Preservation Briefs: 29 The Repair, Replacement And Maintenance Of Historic Slate Roofs

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Preservation Briefs 29, National Park Service, Pad
Thermal And Moisture Protection
Slate Shingles
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The link immediately below connects to the latest version of Preservation Brief 29:

Jeffrey S. Levine

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

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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

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.


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 fireproof

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.  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.


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.  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.  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.

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

Graduated slate roofs were frequently installed on large
institutional and ecclesiastical structures.  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 ridge, and comb ridge,
and for hips, the saddle hip, mitered hip, Boston hip, and fantail
hip.  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.  Snow guards, snow boards,
and various types of gutter and rake treatments also contributed to
the character of historic slate roofs.

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.  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.  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.

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.  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

Slate was typically laid in horizontal courses starting at the
eaves with a standard headlap of 3" (7.5 cm).  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.


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, 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, sculpturing,
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
sculptured, or split along the grain of the slate, to widths
slightly larger than the widths of finished slates.  Sculpturing 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.  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 gram.

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 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, 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
orangebrown, 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

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.


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.  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.  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.  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 personally.  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.  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 to produce a watertight and
aesthetically pleasing roof requires training, much practice, and
the right tools.  The installation and repair of slate roofs should
be entrusted only to experienced slaters.


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.

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.  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.  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.

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
the roof.



Last Reviewed 2012-09-05