss the 

seismic retrofi t approach and to explore potential code 

compliance alternatives. 
12

Remediating Seismic Defi ciencies 

Achieving desired building performance objectives 

depends on the specifi c occupant needs, site, and 

building conditions. The particular approaches to 

achieving this should also be tailored to the unique 

historic character and conditions of each building, and 

the specifi c seismic defi ciencies that need to be remedied 

or mitigated. Most seismic defi ciencies and possible 

required mitigation techniques can be divided into the 

general categories described here. Although not an 

exhaustive list, it is intended to provide an overview of 

the scope of work a building owner or project manager 

might expect during a seismic rehabilitation.

Inadequate Global Strength or Stiffness. Historic 

buildings which were not designed to resist the lateral 

forces of a ground-shaking event often lack adequate 

overall strength, or “global strength.” While this is 

seldom the only defi ciency that needs to be mitigated, 

lack of suffi cient global strength makes a building 

particularly vulnerable during a seismic event. 

Questions to Ask 

These questions should be discussed with the team to 

determine acceptable alternatives. Since there is never a 

single “right” answer, the design team and code offi cials 

should work together to determine the appropriate level 

of seismic retrofi t with the lowest visual impact on signifi -

cant spaces, features, and fi nishes on both the interior and 

exterior of historic buildings. This guide is not intended to 

prescribe how seismic retrofi t should be done, but rather, 

to illustrate that every physical change to a building will 

have some consequence. By asking how impacts can be 

reduced, the owner will have several options from which to 

choose.

• Can bracing be installed without damaging decorative 

details or the appearance of parapets, chimneys, or 

balconies?

• Are the visible features of the reinforcement, such as 

anchor plates/washers or added exterior buttresses 

adequately designed to blend in with the historic 

building?

• Can hidden or grouted bolts be used to tie fl oors and 

walls together, instead of using traditional bolts and 

exposed washers or rosettes if they might detract from a 

building’s historic character?

• Are diagonal frames, such as X- or K-braces or other 

located to have a minimal impact on the primary 

facade? Are they set back and painted a receding color if 

visible through windows or storefronts?

• Can moment frames or reinforced bracing be added 

around historic storefronts in order to avoid exposed 

reinforcement, such as X-braces, within the immediate 

viewing range of the public?

• Can shorter sections of reinforcement be “stitched” into 

the existing building to avoid removing or covering 

large sections of historic materials? This is particularly 

important for the insertion of additional roof framing 

supports.

• Can shear walls be located in utilitarian interior spaces to 

reduce the impact on fi nishes in the primary areas?

• Are there situations where a thinner, applied fi berreinforced coating would adequately strengthen walls 

or supports without the need for heavier reinforced 

concrete?

• Can diaphragms be added to non-signifi cant fl oors in 

order to protect highly decorated ceilings below, or the 

reverse if the fl oor is more ornamental than the ceiling?

• Are there adequate funds to retain, repair, or reinstall 

ornamental features and fi nishes once structural 

reinforcements have been installed?

• Should alternative seismic reinforcement methods such 

as base isolation, wall damping systems, or core drilling 

be considered? Could they protect signifi cant features 

and materials by reducing the amount of intervention 

required?

• Are the seismic treatments under consideration 

“reversible” in a way that allows the most amount of 

historic materials to be retained and allows future repair 

and restoration?

• Could the seismic rehabilitation add excessive 

strengthening that could have unintended negative 

consequences to the historic structure during an 

earthquake?

Approaches to mitigating this defi ciency often require 

reinforcing the existing structure or adding structural 

components to strengthen the building. Mitigating the 

lack of suffi cient overall stiffness, or “global stiffness,” is 

also necessary in many seismic rehabilitations. While the 

methods used to improve stiffness are often the same as 

those used to correct inadequate strength, strength and 

stiffness are considered separately because they relate 

to specifi c building responses. A building may have 

minimum acceptable strength to prevent its collapse, 

but if it is too fl exible the building may be vulnerable 

to excessive movement or drift. Too much drift often 

results in extensive damage to exterior and interior walls 

and nonstructural components.

