The Repair, Replacement, and
Maintenance of Historic Slate Roofs
Jeffrey S. Levine
U.S. Department of the Interior
National Park Service
JUN 29 19W
Slate is one of the most aesthetically pleasing and durable
of all roofing materials. It is indicative at once of the
awesome powers of nature which have formed it and the
expertise and skill of the craftsman in hand-shaping and
laying it on the roof. Installed properly slate roofs require
relatively little maintenance and will last 60 to 125 years
or longer depending on the type of slate employed, roof
configuration, and the geographical location of the
property. Some slates have been known to last over 200
years. Found on virtually every class of structure, slate
roofs are perhaps most often associated with institutional,
ecclesiastical, and government buildings, where longevity
is an especially important consideration in material
choices. In the slate quarrying regions of the country
where supply is abundant, slate was often used on farm
and agricultural buildings as well.
Because the pattern, detailing, and craftsmanship of slate
roofs are important design elements of historic buildings,
they should be repaired rather than replaced whenever
possible. The purpose of this Preservation Brief is to assist
property owners, architects, preservationists, and building
managers in understanding the causes of slate roof
failures and undertaking the repair and replacement of
slate roofs. Details contributing to the character of historic
slate roofs are described and guidance is offered on
maintenance and the degree of intervention required at
various levels of deterioration.
The relatively large percentage of historic buildings roofed
with slate during the late nineteenth and early twentieth
centuries means that many slate roofs, and the 60 to 125
year life span of the slates most commonly used, may be
nearing the end of their serviceable lives at the end of
the twentieth century. Too often, these roofs are being
improperly repaired or replaced with alternative roofing
materials, to the detriment of the historic integrity and
appearance of the structure. Increased knowledge of the
characteristics of slate and its detailing and installation on
the roof can lead to more sensitive interventions in which
original material is preserved and the building's historic
character maintained. Every effort should be made to
replace deteriorated slate roofs with new slate and to
develop an effective maintenance and repair program for
slate roofs that can be retained.
History of Slate Use in the United States
Although slate quarrying was not common in the United
States until the latter half of the nineteenth century, slate
roofing is known to have been used prior to the Revolution.
Archeological excavations at Jamestown, Virginia, have
unearthed roofing slate in strata dating from 1625-1650
and 1640-1670. Slate roofs were introduced in Boston as
early as 1654 and Philadelphia in 1699. Seventeenth
century building ordinances of New York and Boston
recommended the use of slate or tile roofs to ensure
In the early years of the Colonies, nearly all roofing slate
was imported from North Wales. It was not until 1785
that the first commercial slate quarry was opened in the
United States, by William Docher in Peach Bottom
Township, Pennsylvania. Production was limited to that
which could be consumed in local markets until the
middle of the nineteenth century. Knowledge of the
nation's abundant stone resources was given commercial
impetus at this time by several forces, including a rapidly
growing population that demanded housing, advances in
quarrying technology, and extension of the railroad system
to previously inaccessible markets. Two additional factors
helped push the slate industry to maturity: the immigration
of Welsh slate workers to the United States and the
introduction of architectural pattern and style books
(Figure 1). Slate production increased dramatically in the
years following the Civil War as quarries were opened in
Vermont, New York, Virginia, and Lehigh and
Northampton Counties, Pennsylvania. By 1876, roofing
slate imports had all but dried up and the United States
became a net exporter of the commodity.
Figure 1. Architectural pattern books of the mid-nineteenth century
awakened Americans to the availability and quality of slate for roofing
purposes by incorporating slate roofs in their designs. Design XX,
"A French Roof House," in A. J. Douming's Victorian Cottage
Residences is shown.
The U.S. roofing slate industry reached its highest point in
both quantity and value of output in the period from 1897
to 1914. In 1899, there were over 200 slate quarries
operating in 13 states, Pennsylvania historically being the
largest producer of all. The decline of the U.S. roofing
slate industry began c.1915 and resulted from several
factors, including a decline in skilled labor for both the
fabrication and installation of slate and competition from
substitute materials, such as asphalt shingles, which could
be mass produced, transported and installed at a lower
cost than slate. Only recently with the increasing
popularity of historic preservation and the recognition of
the superiority of slate over other roofing materials, has
slate usage begun to increase.
The Character and Detailing of Historic
During some periods of architectural history, roof design
has gone far beyond the merely functional and contributed
much to the character of buildings. Roofs, by their
compelling forms, have defined styles and, by their
decorative patterns and colors, have imparted both dignity
and beauty to buildings. The architectural styles prevalent
during the latter half of the nineteenth and early twentieth
centuries placed strong emphasis on prominent roof lines
and greatly influenced the demand for slate. Slate, laid in
multi-colored decorative patterns, was particularly well
suited to the Mansard roofs of the Second Empire style,
the steeply pitch roofs of the Gothic Revival and High
Victorian Gothic styles, and the many prominent roof
planes and turrets associated with the Queen Anne style.
The Tudor style imitated the quaint appearance of some
English slates which, because of their granular cleavage,
are thick and irregular. These slates were often laid in a
graduated pattern, with the largest slates at the eaves and
the courses diminishing in size up the roof slope, or a
textural pattern (Figure 2). Collegiate Gothic style
buildings, found on many university campuses, were
often roofed with slate laid in a graduated pattern.
