Historic Preservation - Technical Procedures
- Preservation Briefs: 4 Roofing For Historic Buildings
- Procedure code:
- Preservation Briefs 4, National Park Service, Pad
- Thermal And Moisture Protection
- Roof Maintenance And Repairs
- Last Modified:
- Preservation Briefs: 4 Roofing For Historic Buildings
- Last Modified:
PRESERVATION BRIEFS: 4
ROOFING FOR HISTORIC BUILDINGS
Link below connects to the NPS website and latest version of PB #4:
Sarah M. Sweetser
This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:
Superintendent of Documents
P.O. Box 371954
Pittsburgh, PA 15250-7954
Available ONLY in package sets
Briefs 1-14 - $13.00
Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.
***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 turrets 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
But no matter how decorative the patterning or how compelling the
form, the roof is a highly vulnerable element of a shelter that
will inevitably fail. A poor roof will permit the accelerated
deterioration of historic building materials (masonry, 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***
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 fire such 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 made by molding clay over sections of logs, and they
were generally 22" long and tapered in width.
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 1/2", 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.
For specific information relating to clay roofing, see 07321-01-S.
Another practice settlers brought to the New World was slate
roofing. Evidence of roofing slates have been found among the
ruins of mid-17th century Jamestown. 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 in the
late 1600's. 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
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
For specific information relating to slate roofing, see 07315-01-S.
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
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 roofing in America is principally a 19th century phenomenon.
Before then the only metals commonly 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 roof's 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 Morris 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
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
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 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
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 tin roof on "Monticello"
(ca.1770-1802). The Arch Street Meetinghouse (1804) in
Philadelphia had tin shingles laid in a herringbone pattern on a
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
Asphalt shingles and roll roofing were used in the 1890s. Many
roofs of asbestos, aluminum, stainless steel, galvanized 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
***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.
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
Ideally, shingles are split with the grain perpendicular to
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
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 galvanizing.
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.
Perhaps the most durable roofing materials are slate and
tile. Seemingly indestructible, 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
rainwater 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.
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 a new or
replaced roof should be a primary consideration. Remember, some
roofing and flashing 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
If the roof structure appears sound and nothing indicates 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.
In a restoration project, research of documents and physical
investigation of the building usually will establish the roof's
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 buildings 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 roof's 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
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 chinmeys 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 the
other edges of the roof. This primary evidence is essential for a
full understanding of the roof's 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 an
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 ensure 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 Building 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 an 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.
Determining the craft practices used in the installation of an
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 labeled "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.
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 craftmanship, 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.
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.
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
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
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-retardant 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 alternative roofing materials is not new. As early
as the 18th century, fear of fire caused 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 over-ambitious 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.
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