The addition of steel moment frames, braced frames 

of various confi gurations, and concrete shear walls are 

typical methods for increasing the strength and stiffness 

of a building. Careful placement of new structural 

elements is critical to avoid negatively affecting 

windows and doorways, particularly on highly-visible 

elevations. Alterations to signifi cant interior spaces such 
13

and stiffness are some of the most common diaphragm 

defi ciencies. Strengthening methods for diaphragms 

can entail adding sheathing to roofs or fl oors, installing 

tension rods in shallow-vaulted concrete fl oors, and 

various other methods. 

Reinforcement around openings, re-entrant corners, 

and mitigation of other plan irregularities may also 

be necessary. Ensuring proper transfer of the lateral 

earthquake forces spread over the diaphragm to the 

lateral force-resisting system (vertical elements of 

the structure) is also an important component of a 

seismic retrofi t. Adequate diaphragm “collectors,” 

structural elements that collect the lateral force and 

transfer it to the lateral force-resisting system, need 

to be strengthened or added to compensate for any 

defi ciencies in the existing construction. 

Foundations. Foundation defi ciencies can vary greatly 

and include defi ciencies within the foundation system 

itself or with the soil conditions. Foundation defi ciencies 

are independent of building or construction type, and 

remedial work can include replacing or upgrading the 

existing foundation, adding a new foundation next to 

the existing one, or adding new pilings or drilled piers. 

While often expensive and disruptive to correct or 

mitigate, foundation defi ciencies are critical to address. 

Remediating Seismic Defi ciencies According 

to Building Construction Type

Unreinforced Masonry Bearing Wall. Unreinforced 

masonry (URM) buildings are most commonly brick 

or stone without steel reinforcement bars imbedded in 

them. In many historic buildings of this type, masonry 

bearing walls support the weight of the roof and fl oors. 

Joists bear directly on the wall at the perimeter of the 

building and are supported by post and beams on the 

interior. Unreinforced masonry is one of the oldest and 

most diverse construction types and encompasses a 

wide range of materials and wall construction methods 

such as solid clay brick, cut or fi eld stone, hollow-clay 

brick, structural tile, concrete masonry units, and adobe. 

URM buildings are generally considered to be some 

of the most susceptible to earthquake damage. In 

strong earthquakes, the walls may fall outward and 

cause a partial or total collapse of the building. Poor 

connection between the exterior walls and diaphragms 

severely impair the capacity of a URM building to resist 

an earthquake, and thus it is important that this be 

remediated. Other signifi cant hazards that can be life 

threatening are falling debris from parapets, cornices, 

chimneys, or other nonstructural elements. 

A basic seismic rehabilitation program for a URM 

building may involve bracing the parapets and 

chimneys, and securing other elements that can break 

away from the building, as well as securing the walls to 

the fl oors and roof. Engineering analysis of the structure 

as lobbies, assembly spaces, and other character-defi ning 

spaces should likewise be avoided.

Building Irregularities. Plan irregularities such as 

re-entrant corners place extraordinary demands on 

building elements due to torsional or twisting reactions 

to the ground-shaking forces of an earthquake. Vertical 

irregularities create an uneven distribution of the 

mass or stiffness between the vertical elements of the 

building which can be particularly problematic in 

multi-story buildings. The vulnerability to seismic 

damage that these types of irregularities impose was 

seldom considered in the original design of many 

historic buildings, and normally requires retrofi t 

measures to mitigate them. 

Plan and elevation irregularities are too many to 

enumerate here. Mitigation measures for this type of 

defi ciency can be challenging to undertake in historic 

buildings, as some of these irregularities may be 

important to the building’s historic character. Some of 

the most problematic solutions involve adding shear 

walls directly behind window and door openings, 

particularly when they are on highly-visible elevations. 

Solutions that involve inserting a new fl oor or a large 

mezzanine into an architecturally signifi cant, two-story 

space should be avoided. 

Load Path. There should be a positive and continuous 

load path to convey lateral forces. Inadequate connection 

between structural and nonstructural components of 

a building is one of the most critical defi ciencies to be 

addressed in many seismic rehabilitations. Forces acting 

on building components must be able to be transferred 

down to the supporting soil. For example, in order to 

resist the forces of an earthquake, a panel of cladding 

must be adequately connected to the fl oor, walls, frames, 

braces, and other structural members that connect it to 

the building’s foundation. 