The configuration, massing, and style of historic slate
roofs are important design elements that should be
preserved. In addition, several types of historic detailing
were often employed to add visual interest to the roof,
essentially elevating the roof to the level of an ornamental
architectural element. When repairing or replacing a slate
roof, original details affecting its visual character should
Before repairing or replacing an existing slate roof, it is
important to document the existing conditions and
detailing of the roof using written, visual, and physical
evidence so that original features can be identified and
preserved. Documentation should continue through the
repair or replacement process as significant details, long
obscured, are often rediscovered while carrying out these
activities. Local histories, building records, old receipts
and ledgers, historic photographs, sketches, and paintings,
shadow lines and nail hole patterns on the roof deck, and
bits of historic material left over from previous interventions
(often found in eave cavities) are all useful sources of
information which can be of help in piecing together the
original appearance of the roof. Size, shape, color, texture,
exposure, and coursing are among the most important
characteristics of the original slates which should be
documented and matched when repairing or replacing an
historic slate roof.
Historically, three types of slate roofing— standard,
textural, and graduated— were available according to the
architectural effect desired. Standard grade slate roofs
were most common. These are characterized by their
uniform appearance, being composed of slates
approximately 3/16" (0.5cm) thick, of consistent length
and width, and having a smooth cleavage surface. Thirty
different standard sizes were available, ranging from 10"
(25cm) x 6" to 24" x 14" (15cm x 61cm x 35cm). The
slates were laid to break joints and typically had square
ends and uniform color and exposure. Patterned and
polychromatic roofs were created by laying standard slates
of different colors and shapes on the roof in such a way as
to create sunbursts, flowers, sawtooth and geometric
designs, and even initials and dates (Figure 3). On
utilitarian structures, such as barns and sheds, large gaps
were sometimes left between each slate within a given
course to reduce material and installation costs and
provide added ventilation for the interior (Figure 4).
Figure 2. The quaint character of this Tudor style residence is derived,
in part, from its textural roof.
Figure 3. A sawtooth geometric design composed of red, green, and
black slates makes this roof the most visually important feature of the
Textural slate roofs incorporate slates of different
thicknesses, uneven tails, and a rougher texture than
standard slates. Textural slate roofs are perhaps most often
associated with Tudor style buildings where slates of
different colors are used to enhance the effect.
Graduated slate roofs were frequently installed on large
institutional and ecclesiastical structures (Figure 5). The
slates were graduated according to thickness, size, and
exposure, the thickest and largest slates being laid at the
eaves and the thinnest and smallest at the ridge. Pleasing
architectural effects were achieved by blending sizes and
Detailing at the hips, ridges and valleys provided added
opportunity to ornament a slate roof. Hips and ridges can
be fashioned out of slate according to various traditional
schemes whereby the slates are cut and overlapped to
produce a watertight joint of the desired artistic effect.
Traditional slate ridge details are the saddle ridge, strip
saddle ridge, and comb ridge, and for hips, the saddle
hip, mitered hip, Boston hip, and fantail hip (Figure 6).
A more linear effect was achieved by covering the ridges
and hips with flashing called "cresting" or "ridge roll"
formed out of sheet metal, terra cotta, or even slate
(Figure 7). Snow guards, snow boards, and various types
of gutter and rake treatments also contributed to the
character of historic slate roofs (Figure 8).
Two types of valleys were traditionally employed, the
open valley and the closed valley. The open valley is lined
with metal over which slates lap only at the sides. Closed
valleys are covered with slate and have either a continuous
metal lining or metal flashing built in with each course.
Open valleys are easier to install and maintain, and are
generally more watertight than closed valleys. Round
valleys are a type of closed valley with a concave rather
than V-shaped section (Figure 9). Given the broader
sweep of the round valley, it was not uncommon for
roofers to interweave asphalt saturated felts rather than
copper sheet in the coursing in order to cut costs.
Figure 4. Widely spaced open slating was often used on utilitarian
structures where ventilation was desirable. It provided an interesting
texture and visual pattern to often plain structures.
Figure 5. This graduated slate roof is composed of large, thick slates at
the eave which are reduced in size and thickness as the slating
progresses to the ridge.
Although principally associated with graduated and
textural slate roofs, round valleys were infrequently
employed due to the difficulty and expense of their
Common types of sheathing used include wood boards,
wood battens, and, for fireproof construction on
institutional and government buildings, concrete or steel
(Figure 10). Solid wood sheathing was typically
constructed of tongue and groove, square edged, or
shiplapped pine boards of 1" (2.5 cm) or 1 1/4" (3 cm)
nominal thickness. Boards from 6" (15 cm) to 8" (20 cm)
wide and tongue and groove boards were generally
preferred as they were less likely to warp and curl.
Wood battens, or open wood sheathing, consisted of wood
strips, measuring from 2" (5 cm) to 3" (7.5 cm) in width,
nailed to the roof rafters. Spacing of the battens depended
on the length of the slate and equaled the exposure. Slates
were nailed to the batten that transected its mid-section.
The upper end of the slate rested at least 1/2" (1.25 cm) on
the batten next above. Open wood sheathing was employed
primarily on utilitarian, farm, and agricultural structures
in the North and on residential buildings in the South
where the insulating value of solid wood sheathing was
not a strict requirement. To help keep out dust and wind
driven rain on residential buildings, mortar was often
placed along the top and bottom edge of each batten, a
practice sometimes referred to as torching.
Various Roofing Details
Figure 6. Hips are formed at the external angle of two roofing slopes. In (a), the hips at both the roof and the dormer are covered with metal. Note also the
open valley and the built-in gutter. In (b), the dormer hip slates have been laid in a fantail pattern to help shed water. Note also the metal ridge cap.
Figure 7. Ridge caps and cresting can be elaborate. Ridges are formed at the long horizontal juncture of two roofing slopes and capping protects this joint
from moisture. In (a), a terra cotta capping with a decorative profile complements the finial over the various roof peaks. In (b), the mansard roof has a
decorative iron cresting at the break between the lower and the upper roof slopes.
