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The Repair, Replacement, and
Maintenance of Historic Slate Roofs
Jeffrey S. Levine
Introduction
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 handshaping 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 16251650 and 16401670. 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
fireproof construction.
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.
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 Slate Roofs
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 multicolored 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 be retained.
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).
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 colors.
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 rid~e 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 Vshaped 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.
Although principally
associated with graduated and textural slate roofs, round
valleys were infrequently employed due to the difficulty and
expense of their installation.
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 midsection. The upper end of the slate
rested at least ‡" (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.
Steel angles substituted
for the wood battens in fireproof construction. The slates
were secured using wire wrapped around the steel angle, where
it was twistedoff 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 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 broadfaced
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 and grain.
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 MidAtlantic 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 produced a
wide range of colors and qualities and ultimately determine
the character of the slate found in these areas.
Slate is available in a
variety of colors. The most common are grey, bluegrey, 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 blueblack 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 grayishblack 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 finegrained disseminated pyrite. For example,
the "weathering green" or "seagreen" 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 SoftVein slates in excess of
60 years; Pennsylvania HardVein 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. Highgrade 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 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 reused.
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.
Nonferrous 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 watertight and aesthetically
pleasing roof requires training, much practice, and the right
tools (Figure 17). 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 (Figure
18).
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 (Figure 19). 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 reinstalling 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 relaying
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 against
leakage.
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 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 subroof to the
weather. ln 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 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 decisions.
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 nonferrous 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. Sixteenounce 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 modernday 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 access.
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.
The use of artificial,
mineral fiber slate is not recommended for restoration work
since its rigid appearance is that of a manmade 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
tearingoff 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 slate
roof.
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 setup and
breakdown of scaffolding. Tearoff of the existing roof will
require additional time.
Maintenance
Given the relatively high
initial cost of installing a new slate roof, it pays to
inspect its overall condition annually and after several
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 lowermiddle of the exposed portion
of the slate unit.
Conclusion
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.
Further Reading
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):
3843.
Levine, Jeffrey S. "Slate
Roofs For Historic Religious Buildings." Inspired.
Philadelphia: Philadelphia Historic Preservation Corporation,
1987.
----------, "Slate
Quarrying and Shingle Manufacture" Fine Homebuilding No. 71
(Jan. 1992): 6468.
McKee, Harley 1. "Slate
Roofing." APT Bulletin, Vol. 2, Nos. 1-2 (1970): 7784.
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 (1987):
1023.
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. [SIDEBAR]
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
employed.
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 require.
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 ventilation.
4. Check the roof rafters
and sheathing for moisture stains. Poke an awl into the wood
to determine if * is rotted. Remember that very old,
delaminating slates will hold moisture and cause adjacent
wood members to deteriorate even if there are no apparent
leaks.
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
relay them on the roof. If the slates have worn around the
nails holes, it may be necessary to punch new holes before
relaying 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 building. 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.
7. 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 poor condition.
8. 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 replacement
slates.
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.
Acknowledgements
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;
Suzanne Barucco, Martin Jay Rosenblum, R.A. & Associates;
and Fred Walters, John Milner Associates, Inc. All
photographs are by the author unless otherwise noted.
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. Washington, D.C. September, 1992
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