A strong connection between all of the elements of the 

building allows for the transfer of the stressing forces 

down to the foundation, where it is absorbed by the 

soil. Breaks or failures along this load path render 

any existing seismic system ineffective. Methods for 

mitigating load path defi ciencies range from simple 

reinforcement measures such as adding metal straps, 

threaded bolts, and other mechanical fasteners to more 

elaborate engineered connections. Adequate connections 

between walls, roof, fl oors, and foundation are critical, 

as well as between the components of all the vertical and 

horizontal structural components of the building such as 

joist to beams or walls, and columns to beams and fl oors 

or footings. 

Diaphragms. Floors and roofs are commonly referred to 

as “diaphragms” in seismic design. Diaphragms act as 

horizontal “beams” between the vertical elements (walls 

and/or columns) that resist the lateral forces during 

an earthquake. Inadequate shear, bending strength, 
14

Figure 21 Historic wine 

cellar building in Napa, 

California.

Figure 22. Drawing of the 

wine cellar’s front wall 

showing the locations of the 

center cores for continuous 

steel reinforcement rods to 

be inserted and grouted in 

place.

Figure 25. Location of the eye bolts 

after grouting

The seismic rehabilitation of this 

historic wine cellar building shown 

in Figure 21 included center coring 

of the exterior stone walls to add steel 

reinforcement, as illustrated in Figures 

22 and 23. The fl oors were tied to the 

steel reinforcement in the walls using 

eye bolts, as seen in Figures 24 and 

25, eliminating the need for exterior 

washers or plates. Figure 21. Photo: 

Architectural Resources Group. Figures 

22-25. Photos: MKM & Associates.

Figure 24. Eye bolts used to anchor the fl oor to the wall.

Figure 23. View of one of the center cores 

drilled down through the wall.
15

Figure 26. Epoxy-embedded fi berglass mesh is being bonded to the back of the brick pilasters of this 1906 

unreinforced masonry building. New fi nishes matching the appearance and detailing of the original 

fi nishes will be applied over the mesh. Photo: Wiss, Janney, Elstner and Associates, Inc.

may indicate that additional reinforcement of the roof 

and fl oor may be necessary. This may be accomplished 

by adding columns or shear walls in order to adequately 

transfer lateral loads to the ground. 

Preventing the collapse of the walls, particularly loadbearing walls, is critical in URM buildings. This entails 

strengthening the walls through various methods such 

as adding strongbacks, or secondary support members, 

to the wall; adding a layer of steel-reinforced concrete; 

adding shotcrete bonded to the URM wall; core drilling; 

or adding a fi ber composite layer (epoxy embedded 

fi berglass mesh). Adding interior wall bracing or 

inserting vertical steel bars embedded in grout into 

the wall itself are also widely-used wall reinforcement 

methods. These techniques must be carefully evaluated 

to balance building performance and preservation 

objectives (Figures 21-27).

Concrete Frame. Most historic concrete building 

assemblies consist of concrete columns and fl oor 

Figure 27. A new steel stud wall is being added to one 

side of this interior hollow-clay tile wall. The side with 

the least amount of historic fi nishes was chosen as the 

least disruptive location for the added reinforcement. 

The wall will be returned to its original appearance by 

installing new fi nishes matching the historic fi nishes. 

Photo: Wiss, Janney, Elstner and Associates, Inc.

systems of various types (fl at slabs, two-way slabs, and 

beam-and-slab). Exterior and interior walls are often 

unreinforced masonry, commonly called infi ll, and 

are typically nonstructural, presenting the defi ciencies 

described previously. The most important structural 

vulnerability of this construction type is the lack of 

ductility. Prior to 1960, building codes may not have 

required concrete-frame buildings to be designed 

and detailed for ductile performance (i.e., not fail 

when loaded beyond capacity) to resist lateral forces, 

depending on the region. Concrete-frame buildings were 

required to be “ductile” in the 1976 Uniform Building 

Code. In many historic concrete-frame buildings, 

because the connection between the columns and beams 

is not strong enough to resist the lateral loads during an 

earthquake, the buildings are in danger of collapsing. 