Figure 8. Eai>e details include snow guards, snow boards, gutter treatments. Snow guards are generally used in areas where ice and snow accumulate to
avoid dangerous slides from the roof. In (a) the snow guards are set in two staggered rows above a pole gutter. In (b), the copper wire snow guards are set
more frequently up a very steep gable.
Figure 9. Vallei/s are formed at the internal angle of two roofing slopes. Flashing is often placed under the slate to increase moisture protect at this
vulnerable joint. Show in (a) is a closed vallex/ where the slates are held tight to the valley line, (b) illustrates of a round valley where the transition
between the two slopes is a continuous curve. It requires careful workmanship and an experienced roofer.
(a) TYPICAL SLATE
(b) TYPICAL SLATE ON OPEN LATH
(d) SLATE ON IRON PURLIN (NORCROSS)
Figure 10. (a) shows a typical slate; exposure may be calculated by
subtracting the headlap from the total length of the slate and dividing by
two. Slates were typically nailed either to closed wooden decking or to
open laths (b). In the late 19th century, with the concern for fireproof
construction, special fasteners were developed to secure slate to steel
purlins (c) 1881 patent to James G. Hill; (d) 1889 patent to Orlando W.
Steel angles substituted for the wood battens in fireproof
construction. The slates were secured using wire wrapped
around the steel angle, where it was twisted-off tight.
Alternately, any of a variety of special fasteners patented
over the years could have been used to attach the slate to
the steel angle (Figure 10). On roofs with concrete decks,
slates were typically nailed to wood nailing strips
embedded in the concrete.
Beginning in the late nineteenth century, asphalt saturated
roofing felt was installed atop solid wood sheathing. The
felt provided a temporary, watertight roof until the slate
could be installed atop it. Felt also served to cushion the
slates, exclude wind driven rain and dust, and ease slight
unevenness between the sheathing boards.
Slate was typically laid in horizontal courses starting at the
eaves with a standard headlap of 3" (7.5 cm) (Figure 10).
Headlap was generally reduced to 2" (5 cm) on Mansard
roofs and on particularly steep slopes with more than 20"
(50 cm) of rise per 12" (30 cm) of run. Conversely, headlap
was increased to 4" (10 cm) or more on low pitched roofs
with a rise of 8" (20 cm) or less per 12" (30 cm) of
horizontal run. The minimum roof slope necessary for a
slate roof was 4" (10 cm) of rise per 12" (30 cm) of run.
Where Does Slate Come From?
Slate is a fine grained, crystalline rock derived from
sediments of clay and fine silt which were deposited on
ancient sea bottoms. Superimposed materials gradually
consolidated the sedimentary particles into bedded
deposits of shale. Mountain building forces subsequently
folded, crumpled, and compressed the shale. At the same
time, intense heat and pressure changed the original clays
into new minerals such as mica, chlorite, and quartz. By
such mechanical and chemical processes bedded clays
were transformed, or metamorphosed, into slate, whole
Original block is crosscut with a wet diamond blade saw
A hammer and chisel are used to split the block to the proper dimension along the grain
Shingles or tiles are created with a hammer and chisel by systematically halving and
splitting the smaller block along the cleavage plane
Figure 11. Slate roofing tiles or shingles are still manufactured using
traditional methods brought to this country by Welsh immigrants in
the nineteenth century. Shown above are the first 3 steps of cutting,
sculping and splitting. Once the rough slate tile is made it is trimmed
and punched for holes.
geologic ages being consumed in the process. Slates vary
in composition, structure, and durability because the
degree to which their determinant minerals have been
altered is neither uniform nor consistent.
The adaptation of slate for roofing purposes is inextricably
linked to its genesis. The manufacturing processes of
nature have endowed slate with certain commercially
amenable properties which have had a profound influence
on the methods by which slate is quarried and fabricated
(Figure 11), as well as its suitability for use as a roofing tile.
Slate roofing tiles are still manufactured by hand using
traditional methods in a five step process: cutting,
sculping, splitting, trimming, and hole punching. In the
manufacturing process, large, irregular blocks taken from
the quarry are first cut with a saw across the grain in
sections slightly longer than the length of the finished
roofing slate. The blocks are next sculped, or split along
the grain of the slate, to widths slightly larger than the
widths of finished slates. Sculping is generally
accomplished with a mallet and a broad-faced chisel,
although some types of slate must be cut along their
grain. In the splitting area, the slightly oversized blocks
are split along their cleavage planes to the desired shingle
thickness. The splitter's tools consist of a wooden mallet
and two splitting chisels used for prying the block into
halves and repeating this process until the desired
thinness is reached (Figure 12). The last two steps involve
trimming the tile to the desired size and then punching
two nail holes toward the top of the slate using a formula
based on the size and exposure of the slate.
Minerals, the building blocks of rocks, through their
characteristic crystalline structures define the physical
properties of the rocks which they compose. Slate consists
of minerals that are stable and resistant to weathering and
is, therefore, generally of high strength, low porosity, and
low absorption. The low porosity and low absorption of
slate mitigate the deleterious action of frost on the stone
and make it well adapted for roofing purposes. The two
most important structural properties of slate are cleavage
The metamorphic processes of geologic change necessary
to produce slate are dependent upon movements in the
earth's crust and the heat and pressure generated thereby.
For this reason, slate is found only in certain mountainous
regions. The most economically important slate deposits
in this country lie in the Mid-Atlantic and Northeastern
states transversed by or bordering on the Appalachian
Mountain chain. Variations in local chemistry and
conditions under which the slate was formed have
Figure 12. In the splitting area, the slightly oversized blocks are split
along their cleavage planes. The splitter's tools consist of a wooden
mallet and splitting chisels. The process of halving the split portions
is repeated until a tile or shingle of appropriate thinness is obtained.
Figure 13. Paper thin lamination can be. seen flaking off of this
weathered, 120 year piece of Pennsylvania Hard-Vein slate.
produced a wide range of colors and qualities and
ultimately determine the character of the slate found in
Slate is available in a variety of colors. The most common
are grey, blue-grey black, various shades of green, deep
purple, brick red, and mottled varieties. The presence of
carbonaceous matter, derived from the decay of marine
organisms on ancient sea floors, gives rise to the black
colored slates. Compounds of iron generate the red,
purple, and green colored slates.
Generally, the slates of Maine, Virginia, and the Peach
Bottom district of York County, Pennsylvania are deep
blue-black in color. Those of Virginia have a distinctive
lustrous appearance as well due to their high mica
content. The slates of Lehigh and Northampton Counties,
Pennsylvania, are grayish-black in color. Green, red,
purple, and mottled slates derive from the New York-
Vermont district. The slate producing region of New York,
which centers around Granville and Middle Granville, is
particularly important because it contains one of the few
commercial deposits of red slate in the world.
Slates are also classified as fading or unfading according
to their color stability. Fading slates change to new shades
or may streak within a short time after exposure to the
atmosphere due to the presence of fine-grained
disseminated pyrite. For example, the "weathering green"
or "sea-green" slates of New York and Vermont are grayish
green when freshly quarried. Upon exposure, from 20% to
60% of the slates typically weather to soft tones of orange-
brown, buff, and gray while the others retain their original
shade. Slates designated as unfading maintain their
original colors for many years.
Color permanence generally provides no indication of
the durability of slate. Rather, time has shown that the
Vermont and New York slates will last about 125 years;
Buckingham Virginia slates 175 years or more; and
Pennsylvania Soft-Vein slates in excess of 60 years;
Pennsylvania Hard-Vein slates and Peach Bottom slates,
neither of which is still quarried, had life spans of roughly
100 and at least 200 years respectively. The life spans
provided should be used only as a general guide in
determining whether or not an existing slate roof is
nearing the end of its serviceable life.
Ribbons are visible as bands on the cleavage face of slate
and represent geologic periods during which greater
amounts of carbonaceous matter, calcite, or coarse quartz
particles were present in the sediment from which the
slate was formed. Ribbons typically weather more and
were most common in Pennsylvania slate quarries. As
they were not as durable as clear slates, ribbon slate is no
longer manufactured for roofing purposes. Mottled grey
slates from Vermont are the closest match for Pennsylvania
ribbon slate available today.
In recent years, slates from China, Africa, Spain and other
countries have begun to be imported into the United
States, primarily for distribution on the West Coast. The
use of imported slates should probably be limited to new
construction since their colors and textures often do not
match those of U.S. slate.
Deterioration of Slate and Slate Roofs
The durability of a slate roof depends primarily on four
factors: the physical and mineralogical properties of the
slate; the way in which it is fabricated; installation
techniques employed; and, regular and timely
maintenance. The first three of these factors are examined
below. The maintenance and repair of slate roofs are
discussed in later sections of this Brief.
The natural weathering of roofing slate manifests itself as
a slow process of chipping and scaling along the cleavage
planes (Figure 13). Paper thin laminations flake off the
surface of the slate and the slate becomes soft and spongy
as the inner layers begin to come apart, or delaminate.
The nature of the sound given off by a slate when tapped
with one's knuckles or slating hammer is a fair indication
of its condition. High-grade slate, when poised upon the
fingertips and struck, will emit a clear, solid sound.
Severely weathered slates are much less sonorous, and
give off a dull thud when tapped.
The weathering of slate is chiefly due to mineral
impurities (primarily calcite and iron sulfides) in the slate
which, in concert with alternating wet/dry and hot/cold
cycles, react to form gypsum (Figure 14). Because gypsum
Figure 14. The white blotches on these Pennsylvania Soft-Vein slates
indicate areas where gypsum is leaching out onto the surface of the
Figure 15. View of the underside of slates laid on open sheathing shows
that delamination and flaking is just as bad or worse on the underside of
slates as on the exposed surface. This is why most slates cannot be
flipped over for reuse.
molecules take up about twice as much volume as calcite
molecules, internal stresses result from the reaction,
causing the slate to delaminate. This type of deterioration
is as prominent on the underside of the roof as on the
exposed surface due to the leaching and subsequent
concentration of gypsum in this area (Figure 15).
Consequently, deteriorated roofing slates typically cannot
be flipped over and re-used.
The chemical and physical changes which accompany
slate weathering cause an increase in absorption and a
decrease in both strength and toughness. The tendency of
old, weathered slates to absorb and hold moisture can
lead to rot in underlying areas of wood sheathing. Such
rot can go undetected for long periods of time since, often,
there is no accompanying leak. Due to their loss of
strength, weathered slates are more prone to breakage,
loss of corners, and cracking.
Slates with low calcite content tend to weather slowly.
Dense slates, with low porosity, likewise decay slower
than slates with equal calcite, but with a greater porosity.
The pitch of a roof can also affect its longevity. The steeper
the pitch, the longer the slate can be expected to last as
water will run off faster and will be less likely to be drawn
under the slates by capillary action or driven under by
wind forces. Spires and the steep slopes of Mansard roofs
often retain their original slate long after other portions of
the roof have been replaced. Areas of a roof subject to
concentrated water flows and ice damming, such as along
eaves and valleys, also tend to deteriorate more rapidly
than other areas of the roof.