Particularly vulnerable buildings of this type are those 

with relatively few concrete walls and columns that 

are weaker than the beams or slabs. Mitigating this 

condition is typically accomplished by adding a new 

system, such as shear walls (Figure 28).
16

able to withstand strong lateral forces, they are subject 

to signifi cant movement between stories, or interstory 

drifts, and are also prone to pounding on adjacent 

buildings. Their fl exibility can also result in damage to 

structural connections as well as to interior walls and 

cladding.

Large interstory drifts may make steel-frame buildings 

more diffi cult to repair after an earthquake. Typical 

remedies to improve the stiffness of steel-frame 

buildings include adding new concrete shear walls 

or brace frames, or adding steel cover plates to the 

existing steel columns. The latter technique may not 

be recommended if it requires signifi cant removal or 

damage to existing historic features or fi nishes. 

Light Wood Frame. Light wood-frame construction 

is common in much residential and small commercial 

building construction. It includes both post-and-beam 

construction as well as stud-wall construction. Lifesafety performance of light wood-frame construction 

during an earthquake is typically very good. The 

most common seismic defi ciency of this building type 

is the lack of, or poor, anchorage of the walls to the 

foundation, deterioration of existing structural fasteners, 

and/or an insuffi cient number of fastened connections. 

Inadequate strength of the lower stories in multi-story 

buildings may also be a vulnerability. In many dwellings 

this includes structures resting on intermittent wood or 

masonry piers, unfi nished garages, and/or crawlspaces. 

Figure 28. Steel 

reinforcement for the new 

sheer walls, perpendicular 

to the windows, is being 

installed in this concreteframe building. 

Photo: Robert Chattel.

Steel Frame. This type of construction consists of a 

complete frame of steel columns and beams. Floors 

are usually concrete slabs or metal decks fi lled 

with concrete. Exterior walls can be reinforced or 

unreinforced masonry, architectural terra cotta, glass 

curtain walls, or other types of construction. Mitigating 

the seismic defi ciencies of the nonstructural elements 

must be done according to their construction methods. 

Steel-frame construction is used in a wide variety of 

building types such as offi ces, hospitals, government, 

and academic buildings, as well as industrial and other 

utilitarian structures. This type of construction started 

to gain widespread use in the latter part of the 19th 

century. 

There are two general categories of steel-frame 

construction: Steel moment frame, which consists of 

steel columns and beams with rigid connections using 

angles and plates that can be riveted, welded, or bolted 

together; and braced frame. Braced-frame structures 

have diagonal steel members placed in selected bays of 

the structure to improve the lateral force resistance of 

the overall structure, whereas moment frames rely on 

the strength of rigid corner connections to resist lateral 

forces.

Steel-frame buildings, particularly braced-frame 

buildings, have superior seismic performance over other 

construction methods. Moment-frame buildings are very 

elastic during seismic activity, and, although they are 
17

Unbraced cripple walls and poor foundation anchorage 

are common vulnerabilities in many one-and twofamily detached dwellings of one or more stories. In 

these buildings, interior walls usually provide suffi cient 

bracing to resist lateral loads during a seismic event, but 

the crawlspace often only has perimeter framing which 

may become highly stressed and collapse during an 

earthquake. Adequate anchorage to the foundation in 

wood-frame dwellings prevents them from sliding off 

their foundation. Mitigating the lack of adequate bracing 

of cripple walls and proper foundation anchoring are 

high priority seismic rehabilitation measures in light 

wood-frame buildings (Figure 29).

Multi-story, multi-unit residential wood-frame buildings 

with parking, common areas, or commercial uses on 

the ground fl oor are a sub-category of this construction 



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



Figure 29. Cripple-wall reinforcement. Simple approaches such as 

nailing plywood between crawlspace studs, bolting sill plates to the 

foundation, and strapping the cripple wall to the fl oor above can make 

a dramatic difference in protecting a building from seismic damage.

type that is also very susceptible to damage during 

earthquakes. The upper fl oors of these buildings 

typically have a repetitive interior layout which 

gives them uniform strength and stiffness. The lower 

fl oors however, because they typically have larger 

exterior openings and may have more varied interior 

confi gurations and large open spaces, often create a 

“weak story” condition (Figure 30).