Mechanical agents, such as thermal expansion and
contraction and the action of frost, are subordinate in the
weathering of slate, coming into play only after the slate
has been materially altered from its original state by the
chemical transformation of calcite to gypsum. The more
rapid deterioration of slates found on roof slopes with the
most severe exposure to the sun, wind, and rain (typically,
but not always, a southern exposure) may be attributable
to the combined result of the deleterious effects of impurities
in the slate and mechanical agents. Atmospheric acids
produce only negligible deterioration in roofing slate.
It is difficult to assess the procedures by which a piece of
slate has been fabricated without visiting the quarry and
observing the process first hand. The location and size of
nail holes, grain orientation, the condition of corners, and
the number of broken pieces are all things which may be
observed in a shipment of slate to judge the quality of its
fabrication. Nail holes should be clean and with a shallow
countersink on the face of the slate for the nail head; grain
oriented along the length of the slate; and, corners left
whole. An allowance for 10% breakage in shipment is
typically provided for by the quarry.
Installation problems often involve the improper nailing
and lapping of slates. The nailing of slates differs from
that of other roofing materials. Slate nails should not be
driven tight as is the case with asphalt and wood shingles.
Rather, they should be set such that the slate is permitted
to hang freely on the nail shank. Nails driven too far will
crack the slate and those left projecting will puncture the
overlying slate (Figure 16). Nail heads left exposed accelerate
roof deterioration by providing a point for water entry.
Non-ferrous slater's nails, such as solid copper or stainless
steel, should always be used since plain steel and galvanized
nails will usually rust out long before the slate itself begins
to deteriorate. The rusting of nineteenth century cut nails
is a common cause of slate loss on historic roofs.
When joints are improperly broken (i.e., when slates lap
the joints in the course below by less than 3" [7.5 cm]), it
is possible for water to pass between the joints, through
the nail holes and ultimately to the underlying felt, where
it will cause deterioration and leaks to develop. Insufficient
headlap can also result in leaks as water entering the joints
between slates may have a greater tendency to be wind
blown beyond the heads of the slates in the course below.
Occasionally, individual slates are damaged. This may be
caused by falling tree limbs, ice dams in gutters, valleys,
and chimney crickets, the weight of a workman walking
on the roof, or a naturally occurring fault in the slate unit.
Whatever the form of damage, if it is caught soon enough,
the roof can usually be repaired or selectively replaced
and deterioration mitigated.
The ability to lay slate properly so as to produce a water-
tight and aesthetically pleasing roof requires training,
much practice, and the right tools (Figure 17). The
Figure 16. Detail view of a slate which has been punctured by the head
of a nail used to secure the slate in the course below. Likely, the nail was
not hammered in far enough when originally installed.
Figure 17. Slater's Tools. The cutter (a) is used to trim slate edges; the slate hammer (b) is used for hammering trails, trimming, cutting and punching
holes in slates, and pulling roofing nails; the steak (c) is a T-shaped piece of iron upon which the slater places the edge of a slate to be trimmed; and the
ripper (d) is slid under the slates to pull out the nails.
Figure 18. A method of securing replacement slates is shown. The copper strip is nailed to the roof sheathing just above the head of the slate in the
course below. The replacement slate is then inserted and the copper tab folded over its tail. This type of repair is not recommended for northern climates
where snow and ice can cause the copper tab to fold over, allowing the replacement slate to slide out of position as occurred here on the left.
(a) A ripper is used to remove the nails from the deteriorated slate.
(b) A replacement slate is slid into place.
(c) A new slate is secured in place with a copper nail.
(d) A copper bib is formed to protect the newly created nail hole.
(e) The bib is cut along its edges and bent into a concave shape to create a if) The slate hammer is used to push the bib in place over the nail head,
Figure 19. Above is a repair sequence for replacing a damaged slate.
The installation and repair of slate roofs should be entrusted
only to experienced slaters.
Repairing Slate Roofs
Broken, cracked, and missing slates should be repaired
promptly by an experienced slater in order to prevent
water damage to interior finishes, accelerated deterioration
of the roof and roof sheathing, and possible structural
degradation to framing members (Figures 18 and 19).
The damaged slate is first removed by cutting or pulling
out its nails with a ripper. If steel cut nails, rather than
copper nails, were used in laying the roof, adjacent slates
may be inadvertently damaged or displaced in the ripping
process, and these, too, will have to be repaired. If the
slate does not slide out by itself, the pointed end of the
slate hammer can be punched into the slate and the slate
dragged out. A new slate, or salvaged slate, which should
match the size, shape, texture, and weathered color of the
old slate, is then slid into place and held in position by
one nail inserted through the vertical joint between the
slates in the course above and approximately one inch
below the tail of the slate two courses above. To prevent
water penetration through the newly created nail hole, a
piece of copper with a friction fit, measuring roughly 3"
(7.5 cm) in width and 8" (20 cm) in length, is slid lengthwise
under the joint between the two slates located directly
above the new slate and over the nail. Alternate methods
for securing the replacement slate include the use of metal
hooks, clips, and straps that are bent over the tail end of
the slate. The application of roofing mastic or sealants to
damaged slates should not be considered a viable repair
alternative because these materials, though effective at first,
will eventually harden and crack, thereby allowing water to
enter (Figure 20). Mastic also makes future repairs more
difficult to execute, is unsightly, and, when applied to
metal flashings, accelerates their corrosion.
When two or more broken slates lie adjacent to each
other in the same course, or when replacing leaky valley
flashings, it is best to form pyramids (i.e., to remove a
diminishing number of slates from higher courses) to keep
the number of bibs required to a minimum. When
re-installing the slates, only the top slate in each pyramid
will need a bib. Slates along the sides of the pyramid will
receive two nails, one above the other, along the upper
part of its exposed edge.
When many slates must be removed to effect a repair, the
sheathing should be checked for rotted areas and projecting
nails. Plywood is generally not a good replacement
material for deteriorated wood sheathing due to the
relative difficulty of driving a nail through it (the bounce
produced can loosen adjacent slates). Instead, new wood
boards of similar width and thickness to those being
replaced should be used. Because the nominal thickness
of today's dimension lumber is slightly thinner than that
produced in the past, it may be necessary to shim the new
wood boards so that they lie flush with the top surface of
adjacent existing sheathing boards. Pressure treated
lumber is not recommended due to its tendency to shrink.
This can cause the slates to crack and become displaced.
To permit proper re-laying of the slate, the new roof
sheathing must be of smooth and solid construction. At
least two nails should be placed through the new boards
at every rafter and joints between the ends of the boards
should occur over rafters. Insufficient nailing will cause
the boards to be springy, making nailing of the slates
difficult and causing adjacent slates to loosen in the
process. Unevenness in the sheathing will show in the
finished roof surface and may cause premature cracking of
the slate. Roof sheathing in valleys and along hips, ridges,
and eaves may be covered with waterproof membrane
underlayment rather than roofing felt for added protection
In emergency situations, such as when severe hurricanes
or tornadoes blow numerous slates off the roof, a
temporary roof covering should be installed immediately
after the storm to prevent further water damage to the
interior of the building and to permit the drying out
process to begin. Heavy gauge plastic and vinyl tarpaulins
are often used for this purpose, though they are difficult
to secure in place and can be blown off in high winds. Roll
roofing, carefully stitched in to areas of the remaining
roof, is a somewhat more functional solution that will
allow sufficient time to document the existing roof
conditions, plan repairs, and order materials (Figure 21).
Slate roof repair is viable for localized problems and
damaged roofs with reasonably long serviceable lives
remaining. If 20% or more of the slates on a roof or roof
slope are broken, cracked, missing, or sliding out of
position, it is usually less expensive to replace the roof
than to execute individual repairs. This is especially true
of older roofs nearing the end of their serviceable lives
—r— -^ • — f ™ \ -r ■ - ■ . ' —- <*
■/JW^ ^ i— ■ * ■■ ■ s . — ' =-*— — i — * ■ — • ' III > ) l
Figure 20. This roof has been poorly repaired numerous times in the
past. The installation of mastic to seal out moisture has orily exacerbated
the problem. A timely repair and good maintenance could have extended
the life of this roof.
Figure 21. As a result of hurricanes and other disasters, it may be necessanf to temporarily stabilize a roof until materials can be obtained and a qualified
roofing contractor hired. Heai>y roofing felt was stitched into this slate roof to stop moisture penetration until matching slate was obtained for repair.
Significant slate roofs should not be stripped off and replaced with asphalt shingles. Photograph courtesy of the National Park Service.
because even the most experienced slater will likely
damage additional slates while attempting repairs.
Depending on the age of the slate, its expected serviceable
life, and the cause(s) of deterioration, it may or may not be
cost effective to salvage slates. Where deteriorated nails or
flashings are the cause of the roof failure, salvage of at
least some slates should be possible for use in repairs.
When salvaging slates, each must be sounded to discover
cracks and faults and the degree to which it has
weathered. It is usually wise to salvage slates when only
a portion of the roof is to be replaced. In this way, the
salvaged slates may be used for future repairs to the
remaining sections of the roof.
The Replacement of Deteriorated Roofs
Historic slate roofs should be repaired rather than
replaced whenever possible. Before replacing a slate roof,
check for isolated damage, corroded and worn flashings,
leaky gutters, poor ventilation in the attic, and other
possible sources of moisture. All too often slate roofs are
mistakenly replaced when, in fact, they could have been
effectively repaired. Deciding whether an historic slate
roof should be repaired or replaced can be difficult and
each roof must be judged separately (see guidance in
shaded box on page 16).
If repair is not possible and a new slate roof must be
installed, it is important to remember that more than just
the replacement of the slate is involved (Figure 22). The
old slate should be removed to prevent overloading of the
roof timbers. Stripping should be done in sections, with
felt installed, to avoid exposing the entire sub-roof to the
weather. In the process, rotted wood sheathing should be
replaced and the roof timbers checked for signs of stress,
including deflection, cracking, and twisting. If such
conditions are found, a structural engineer experienced in
working with older buildings should be consulted. Other
repairs, such as chimney repointing, which may require
access to the roof should be completed before the new
roof is put on.
Drawings and specifications for a new slate roof should be
prepared by a restoration architect, especially if the project
is going to be competitively bid or if the roof is particularly
complex. Standard specifications, like those published in
1926 by the National Slate Association may be used as a
basis for developing specifications appropriate for a
particular project. The specifications and drawings should
contain all the information necessary to replicate the
original appearance of the roof as closely as possible.
Certain changes may have to be accepted, however, since
several types of slate once prominent in this country, such
as ribbon slate, are no longer quarried. It is wise to
(a) Historic documentation was necessary to determine the historic
(b) Scaffolding was installed early to document existing conditions to
determine extent of work.
(c) Slate zoas removed in sections to avoid stress on timber framing.
(d) Deteriorated sheathing zoas replaced and rafter tails were reinforced.
V iiE j
(e) Roofing felt was installed over decking; a rubberized membrane was used (f) Lead coated copper flashing zoas installed throughout. Seen is th
selectively at the eaves and under some flashing. offset between the portico roof and the main roof in progress.