Seismic rehabilitation requirements for one-and 

two-family residential and other small-scale buildings 

commonly include securing chimneys and correcting 

other life-safety nonstructural defi ciencies, such as 

bolting the sill plate to the foundation and adding 

plywood sheathing to reinforce knee walls. Structures 

built over intermittent wood or masonry piers are also 

vulnerable during earthquakes. Providing connection 

between the piers or installing a new continuous support 

element of adequate strength under the walls is often 

required. Large exterior openings such as garage doors 

or structural irregularities such as cantilevers must also 

be evaluated and mitigated. In the case of multi-story 

apartment buildings with a soft or weak story, additional 

steel frames or masonry shear walls may be necessary 

to give adequate strength to the ground fl oor (see FEMA 

P-807).

Seismic Isolation and Energy Dissipation 

Systems

Seismic isolation and energy dissipation systems are 

relatively new, highly sophisticated methods that require 

extensive design and engineering analysis. Seismic 

isolation systems involve disconnecting, or greatly 

reducing, the transmission of the seismic forces from 

the ground to the building. Energy dissipation methods 

involve either passive or active energy dissipation 

devices that dampen the effects of the lateral and vertical 

motion of an earthquake. Energy dissipation systems 

are sometimes used in tall buildings where installing 

seismic isolation systems may not be feasible. However, 

implementing either of these systems is often too costly 

or impractical for most seismic rehabilitations of historic 

buildings.

Important Preservation Considerations

Reinforcing the connection between various structural 

and nonstructural elements, installing new structural 

elements, and performing other seismic rehabilitation 

work may require selective removal of historic wall and 

ceiling fi nishes. Attention should be given to minimize 

the amount of historic material removed, as well as 

performing the necessary repair or replacement of the 

historic features and fi nishes after the seismic work is 

completed. 

Completely removing a character-defi ning feature such 

as a chimney, cornice, or parapet in order to mitigate its 

Figure 30. Large perimeter openings and a limited number of interior 

partitions on the ground fl oor can create a “weak story” condition 

that makes buildings particularly vulnerable to earthquake damage. 

Photo: USGS.
18

seismic vulnerability is not a recommended treatment. 

Instead, such building elements should be braced and 

secured whenever possible. If a signifi cant architectural 

feature has sustained earthquake damage, it should 

be repaired rather than removed. If the damage is 

so extensive that it requires complete replacement, 

using in-kind replacement materials is generally the 

recommended approach. In some circumstances, 

however, a compatible substitute material that meets 

technical performance requirements may be appropriate, 

such as a lighter weight material that matches the 

original in design and appearance. 

Historic corridor walls constructed of unreinforced 

masonry, hollow-clay tile in particular, need careful 

consideration. Every effort should be made to retain 

the historic materials while taking measures to secure 

the wall and the historic materials to prevent them 

from the possibility of injuring occupants or blocking 

an egress path. Every available reinforcement method 

and technique that allows the corridor walls and their 

historic materials and detailing to be preserved in place 

should be considered. If retention of the core masonry 

elements is not feasible, but there are extant features 

and fi nishes, such as a marble wainscot, doors and 

transoms, wood trim, or other features, they should be 

retained, even if they have to be reinstalled over a new 

framing structure for the corridor that replicates the 

ceiling height, corridor width, and the relationship and 

installation details of the historic features. 

Post-Earthquake Issues 

In most municipalities, a survey or inspection, usually 

by professionals or trained volunteers, will be conducted 

as soon as possible after an earthquake, and buildings 

will be tagged on the front with a posted notice stating 

whether they are safe to enter. Typically red, yellow, 

and green tags are used to indicate varying levels 

of damage—no entry, limited entry, and useable, 

respectively—and their relative safety. Heavilydamaged areas are often secured and determined 

off-limits, and, unfortunately, many red-tagged, but 

repairable, buildings have been torn down unnecessarily 

because owners were unable to evaluate and develop 

a stabilization plan in time. Owners or members of 

the preservation community may engage their own 

engineers with specialized knowledge to challenge a 

demolition order. 