Figure 22. Installing a nezv roof involves more than just slate. Above is a sequence of views of the roof replacement at Arlington House. Photographs
courtesy of the National Park Service. (Sequence continued on page 14.)
(g) Masonry chimneys were repaired and metal crickets were fabricated
at the chimneys.
(h) This installation pattern allows the slates to be laid in courses
leaving a temporary path of travel to avoid stepping on installed slates.
(i) Although the gutters and snow boards were the last elements
installed, their support brackets were installed as the slates were laid.
anticipate the replacement of older roofs so that proper
planning can be undertaken and financial resources set
aside, thereby, reducing the likelihood of rash last minute
Roofing slate is sold by the square in the United States.
One square is enough to cover 100 square feet (13.3 square
meters) of plain roof surface when laid with a standard
headlap of 3" (7.5cm). When ordering slate, considerable
lead time should be allowed as delivery may take
anywhere from 4 to 12 weeks and even as long as 1 year
for special orders. Orders for random widths of a
particular slate can generally be filled more quickly than
orders for fixed widths. Once on site, slates should be
stored on edge, under cover on pallets.
A roof and its associated flashings, gutters, and
downspouts function as a system to shed water. Material
choices should be made with this in mind. For example,
use a single type of metal for all flashings and the
rainwater conductor system to avoid galvanic action.
Choose materials with life spans comparable to that of
the slate, such as non-ferrous nails. Use heavier gauge
flashings or sacrificial flashings in areas that are difficult
to access or subject to concentrated water flows.
Flashings are the weakest point in any roof. Given the
permanence of slate, it is poor economy to use anything
but the most durable of metals and the best workmanship
for installing flashings. Copper is one of the best flashing
materials, and along with terne, is most often associated
with historic slate roofs. Copper is extremely durable, easily
worked and soldered, and requires little maintenance.
Sixteen-ounce copper sheet is the minimum weight
recommended for flashings. Lighter weights will not
endure the erosive action of dust and grit carried over the
roof by rain water. Heavier weight, 20 oz. (565 grams) or
24 oz. (680 grams), copper should be used in gutters,
valleys, and areas with limited accessibility. Lead coated
copper has properties similar to copper and is even more
durable due to its additional lead coating. Lead coated
copper is often used in restoration work.
Terne is a less desirable flashing material since it must be
painted periodically. Terne coated stainless steel (TCS) is a
modern-day substitute for terne. Although more difficult
to work than terne, TCS will not corrode if left unpainted;
a great advantage, especially in areas that are difficult to
Once a metal is chosen, it is important to use it throughout
for all flashings, gutters, downspouts, and metal roofs.
Mixing of dissimilar metals can lead to rapid corrosion of
the more electronegative metal by galvanic action. Where
flashings turn up a vertical surface, they should be covered
with a cap flashing. Slates which overlap metal flashings
should be nailed in such a manner as to avoid puncturing
the metal. This may be accomplished by punching a
second hole about 2" (5cm) above the existing hole on the
side of the slate not overlapping the metal flashing. It is
important that holes be punched from the back side of the
slate. In this way, a shallow countersink is created on the
face of the slate in which the head of the nail may sit.
I Continuation of the roof repair sequence of Arlington House from page 13.
Figure 23. Slate roofs should not be walked on if at all possible. For
large projects, lifts can be used to inspect roofs periodically to assess
maintenance needs. Photograph courtesy of the National Park Service.
The use of artificial, mineral fiber slate is not recommended
for restoration work since its rigid appearance is that of a
man-made material and not one of nature. Artificial slates
may also have a tendency to fade over time. And, although
artificial slate costs less than natural slate, the total initial
cost of an artificial slate roof is only marginally less than a
natural slate roof. This is because all the other costs
associated with replacing a slate roof, such as the cost of
labor, flashings, and tearing-off the old roof, are equal in
both cases. Over the long term, natural slate tends to be a
better investment because several artificial slate roofs will
have to be installed during the life span of one natural
Clear roof expanses can be covered by an experienced
slater and one helper at the rate of about two to three
squares per day. More complex roofs and the presence of
chimneys, dormers, and valleys can bring this rate down
to below one square per day. One square per day is a
good average rate to use in figuring how long a job will
take to complete. This takes into account the installation of
flashings and gutters and the set-up and break-down of
scaffolding. Tear-off of the existing roof will require
Given the relatively high initial cost of installing a new
slate roof, it pays to inspect its overall condition annually
and after severe storms. For safety reasons, it is
recommended that building owners and maintenance
personnel carry out roof surveys from the ground using
binoculars or from a cherry picker (Figure 23). Cracked,
broken, misaligned, and missing slates and the degree to
which delamination has occurred should be noted, along
with failed flashings (pin holes, open seams, loose and
misaligned elements, etc.) and broken or clogged
downspouts. A roof plan or sketch and a camera can aid
in recording problems and discussing them with
contractors. In the attic, wood rafters and sheathing
should be checked for water stains and rot. Critical areas
are typically near the roof plate and at the intersection of
roof planes, such as at valleys and hips. Regular
maintenance should include cleaning gutters at least twice
during the fall and once in early spring, and replacing
damaged slates promptly. Every five to seven years
inspections should be conducted by professionals
experienced in working with slate and steep slopes. Good
record keeping, in the form of a log book and the
systematic filing of all bills and samples, can help in
piecing together a roof's repair history and is an important
part of maintenance.