During times of emergencies, many communities, 

banks, and insurance agencies will not be in a position 

to evaluate alternative approaches to dealing with 

damaged historic buildings. Therefore, they often 

require full compliance with codes for new construction 

for the major rehabilitation work required. Because 

seismic aftershocks may further damage a weakened 

building, the inability to act quickly—even to shore up 

or stabilize the structure on a temporary basis—can 

result in the building’s demolition. 

Penetrating rain, uneven settlement, vandalism, 

and continuing aftershocks can easily undermine a 

building’s remaining structural integrity. Moreover, 

the longer a building is unoccupied or non-incomeproducing, the sooner it is likely to be torn down in a 

negotiated settlement with the insurance company. All 

of these factors work against saving buildings damaged 

in earthquakes. Thus, having a recovery plan already in 

place is highly recommended.

Having an established emergency plan, complete with 

access to plywood, tarpaulins, bracing timbers, and 

equipment, will allow quick action to save a building 

following an earthquake. Technical assistance programs 

are available from the federal government after a natural 

disaster. Grant funds or low-cost loans from federal, 

state, and Congressional special appropriations are 

targeted for qualifi ed properties, which can help offset 

the cost of rehabilitation (see information about FEMA).

Summary

Recognizing the seismic vulnerability of historic 

buildings is an important step toward saving and 

protecting the built heritage in earthquake-prone areas. 

Vulnerability resulting from lack of maintenance and 

improper repairs that weaken the structural integrity of a 

building must not be overlooked. Even prior mitigation 

work can become ineffective over time if buildings are 

not adequately maintained. Unlike many other natural 

disasters, earthquakes come with no warning and can 

result in devastating loss of life and property. Therefore, 

preparedness cannot be overemphasized.

Damage to historic buildings after an earthquake can 

be as great as the initial damage from the earthquake 

itself. The ability to act quickly to shore up and stabilize 

a building and to begin its sensitive rehabilitation is 

imperative. Communities without earthquake-hazardreduction plans in place put their historic buildings—as 

well as the safety and economic well-being of their 

communities—at risk.

Simple measures such as bracing parapets and 

chimneys, tying buildings to foundations, and anchoring 

brick walls to fl oors and the roof, are extremely effective 

mitigation measures. However, even simple measures 

such as these can cause damage to historic materials 

or impact the visual qualities of a historic building 

when not properly executed. For this reason, engaging 

qualifi ed and experienced professionals and workers 

when undertaking seismic retrofi t work is important. 

Finally, modern research has helped develop various 

new seismic retrofi t techniques that, added to more 

traditional methods, provide many approaches to 

strengthen buildings in earthquake-prone areas. These 

techniques must be carefully evaluated and chosen so 

that the process of mitigating potential seismic damage 

avoids unnecessary removal of historic materials and 

retains the character of historic properties.
19

Glossary 

BASE ISOLATION: Also referred to as Seismic Base Isolation 

or Base Isolation System, is a technique aimed at isolating or 

separating a building or structure from the movement caused by 

earthquakes by means of a fl exible layer between the foundation 

and the vertical supports.

BRACED FRAME: Essentially, a vertical truss, or its equivalent, 

added to a building frame to resist lateral forces.

COLLECTOR: A member or element provided to transfer lateral 

forces from a portion of a structure to vertical elements of the 

lateral force-resisting system (also called a drag strut).

CORE DRILLING: A vertical reinforcement system that relies 

on drilling a continuous vertical core that is fi lled with steel 

reinforcing rods and grouting to resist in-plane or out-of-plane 

bending.

CRIPPLE WALL: A short wall between the foundation and the 

fi rst-fl oor framing.

DAMPING: The internal energy absorption characteristic of a 

structural system that acts to attenuate induced free vibration.

DIAGONAL BRACES: Inclined components designed to carry 

axial loads, enabling a structural frame to act as a truss to resist 

lateral forces.

DIAPHRAGM: A horizontal, or nearly horizontal, system 

designed to transmit lateral forces to the vertical elements of the 

lateral force-resisting system. The term “diaphragm” includes 

horizontal bracing systems.

FIBER WRAP REINFORCEMENT: A synthetic compound of 

fi laments that increase the shear capacity of structural members. 

DUCTILITY: The ability of a structure or element to dissipate 

energy inelastically when displaced beyond its elastic limit 

without a signifi cant loss in load-carrying capacity.

FAULT RUPTURE: A break in the ground along the fault line 

during an earthquake.

GLOBAL STRENGTH: The lateral strength of the vertical 

oriented lateral force-resisting system at the effective global yield 

point.

GLOBAL STIFFNESS: The stiffness of the entire lateral forceresisting system.

GROUTED BOLTS: Anchor bolts set in a grout mixture. 

INTERSTORY DRIFT: The displacement of one fl oor level 

relative to the fl oor level above or below.

LATERAL FORCE-RESISTING SYSTEM: The part of the 

structural system assigned to resist lateral forces.

LIQUEFACTION: A condition where the soil underneath or 

around the building loses cohesive strength and behaves like a 

liquid during an earthquake.

LIFE SAFETY: Providing a level of assurance that risk of loss 

of life is kept to minimal levels. For buildings, this includes 

strengthening to reduce l) structural collapse, 2) falling debris, 

3) blocking exits or emergency routes, and 4) prevention of 

consequential fi re. 

MOMENT FRAME: A structural frame system in which seismic 

shear forces are resisted by shear and fl exure in members and 

joints of the frame.

POUNDING: The action of two adjacent buildings coming 

into contact with each other during an earthquake as a result 

of their close proximity and differences in dynamic response 

characteristics.

RE-ENTRANT CORNER: A corner on the exterior of a building 

that is directed inward such as the inside corner of an L-shaped 

building, where the two perpendicular portions of the building 

meet.

SEISMIC REHABILITATION: Modifi cations to existing 

components, or installation of new components, that correct 

defi ciencies identifi ed in a seismic evaluation to achieve a selected 

rehabilitation objective.

SHEAR WALL: A wall, bearing or nonbearing, designed to resist 

lateral forces acting in the plane of the wall.

SHOTCRETE: Concrete that is pneumatically placed on vertical 

or near vertical surfaces, typically with a minimal use of forms.

SOFT STORY: A story in which the lateral stiffness is less than 70 

percent of the stiffness of the story above.

STRUCTURE: An assemblage of framing members designed to 

support gravity loads and resist lateral forces. Structures may be 

categorized as building structures or non-building structures.

WEAK STORY: A story in which the lateral strength is less than 

80 percent of that in the story above.

Federal Emergency Management Agency (FEMA)

In March 2003, FEMA and 22 other federal agencies, programs 

and offi ces became part of the Department of Homeland Security. 

FEMA’s role and mission continues to focus on building, 

sustaining, and improving the nation’s capacity to prepare 

for, protect against, respond to, recover from, and mitigate all 

hazards. FEMA offers a variety of earthquake-related resources 

including reports, handbooks, guides, manuals, software, webbased tools, and instructional materials. These documents contain 

nationally-applicable technical criteria intended to ensure that 

buildings will withstand earthquakes better than before. There is 

a great deal of information that is applicable to historic buildings, 

although they are not necessarily identifi ed as a separate category. 

Most of the information is available online at www.fema.gov/

earthquake. Information on how to obtain hard copies may be 

found on the web site. 

National Earthquake Hazards Reduction Program (NEHRP)

The National Earthquake Hazards Reduction Program 

(NEHRP) leads the federal government’s efforts to reduce the 

fatalities, injuries, and property losses caused by earthquakes. 