As part of regular maintenance, an attempt should be
made to keep foot traffic off the roof. If maintenance
personnel, chimney sweeps, painters, or others must walk
on the roof, it is recommended that ladders be hooked
over the ridge and that the workmen walk on the ladders
to better distribute their weight. If slates are to be walked
on, it is best to wear soft soled shoes and to step on the
lower-middle of the exposed portion of the slate unit.
Slate roofs are a critical design feature of many historic
buildings that cannot be duplicated using substitute
materials (Figure 24). Slate roofs can, and should be,
maintained and repaired to effectively extend their
serviceable lives. When replacement is necessary, details
contributing to the appearance of the roof should be
retained. High quality slate is still available from reputable
quarries and, while a significant investment, can be a cost
effective solution over the long term.
Copper And Brass Research Association. Copper Flashings. 2nd ed.
New York: Copper And Brass Research Association, 1925.
Dale, T. Nelson, and others. Slate In The United States, Bulletin
586. Washington, D.C.: U.S. Department of the Interior, United
States Geological Survey, 1914.
Heim, David. "Roofing With Slate." Fine Homebuilding, No. 20
(April/May 1984): 38-43.
Levine, Jeffrey S. "Slate Roofs For Historic Religious Buildings."
Inspired. Philadelphia: Philadelphia Historic Preservation
"Slate Quarrying and Shingle Manufacture." Fine Homebuilding,
No.71 (Jan. 1992): 64-68.
McKee, Harley J. "Slate Roofing." APT Bulletin, Vol. 2, Nos. 1-2
National Slate Association. Slate Roofs. 1925. Reprint. Fair Haven,
Vermont: Vermont Structural Slate Co., Inc., 1977.
Pierpont, Robert N. "Slate Roofing." APT Bulletin, Vol. 19, No. 2
Sweetser, Sarah M. "Roofing for Historic Buildings." Preservation
Briefs, No. 4. Washington, D.C: U.S. Department of the
Interior, Technical Preservation Services Division, 1975.
Repair/ Replacement Guideline
The following guideline is provided to assist in the
repair/replace decision making process:
1. Consider the age and condition of the roof versus
its expected serviceable life given the type of slate
2. Calculate the number of damaged and missing
slates. Is the number less than about 20%? Is the
roof generally in good condition? If so, the roof
should be evaluated for repair rather than
replacement. Also, keep in mind that the older a
roof becomes, the more maintenance it will likely
3. Determine if there are active leaks and what their
source may be. Do not assume the slates are
leaking. Gutters, valleys and flashings are more
likely candidates. "False leaks" can be caused by
moisture condensation in the attic due to improper
4. Check the roof rafters and sheathing for moisture
stains. Poke an awl into the wood to determine if it
is rotted. Remember that very old, delaminating
slates will hold moisture and cause adjacent wood
members to deteriorate even if there are no
5. Are many slates sliding out of position? If so, it
may be that ferrous metal fasteners were used and
that these are corroding, while the slates are still in
good condition. Salvage the slates and re-lay them
on the roof. If the slates have worn around the
nails holes, it may be necessary to punch new
holes before re-laying them.
6. Consider the condition of the roof's flashings.
Because slate is so durable, metal flashings often
wear out before the slate does. Examine the
flashings carefully. Even the smallest pinhole can
permit large quantities of water to enter the
7. Is the deterioration of the slate uniform? Often this
is not the case. It may be that only one slope
needs replacement and the other slopes can be
repaired. In this way, the cost of replacement can
be spread over many years.
8. Press down hard on the slates with your hand.
Sound slates will be unaffected by the pressure.
Deteriorated slates will feel brittle and will crack.
Tap on slates that have fallen out or been removed.
A full, deep sound indicates a slate in good
condition, while a dull thud suggests a slate in
9. Are new slates readily available? Even if replacement
is determined to be necessary, the existing roof
may have to be repaired to allow time for
documentation and the ordering of appropriate
3 1604 011 830 546
Figure 24. Although slate replacement roofs are expensive, the superiorit
of materials and craftsmanship will give years of continued service. If
amortized over the life of the roof, the replacement cost can be very
reasonable. Photograph courtesy of the National Park Service.
Note: measurements in this publication are given in both
U.S. Customary System and International (Metric) System
for comparative purposes. Metric conversions are in some
cases approximate and should not be relied upon in
preparing technical specifications.
The author, Jeffrey S. Levine, is an Architectural Conservator with John Milner
Associates, Inc., and gratefully acknowledges the technical review of this
publication by the following: Russel Watsky, Watsky Associates; Kenton Lerch,
The Structural Slate Company; Matt Millen, Millen Roofing Co.; Alex
Echeguren, Echeguren Slate Company; Bill Markcrow, Vermont Structural Slate
Company; and Dick Naslund, Department of Geological Sciences, State
University of New York at Binghamton. In addition, invaluable comments were
provided by Sharon Park, Doug Hicks and Michael J. Auer, National Park
Service; Suzanna Barucco, Martin Jay Rosenblum, R.A. & Associates; and Fred
Walters, John Milner Associates, Inc. All photographs are by the author unless
Sharon C. Park, AIA, Senior Historical Architect, Preservation Assistance
Division, National Park Service, is credited with directing the development of
this publication and with its technical editorship. This publication has been
prepared pursuant to the National 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 H. Ward Jandl, Chief, Technical Preservation Services
Branch, Preservation Assistance Division, National Park Service, P.O. Box 37127,
Washington, DC. 20013-7127. Drawings for this publication were prepared by
Karin Murr Link. 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.
Cover Photograph: A portion of an advertisement for Slatington-Bangor
Slate Syndicate (Slatington, PA) which appeared in the July 1910 issue
of Building Age (Vol.32 No.7).