Congress established NEHRP in 1977, directing that four 

federal agencies coordinate their complementary activities to 

implement and maintain the program. These agencies are FEMA, 

the National Institute of Standards and Technology (NIST), the 

National Science Foundation (NSF) and the U.S. Geological 

Survey (USGS). More information about NEHRP is available 

online at www.nehrp.gov.
Acknowledgements

Antonio Aguilar, Senior Historical Architect, Technical Preservation Services, National Park Service, revised Preservation Brief 41: The 

Seismic Retrofi t of Historic Buildings: Keeping Preservation in the Forefront, originally written by David W. Look, AIA, Terry Wong, PE, 

and Sylvia Rose Augustus and published in 1997. The revised Brief contains expanded and updated information as well as new color 

photographs describing the general issues and needs associated with the seismic rehabilitation of historic buildings. 

The author wishes to thank the following: Melvyn Green, SE, Melvyn Green & Associates, Inc., for his guidance and expertise in the 

revision of the Brief; Stephen Day, AIA, Stephen Day Architecture, Andrew Phillips, Associate Architect, SMR Architects, and Meredith 

Wirsching, NCARB, for their input in the initial planning of the revision; Alan Dreyfuss, AIA, Wiss, Janney, Elstner Asssociates, 

Inc., Loring A. Wyllie Jr., Senior Principal, Degenkolb Engineers, Timothy Brandt, AIA, Senior Restoration Architect, and Mark C. 

Huck, AIA, Restoration Architect, from the California Offi ce of Historic Preservation, Nicholas Vann, AIA, State Historical Architect, 

Washington Department of Archaeology & Historic Preservation, Sueann Brown, Regional Historical Architect, Pacifi c West Region, 

and Jason Hagin, Historical Architect, Golden Gate National Recreational Area, National Park Service, for their assistance in reviewing 

various drafts of the brief; and to Charles Fisher, Brian Goeken, Anne Grimmer, Elizabeth Milnarik, and Gary Sachau, Technical 

Preservation Services, National Park Service, for their help in the fi nal edits of the publication. Illustrations not specifi cally credited 

are from National Park Service fi les. Front cover image: Damage to a ca. 1910 building in Napa, California caused by the South Napa 

ea rthquake in 2014. Photo: Wiss, Janney, Elstner and Associates, Inc.

This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which directs the Secretary 

of the Interior to develop and make available information concerning historic properties. This publication is available from the 

Technical Preservation Services website at http://www.nps.gov/tps/ or hard copies may be purchased from the Government Printing

Offi ces at the U.S. Government Bookstore. 

Comments about this publication should be addressed to: Technical Preservation Services, National Park Service, 1849 C Street, NW, 

Washington, DC 20240. This publication is not copyrighted and can be reproduced without penalty. Normal procedures for credit to 

the authors and the National Park Service are appreciated. The photographs used in this publication may not be used to illustrate other 

publications without permission of the owners.

April 2016

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Migration Measures for Art Objects.” Getty Conservation Institute 

Scientifi c Program Report (1990).

American Society of Civil Engineers. Seismic Rehabilitation of Existing 

Buildings. ASCE/SEI Standard 41-13 (2014). Reston, VA.

California Historical Building Code, California Code of Regulations. 

Title 24, Part 8. California Building Standards Commission, 2010.

Federal Emergency Management Agency (FEMA). A Benefi t-Cost 

Model for the Seismic Rehabilitation of Buildings. FEMA 227 (1992). 

Washington, DC.

FEMA. Engineering Guideline for Incremental Seismic Rehabilitation.

FEMA P-420 (2009). Washington, DC.

FEMA. Incremental Seismic Rehabilitation of Retail Buildings. FEMA 399 

(2014). Washington, DC.

FEMA. Incremental Seismic Rehabilitation of School Buildings. FEMA 395 

(2003). Washington, DC.

FEMA. Seismic Evaluation and Retrofi t of Multi-Unit Wood-Frame 

Buildings With Weak First Stories. FEMA P-807 (2012). Washington, DC.

FEMA. Techniques for the Seismic Rehabilitation of Existing Buildings.

FEMA 547 (2007). Washington, DC. 

International Code Council (ICC). International Existing Building Code

(2015). Falls Church, VA.

National Park Service (NPS). Museum Handbook, Part I, Museum 

Collections, Chapter 10, Emergency Planning. http://www.nps.gov/

museum/publications/MHI/CHAP10A-B.pdf